Plasma Wakefield Accelerator Research 2019 - 2040: A community-driven UK roadmap compiled by the Plasma Wakefield Accelerator Steering Committee (PWASC)
Bernhard Hidding, Simon Hooker, Steven Jamison, Bruno Muratori, Christopher Murphy, Zulfikar Najmudin, Rajeev Pattathil, Gianluca Sarri, Matthew Streeter, Carsten Welsch, Matthew Wing, Guoxing Xia
PPlasma Wakefield Accelerator Research2019–2040
A community-driven UK roadmap compiled by thePlasma Wakefield Accelerator Steering Committee (PWASC)
March 2019
Bernhard Hidding, Simon Hooker, Steven Jamison, Bruno Muratori, Christopher Murphy,Zulfikar Najmudin, Rajeev Pattathil, Gianluca Sarri, Matthew Streeter, Carsten Welsch,Matthew Wing, & Guoxing Xia a r X i v : . [ phy s i c s . acc - ph ] A p r K Roadmap for Plasma Wakefield Accelerator Research
CONTENTS
Contents
UK Plasma Wakefield Accelerator Steering Committee iK Roadmap for Plasma Wakefield Accelerator Research
CONTENTS
10 Summary 29A Consultation 31B List of abbreviations used 31
UK Plasma Wakefield Accelerator Steering Committee iiK Roadmap for Plasma Wakefield Accelerator Research
The acceleration gradients generated in a laser- or beam-driven plasma wakefield accelerator are typically three ordersof magnitude greater than those produced by a conventional accelerator. Plasma accelerators can therefore open aroute to a new generation of very compact accelerators in a technological transformation comparable to that enabledby the switch from bulky vacuum tubes to transistors. In addition, plasma-based accelerators can generate beamswith unique properties, such as tens of kiloamp peak currents, attosecond bunch duration, ultrahigh brightness andintrinsic particle beam-laser pulse synchronization.Plasma wakefield accelerators have already generated electron beams with GeV energies — comparable to thebeam energy used in a stadium-sized synchrotron — from accelerator stages as short as a few centimetres. Thesebeams have been used to produce femtosecond-duration pulses of radiation from visible to hard X-ray wavelengthswhich themselves have been used, for example, to make tomographic images of biological samples and to captureshock propagation in laser-shocked materials.It is clear that plasma accelerators have the potential to be a disruptive technology with broad impact in science,medicine and technology. The ability to drive multiple, synchronised sources of energetic, high-brightness particlesand X-ray or γ -ray photons from compact machines will enable new opportunities in ultrafast science and offers thepotential, for example, to provide medical diagnosis and treatment from a single, laboratory-scale facility. Generationof neutral electron–positron plasmas offers the prospect of recreating extreme astrophysical environments and thestudy of these spectacular events in the laboratory. In the longer term, plasma accelerators could provide a way torealise high-energy physics colliders required at the forefront of physics.These transformative prospects motivate the rapidly increasing number of laboratories around the world whichnow engage in plasma accelerator R&D, with major projects in Europe, the USA, China, Korea, and Japan. The UKgroups have very high international standing in this field, which is based on an impressive, decades-long record ofachievement. UK groups continue to be at the forefront of new developments in plasma accelerators, and for thisreason UK scientists play major roles in almost all of the international research projects in this field. Nevertheless,this position is threatened by a known lack of investment in national and university-scale research facilities, and bythe increase in research investment by other countries.Plasma accelerator research in the UK grew out of pioneering research by university groups, in partnershipwith national laboratories. As plasma accelerators mature and move from being the object of study to a driver ofapplications, as the number of UK groups working in the field has grown, and as merging with conventional acceleratorscience and the influence on other fields of science picks up pace, it has become clear that work on plasma acceleratorsrequires more coordination. The research community has responded to this need by the establishing the UK-widePlasma Wakefield Accelerator Steering Committee (PWASC, http://pwasc.org.uk/) to help coordinate the activitiesof key stakeholders. Members of PWASC are drawn from UK research groups at universities, the Central LaserFacility, the Accelerator Science and Technology Centre, and the two Accelerator Science Institutes.The need for a roadmap for plasma accelerator research was identified, a draft roadmap was produced by membersof PWASC and other members of the UK community; feedback and further input on this was gathered at a CommunityMeeting held in January 2018, and feedback on a second full draft was obtained via email at the end of 2018.Community members also contributed to STFC’s 2017 Accelerator Strategic Review Report, the consultation periodfor which coincided with the development of the present roadmap. While the STFC report covers all aspects ofaccelerator science, the present roadmap provides a more detailed consideration of plasma accelerator research. Itshould also be emphasised that this roadmap focuses on plasma wakefield acceleration and does not discuss in detailvery important, and closely related, fields such as ion acceleration and dielectric accelerators.In the sections below we review the state of the art, outline potential applications, describe the research anddevelopment required to enable those applications, and discuss synergies with related areas of research. We alsoset-out the resources required to realise these ambitions and provide a timeline for advances in the key areas.This roadmap contains the following recommendations, with the ordering as they appear in the main text and noranking implied. UK Plasma Wakefield Accelerator Steering Committee
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Recommendations
1. A facility with target areas and beamlines dedicated to research on laser- and hybrid laser-driven plasmaaccelerators and their applications should be developed at CLF; these facilities should be state of the art interms of beam stability, and should take advantage of the UK’s lead in high-average power laser technology toallow operation at repetition rates above 10 Hz.2. The development and operation of UK university-scale labs for laser-plasma accelerator research and applica-tions, including e.g. SCAPA beamlines, should to be supported and exploited.3. The UK should aim to play a key role, and support the pan-European EuPRAXIA initiative for a EuropeanPlasma Research Accelerator with eXcellence In Applications.4. A dedicated plasma wakefield acceleration beamline should be developed at CLARA/ASTeC.5. The UK should provide substantial, and increased, investment in the AWAKE Run 2 (2021–4) programme.6. Programmatic funding of plasma wakefield accelerators should be provided to optimise the quality and stabilityof plasma-accelerated particle bunches, to increase pulse repetition rates, and to enable key applications in theindustrial sector and in fundamental science.7. UKRI should develop mechanisms for providing cross-council support for the wide range of research, in avariety of settings, necessary to drive advances in plasma accelerators; this range includes fundamental research(e.g. plasma physics), technology development (e.g. novel lasers), and application development (e.g. medicalimaging).8. Mechanisms should be sought to allow UK groups to play leadership roles in international high-visibility col-laborations such as SLAC FACET-II, Helmholtz ATHENA, Laserlab Europe, ELI, ARIES etc., and to exploitthese.9. A national scheme should be developed to enable mobility and knowledge transfer within UK institutions, toincrease beam access, and to sustain collaborative efforts; this would provide a means to test new concepts,train students, and prepare for beam time at national and international facilities.10. A new "Novel Accelerator Fellowship" scheme should be developed and the support available for training PhDstudents in novel accelerators should be increased.
UK Plasma Wakefield Accelerator Steering Committee
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Particle accelerators play a central role in numerous areas of science and technology. They have been instrumental inkey discoveries in particle physics that have enabled our understanding of the building blocks of the universe, and inthis sphere alone they have played a vital role in discoveries which led to tens of Nobel prizes at a rate of approximatelyone every 3 years. Accelerators drive the latest generation of light sources, which deliver unprecedented informationabout the structure of materials from the structure of complex proteins, to tumour identification, and understandingthe function of high performance materials. In medicine, particle accelerators are used for imaging and treatment, aswell as to generate radio-isotopes. In the realm of national security, particle accelerators are used for cargo inspectionand in airport scanners.Plasma-based accelerators are important because they can accelerate particles to high energies in distances athousand times shorter than conventional radio-frequency (RF) accelerators (see Fig. 1). This fact alone promisesa technological transformation comparable to that enabled by the switch from bulky vacuum tubes to transistors.Plasma-based accelerators are also important because they generate beams with unique properties, such as high peakcurrents, femtosecond to attosecond bunch duration, low emittance and ultrahigh brightness. They therefore offerthe prospect of a new generation of very compact accelerators with several potential near-term applications, such asdriving next generation light sources or studying high field effects; in the longer term they may offer a route to thebeam energies required for future particle colliders.This roadmap has been prepared by the UK Plasma Wakefield Accelerator Steering Committee (PWASC), withinput from all those working in the UK on this topic (Appendix A describes how input from the community wasobtained). In this roadmap we review the state of the art, outline potential applications, describe the research anddevelopment required to enable those applications, and discuss synergies with related areas of research. We alsoset-out the resources which would be required to realise these ambitions and provide a timeline for advances inthe key areas. The roadmap concludes with recommendations for research funding, provision of experimental andcomputational facilities, training, and international collaboration.
It is important to emphasise that the remit of PWASC is plasma wakefield acceleration. As such theroadmap focuses on this area and does not discuss in detail very important, and closely related, synergisticfields such as ion acceleration and dielectric accelerators.
Plasma wakefield acceleration can be driven by intense laser pulses or by particle bunches. In contrast to conventionallinacs, where a series of static cavities accelerate particles, plasma-based cavities co-propagate with the driving pulseswith approximately the speed of light. There are several flavours of plasma wakefield acceleration, each with its ownadvantages and challenges. This variety allows many of the major goals of accelerator R&D, and their applications,to be addressed. The transformative prospects of plasma acceleration are reflected by the increasing number of labsaround the world, including national facilities, which now work on plasma acceleration, as summarised in Fig. 2.There are two major approaches of plasma wakefield acceleration: laser-driven plasma wakefield aocceleration andparticle-beam driven plasma wakefield acceleration. These are discussed briefly below.
In laser wakefield accelerators (LWFAs) the plasma wave is driven by one or more intense pulses of laser radiation. Laserwakefield acceleration has made rapid technical and conceptual progress, not least since the milestone demonstrationsof quasi-monoenergetic beams, the first generation of GeV-scale beams and first LWFA-based undulator radiation —all examples of world-leading breakthroughs made with significant involvement from UK groups.
The driving lasersystems are reasonably compact, and (for low pulse repetition rates) are commercially available. It is also relativelyeasy to inject electrons from the plasma itself, and to accelerate them to energies of tens of MeV to a few GeV ingas jets, gas cells, or plasma channels.A defining feature of LWFAs is that the driving laser pulse propagates at its group velocity, which is slightlybelow c . As such relativistic particle bunches gradually overtake the plasma wakefield and eventually move into adecelerating phase, a process known as “dephasing”.In most regimes the dephasing length is much longer than the distance over which the laser pulse naturallydiffracts, i.e. the Rayleigh range. For example, for laser focal spot sizes in the range − µ m , the Rayleigh range UK Plasma Wakefield Accelerator Steering Committee
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Figure 1: Field gradients of different accelerator technologies and their relation to wavelength and frequency of theaccelerating field.is 0.3 – 30 mm. In comparison, a 10 GeV accelerator requires a plasma length of around 1 m.Broadly speaking LWFAs operate in one of two regimes. In the quasi-linear regime the plasma wave is approxim-ately sinusoidal which means that it can provide nearly equal size phases for accelerating and focusing particles ofpositive or negative charge. In this regime the driving laser pulse must be guided over the length of the acceleratorby an external waveguide. To date, guiding by grazing-incidence reflections in a hollow capillary or gradient refractiveindex guiding in plasma channels has been employed. LWFAs driven in 200 mm long plasma channels have been usedto reach an electron energy of 7.8 GeV.
