Towards compact Free Electron Laser based on laser plasma accelerators
TTowards compact Free Electron Laser based on laser plasma accelerators
Marie Emmanuelle Couprie
Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, 91 192 Gif-sur-Yvette, France
Abstract
The laser invention more than fifty years ago was a major scientific revolution. Among the di ff erent possible gain media, the FreeElectron Lasers (FEL) uses free electrons in the period permanent magnetic field of an undulator, covering wavelengths from farinfra red to X-ray, with easy tuneability and high peak power. Nowadays, the advent of tuneable intense (mJ level) short pulseFELs with record peak power (GW level) in the X-ray domain sets a major step in laser development, and enables to explore newscientific areas, such as deciphering molecular reactions in real time, understanding functions of proteins. Besides, lasers have alsobeen considered for driving plasma electron acceleration. A high-power femtosecond laser is focused into a gas target and resonantlydrives a nonlinear plasma wave in which plasma electrons are trapped and accelerated with high energy gain of GeV / m. Nowadays,laser wakefield acceleration became reality and electron beams with multi-GeV energies, hundreds pC charge, sub-percent energyspread and sub-milliradian divergence can be produced. It is relevant to consider a FEL application to quality these laser plasmaproduced electrons. After having described the FEL panorama, the strategies towards laser plasma based acceleration based FELswill be discussed, including the mitigation of the large energy spread and divergence of these beams should be mitigated. Keywords:
Free Electron Laser, Laser Plasma Acceleration, Undulator
1. Introduction : the origins of the Free Electron Laser
In 1927, Einstein (1879-1955, Nobel prize in 1921) predictedthe energy enhancement by atom desexcitation [1] in the anal-ysis of the black-body radiation, while absorption and spon-taneous emission were the known light matter interactions atthat time. This process was named in 1924 stimulated emis-sion [2, 3]. First, a photon is absorbed and drives an atom toan excited state. The excited atom being unstable, it emits aspontaneous photon after a duration depending on the lifetimeof the excited level. Besides, when a photon is absorbed by anexcited atom, two photons with identical wavelength, direction,phase, polarization are emitted, while the atom returns to itsfundamental state. This stimulated emission emission was seenas addition of photons to already existing photons, and not asthe amplification of a monochromatic wave with conservationof its phase, without the notion of light coherence.
The electron beam in vacuum tubes witnessed a rapid andspectacular development in the beginning of the twentieth cen-tury for the current amplifier applications such as radiodi ff u-sion, radar detection, where high frequency oscillations wereneeded. In vacuum tubes, a free electron of relativistic factor γ given by γ = Em o c (with E its energy, m o the electron mass, e theparticle charge and c the speed of light) interacts with an elec-tromagnetic wave of electric field (cid:126) E : (cid:126) E = (cid:126) E sin ( ks − ω. t ) with k the wave number and ω the pulsation according to d γ dt = e (cid:126)β.(cid:126) Emc with β the normalised electron velocity. In a klystron [4] (see Synchrotron radiation and Free Electron Laser 25 d ⌅ dt = e ⇥ ⇤ . ⇥ Emc (69)with ⇥ ⇤ the normalised velocity of the electrons, with ⌅ = ⇤ ⇤ The magnetron is a high power vacuum tube where electron bunches passing through open cavities excite RF wavesoscillations by interaction with the magnetic field, the frequency being determined by the geometry of the cavity. Itcan act only as an oscillator for the generation of microwave signal from the direct current supplied to the tube. Thisdevice can not amplify the RF signal. It is now used for microwave ovens.The klystron, invented by the Russel and Sigurd Varian brothers [207] consists of two cavities (metal boxes alongthe tube), as shown in Fig. 19a. . In the first one, an electric field oscillates on a length s at a frequency ⌃ = ⌥ f ranging between 1 and 10 GHz (i.e. with corresponding wavelengths of 30-3 cm ). The electrons, generated at thecathode, enter in the first cavity where the input RF signal is applied. They can gain energy according to : W = s ⇥ ⇤ . ⇥ E dt ⇥ E . ⇤ cos t . s ⇤ = E . s . cos t (70)The XXX revoir equation XXXThe sign of W depends on the moment t when the electron arrives inside the cavity. W is modulated in timeat a temporal period T = ⌥ or spatial period ⇧ ⇤ . In average over the electrons, W = Fig. 19 klystron principle : a) klystron scheme, b) electron bunching by energy modulation in the klystron drift space, electrons accumulatein bunches, c) Phased electron in the second klystron cavity, electric field in red