In the highly non-linear, or “bubble”, regime the laser intensity is so high that essentially all the plasma electronsare expelled from the region immediately behind the driving laser pulse. In this regime the laser pulse is self-guided bythe relativistic response of the plasma and the transverse density gradients within the bubble structure. Self-guidingover tens of millimetres of plasma to generate few-GeV electron beams has been demonstrated.The particle bunches generated by LWFAs have pulse durations of order a few femtoseconds, charges in therange 10 – 1000 pC, a normalised emittance of order 1 mm mrad, and a relative energy spread in the range 1 –10 %. However, since in most work to date the electrons are injected into the wakefield by stochastic processes,the shot-to-shot jitter in these parameters can be high. There is therefore considerable effort underway to developmethods for controlling the processes by which electrons are injected and trapped in the plasma wave; the ability tocontrol the injection process will not only reduce shot-to-shot jitter, but also improve (and enable control of) thebunch parameters.Owing to dephasing, diffraction and/or energy depletion of the laser driver, it is likely that multiple LWFAs willneed to be coupled together to reach particle energies much in excess of 10 GeV. In order to reduce the length of theoptical system used to couple in the driving laser pulse into each stage, tape-based plasma mirror systems have beendeveloped. In these, a thin tape is placed between the two stages; particles from the previous stage pass throughthis, but the drive pulse for the second stage is reflected into the second stage by the plasma it forms on the surfaceof the tape. It has been demonstrated that this approach — together with the use of compact active plasma lenses,based on capillary discharges — can couple two 100 MeV-scale plasma stages in a distance of a few centimetres (see§5.3).
In plasma wakefield accelerators (PWFAs) the driver is an intense bunch of particles. An important feature of PWFAsis that acceleration can be maintained over a long distance since, unlike with a laser driver, dephasing is not relevant
UK Plasma Wakefield Accelerator Steering Committee
4K Roadmap for Plasma Wakefield Accelerator Research (for relativistic electron beam drivers) and the driving beam does not diverge strongly. Further, building on decadesof development of conventional accelerators, high-energy particle beam drivers are available which can operate athigh repetition rates, with excellent stability, and with good wall-plug efficiency. Indeed, to their mutual benefit,advances in conventional accelerators provide a technological push which is matched by the scientific pull providedby the demands of PWFAs.A distinct challenge faced by PWFAs is that the rate of progress is limited by the small number of facilitiesavailable since large scale conventional accelerators and complex infrastructure required.PWFAs can be driven by bunches of electrons, positrons, or protons. Experiments with electron or positron drivershave been undertaken at several facilities, mainly in the USA. Electron-driven PWFAs are the most prevalent type ofPWFA since its inception and first demonstration. For example, in 2007 experiments at SLAC demonstrated doublingthe energy of a 42 GeV electron beam, and efficient acceleration of a separate witness bunch has been demonstratedat the same facility. Positron beams have been used not only as drivers, but also have been accelerated.
Complemented by further programmatic R&D on fundamental beam-plasma interaction studies, this fuels prospectsfor a potential path towards a high energy physics collider, next to various key applications such as advanced lightsources.More recently a hybrid approach has been proposed which aims to combine many of the advantages of LWFAsand PWFAs. In the hybrid LWFA → PWFA, a LWFA generates an electron bunch which then drives a subsequentPWFA stage. This provides a very compact particle beam driver, with inherent synchronization between the laserand electron pulses. Additional laser beams can be used for preionization, injection, manipulation and plasma andbunch diagnostics.Proton beams as drivers for PWFAs were considered as early as 1986. In 2009, a more thorough theoretical studyinvestigated the use of LHC-scale proton beam drivers to generate TeV-scale beams. The CERN Advanced WakefieldExperiment (AWAKE) has made huge steps forward, and very recently this approach demonstrated accelerationof an electron witness bunch for the first time. At present the proton bunches available are much longer thanthe plasma period, and hence it is necessary to modulate them by seeding a self-modulation process. Ultimately itmay be possible to avoid this step by generating short proton bunch drivers. Other challenges for p-PWFAs are thedevelopment of long ( >
10 m ) plasma cells, and the development of short-bunch electron injectors.Current research into PWFAs is focused on: controlling injection and trapping of particles into the wakefield, e.g.by plasma cathodes, photocathodes and related approaches; improving the output beam quality and stability;and reducing the footprint of the drivers.
Figure 2 shows laboratories around the world working on experimental plasma wakefield acceleration. The growth ofthis research area is evidenced by the fact that many of these laboratories started work in this field only recently.
Today, LWFAs constitute the largest part of the research worldwide on plasma accelerators, partly due to the availab-ility of compact high-power laser systems (typically Ti:sapphire lasers) and the relatively low technological thresholdfor initiating research programmes in this area. There are many multi-institutional national and international projectsand initiatives devoted to LWFAs.LWFA research is very strong in Europe. Here, Laserlab Europe plays an important role in enabling campaign-based access to high-power laser facilities across Europe; EuPRAXIA is an EU H2020 project across LWFAs ande-PWFAs to design a European facility based on plasma accelerators; and the Extreme Light Infrastruture (ELI) is a European Strategy Forum on Research Infrastructures (ESFRI) project for the investigation of light-matterinteractions at the highest intensities and shortest time scales. There are particularly strong research groups basedin Germany, France, Italy, and Portugal.The USA has a large number of groups working on LWFAs. Of these, the biggest is the BELLA programme basedat Lawrence Berkeley National Laboratory (LBNL), but there is a large number of other significant research groupsbased at other national laboratories and in universities.There is significant and important research on LWFAs undertaken by the large number of groups based at nationalfacilities and university laboratories in China, India, Korea and Japan. UK Plasma Wakefield Accelerator Steering Committee
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Figure 2: Non-exhaustive overview of laboratories working on (or with the capacity to work on) laser-driven (black)and particle beam-driven (green) plasma wakefield R&D. Based in part on the map of high-power laser laboratoriesproduced by the International Committee on Ultra-high Intensity Lasers (ICUIL). Research on PWFAs was initially restricted to a handful of large research centres, but the intensity of research is nowstrongly ramping up as more laboratories engage. In the field of e-PWFAs, there are several important multi-nationalcollaborations providing programmatic or campaign-based R&D at, for example, SLAC FACET(-II), BNL ATF(-II),DESY FLASHForward, INFN SPARC-X and elsewhere. Research on p-PWFAs, through the AWAKE experiment, isstrongly supported by CERN and involves a large collaboration by the standards of plasma wakefield acceleration.As mentioned above, the EuPRAXIA project includes a programme on linac-driven plasma wakefield acceleratorconcepts.
The UK has several internationally leading groups; these are mostly university-based, several of which are also affiliatedwith one of the two Accelerator Institutes. The UK groups have made major contributions to fundamental researchon LWFAs. These include the first demonstration of the generation of narrow energy spread beams; pioneeringdemonstrations of acceleration to the GeV range in externally-guided and self-guided geometries; the developmentof novel plasma channels; studies of novel methods for controlling electron injection via ionization of dopantspecies; and measurements of the duration and emittance of laser-accelerated electron bunches. UK groupshave also played leading roles in demonstrating applications of LWFAs, including: the generation of visible and extremeultraviolet undulator radiation from laser-accelerated electrons; the generation of bright betatron radiation withphoton energies in the keV to MeV range; the application of betatron radiation to tomographic imaging of humanbone; and applications to fundamental physics, such as studies of the radiation reaction. To date most experimental work by the UK groups has been performed at the Central Laser Facility (CLF)at the Rutherford Appleton Laboratory (RAL), or at laser facilities outside the UK. The Astra-Gemini TA3 laser,commissioned in 2008 at RAL, was a major advance for laser driven particle acceleration in the UK. This is notonly because it features an ultrashort high-intensity pulse, which is ideal for laser wakefield acceleration, but alsobecause it operates at relatively high-repetition rate of 1 shot every 20 seconds, compared to the few shots per hourof previous petawatt-scale laser facilities. The Gemini laser increased by a factor of 10 the laser energy provided bythe Astra TA2 laser which was used in the first demonstration of the generation of monoenergetic, self-injected beamexperiments. The increased pulse energy available from the Gemini laser allows a laser wakefield to be driven to
UK Plasma Wakefield Accelerator Steering Committee
6K Roadmap for Plasma Wakefield Accelerator Research close to wavebreaking at lower density, increasing the phase velocity of the wakefield, and thereby allowing electronsto reach a higher energy before being dephased.The CLF is heavily oversubscribed, partly due to its reliance on a single, multi-purpose target area, which limitsprogrammatic and cutting-edge R & D in LWFAs. In order to keep international leadership, a new high-energy, high-repetition rate, state-of-the-art facility is required.
Recommendation 1
A facility with target areas and beamlines dedicated to research on laser- and hybrid laser-driven plasmaaccelerators and their applications should be developed at CLF; these facilities should be state of theart in terms of beam stability, and should take advantage of the UK’s lead in high-average power lasertechnology to allow operation at repetition rates above 10 Hz.
Small- and medium-sized laser systems — such as those based at Imperial College, Oxford, Strathclyde andQueen’s University Belfast (QUB) — play an increasingly important role in allowing groups to develop new concepts,to prepare for experiments at national facilities, and to train students. It is vital that these laser systems are updatedand supported in order to have a sustained activity in this area in the UK.
Recommendation 2
The development and operation of UK university-scale labs for laser-plasma accelerator research andapplications, including e.g. SCAPA beamlines, should to be supported and exploited.
The UK groups make substantial contributions to European programmes, such as EuPRAXIA, as well as to othercollaborations within Europe, often via joint experiments at Laserlab Europe. They also work with many institutionsoutside Europe; although these are often funded on an ad hoc basis, these collaborations play an important role inenabling the UK groups to undertake world-class research.
Recommendation 3
The UK should aim to play a key role, and support the pan-European EuPRAXIA initiative for aEuropean Plasma Research Accelerator with eXcellence In Applications.
To date the UK groups have contributed to PWFA R&D mainly via experiments at SLAC FACET and at CERNAWAKE. The CERN AWAKE programme has received funding from the institutes and the STFC since 2012; UKgroups play a significant role in AWAKE and constitute approximately 20% of the authors. At SLAC FACET,the E210 and E203 experiments have been led by UK researchers, UK groups have significant involvement withupcoming PWFA research at DESY, and in a Europe-wide hybrid LWFA → PWFA collaboration. UK groups are thelargest fraction of non-US groups in the currently commencing exploitation period of SLAC FACET-II. This includesleadership on several flagship experiments. A UK-US support pathway is required to exploit these opportunities. TheUK groups play a major role in the EuPRAXIA project on LWFAs and PWFAs, representing 6 of the 16 partners andreceiving 21% of the total funding.This multi-year experience, partially obtained without any funding from the UK RC’s, demonstrates:(a) There is strong intellectual leadership of UK researchers and institutes on both electron-driven as well as proton-driven PWFAs.(b) There is an urgent need to support these activities with significant funding commitment.
UK Plasma Wakefield Accelerator Steering Committee
7K Roadmap for Plasma Wakefield Accelerator Research (c) There is need to develop experimental PWFA capabilities in the UK, in addition to commitment to internationalcollaboration.Regarding c), there are promising opportunities in the UK for both electron-linac driven PWFAs as well as forhybrid LWFA → PWFA in addition to the partially established R&D capabilities to support proton-driven PWFAs atCERN in the AWAKE project.CLARA, the Compact Linear Accelerator for Research and Applications, offers R&D opportunities for electron-linac driven PWFAs. CLARA is a dedicated R&D facility for developing FEL R&D and to prepare and support UKX-FEL capabilities. CLARA currently produces 50 MeV electron beams, and will eventually provide 250 MeV beams.While not currently funded, there is the potential to extract the 250 MeV beam into a beamline directed to scienceexperiments requiring short pulses of relativistic beams; such a capability may be of interest to the plasma accelerationcommunity.
Recommendation 4
A dedicated plasma wakefield acceleration beamline, with synergies to other novel accelerator schemes,should be developed at CLARA/ASTeC.
As in AWAKE, which relies on self-modulation of the long proton beam, it has been proposed that long electronbunches, such as those use at Diamond, could be modulated. Using LWFAs to seed the modulation of the Diamondbeam leads to short bunches that are radially polarised on creation. The oscillation of the micro-bunches leads to thegeneration of radially polarised X-ray pulses of much higher brightness and over a wider photon energy range thanthe Diamond facility. This has been studied in detail in simulations and provides an opportunity for an experimentalprogramme which could then lead to upgrades for light sources world-wide.The UK is in principle in a strong strategic position with respect to hybrid LWFA → PWFA. Such research could beundertaken, for example, at the CLF and at SCAPA. A dedicated hybrid LWFA → PWFA beamline could potentiallybe implemented at one of the (up to 7) beamlines of SCAPA, thereby allowing programmatic development. Suchprogrammatic R&D at laser powers accessible at SCAPA could be complemented by campaign-based R&D at theCLF with the Gemini laser. Here, the higher laser power levels, and in particular the two-laser beam capability, whichis a strong asset for hybrid plasma acceleration, could be exploited, and may then also lead to a dedicated beamlinein future CLF upgrades.In 2018, AWAKE completed data-taking for its Run 1 and achieved its major milestones. The experiment demon-strated the self-modulation of the long proton bunches and the acceleration of electrons up to 2 GeV in the wakefieldsdriven by the microbunches. A future AWAKE programme is being developed in which electrons are expected to beaccelerated to about 10 GeV over about 10 m, with high bunch quality. This should be a reproducible and scalableprocess such that first applications of the AWAKE scheme could be considered. There are significant opportunit-ies for UK contributions to the electron source, plasma technology and general aspects of the experiment such asdiagnostics. The UK should also continue its leading efforts on applications of the AWAKE scheme to high energyphysics experiments.
Recommendation 5
The UK should provide substantial, and increased, investment in the AWAKE Run 2 (2021–4) pro-gramme.
Advances in LWFAs have often been driven by advances in laser technology, and this will continue to be the case. Theapplication of plasma accelerators to driving useful radiation and particle sources is constrained by the low repetition
UK Plasma Wakefield Accelerator Steering Committee
8K Roadmap for Plasma Wakefield Accelerator Research rate ( < for a petawatt-class laser) and wall-plug efficiency ( < . ) of the Ti:Sapphire driving lasers whichare typically used today. These limitations are imposed by the relatively low quantum efficiency of the Ti:sapphiregain medium, and the use of flashlamp pumping, both of which lead to significant deposition of heat that must beremoved between laser shots.Increasing the repetition rate to 10s, 100s, or 1000s of pulses per second requires the development of new lasertechnologies, and possibly the development of new strategies for driving the wakefield. The UK has made significantcontributions in both areas: CLF’s DiPOLE project has led to the development of new pump lasers able to provide >
100 J pulses at 10 Hz; and UK groups have proposed that plasma wakefields could be driven by trains oflow-energy laser pulses, potentially enabling the wakefield to be driven by emerging laser technologies.In order to realise the scientific goals of laser-driven wakefield accelerators mentioned in this roadmap, high powerlasers based on advanced technology would be necessary. In particular, to drive applications of laser-based secondarysources, high-repetition-rate, high-energy (including petawatt-class) lasers will be required. Diode-pumped petawatt lasers
The overall efficiency of laser pump diodes is close to , and hence replacingflashlamps with laser diodes would substantially increase the laser efficiency and allow significant increases in repetitionrate. This diode pumped solid state laser (DPSSL) technology will therefore bring a paradigm shift in the performanceof high power lasers, combining high peak power ( > ) with high repetition rate (10 Hz – 1 kHz) and high overallefficiency ( > ). Such lasers are being developed at a few places around the world, including STFC’s CLF.A significant part of the work described in this roadmap would rely on a petawatt laser (30 J, 30 fs) driven at10 Hz in the short-to-mid term. For future-proofing, it is imperative that this laser is based on DPSSL technology.For example, the DiPOLE100 system developed at CLF would have enough pump power to drive a petawatt laser at10 Hz. High-peak-power, kilowatt mean power lasers
Thin-disk and fibre lasers pumped by DPSSLs are being invest-igated as technologies for driving high-peak-power lasers at 100 Hz repetition rates and beyond. For example, CLF isinvestigating 100 TW, 100 Hz lasers; and the iCAN project plans to coherently combine thousands of low-power fibrelasers to generate high-power laser pulses at multi-kHz repetition rates at wall-plug efficiencies significantly above .An alternative scheme is the multi-pulse LWFA (MP-LWFA) approach being developed by UK groups: here thedriving laser energy is delivered over tens of plasma periods in a train of low-energy pulses or a long, modulated pulse.Delivering the drive energy over a longer interval (1 – 10 ps) allows the use of different laser technologies (e.g. fibreor thin-disk lasers), capable of multi-kilohertz repetition rates and high wall-plug efficiencies. Many of these ideascan be tested using laser systems available in university laboratories, and at CLF, and there is a clear opportunity forthe UK to build on its world lead in this area. Mid and far IR lasers
Today most LWFAs are driven by lasers operating at wavelengths at around 1 µ m. However, high-power laser systemsoperating at longer wavelengths ( λ = 5 − µ m ) could expand the parameter range of LWFAs considerably. For example, since the force responsible for driving the wakefield, the ponderomotive force, scales as Iλ , increasingthe wavelength of the driver allows the laser intensity to be reduced. This in turn allows cold electrons to be injectedinto the wakefield via tunneling ionization of a dopant species by a low-intensity injection pulse, which could lead tolow emittance electron bunches similar to the PWFA-based plasma photocathode or Trojan Horse approach. Longwavelength laser pulses also may be useful to preionise comparably broad plasma channels for PWFA systems.R&D on these approaches both conceptually and as regards mid to far IR laser systems such as CO lasers isrequired to assess and develop these scenarios. Intense particle beams are required to excite intense plasma oscillations and to drive intense plasma waves overextended distances. Short beams with high peak currents are ideally suited to drive resonant plasma wakes intothe nonlinear blowout regime; and high energy, low emittance beams are necessary to realise extended accelerationdistances. It is a fortunate synergy that substantial R&D has been put into the development of photocathodes andadvanced compression schemes in order to drive hard X-ray free-electron lasers such as the LCLS and EuropeanXFEL. Nevertheless, further optimization is necessary to produce an ideal driver for PWFAs: for example, a highpeak current is of even higher importance for PWFAs than for FELs; whereas, in contrast, a small energy spread is
UK Plasma Wakefield Accelerator Steering Committee
9K Roadmap for Plasma Wakefield Accelerator Research crucial for a FEL, but is less important for driving a PWFA stage.Trapping of electrons from the background plasma requires high peak driver currents > , although this canbe reduced to or less using a density downramp, plasma torch downramps, or a downramp-assisted plasmaphotocathode. Dedicated efforts are now underway at SLAC FACET-II to produce beams from linacs with peakcurrents exceeding tens of kA. The optimum parameters of the driver depend strongly on the density of the plasma. Lower density plasmascan be driven by longer drive beams, likely improving the shot-to-shot stability and uncorrelated energy spread ofthe bunch. On the other hand, very short (few or sub-fs) electron beam drivers could drive
TV m − stages at highplasma density.The ability to shape electron beams from photocathode linacs is also important. For example, triangular bunchcurrent shapes can be used to harness higher transformer ratios, or to develop reduced energy spreads via directbeam loading. Other parameters of the driver which require optimization include energy spread and emittance;finding the best trade-off between these, and other, parameters of the driver requires dedicated machine developmenttime. We note that CLARA offers outstanding opportunities for develping electron PWFA drivers since it offers astate-of-the-art photocathode with the potential to operate at a repetition rate up to 400 Hz.Finally, high repetition rates are important, which can for example be produced by superconducting linac techno-logy. More research will then also be required to explore the plasma response at such high repetition rates. Proton-driven PWFAs
A special approach is required for proton-driven PWFAs since short, intense high energyproton beams are not readily available from linacs. Here the approach is to exploit self-modulation of long protonbunches, such as those produced at the LHC. The possibility of pre-modulating the long proton bunches or generatingshorter drive bunches are areas of interest for proton-driven PWFAs. plasma.
Hybrid LWFA–PWFA
Another possible next-generation driver could be electron bunches generated from a LWFA.These are particularly attractive for PWFAs since they are of short duration and have peak currents exceeding tens ofkA. Although LWFA bunches often have high relative energy spread (tens of percent), for driver bunches (cid:38)
100 MeV all the electrons propagate at a speed close to c . This hybrid LWFA–PWFA allows PWFAs to be achieved with anyLWFA system and offers several advantages, including: elimination of dephasing; and excellent laser–electron bunchsynchronization.Significant progress has been made with this approach, much under UK leadership. However, it has received nosignificant UK research council funding. Substantial R&D is required to explore this area, which could be undertakenat UK laser centres such as SCAPA, CLF, and university-based labs. A core component of a plasma accelerator is the plasma itself. For both PWFAs and LWFAs this must comprisea region of plasma in which the species, ionization state, density, uniformity, and length are all well defined. Sinceplasma does not exist at room temperature, the plasma must be created by the driving beam itself, or by auxilliarybeams or electrical discharges.For more advanced plasma accelerators it may be necessary to control the longitudinal and/or transverse profileof the plasma density. For LWFAs, control of the longitudinal profile allows the possibility of overcoming dephasing,increasing the energy gain per stage. The development of hollow, or near-hollow, plasma channels is of significantcurrent interest for LWFAs and PWFAs since the focusing plasma wakefields are weak, which prevents emittancegrowth. Hollow channels could be particularly important for positron acceleration since the accelerating fields havea larger amplitude, and occupy a greater proportion of the plasma wave, than those generated in uniform plasma.Some requirements of the plasma source are specific to the driver. For laser-driven plasma accelerators theintensity of the driving laser is nearly always more than sufficient to ionise a target gas to create the plasma andhence auxiliary ionization systems are not usually required. For LWFAs the length of the plasma source shouldmatch the shorter of the dephasing or pump-depletion lengths. Alternatively, in advanced schemes, dephasing can beovercome partially by controlling the longitudinal profile of the plasma density. The length of the transition regionbetween the body of the plasma and the surrounding vacuum is important for laser-driven plasma accelerators sinceit determines the extent to which the laser is defocused as it is coupled into the plasma. Finally, in the quasi-linearregime it is necessary to guide the driving laser pulse over the length of the accelerator stage using an externalwaveguide capable of withstanding laser intensities of order W cm − .Particle-driven plasma accelerators typically require longer (up to several metres), lower-density ( n e ≈ − UK Plasma Wakefield Accelerator Steering Committee
10K Roadmap for Plasma Wakefield Accelerator Research cm − ) plasma than the laser-driven case. For PWFAs it may be necessary to use an auxiliary laser to ionise thesource species since the electric fields of the driver may be too low for field ionization. On the other hand, the designof the plasma source in this case is simplified by the fact that neither defocusing of the driver or dephasing occursto a significant extent.For both LWFAs and PWFAs, important practical considerations include the operating lifetime and shot-to-shotreproducibility of the plasma source. In some cases, it will also be important to be able to provide diagnostic accessto the plasma, or to isolate it from other parts of the beam line and/or vacuum system.The development of practical plasma accelerators will require a parallel development of plasma sources. ForLWFAs self- or external-guiding of higher pulse energies, through longer lengths of lower density plasma is needed toincrease the energy gain per stage above 10 GeV. The development of hollow plasma channels could be crucial foracceleration of positrons. For PWFAs driven by modulated proton beams, the development of long, uniform plasmacells will be vital.As the energy gain of the accelerator is increased, the plasma target will need to handle large particle beam orlaser pulse energies without damage. For plasma accelerators to drive applications, the plasma source will need tobe reproducible and highly reliable; and as the repetition rate is increased this will need to be the case for millions ofshots per day.Plasma sources play a central role in both laser- and particle-beam-driven plasma accelerators, and their continueddevelopment is necessary and important if plasma accelerators are to drive applications. The development of plasmasources should therefore be recognised by the research councils as a priority research area. Future plasma-based electron–positron colliders will require a source of high-quality positron bunches for injection intothe plasma wakefield. Although great strides have been made in plasma-based acceleration of electrons, accelerationof positrons in plasma accelerators is still in its infancy. One reason for this is that only one RF facility in the world(FACET-II) can provide positron beams of sufficient quality.A potential solution to this problem is the generation of high-quality, ultra-relativistic positrons by interactionof a laser-accelerated electron beam with a solid target. In a nutshell, the positrons are generated as a result of aquantum cascade initiated by a laser-driven electron beam propagating through a high-Z solid target. For sufficientlyhigh electron energy and thin converter targets, the source size, divergence and duration of the generated positronbunches resemble those of the parent electron beam. The positron beams generated this way are therefore attractivefor injection into further plasma accelerator stages, with the important bonus that they are naturally synchronisedwith a high-power laser system. UK-led groups have recently used this approach to demonstrate the generation offs-scale, narrow divergence positron beams, together with the first ever generation of a neutral matter–antimatterplasma in the laboratory.