In the second cavity, the electrons have the same phase with respect to the electromagnetic wave in the cavity, sincethey have been bunched (see in Fig. 19c). The second energy exchange is given by : W = electrons l ⇥ ⇤ . ⇥ E dt = N e EL cos t (71)with N e the number of electrons, L the interaction region in the second cavity, E the electric field. The phase ofthe electrons in the second cavity is ruled by the electrons themselves. The gain in electric field can be very high(practically, 10 ) . Figure 1: Klystron principle : klystron scheme (electron bunching by energymodulation in the klystron drift space, electron accumulation in bunches, andRF field amplification due to phased electron in the second klystron cavity
Fig. 1), electrons generated on a cathode enter in a first metalliccavity where an input GHz electric field is applied (an electricfield oscillating at a frequency ν = π f of several GHz ). Thesign of their energy gain ∆ W (cid:39) ecE . ∆ s . cos ( ω t ) depends onthe moment t when they arrive inside the cavity, ∆ W is thusmodulated in time at a temporal period T = πω or spatial pe-riod 2 π c β/ω . < ∆ W > electrons = ff erent phases. Then, the electrons enter into the drift spaceand accumulate in bunches. In the second cavity of interactionregion L , the bunched electrons have the same phase with re-spect to the electromagnetic wave. The second energy exchange ∆ W = N e ecEL cos ( ω t ) leads to very high electric field gains,from 3 to 6 orders of magnitude. Preprint submitted to Elsevier February 20, 2018 a r X i v : . [ phy s i c s . acc - ph ] F e b .3. Synchrotron radiation Synchrotron radiation, the electromagnetic radiation emittedby accelerated charged particles, is generally produced artifi-cially in particle accelerators. Its theoretical foundations estab-lished at the end of the nineteenth century [5, 6] were developedfurther [7, 8, 9, 10, 11, 12, 13]. After the first particle energyloss on a 100
MeV betatron [14], the first synchrotron radiationwas observed in the visible tangent to the electron orbit one yearlater on the 70
MeV
General Electric synchrotron, of 29 . m ra-dius and 0 . T peak magnetic field [15]. Radiation is emitted ina narrow cone of aperture 1 /γ .Radiation emitted by relativistic electrons performing trans-verse oscillations was first considered [16]. The electromag-netic field created by a relativistic particle in a periodic per-manent magnetic field (produced by undulators, consisting ofa succession of alternated poles) [17, 18] was calculated andobserved [19, 20]. For a planar undulator generating a sinu-soidal magnetic field (cid:126) B u = B u cos (cid:16) πλ u s (cid:17) (cid:126) z = B u cos( k u s ) (cid:126) z withthe undulator wavenumber k u : k u = πλ u , the emitted radiationalong the undulator periods interfere constructively accordingto λ n = λ u n γ (1 + K u / + γ θ ) with the deflection parameter K u = eB u λ u π m o c and θ the observation angle. The radiation is tune-able by changing the magnetic field or the electron energy. For doing a quantum microwave source using stimulatedemission in molecules instead of the amplification by an elec-tron beam, an excited molecule is introduced in a microwavecavity resonant at the frequency of the molecule transition. In1954, the first MASER (Microwave Amplification by Stimu-lated Emission of Radiation) is operated in the micro-waves[21] at Columbia Univ with NH molecule. For reaching theoptical spectral range, an open Fabry-Perot type resonant cav-ity (In a cavity in which the light makes round trips betweenthe two mirrors on which it is reflected) [22] is used insteadof a resonant cavity on its fundamental mode that would be-coming extremely small ( ∼ µ m ). These ”optical lasers” werenamed LASER for Light Amplification by Stimulated Emissionof Radiation [23]. ”For wavelengths much shorter than those ofthe ultraviolet region, maser-type amplification appears to bequite impractical. Although using of a multimode cavity is sug-gested, a single mode may be selected by making only the endwalls highly reflecting, and defining a suitably small angularaperture. Then extremely monochromatic and coherent lightis produced [22].” Lasers were achieved experimentally, withRuby [24, 25], He–Ne [26], AsGa [27] and others [28]. Limitsin extending lasers towards very short wavelengths were un-derlined : “As one attempts to extend maser operation towardsvery short wavelengths, a number of new aspects and