UK researchers initiated this area of research and retain a world-leading position, and as a consequence theycurrently play a significant role in developing positron sources for the EuPRAXIA and ALEGRO projects.Further optimization of these sources will require the charge of the positron bunch to be increased and its emittanceto be decreased, both of which can be achieved by increasing the charge and energy of the parent electron beam.We note that the generation of neutral electron–positron plasmas presents an opportunity to recreate in thelaboratory extreme astrophysical environments such as the atmosphere of quasars and active galactic nuclei. Thephysical understanding of these environments is based on astronomical observation and numerical modelling only, andis therefore somewhat speculative. Recreating similar conditions in the laboratory will unlock the physics involved insome of the most spectacular events in the known Universe, such as gamma-ray bursts and astrophysical jets.
The development of techniques for controlling the transverse and longitudinal phase space of particle bunches in con-ventional accelerators took decades, and although many of these methods can also be used with plasma accelerators,the unique properties of the particle bunches they generate will require novel approaches to be developed. Manyimportant applications of plasma accelerators require improvements in, or control of, the emittance, energy spread,or energy chirp of the particle bunch. The development of methods for phase-space control in plasma accelerators istherefore vital.
UK Plasma Wakefield Accelerator Steering Committee
11K Roadmap for Plasma Wakefield Accelerator Research
Recommendation 6
Programmatic funding of plasma wakefield accelerators should be provided to optimise the quality andstability of plasma-accelerated particle bunches, to increase pulse repetition rates, and to enable keyapplications in the industrial sector and in fundamental science.
The transverse emittance is a key parameter both for light source and high energy physics applications. The nor-malised emittance (cid:15) n = γ(cid:15) , where (cid:15) is the geometrical emittance, is a crucial parameter for FELs since the lasingthreshold diffraction limit for a FEL operating at a wavelength of λ rad is (cid:15) = (cid:15) n /γ < λ rad / (4 π ) ∝ λ / , wherethe proportionality holds for a given undulator. Hence FEL operation at shorter wavelengths requires decreasednormalised emittance, which becomes increasingly challenging . The normalised emittance also determines thebeam brightness B ∝ (cid:15) n , which in turn determines the FEL gain. For high energy physics (HEP) applications, thenormalised emittance is a vital parameter since it determines the event rate: a low emittance enables a final focusof small area, σ x σ y , and hence a high luminosity L = fN πσ x σ y , where f is the frequency and N is the number ofparticles.Although measurement of emittance is difficult, especially for beams with significant shot-to-shot jitter and energyspread, there is solid evidence that laser–plasma accelerators today routinely produce beams with (cid:15) n ≈ ,depending on the electron injection method and laser–plasma parameters. These emittance values put LWFA-generated electron beams at the same level as those used in state-of-the-art X-ray FELs. Nevertheless, in view of thefundamental importance of emittance for key applications, and the potential for significant further improvements,substantial R&D is required on methods to further improve and characterise the emittance. This includes thedevelopment of novel measurement techniques, since emittance is a rather indirect observable and the measurementof low emittance beams is non-trivial.UK groups have played a leading role in developing new approaches which promise to decrease the obtainableemittance by three orders of magnitude to the nm rad level. These approaches, known as plasma photocathodesor Trojan Horse injection, are based on injection of electrons field-ionised from ions in the plasma by a trailing(injection) laser pulse. Achieving the lowest possible emittance requires that the intensity of the injection pulse islow, in order to minimise the transverse momentum gained by the ionised electrons, and that the electric fields of thedriver are lower than that of the injection pulse, to avoid ionization by the driver. These conditions favour PWFAssince particle beam drivers excite strong wakefields at electric field values of a few GVm − compared to the TV m − driver fields typical of LWFAs. The plasma photocathode approach has been recently realised for the first time in theE210: Trojan Horse PWFA experiment at SLAC FACET.Plasma photocathodes could also be realised in LWFAs by using long-wavelength or multi-pulse laser drivers, and shorter wavelength injection pulses. These approaches deserve significant attention, including the developmentof high-power long wavelength laser pulses. In the UK, the high-power laser centres and CLARA offer promisingfacilities to develop this future class of plasma photocathodes. A key parameter in determining the quality of a relativistic electron beam is its spectral bandwidth, or relative energyspread. It is usually advantageous for the energy spread to be as small as possible. For example, a narrow energyspread is important for efficient acceleration in subsequent accelerator stages, and for subsequent beam manipulationand focusing. Narrowband electron beams are particularly important for driving light sources: for example, hardX-rays FELs require relative energy spreads (cid:46) . , which at present can only be achieved for multi-GeV beams byconventional accelerators.Both LWFAs and PWFAs can generate multi-GeV electron beams with relative energy spreads typically of a fewpercent. A reduction in the energy spread by one or two orders magnitude is therefore required. To a largedegree the comparably large energy spread of plasma accelerators is caused by the high accelerating fields and smallplasma cavity size; these features also cause the bunch to develop a negative longitudinal energy chirp in addition touncorrelated energy spread arising from the injection process. The total energy spread is usually dominated by thecorrelated energy spread, and this can be reduced by various approaches including: longitudinal phase space rotationin a beam transport section; periodically localizing the bunch at different phases of the accelerating wakefield; or by
UK Plasma Wakefield Accelerator Steering Committee
12K Roadmap for Plasma Wakefield Accelerator Research
Figure 3: The 6D brightness of electron beams generated by current and future generations of plasma accelerators.beam loading. These approaches are generally viable both for LWFAs and PWFAs, while in LWFAs dephasing is anadditional complexity which needs to be taken into account. The compensation of local electric field gradient bymatched beam loading or counter-rotation of longitudinal phase space by overloading the wake is directly connectedto injection processes and control of these processes.Reducing both correlated and uncorrelated energy spread requires control of the injection of particles in theplasma wakefield. A wide variety of schemes has been proposed and are currently being studied experimentallyand theoretically. A long-term scientific and technological development is certainly required in this area, includinghigh-precision tailoring and stability of plasma sources, fine control of the spatio-temporal properties of the (laser orparticle) driver beam, and the development of detailed control of the injection dynamics. Numerical and analyticalwork suggests that substantial improvements can be achieved in this area, down to relative energy spreads below0.01% for few-GeV electron beams. The brightness of a particle beam is a key performance parameter of accelerator output and particularly importantfor light sources. It combines the particle beam current I and the emittance B = I/(cid:15) n and in the so-called6D-brightness also includes the relative energy spread. Plasma wakefield acceleration intrinsically generate ultrashortbunches with very high current, and can produce very low emittance values. Figure 3 shows the 6D brightness ofpresent and future generations of plasma accelerators as a culmination of prospects which are within reach of theapproaches described in this roadmap.At these extreme values, a notable challenge in particular for beams with significant energy spreads at the endof the plasma is extraction and transport of beams under preservation of beam quality, which can be addressed bybeam manipulation via plasma-based devices. These plasma-based building blocks also allow the compactnessand relatively low cost of the overall system to be preserved.High brightness is particularly important for key applications, such as light sources, as it may allow realizationof high-gain coherent light sources with extreme performance in the hard x-ray range. Low emittance and energyspread are requirements shared with high energy physics applications of ultra-relativistic particle beams in the formof beam luminosity, which is of fundamental importance for colliders.As discussed in §5.1.1, considerable effort world-wide is aimed at developing high-power laser systems capableof operation with repetition rates in the 0.1 – 10 kHz range, and we note that the development of 1 kHz plasmaaccelerators is one of the main long-term scientific goals of the US accelerator roadmap. Such developments wouldbe needed to enhance both the average brightness and luminosity of plasma-accelerated particle beams and willdramatically boost the range of applications of laser-driven particle accelerators. UK Plasma Wakefield Accelerator Steering Committee
13K Roadmap for Plasma Wakefield Accelerator Research
Since the accelerating structure typically travels at very close to the speed of light, in order to be trapped in a plasmaaccelerator, injected particles must rapidly achieve relativistic velocities. The simplest method of achieving injectionis to drive the plasma wave to wave-breaking so that some background plasma electrons can enter the acceleratingstructure and become trapped. In LWFAs this mechanism has been widely studied for more than a decade and isresponsible for the highest observed electron energies to date.
However, this self-injection process is very sensitiveto small variations of the laser–plasma parameters and is therefore usually associated with large shot-to-shot jitter ofthe bunch parameters.Reducing shot-to-shot jitter, and improving or controlling the 6D phase space of the particle bunches, requirescontrol of the particle injection process. A wide variety of techniques for controlling electron injection has beeninvestigated to date. These include: locally reducing the wake velocity at density down-ramps; ionizing electronsfrom dopant species, by the driver or additional pulses; localised stochastic heating with one or more collidinglaser pulses. It should be noted that it is likely that no single method will be best suited for all applications. For example,radiation hardness testing requires high average charge, but beam quality is less important, whereas high bunchquality is vital for driving radiation sources.Due to their experimental complexity, the more advanced injection mechanisms have been investigated less in-tensively, especially for the highest power laser systems. This area of research will become increasingly importantas applications of plasma accelerators are developed, and progress will require dedicated beam time both to developand optimise controlled injection techniques and to make challenging measurements of the bunch quality.
Plasma wakefield accelerators have the potential to offer wide tunability. For example, in a LWFA the particle energycan be tuned from keV to few GeV by adjusting the laser power and/or the length and density of the plasma; thebunch charge can vary from fC to nC, and consequently the current can be ramped up to the tens of kA scale. Thatsaid, the bunch parameters are not independent; for example, the peak energy and energy spread depend on thebunch charge via beam loading.
The successful incorporation of plasma accelerator concepts into scientific, engineering and industrial applications willin many cases require the controlled capture and transport of beams as they exit the plasma medium. Drawing oncomparisons from advanced RF-driven particle accelerators, the transport challenges may be a limiting factor in theutilisation of the beams, as well as being an important factor determining the overall scale of any plasma acceleratorfacility. For example, the particle-beam transport optics and phase-space manipulation section in state-of-the-artFELs and high-energy physics machines constitute a substantial part (and cost) of the overall system footprint.Plasma wakefield accelerated beams present additional challenges beyond those of RF-driven accelerators. Thesub-micron transverse dimensions together with the very high divergence on exiting the plasma medium requirefocusing magnetic optics with strengths beyond the magnetic saturation levels of Fe-based electromagnets. Ceramicpermanent magnets may be a solution since they provide high field gradients, but further development is needed tomeet the field-quality levels, such as field flatness, required for many applications. Other approaches, such as activeplasma lenses are being explored by UK and international groups.. More exotic concepts such as large energyacceptance ‘neutrino horns‘ may also offer innovative solutions.At few-percent energy spread levels, much tighter control of chromatic aberrations than generally encountered inaccelerator applications is required. Corresponding setups may require sextupoles supplemented with octopole andhigher order magnets. Minimization of the energy spread before extraction from the plasma stage is therefore crucialto reduce the complexity and challenges of beam transport.The intrinsically ultrashort duration and high current of plasma generated beams is a highly attractive feature,but also a substantial challenge to beam control. The associated high current density is known to cause significantcoherent synchrotron radiation (CSR) during transport through dispersive elements, which causes back-reactionsin the particle beam phase-space. Related deleterious processes such as the excitation of the CSR microbunchinginstability may occur, although the growth of such instabilities may be damped by the large energy spread of thebeams; successful efforts to reduce the energy spread for applications will give rise to increased need to addresscoherent and collective instabilities in the transport of short duration high-current beams.