problemsarise, which require a quantitative reorientation of theoreticaldiscussions and considerable modification of the experimentaltechniques used” ; “These figures show that maser systems canbe expected to operate successfully in the infrared, optical, andperhaps in the ultraviolet regions, but that, unless some radi-cally new approach is found, they cannot be pushed to wave-lengths much shorter than those in the ultraviolet region” [22]. J. M. J. Madey (1943–2016) considered that “A. Schawlowand C. Townes descriptions of masers and lasers coupled withthe new understanding of the Gaussian eigenmodes of freespace o ff ered a new approach to high frequency operation thatwas not constrained by the established limits to the capabili-ties of electron tubes” [29] and examined whether there was“a Free Electron Radiation Mechanism that Could Fulfill theseConditions” [30], considering di ff erent possible radiation pro-cesses. Stimulated Compton Scattering appeared very promis-ing [31]. Already investigated earlier [32, 33, 34, 35] TheCompton backscattering (CBS) radiation resulting from a head-on collision between a laser pulse and a bunch of relativisticparticles has a high energy E CBS : E CBS = γ E ph + ( γθ ) with E ph theenergy of the initial photon beam, θ the angle between the CBSphotons and the electron beam trajectory. The CBS radiationcould be tuneable by a change of the energy of the relativisticelectrons. The scattered radiation presents a low divergence forrelativistic electron beams (i.e. γ (cid:29) /γ . J. M. J. Madey had the idea to make thephenomenon more e ffi cient by using the magnetic field of anundulator [36]: “Relativistic electrons can also not tell the dif-ference between real and virtual incident photons, permittingthe substitution of a strong, periodic transverse magnetic fieldfor the usual counter-propagating real photon beam” [30].His proposed scheme (see Fig. 2) includes thus the electronbeam in the undulator field as the gain medium, and the opticalresonator, as for lasers. After first gain calculations in quan-tum mechanics [36], theory was developed in with various ap-proaches : plasma [37], distribution functions [38], relativisticmotion of the electrons in the undulator and energy exchange[39]. The electron beam progressing in the undulator emits syn-chrotron radiation, which is stored in the optical resonator. Anenergy exchange between the optical wave and the electronstakes place, leading the microbunching of the electron bunch at λ n separation (electron position being depending of the energy).The electrons are thus set in phase, radiate coherently and thelight is amplified. Saturation takes place by enhancement of en-ergy spread, or the undulator resonance condition unsatisfied. Figure 2: Scheme of the FEL oscillator with the gain medium consisting ofrelativistic electrons in the undulator
2. Free Electron Laser development
The first experimental demonstration of the FEL amplifica-tion (single pass gain of 7%) in the infra-red was performed in2 igure 3: Short wavelength FEL oscillators
Stanford in 1976 [40] on the super-conducting linear accelera-tor. The first FEL oscillation was achieved at 3.4 µ m in 1977360 mW average power, corresponding to an estimated 7 (500intracavity) kW peak power [41]. The second worldwide FELoscillation was then obtained in 1983 on the ACO storage ring,in the visible [42], and then followed by Coherent harmonicgeneration [43, 44] in the UV and VUV usng a Nd–Yag laser.The Stanford FEL has been operated with a tapered undula-tor in order to enhance the e ffi ciency [45], enabling the outputpower to be enlarged [46]. FEL oscillation was obtained at LosAlamos in 1983 at 9 − µ m , with nine orders of magnitude ofpower growth and a net gain of 17 %, leading to an intra-cavitypeak power of 20 MW [47]. A 4% e ffi ciency [48] was reachedwith undulator tapering. Besides linear accelerators and stor-age rings [49, 50], FEL oscillators were then developed on var-ious types of accelerators, such as Van de Gra ff , microtrons,energy recovery linacs. The developed FEL oscillators enabledto cover from infra-red to VUV spectral range (Fig. 3) withthe shorter wavelength obtained on the ELETTRA storage ringFEL [51, 52]. The limit is set somehow by the gain value com-pared to the mirror losses [53] submitted to drastic irradiationconditions [54]. FEL oscillators present a very high degree ofcoherence, both in transverse thanks to the optical resonator andin longitudinal close to the Fourier limit thanks to multi-passes. Figure 4: FEL Self Amplified Spontaneous Emission (SASE) configuration :spontaneous emission emitted along the undulator amplified in one single pass.
In parallel to high gain FEL studies [55, 56, 57, 58, 59], theproduction of coherent radiation from a self-instability, withoutthe use of an optical resonator was considered [60, 61, 62] andeven a short wavelengths [63]. The system starts from noisewith the undulator spontaneous emission which is amplified itin the high gain regime until saturation (see Fig. 4). More pre-cisely, the electrons communicate with each other through theradiation and the space charge field; they ”self bunch” on thescale of the radiation wavelength periods. A collective instabil-ity occurs where the electrons have nearly the same phase andemit collectively coherent synchrotron radiation [64, 65] over acooperative length. After a ”lethargy” period required for the initial pulse to build up, the light is then amplified exponen-tially with a gain length L go = √ k u ρ FEL (1 + ( σ γ /ρ FEL ) with ρ FEL the so-called Pierce parameter and σ γ the energy spread.This regime is called Self Amplified Spontaneous Emission(SASE), in reference to the Amplified Spontaneous Emissionin conventional lasers. The SASE spectral bandwidth is givenby the Pierce parameter ∆ λλ = ρ FEL , and the saturation powerby P sat = ρ FEL EI p with I p the peak current. Typically, the sat-uration power is reached after roughly 20 gain lengths, at thesaturation length L s . The interaction between the electrons isonly e ff ective over a cooperation length, the slippage (slippingof the emitted by one electron moves ahead by one wavelengthper undulator period) in one gain length, as L coop = λ √ ρ FEL [66]. The uncorrelated trains of radiation, which result from theinteraction of electrons progressing jointly with the previouslyemitted spontaneous radiation, lead to spiky longitudinal andtemporal distributions and poor longitudinal coherence, apartfrom single spike operation for low charge short bunch regime[67, 68].SASE experimental results were first obtained at long wave-length the mid eighties (saturated high gain amplification inthe mm waves (34 . GHz ) in (Lawrence Livermore NationalLaboratory / Lawrence Berkeley Laboratory (USA) collabo-ration) [69], superradiant emission at 640 µ m at MIT (USA)[70], observation of bunching at 8 mm [71] and SASE [72])at CESTA (France). Then, SASE was observed in the infra-red (20 − µ m at ISIR (Japan) [73], at SUNSHINE (USA)[74], at CLIO (France) in the mid-infrared (5 − µ m ) [75], atBNL (USA) at 1064 and 633 nm [76], at Los Alamos (USA)at 15 µ m [77]). Then five orders of magnitude of amplifica-tion and saturation at 12 µ m have been achieved (UCLA, LosAlamos, Stanford, Kurchatov collaboration) on the AdvancedFree Electron Laser (AFEL) linac at the Los Alamos NationalLaboratory [78, 79]. Saturation at 845 nm has been observedon the VISA (Visible to Infrared SASE Amplifier) experimenton the Accelerator Test Facility (ATF) at Brookhaven NationalLaboratory (USA) [80, 81].The beginning the the twentieth century saw the advent ofthe saturated SASE in the visible and UV (530 and 385 nm)in 2000 at Argonne National Laboratory (USA) [82, 83] on theLow-Energy Undulator Test Line (LEUTL). In parallel, non-linear harmonics at 423 and 281 nm were observed using theVISA SASE FEL at saturation [80]. In the same years, a majorstep was achieved with the observation of SASE radiation inthe VUV spectral range, with a 233 MeV electron beam fromthe Tesla Test Facility (Germany) presently called FLASH us-ing a photo-injector and superconducting accelerator modules(6 π mm . mrad emittance, 0 . kA peak current, 0 .