UK Plasma Wakefield Accelerator Steering Committee
14K Roadmap for Plasma Wakefield Accelerator Research
Some applications may benefit from non-conventional transport optics. An example occurs in efforts to obtaincoherent gain and free-electron laser action; lasing requires very small uncorrelated energy spreads, to limit Landaudamping in the microbunching formation. This stringent constraint can however be mitigated though a combinationof transverse dispersion of the beams, coupled with transverse gradient undulators. Such concepts are currently beingpursued experimentally by several groups internationally, and are likely to be of interest in UK efforts in FEL actionwith PWFA beams.
Laser-driven plasma accelerators have demonstrated multi-GeV energy gain in a single accelerator stage (i.e. a singletarget, a single driver beam). Although it will be possible to extend beyond the 10 GeV range with planned facilities(such as ELI, Apollon, CLF 20 PW), large laser installations of this kind are currently limited to low repetition rates,which limits the potential applications and makes it difficult to maintain long-term accelerator stability.An alternative approach to obtaining higher beam energy is staging of multiple plasma accelerator stages, eachdriven by its own laser pump. This rephases the accelerating structure with the accelerated beam, eliminating theeffects of dephasing or pump energy depletion. In principle, the particle beam energy can then be increased limitlesslyby increasing the number of stages. Staging presents three main challenges: (i) transporting the particle beambetween stages; (ii) coupling in the driver beam into each stage; and (iii) stability.As discussed in §5.3.1, transporting the particle beam between stages necessarily increases the total length of theaccelerator and novel methods need to be developed to deal with the challenging parameters of plasma-acceleratedbeams. To meet this challenge, active areas of research include the development of active plasma lenses, andshaping of the plasma to directly couple beams from one stage to another.Coupling the laser pulse into each stage requires an optic of several metres focal length which would decreasethe average acceleration gradient substantially if the drive focusing and accelerator stages were arranged in a lineargeometry. A much better option would be to fold the drive beamline so that most of its length is perpendicular tothe axis of the accelerator, and only the last few centimetres are directed along the accelerator axis by a mirror. Thismirror must withstand high laser intensities and allow transmission of the particle beam from the previous stage. Apromising solution is a plasma mirror: this comprises a thin tape which allows transmission of the particle beam withminimal emittance growth; the incident laser pulse converts the surface of the tape into a dense plasma, and henceis reflected with high efficiency. Plasma mirrors have been used to couple two plasma stages to yield electron beamsin the 100 MeV range, although implementation in the GeV-range with higher efficiency has yet to be achieved.Innovative solutions such as curved plasma channels for staging are therefore also being developed.The small size of the wakefield structure means that coupling driver and particle beams into each stage requiresachieveing, and maintaiting, alignment to the micron level or even better. Achieving this will require active pointingstabilization of both the laser system and the plasma stages.Although dephasing is not a relevant limitation in PWFAs and tens of GeV energies have been realised in singlestages, staging is required if aiming at electron energies which exceed low multiples of the driver beam energy,determined by the transformer ratio. Although conventional transport systems could be used to couple driver andwitness beams into each stage, the distances required will reduce the mean accelerating gradient. Novel approaches,such as off-axis injection, are therefore being investigated to maintain a high average acceleration gradient.One of the principal motivations for proton-driven PWFAs is the potential to accelerate electrons from the GeV tothe TeV scale in one stage due to the high stored energy of the drive bunches in the CERN SPS or LHC accelerators.In this this way, it is possible to avoid the issues of staging. The characterisation of particle beam phase space is an important aspect for developing acceleration processes,benchmarking with simulation codes, and for exploitation of plasma accelerated beams for applications. The accessibleparameter regime goes well beyond that known from RF-accelerated beams; or where measurement techniques existin the conventional accelerator realm, the techniques may not be applicable to the facilities and environment ofPWFA experiments. The measurement of particle phase-space with femtosecond time resolution or better, andultralow emittance, are particular challenges. In RF-driven accelerator facilities temporal characterisation is achievedwith RF powered transverse deflection structures (TDS) providing a temporal streak to the beam. TDSs are capableof measuring the slice-parameters of energy vs time, or emittance vs time, with few femtosecond resolution. Themeasurement, and the time resolution, are however entwined with beam transport, and typically require tens of metrestransport and magnetic focusing and defocusing optics. The RF infrastructure requirements are large, in the region
UK Plasma Wakefield Accelerator Steering Committee
15K Roadmap for Plasma Wakefield Accelerator Research of a few million pounds. To date TDS measurements of LWFA beams have not been demonstrated. Beam-drivenplasma accelerators are more amenable to incorporating such diagnostics.Coherent optical emission has been more widely explored for temporal measurements of charge density. Whileambiguities exist in inferring temporal information from optical spectral emission, these techniques have demonstratedcapability in determining plasma acceleration physics, e.g. through observation of multiple-bucket acceleration.Coherent emission techniques are not currently amenable to slice-parameter characterisation.There is a need for new concepts and techniques for accessing temporal phase-space information. Candidatesinclude THz-driven streaking of electron beams, which are being developed for RF-driven beams, and in the streakingof X-ray-liberated photoelectrons for fs X-rays diagnostics, or by exploitation of the plasma afterglow response. Fora sufficiently large project directed at plasma acceleration there is potential to include a RF driven TDS diagnosticfor plasma accelerated beams.There is also a need for new concepts and techniques for transverse phase space measurements to resolve thenm rad-scale normalised emittance values, and corresponding brightnesses, which may be achievable by novel bunchgeneration methods as outlined in §5.2.
At present laser–plasma accelerators typically exhibit relatively large shot-to-shot jitter of the bunch parameters,including energy, charge and energy spread. For example, shot-to-shot variations in these bunch parameters oftenrange up to tens of % and typically also are combined with pointing variations which are much higher than those ofconventional accelerators.There are two reasons for the relatively poor stability typically demonstrated to date for laser-plasma accelerators:(i) in most work to date, electrons are self-injected into the wakefield by stochastic processes which are thereforeprone to large shot-to-shot variations; (ii) the parameters of the driving laser, and those of the plasma target tosome extent, themselves show large jitter and drift. As discussed in §5.2.4, the solution to (i) is to develop methodsfor controlling the injection (and trapping) of electrons into the wakefield, and ideally to decouple it from driverand plasma shot-to-shot variations as far as possible. The solution to (ii) includes general advances in driver pulsetechnology and to develop feedback and control systems.We note that the electron and proton drive bunches used in PWFA have well-defined energies, with little variationand a small energy spread. These positive features will help ensure that the shot-to-shot stability of PWFAs is high,and could be improved further by incorporating feedback systems.
Feedback and control
Lasers capable of driving GeV class plasma wakefield accelerators are now able to operate at repetition rates at 1 Hz,and are expected to develop to >
10 Hz in upcoming facilities. The increased repetition rate will allow for enhancedactive feedback on laser and target parameters for the optimisation and stabilisation of LWFAs.There are many causes of drift and shot-to-shot jitter in the bunch parameters of a LWFA including, diurnalvariation of the environment, slow heating of the laser systems with use, vibration, powerline fluctuations, and driftor fluctuations in the density of the plasma source. The stability of LWFAs against jitter and drift could be improvedsignificantly by implementing feedback systems for the drive laser and plasma stages. Control systems could includefeedback loops for the laser energy, beam position, and beam pointing, together with active control of the positionand density of the plasma stage. Control and feedback systems of this type would be relatively straightforward toimplement, but to date they have not been deployed at national laser facilities (such as CLF), since these are generalpurpose facilities which need to cater to a wide range of experiments.At a higher-order level, feedback and control could be used to adjust the laser and plasma parameters pro-activelyto stabilise the output bunch parameters. This requires knowledge of the matrix connecting the output bunchparameters with the laser and plasma parameters.Feedback loops can also be used to optimise the output beam parameters. Proof-of-principle experiments havealready demonstrated the use of this approach to optimise the charge and divergence of sub-MeV electron beams at1 kHz, and of ∼ MeV beams at 5 Hz.We note that PWFAs could operate at repetition rates up to the MHz range. The high repetition rate is expectedto make it easier to develop feedback and control systems, and these can draw on decades of development forconventional accelerators. The development of high repetition rate feedback and control systems for PWFAs will bebeneficial for all plasma wakefield research.
UK Plasma Wakefield Accelerator Steering Committee
16K Roadmap for Plasma Wakefield Accelerator Research
Plasma accelerators have the potential to be a disruptive technology which could fundamentally impact many areasof science, technology and medicine, as illustrated in Fig. 4.In order for plasma accelerators to be developed and deliver solutions for industry at the appropriate TechnologyReadiness Level (TRL), industrial representatives are needed to champion the emerging disruptive technologies and toprovide the industry pull and guidance. We have identified collaborative opportunities in the space technology area,energy, aerospace, nuclear, advanced manufacturing, new materials, security and defence sector and medicine. Someindustrial links ranging from SME’s to major players have already been established. In view of the greater appetitefor industry collaboration, and increased industry interest as the area matures, it is recognised that challenges led byindustry consortia are of great importance.The plasma wakefield community will benefit from strong links and knowledge transfer with other areas ofacademia to influence and inspire future science and development areas. Academic groups such as accelerator science,particle physics, medical/radiobiology and materials engineering are identified as core opportunities for interdisciplinaryresearch activity and collaboration. Cross-council funding mechanisms which can support these important activitiesare urgently needed. The emergence of the Industrial Strategy Challenge Fund (ISCF) is welcome, although thecommunity has had very limited influence on this so far. Owing to the fundamental character of plasma acceleratorresearch, and the wide TRL range of their various applications, a targeted sector initiative may be required.
Recommendation 7
UKRI should develop mechanisms for providing cross-council support for the wide range of research, in avariety of settings, necessary to drive advances in plasma accelerators; this range includes fundamentalresearch (e.g. plasma physics), technology development (e.g. novel lasers), and application development(e.g. medical imaging).
A near term application of plasma accelerators is driving compact radiation sources. Undulator radiation with photonenergies of about 100 eV has been generated from laser-accelerated electrons, and the betatron motion of theparticle bunch has already been used to generate incoherent X-rays in the 10 keV range, which can be increasedbeyond 100 keV by driving the oscillation of the electron bunch with a laser pulse. Incoherent Thomson- or Compton-scattered radiation in the 10 keV to 1 MeV range has been generated by colliding plasma-accelerated electronbeams with an intense, counter-propagating laser pulse.An attractive feature of radiation sources driven by plasma accelerators is that the short duration of the electronbunches leads directly to the generation of femtosecond-duration radiation pulses. For LWFAs the radiation willalso be naturally synchronised to an ultrafast laser system able to generate auxiliary pump or probe pulses, therebyproviding an unprecedented and versatile tool for ultrafast science. Radiation sources driven by plasma acceleratorscould be sufficiently compact and cheap to bring ultrafast imaging and probing techniques from large-scale facilitiesinto small-scale labs, thereby making them ubiquitous and opening new horizons in many areas of the medical,biological and physical sciences.Plasma-accelerator-driven radiation sources are now starting to be used in applications. For example, betatronX-rays have been used to produce tomographic images of biological samples (see Fig. 5), and to capture thepropagation of shock fronts in laser-shocked materials.To date, radiation sources driven by plasma accelerators have been incoherent. A medium term goal is drivingcompact FELs with a peak spectral brightness ten orders of magnitude brighter than incoherent sources. X-rayFELs (XFELs) driven by kilometre-scale conventional accelerators have transformed many areas of ultrafast scienceand are producing a torrent of high-impact results across the physical and life sciences. However, their large scale,and high cost (of order £1 billion) means that only a handful of such facilities exist worldwide. The impact ofdeveloping XFELs driven by plasma accelerators is self-evident. The challenge to be overcome is the generationof electron bunches of sufficiently high quality for free-electron gain to occur at short wavelengths. This problemis being pursued very actively, particularly in Europe, Japan, and the USA, and it seems likely that the first gaindemonstration of a plasma-accelerator-driven FEL can be achieved in the next five years. The first demonstrationswill probably be at relatively low photon energies, but further improvements in the quality of the bunches generated UK Plasma Wakefield Accelerator Steering Committee
17K Roadmap for Plasma Wakefield Accelerator Research
Figure 4: Schematic diagram showing a subset of potential applications of plasma wakefield accelerators, with impactacross various areas of science and industry.by plasma accelerators will lead to FEL operation into the X-ray spectral range.We note that future advanced radiation sources driven by conventional accelerators are likely to employ ultra-shortelectron bunches, which in turn will require the development of new diagnostics for characterizing electron bunches ofultra-short duration and low emittance. The bunches generated by plasma accelerators could provide an ideal sourcefor testing and characterizing the new diagnostics which will be required. Diagnostics are therefore another area, inaddition to driver technology, where conventional and novel plasma accelerator mechanisms merge and cross-fertiliseeach other.
Plasma-based accelerators, especially those driven by laser fields have the key advantage of being able to drivemultiple bright sources of energetic particles and high energy X-rays beams from a single machine. With appropriateoptimisation, these beams have the potential to provide a unique capability for biomedical imaging, cancer diagnosisand therapy from a single facility.
Early detection of cancer is one of the crucial factors determining the probability of surviving it. Existing screeningmethods such as digital mammography and CT have poor discrimination between glandular and tumour tissuesbecause of their similar X-ray attenuation. A new imaging method sensitive to the phase of X-rays, rather than justtheir absorption, yields enhanced intra-tumour soft-tissue contrast and improved visualization of cancerous structures,especially in soft tissues, and can potentially be achieved with a lower dose to the patient. However, improved patientoutcome so far has only been demonstrated at large and expensive synchrotron sources, which have limited access formedical use. Compact betatron radiation sources driven by plasma accelerators could make earlier diagnosis routinelyavailable, transforming treatment planning, delivery, and monitoring.The high flux of LWFA-driven betatron sources means that images can be acquired in a single laser pulse, over-coming the disadvantage of a micro-CT system which require a long exposure time for a high-resolution tomographicscan. Figure 5 shows the results of phase contrast and tomographic imaging of human tissue samples obtainedat CLF by UK groups. The small size of the laser-based source makes them suitable for deployment in a hospital
UK Plasma Wakefield Accelerator Steering Committee
18K Roadmap for Plasma Wakefield Accelerator Research (a) (b)
Figure 5: Medical imaging with laser-driven plasma accelerators. (a) Phase contrast image of prostate tissue; (b)tomographic image of a human trabecular bone using laser-driven accelerators.environment, which is much preferred for clinical applications.
A new radiotherapy (RT) modality uses so-called very high-energy electrons (VHEEs, electron energy >100MeV) toprovide an alternative treatment than X-rays for deep-seated tumours; this approach exploits the large momentum ofVHEEs. Experiments and simulations have shown that VHEEs can have improved dose distributions over X-rays andcan offer enhanced relative biological effectiveness (RBE). Conformal irradiation makes VHEEs ideal for targetingradio-resistant tumours. However, few VHEE-RT experimental studies have been undertaken because conventionalaccelerators are expensive.Laser-based VHEE radiotherapy is potentially disruptive technology that could cut the cost of cancer treatment.Initial studies performed at ALPHA-X/SCAPA indicate the suitability of VHEE for treating deep-seated tumours. These studies have also shown that VHEE beams have similar biological effects to X-rays but that they have verydifferent dose rate and dose distribution.
Industrial applications are an acknowledged strength of plasma accelerators.
The flexibility of plasma acceleratorsand the ability to produce electron, proton, ion and photon beams, combined with their capability to be realised in“table-top” (or even portable) setups, makes them ideal instruments for industrial exploitation and for promotingtransformative applications. The research and development of plasma accelerators meets most of the ten IndustrialStrategy Pillars:Pillar 1: “Investing in science, research and innovation” is met by all plasma accelerator activities.Pillar 2: “Developing skills” is promoted by the interdisciplinary (lasers, beams, plasmas, high-end electronics) natureof novel accelerator science & engineering training, e.g. at the accelerator institutes and universities.Pillar 3: “Upgrading infrastructure” is a central need for the plasma wakefield accelerator community (see §9.1).Pillar 4: “Supporting businesses to start and grow” is achieved through the technology and innovation centres andincubators e.g. at Daresbury, Harwell, and at universities.Pillar 5: “Improving procurement” is driven by the need, for example, for highly specialised, high performance electronics,nano-fabricated structures, and laser systems for accelerator applications; this demand can provide commer-cialization opportunities which can put companies in a leading position before the need for mass production.
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Pillar 8: “Cultivating world-leading sectors” is an opportunity to build on the world-leading, and highly respected con-tribution to plasma accelerator research by the UK groups.Pillar 9: “Driving growth across the whole country” is promoted by a distributed novel accelerator infrastructure whichencompasses university groups and application-oriented centres distributed across the country. The latter in-clude: CALTA (Harwell), SCAPA (Glasgow), and VELA/CLARA (Daresbury). The expansion of these researchprogrammes and facilities would foster growth in industrial development, with a high return on investment. Pillar 10: “Creating the right institutions to bring together sectors and places” can be achieved via a distributed acceleratorinfrastructure which builds on existing links between between national laboratories and facilities, universitiesand industry.Figure 6 maps selected plasma accelerator applications to the 10 Industrial Strategy Pillars.
The properties and behaviour of materials depends on their structure at atomic and macroscopic scales. The study ofmaterial properties at these scales is therefore important for understanding fundamental physics as well for developingapplications in industry. Materials science is an exciting potential application for plasma-accelerator-driven radiationsources since they can be used to probe the composition, structure and state of materials during processing or stresstesting.The industrial sector has shown considerable interest in absorption and phase-contrast imaging measurementswith LWFA-driven X-ray beams, and this is seen as a near-term opportunity for societal impact. The potentialof LWFA-driven betatron sources to produce hard X-ray radiation over a wide bandwidth allows for single-shotmeasurements of X-ray absorption spectra, thereby probing the atomic state of materials. The short pulse durationof these sources, and their tight synchronization to a femtosecond laser system, offers opportunities for pump-probe experiments with unprecedented temporal precision. Indeed, penetrative imaging and X-ray spectroscopy (e.g.XANES or NEXAFS) with femtosecond resolution is beyond even the capabilities of state-of-the-art conventionallight sources and is therefore an exciting near term application. Extension to higher photon energies, for example viabremsstrahlung or inverse Compton scattering (ICS), also provides the possibility of probing inner-atomic transitionsfor penetrative imaging of bulk (
10 + cm thick) materials.With further optimisation of the spectral brightness of plasma-accelerator-driven photon sources it will be possibleto perform X-ray diffraction experiments with crystalline and poly-crystalline samples. Determining the lattice struc-ture, especially in pump probe experiments, is of great importance for understanding and developing novel materials,such as topological insulators. This work would require X-ray optics to transport the beam to the sample. Analternative approach would be to use diffraction of electrons with energies in the 10s –100s MeV range, which couldpotentially be driven by lower energy, high repetition-rate laser systems. In this case, high charge, low energy spread,low emittance electron beams will be required, with suitable electron beam transport optics; as for the case of X-raydiffraction, the temporal resolution could be in the femtosecond range.Both LWFAs and PWFAs can be purposefully used to generate broadband, exponential/power-law spectra particlebeams to reproduce the spectral features of space radiation. This approach can be used for advanced radiationhardness assurance of space electronics and for nuclear environments. The key technical requirements for realising the near- and medium-term opportunities in materials science couldbe met by the development of dedicated plasma accelerator based X-ray/electron beamlines.
For the security and defence sector, LWFAs provides bright, highly penetrating sources of X-rays, electrons and neut-rons for non-destructive evaluation (NDE), radiation hardness and damage testing and directed energy applications.A long standing programme of collaboration between the CLF user community and DSTL is testament to the interestfrom this sector and the potential for impact of laser accelerators in this area.Laser-driven X-ray radar has attracted great interest for security applications. This patented technique is enabledby the ultrashort pulse length and highly penetration capability of LWFA electrons and has been applied for depthprofile or “through barrier single-sided” imaging. In this application, an important feature of laser-driven sources isthe ability to switch easily between different modes of radiation and particle beam generation, thus providing a highlyversatile system with the potential for fast data acquisition. High resolution radiography is of great interest in thissector, from phase contrast imaging of composite materials used in armour to radiography of large components. The
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Figure 6: Connections between applications of plasma wakefield accelerators and the 10 Industrial Strategy Pillars.
UK Plasma Wakefield Accelerator Steering Committee
21K Roadmap for Plasma Wakefield Accelerator Research short pulse duration and high pulse brightness of LWFA-driven sources means that single shot exposure combined withhigh resolution imaging of objects is possible, which is highly valuable for material damage studies. Laser wakefieldbeams are also being developed to generate MeV energy bursts of X-rays for interrogation of sealed containers togive element identification and to provide a method for rapid identification of special nuclear materials.
Historically, the primary motivation for plasma wakefield acceleration has been its application to an electron-positronlinear collider with energies in the TeV scale. Given the size of conventional RF accelerators operating in this energyrange (30 – 50 km), the desire to significantly reduce the size and cost of particle colliders is strong. This is backedup by the large energy gains observed in LWFAs and PWFAs, indicating that the accelerating section can be an orderof magnitude (or more) shorter.However, given that many cross sections fall with increasing energy and exotic physics beyond the Standard Modelof particle physics is also expected to have a low production rate, colliders generally need to have high luminosityas well as high energy. Building a plasma wakefield collider would therefore be challenging since it would require: abeam energy of hundreds of GeV, with an energy spread of 0.1%; repetition rates of >
10 kHz ; bunches with particles and of nanometre transverse extent; and the generation of positrons with the same characteristics. Further,the production of such beams needs to be reliable and reproducible so that a high luminosity, as well as high energy,can be achieved.Achieving high luminosity is a challenge for plasma wakefield acceleration and given its current status, there hasnot been a great deal of discussion amongst the HEP community on the possible applications. The developmentof particle colliders driven by plasma accelerators is therefore a goal which must be considered to be a long-termobjective. A significant effort over an extended period, with extensive international collaboration, will be required tomeet these considerable challenges. Several international programmes are coordinating efforts in this area, such as theAWAKE, Helmholtz Virtual Institute for plasma wakefield acceleration, and EuPRAXIA projects; the ICFA panel onAdvanced and Novel Accelerators (ANA) has hosted several workshops on developing roadmaps for plasma acceleratorcolliders and has formed the Advanced LinEar collider study GROup (ALEGRO), to co-ordinate the preparation of aproposal for an advanced linear collider in the multi-TeV energy range. The UK has strong representation in all theseefforts, and in order to exploit its strength in this area it will be important that the UK groups are able to continue tocontribute to these and other collaborative efforts. The US LWFA and PWFA communities have produced a similarroadmap with focus on developing both of these technologies towards a high-energy, high-luminosity linear e + e − collider with the goal of technical design reports by the mid-2030s.Although, having an electron–positron linear collider as an ultimate aim for plasma wakefield acceleration andworking towards achieving a number of its parameters is very valuable, it is prudent to consider other first applications.One should however distinguish between a stand-alone all plasma acceleration collider, and an upgrade of conventionalcollider with plasma acceleration. While it is at this moment inconceivable to suggest an all-plasma e + e − collider,it is reasonable to consider plasma acceleration upgrades for either the ILC or CLIC colliders if construction of thefirst Higgs-factory or top-factory stage of either is approved. Given the rate of the progress of plasma accelerationtechnology, it is entirely possible to consider TeV upgrades based on plasma acceleration. Experiments in which high-energy electrons hit a target give insight into the fundamental structure of matter. TheHERA ep collider had a centre of mass energy of 320 GeV and was the only such collider; all other experiments had afixed target. With a high energy electron beam ( >
50 GeV ), further fixed-target experiments could be performed. Asmany have been performed previously, one would need to take advantage of improved detector instrumentation andconsider kinematic regions or physics still poorly understood, e.g. measurements which would have a strong impactfor LHC physics. This needs study and development: consideration of beam structure, review of past experiments,detector requirements and physics goals.The Large Hadron electron Collider (LHeC) is a proposed ep collider using the 7 TeV proton beam and electrons ofabout 60 GeV. One could consider generating the 60 GeV electron bunches by plasma wakefield acceleration. Currentdesigns of this would lead to a luminosity to lower than the conventional scheme. One could thereforeconsider a low-luminosity LHeC, in particular if the LHeC does not go ahead, called Plasma Electron Proton and IonCollider (PEPIC), but potentially at much lower cost, as well as trying to increase the luminosity.A more radical idea is for a very high energy electron proton (VHEeP) collider with a centre of mass energy of9 TeV, but modest luminosity. This uses the 7 TeV proton bunches to collide and to generate the wakefield to provide
UK Plasma Wakefield Accelerator Steering Committee
22K Roadmap for Plasma Wakefield Accelerator Research a 3 TeV electron beam. Higher luminosity is always valuable, but the extra energy reach opens up interesting particlephysics avenues even with modest luminosity. A kinematic region can be investigated where limited luminosity isneeded and where QCD and the structure of matter is not at all understood. A workshop looking further at thephysics opportunities took place in June 2017. However, the physics potential of low luminosity HEP experiments needs to be considered along with the potentialsaving on accelerator size.
As well as fixed-target experiments to investigate the (QCD) structure of matter, these and beam-dump experimentscan be used to look for exotic physics such as “dark photons” which could be an explanation for dark matter anda number of other puzzles in physics. Dark photons are expected to weakly couple between the Standard Modeland a new Hidden Sector. An experiment at CERN, NA64, is currently looking for dark photons using a 50-100 GeVelectron beam. If this beam energy could be achieved with an AWAKE scheme, a factor of 1000 more electrons ontarget could be produced, greatly increasing the sensitivity to dark photons. This is an ideal type of experiment as afirst application as there are less strict constraints on beam quality.Some of the above are being considered as a part of a CERN study on “Physics Beyond Colliders”, althoughother applications could exist. High energy electrons beams are valuable tools for characterisation of detectors for high energy physics as well asaccelerator systems and diagnostics. As there are so few around the world, having another facility could be of potentialvalue. The ability to generate short bunches could be useful for testing modern HEP detectors, which often requirepicosecond timing. Further, the high particle flux per bunch could enable testing of saturation effects in detectorsand an assessment of detector resilience under conditions of high occupancy and particle pile-up. Plasma-acceleratedpencil beams could also be used to test ultra-high granularity calorimeters. ultralow emittance beams could be usedas test beams for emittance preservation in staged approaches. These areas could prove to be near-term applicationsof plasma wakefield acceleration; indeed it is one of the potential applications being considered by the EuPRAXIAproject.
Uniquely, LWFAs combine ultra-relativistic electron beams with high-intensity electromagnetic fields. The generationof GeV-scale electron beams ( γ > ), together with focused laser intensities of − W cm − results inelectric fields in the rest frame of the electron which are a significant fraction of the critical field of electrodynamics,i.e. the Schwinger field. Several fascinating phenomena are predicted to occur in this high-field regime includingvacuum polarisation, pair production from a pure photon–photon collision, and radiation reaction. These phenomenaare not readily accessible by conventional accelerators and to date only one experiment at SLAC has been able tostudy electron–laser interactions at a significant fraction of the Schwinger field. Recent experiments with the Geminilaser, using laser-accelerated electron beams, have hinted at quantum effects. Accessing this regime will help understanding the largely unprobed non-linear aspects of quantum electrodynamics(QED), one of the most advanced theories in modern physics. This highly non-linear regime of QED is interestingnot only in its own merit, but also for the far-reaching implications that it has in a wide range of physics, such asparticle physics, plasma physics, astrophysics, and cosmology. For example, strong field QED is needed to explainthe radiative properties of ultra-massive objects, and is now included in models of particle acceleration with thenext generation of high-intensity lasers, which will provide peak powers approaching 10 PW and focal intensitiesexceeding W cm − . These lasers (ELI, Apollon, Vulcan 20PW, EXCELS...), which are expected to be fullyoperational within the next few years, will have a transformational effect in laser–plasma physics and laser-drivenparticle acceleration.There is still a substantial level of ambiguity in the theoretical models of laser–plasma interactions at these ultra-high intensities, where QED effects cannot be neglected. Moreover, one can envisage the possibility of exploitinglaser-driven ultra-relativistic electron beams to generate, via bremsstrahlung, high energy photon beams. One canthen imagine the construction of the first pure photon–photon colliders in which GeV-scale photons are collided withvisible photons, allowing the study of exotic phenomena such as pure photon conversion into matter and vacuumpolarisation.In order to extract meaningful data from experiments in this regime it will be necessary to increase the repetition UK Plasma Wakefield Accelerator Steering Committee
23K Roadmap for Plasma Wakefield Accelerator Research rate of LWFAs, reduce the energy spread and divergence, and increase the bunch charge.
Research on plasma wakefield acceleration has significant synergies with R&D in novel acceleration, light sources,high energy density, and high field physics. These research areas are connected on many levels: to a substantial degreethey use the same facilities and systems, and many groups engage in these topics in addition to their work on plasmaaccelerators. Hence, investment in the infrastructure for LWFA research would also significantly benefit research onlaser-driven proton and ion acceleration, and in high energy density physics. Investment into electron-beam drivenplasma wakefield acceleration would also support dielectric structure wakefield acceleration and applications; and co-located laser–plasma particle and light sources for manipulation and probing of matter benefit FEL-enabled research,as seen for example at the Matter in Extreme Conditions (MEC) station at LCLS, and as planned for the HelmholtzInternational Beamline for Extreme Fields (HiBEF). The science underpinning plasma accelerators lies at the intersection of optics, lasers, plasmas, and acceleratorscience; and their applications span the physical, medical, material and life sciences as well as industrial and defenceapplications. It is no surprise, then, that funding this area has proved challenging for funding bodies worldwide. Forexample in the US, plasma wakefield acceleration is largely funded by DoE’s High Energy Physics programme, whichdoes not directly include the development of novel light sources. In the UK, plasma wakefield acceleration spansthe EPSRC and STFC remits. A natural consequence of the cross-disciplinary nature of plasma accelerator researchareas is that it does not lie at the centre of gravity of any individual research council’s remit, which can make it moredifficult to obtain funding in this research area.The applications of plasma wakefield accelerators identified in §6 illustrate the potential for commercial exploitationof opportunities which will emerge over the next several years. This will require investment in capabilities andcapacities of UK facilities and laboratories, but also the development of funding mechanisms which can follow anapplication as it grows from concept to product. These opportunities match very well the increased UK emphasis onindustrialization of scientific research, such as the ISCF (see §6.3 and Fig. 6).The compactness, versatility and relative cost-effectiveness of plasma wakefield accelerators make them interestingfor ODA activities in context of the GCRF. Figure 7, also used for the STFC2017/2018 Accelerator Strategy Review,shows that various established and increasingly maturing applications of plasma wakefield accelerators fit well to UK’sODA strategic objectives and UN global goals.The need to foster multidisciplinary work, and to track potential applications from concept to product, wouldseem to make plasma wakefield accelerator research ideally suited to support through the recently established UKRI.Various avenues may be envisaged to provide the UK plasma wakefield accelerator community with the increase infunding needed for them to exploit their internationally leading position. This could, for example, be achieved byan increase of the STFC Accelerator Programme budget, dedicated EPSRC funding programmes, and/or sustainablecross-council funding. In the recent STFC Accelerator Strategy Review Roadmap it was proposed to establish a cross-council Novel Accelerator Action Plan, which includes plasma accelerators, (non wakefield) plasma-based proton/ionacceleration, and dielectric and structure-based electron and laser-beam acceleration. This approach has been furthersubstantiated by the accelerator community in the form of a selected potential large STFC “priority project” calledUKNOVA.It is hoped that the present roadmap, together with community efforts such as the establishment of the PWASC,will help the UK Research Councils appreciate the potential impact of plasma wakefield accelerators, and to developsuitable mechanisms to enable the UK to maintain international leadership in this area.
Plasma accelerators are experiencing worldwide growth, with many research projects and several facilities based onplasma accelerators and their applications being planned. Many of the breakthroughs achieved in this field have beenled by UK research groups. However, as is often commented, the UK community has not always been successful tomake the most of its own creativity, partly due to the difficulties of realizing appropriate funding levels discussed in§7.1. A major objective of this roadmap is to help ensure that the UK builds on its achievements to produce a new
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Figure 7: Correlations between applications of plasma accelerators and the ODA Strategic Objectives and UN globalgoals.generation of dependable plasma accelerators for fundamental research and applications.Although proof-of-principle experiments showcasing the capability of plasma accelerators for several applicationshave already been performed, translating these into the real world requires improvements in beam stability and quality.The UK groups will focus on this in the immediate future. Since some of this work involves technology developmentrather than just high impact science, programmatic access slots are required in the national facilities such as at CLF.University labs and centres could also play an important role in this area. It is worth emphasizing that programmaticaccess will enable improvements to bunch quality and accelerator reliability which, although not obviously of the mostimmedidate scientific importance, will drive future scientific and technological advances with very high impact.In Fig. 8 we identify major technological and scientific milestones, divided into high-power laser and linac facilities;driver technologies; wakefield accelerator development; and applications.It should be noted that the particle beam parameters given in Fig. 8 will not necessarily be available simultaneouslyor from the same technology. As research progresses the most appropriate approach for delivering each of the keybunch parameters will be established which in turn will determine the most suitable applications of LWFAs andPWFAs.Achieving the milestones identified in Fig. 8 will maintain the UK’s world-leading position in plasma accelerators.However, doing so will require investment in university groups and national facilites, as described in the followingsection.
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It has been recognised for many years now that “there is some evidence that the UK laser plasma wakefield accel-erator community is losing leadership due to relatively modest investment in this area compared with internationalcompetitors in the US and Europe”. This, coupled with the fact that other nations have increased their investmentsin this area, threatens to result in the UK losing leading its position.In order to allow the UK to maintain world leadership in this important field, and to meet the scientific challengesidentified above, it will be necessary to provide increased support for upgraded infrastructure and facilities, coupledwith a strong commitment to training the next generation of scientists in this field. These investments are expectedto have high return through industrial exploitation and by driving advances in other scientific disciplines.It has previously been suggested that structures for coordinating work in this area should be established. Thecommunity has responded e.g. by forming the PWASC, and by developing the current roadmap. These structurescould form the basis for efficient planning and management of enhanced investment in this research area.
The availability of state-of-the-art laser systems at the CLF has played a vital role in maintaining the UK’s world-leading standing in LWFA research. However, in recent years, a lack of timely upgrades to the facilities has severelylimited the rate of development. It is clear that other countries (notably Germany, France, China, US, and Korea)invest far more into plasma wakefield accelerators than the UK: for example, in Germany there are now over 10 lasersystems at the 100 TW to 1 PW level, with funding which includes beam-time costs and programmatic R & D.Currently, the preeminent centre for UK LWFA research is the Astra-Gemini laser at the CLF. The Astra-Geminilaser is one of the most mature tools for LWFA research in the world. However, its world-leading position is severelythreatened since: (i) it is a multi-purpose laser system, with only a single target area, which limits its availability forLWFA research to typically only six weeks per year; and (ii) it has not been upgraded for a decade.The limited available beam time means that the Astra-Gemini laser is significantly oversubscribed. The intensecompetition for beam time means that, understandably, experiments with potential for immediate high impact arefavoured by the Facility Access Panel, and that programmatic work aimed at improving the stability and quality ofplasma accelerators is less likely to be given beam time.In order to achieve significant improvements in the quality and stability of the bunches generated by laser–plasmaaccelerators it is imperative that the UK community has available dedicated facilities that support plasma acceleratorresearch. We note that these improvements to bunch quality are vital for developing the applications of plasmaaccelerators, and in due course this investment will lead to high impact results. We note also that many applicationsof LWFAs, such as betatron emission for high-resolution imaging, require operation at high pulse repetition rates.Hence to move from proof-of-principle tests to real-world applications it will be necessary to increase the laserrepetition rate above 10 Hz using the diode-pumped solid-state laser (DPSSL) technology.
UK Plasma Wakefield Accelerator Steering Committee K R o a d m a p f o r P l a s m a W a k e fi e l d A cce l e r a t o r R e s e a r c h R E Q U I R E D R E S O U R C E S Figure 8: Timeline for scientific and technological research and development on plasma wakefield accelerators. U K P l a s m a W a k e fi e l d A cce l e r a t o r S t ee r i n g C o mm i tt ee K Roadmap for Plasma Wakefield Accelerator Research
Research at CLF is strongly reinforced by university-scale laser centres in Strathclyde (SCAPA), Queen’s Belfast(Taranis-X), Oxford, and at ICL. These systems are used for testing novel concepts, preparation for periods of accessat national and international facilities, and, importantly, training the next generation of researchers. Investment inuniversity-based centres will therefore be vital for maintaining the health of the research field and to enable sustainedprogress towards the roadmap goals.There is currently no facility in the UK for particle-driven plasma accelerator research. So while the UK groupshaving leadership roles in large international campaigns and programmes (e.g. AWAKE, SLAC FACET), most of theexperimental work is naturally undertaken overseas. However, with the appropriate support and development of theCLARA facility in Daresbury Laboratory this situation could be changed, and in addition to provision of PWFA R&Dopportunities international groups and intellectual output could be attracted .For the particle-driven plasma accelerator research in the UK, it is important to continue to develop CLARA andestablish it as a full-fledged facility, capable of driving 250 MeV (or beyond) beams with high currents. The UKshould continue to have strong participation in the AWAKE campaign at CERN and at international facilities such asat SLAC FACET-II, DESY FLASHForward and Helmholtz VI, INFN and CERN CLEAR, along with exploiting hybridschemes with dedicated beam lines at CLARA and SCAPA.
Recommendation 8
Mechanisms should be sought to allow UK groups to play leadership roles in international high-visibilitycollaborations such as SLAC FACET-II, Helmholtz ATHENA, Laserlab Europe, ELI, ARIES etc., andto exploit these.
In the medium term, it is clear that the advancements in this field will require next-generation facilities. Tomaintain leadership in this area, we envisage that the UK would require a 100-PW-class laser, along with a 100 Hz,PW-class laser facility driving applications in 10–15 years. It is foreseeable that R&D advancement in this area wouldenable a plasma-accelerator-based FEL on these timescales. The community strongly believes that a EuPRAXIA-likefacility would provide an appropriate staging to de-risk the process.
Modelling and simulations with high performance computing (HPC) facilities is a large and crucial part of all plasmaaccelerator R&D, particularly for particle-in-cell (PIC) simulations. CLF provides a limited access to HPC facilities(SCARF) for users, Daresbury Laboratory offers Hartree, EPSRC manages ARCHER and many universities have theirown HPC facilities (with access fees, or with limited free access). However, these opportunities are not universal, anda programmatic, low-threshold access path for Novel Acceleration simulation work should be established in the UK.A wide range of codes are used by the community, ranging from commercial codes (e.g. VORPAL/VSim, developedby Tech-X) to open-source or self-developed codes. The UK community is particularly fortunate to be able to drawon the use of the EPOCH PIC code for laser/beam/plasma simulations. EPOCH was developed via an EPSRCgrant, and is free for academic users. It is currently supported by two EPSRC grants: until September 2019 by grantEP/P02212X/1; and until June 2022 via the Plasma-HEC Consortium (EP/R029148/1), which provides 40% FTEsupport for EPOCH. The EPOCH developers also run training workshops (funded by the CLF), which provide trainingin EPOCH as well in wider HPC skills. Although EPOCH is a mature code, continued development and advancesin capability and performance are needed to support the need for more sophisticated and/or more computationallyintensive simulations. To date EPOCH support for plasma accelerator work has been undertaken as part of widercode development; direct funding of an EPOCH developer (either partially or fully) to support Novel Acceleratorwork, would enable the implementation of new capabilities and faster resolution of bugs or technical problems, andwould ensure that this UK code remains internationally competitive.As noted in §5.3, as plasma accelerators mature it becomes increasingly important to develop beam transportsystems capable of handling plasma-accelerated beams. The links between groups working on start-to-end simulationsof RF-driven systems and those working on plasma wakefield simulations and modelling capabilities should thereforebe strengthened; provision of programmatic funding of high-performance computing could foster this.
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10 SUMMARY
The university groups, the accelerator institutes (the CI and JAI), and the CLF offer extensive, high-quality trainingprogrammes in all aspects of accelerator science which play a vital role in developing the next generation of scientistsand engineers. These courses attract a significant number of high quality graduate students from around the worldwho are an important factor in enabling the UK to maintain an internationally leading position in plasma acceleratorresearch and development.The UK’s training programmes in conventional and novel accelerator science are world leading. Since 2013 morethan 100 PhD students have graduated from the CI and JAI. This impressive number is enabled through a total oftypically 7–8 STFC quota studentships per annum, plus a similar number of studentships funded by other researchcouncils, scholarships, and research projects. The graduate courses offered by the accelerator institutes cover allaspects of conventional and novel accelerator science, such as beam dynamics, RF cavities, lattice design, magneticinsertion devices, and computational methods — as well as courses on laser physics, plasma physics, and applicationsof accelerators. In addition to lectures and classes, students receive training through seminars and design projects.It should be emphasised that only about 25% of the accelerator institutes’ graduate students work on plasmawakefield accelerators, i.e. typically 3–4 graduate students p.a. This number is too small to meet the current level ofresearch activity, and the number of graduate studentships in this area will need to be increased significantly to helprealise the opportunity for rapid growth in this research field.In many cases a key part of a graduate students work is undertaken at national facilities such as CLF, the DiamondLight Source, VELA, and CLARA. Since the beam time available at these facilities is very limited, university scalefacilities play a vital role in training students, as well as allowing them to undertake small-scale studies and to preparefor beam time at a national facility. The role played by in-house facilities could be enhanced by providing a meansto allow PhD students to undertake training and short periods of work at UK facilities outside their own institution:
Recommendation 9
A national scheme should be developed to enable mobility and knowledge transfer within UK institu-tions, to increase beam access, and to sustain collaborative efforts; this would provide a means to testnew concepts, train students, and prepare for beam time at national and international facilities.
These ambitious developments require increased support for young scientists working in these research areas.However, a current problem is that work on novel accelerators falls between the major areas of expertise on researchcouncil fellowship applications panels. To address this issue we propose the creation of “Novel accelerator fellowships”aimed at retaining our most able young scientists in this area of national importance. It is also vital that the numberof PhD students working in this area is increased substantially; one mechanism for achieving this could be increasingsupport for training PhD students by the UK accelerator institutes.
Recommendation 10
A new "Novel Accelerator Fellowship" scheme should be developed and the support available fortraining PhD students in novel accelerators should be increased.
10 Summary
Laser- and beam-driven plasma accelerators have made rapid advances in recent years, not least as the result ofworld-leading research by researchers based in the UK. Plasma accelerators can today generate electron beams withGeV-scale energies. They have been used to generate femtosecond-duration pulses of radiation from the visible to hardX-ray wavelengths, which points the way to a new generation of compact, synchronised sources of energetic particlesand visible-to-X-ray photons; and they lie at the heart of recent experiments to generate neutral electron–positronplasmas, which offers the enticing prospect of recreating extreme astrophysical environments in the laboratory. Inthe longer term, plasma accelerators could provide a way to reduce the size and cost of high-energy particle collidersused at the forefront of physics. It is clear that plasma accelerators have the potential to be a disruptive technology
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10 SUMMARY with many potential impacts in science, technology, and medicine.The UK has high international standing in this field, based on decades of impressive achievements within universit-ies, the accelerator institutes, and at national facilities. The UK continues to be at the forefront of new developmentsin plasma accelerators, but this position is threatened by a lack of investment in national and university-scale researchfacilities, as well as by the increase in research investment by other countries.In this roadmap we have summarised the state-of-the-art, provided a national and international perspective of thefield, and outlined the research and development needed further to advance the fields and to develop applications.We have also stated where continued or additional investment will be needed to develop plasma accelerators andtheir applications.The plasma accelerator community looks forward to working with the Research Councils, the Government, nationalfacilities, and industry to build on the achievements of UK scientists and realise the enormous promise of this excitingtechnology.
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B LIST OF ABBREVIATIONS USED
Appendix A Community consultation and roadmap input
The development of this roadmap was initiated by the UK Plasma Wakefield Accelerator Steering Committee(PWASC). This committee was established to represent UK groups working on plasma accelerators, and to helpcoordinate their activities, and its members are drawn from UK research groups, the Central Laser Facility, and thetwo Accelerator Science Institutes.The PWASC organized a Community Meeting to discuss, and receive feedback on, the first full draft of theroadmap. This meeting was held on 26th January 2018 and was attended by 30 representatives from universities,national facilities, and industry.The input received was used to compile a second full draft which was circulated to the community in December2018 for further comments.
Appendix B List of abbreviations used
Abbreviation Meaning
ALEGRO Advanced LinEar collider study GROupANA (ICFA panel on) Advanced and Novel AcceleratorsAWAKE Advanced WAKEfield experimentBNL Brookhaven National LaboratoryCALTA The Centre for Advanced Laser Technology and Applications, Rutherford Appleton LaboratoryCI Cockroft InstituteCLARA the Compact Linear Accelerator for Research and Applications, Daresbury LaboratoryCLEAR CERN Linear Electron Accelerator for ResearchCLF Central Laser Facility, Rutherford Appleton LaboratoryCSR Coherent synchrotron radiationDESY Deutsches Elektronen-SynchrotronDPSSL Diode-Pumped Solid-State LaserFACET Facility for Advanced Accelerator Experimental Tests, based at SLACFEL Free-electron laserHEP High Energy PhysicsHPC High Performance ComputingHPL High Power LaserICFA International Committee for Future AcceleratorsICL Imperial College LondonICS Inverse Compton ScatteringINFN Istituto Nazionale di Fisica Nucleare, ItalyJAI John Adam’s Institute for Accelerator ScienceLWFA Laser Wakefield AcceleratorNEXAFS Near Edge X-ray Absorption Fine StructurePIC Particle-In-CellPWFA Plasma Wakefield AcceleratorQED Quantum ElectrodynamicsRBE Rrelative biological effectivenessRF Radio-FrequencyRT RadiotherapySCAPA Scottish Centre for the Application of Plasma-based AcceleratorsTDS Transverse Deflection StructuresTRL Technology Readiness LevelVELA the Versatile Linear AcceleratorXANES X-ray absorption near edge structure
UK Plasma Wakefield Accelerator Steering Committee
31K Roadmap for Plasma Wakefield Accelerator Research
REFERENCES
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32K Roadmap for Plasma Wakefield Accelerator Research
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33K Roadmap for Plasma Wakefield Accelerator Research
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34K Roadmap for Plasma Wakefield Accelerator Research
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