13 % relativeenergy spread), enabling a gain of 3000 at 109 nm [84] in 2000and then saturation [85] in 2001, i.e. twenty- five years after theFEL invention. Tunability in 80 − nm range was demon-strated, and a very high degree of photon beam transverse co-herence was observed. With higher peak current, the GW level(close to 1 µ J energy) had been reached in the 95 − nm spectral range [86] with a gain length of 67 cm . These resultscompeted the shortest wavelength achieved on a FEL oscilla-3ors (on a storage ring), making a turning point in the choiceof the type of the FEL accelerator driver and configuration.The path towards the X-ray domain with SASE radiation waspaved with new achievements, such as the SASE radiation inthe 60 − nm spectral range with an energy of 30 mJ onSCSS Test Accelerator (Japan) [87, 88], 6 . nm [89] and 4 . nm [90], i.e. in the water window on the fundamental at FLASH.Then, a new area started with the advent of hard X-ray FELs,with first LCLS in Stanford (USA) at 0 . nm , with saturationafter 60 m of undulators [91], followed by SACLA (Japan) in2011 down to 0 . nm [92], then PAL FEL (Korea) in 2016[93], Swiss FEL (Switzerland) [94] and European X FEL (Ger-many) [95] in 2017, while new projects are under development.Operation at short wavelengths requires high beam energiesfor reaching the resonant wavelength, and thus long undula-tors (0 . − km for 0 . nm ) and high electron beam density(small emittance and short bunches) for ensuring a su ffi cientgain. The obtention of SASE radiation at shorter wavelengthbenefited largely from the improvements of the linac elec-tron beam performance, thanks to the development of photo-injectors [96, 97, 98, 99] and more generally of the acceleratordevelopments towards colliders. Figure 5: Seeding scheme
Besides the spectacular advent of the powerful tuneable FELs(mJ energy per pulse), the FEL pulse spiky spectral and tem-poral distributions with the associated jitter still provide somelimitations for FEL use. Besides using low-charge short elec-tron bunches [100]], a chirped electron bunch associated withan undulator taper [101], the longitudinal coherence of a sin-gle pass FEL can be significantly improved by seeding with anexternal laser spectrally tuned on the undulator fundamental ra-diation, while intensity fluctuations are reduced and saturationis reached earlier (see Fig. 5). Non linear harmonics can also bee ffi ciently generated [102, 103, 104] in di ff erent configurationssuch as the High Gain harmonic Generation [105, 106, 107] :A small energy modulation is imposed on the electron beam byits interaction with a seed laser in a first undulator (the modu-lator) tuned to the seed frequency, it is is then converted intoa longitudinal density modulation thanks to a dispersive sec-tion (chicane) and in a second undulator (the radiator), whichis tuned to the nth harmonic of the seed frequency, the micro-bunched electron beam emits coherent radiation at the harmonicfrequency of the first one, which is then amplified in the radiatoruntil saturation is reached [108]. High order harmonics gener-ated in gas can also be used as a seed [109]. Such a scheme canbe put in cascade for wavelength reduction. According to theseed characteristics with respect to that of the electron bunch,di ff erent regimes such as super radiance [110], pulse splitting[111, 112] can be observed.FERMI@ELLETRA, the first seeded FEL users facility inTrieste (Italy) consists of two FEL branches, FEL 1 in the 100 − nm via a single cascade harmonic generation, andFEL 2 in the 20 − nm via a double cascade harmonic gener-ation [113, 114, 115]. Up-frequency conversion by a factor of192 [116]. The Dalian FEL (Dalian, China) covers 50 − nm [117]. Seeding with the FEL itself is also considered [118, 119]and is of particular interest for the X-ray domain: a monochro-mator installed after the first undulator spectrally cleans the ra-diation before the last amplification in the final undulator. Re-cently, self-seeding with the spectral cleaning of the SASE radi-ation has been experimentally demonstrated at LCLS [120, 121]and at SACLA [122]. Figure 6: Achieved FEL wavelengths versus year for various configurations(oscillators, coherent harmonic generation, SASE, seeding)
Fig. 6 shows the trend in FEL wavelength decrease versusyears : up to the century change, FEL oscillators were the mostsuitable candidates, while afterwards, single pass FEL such asSASE with their improved versions in terms of temporal coher-ence (seeded FEL) appeared the most adequate. This turn ismainly due to the improvement of the linear accelerator tech-nology, FEL community being benefiting from the develop-ments of high brightness electron beams required for future lin-ear colliders. The path has been long towards these unique tun-able intense X-ray FELs, with some projects that did fail. Morethan forty years have been spend between the first FEL in theinfra-red and the first X-ray FEL, both in Stanford.Present developments are oriented in providing further ad-vanced properties [123].The two-colour FEL concept can beapplied to the X-ray domain in the SASE regime, eithertuning the two series of undulators at di ff erent wavelengths[124, 125, 126], the delay being adjusted by a chicane, or byusing twin bunches at di ff erent energies [127], enabling also op-eration in the self-seeded case. In the external seeding case, onecan take advantage of the pulse splitting e ff ect [111] combinedwith chirp [112, 128], or apply a double seeding [129, 130].Several strategies are investigated in the quest towards in at-tosecond pulses and high peak power. The Echo Enabled Har-monic Generation [131] (EEHG) enables e ffi cient up-frequencyconversion by imprinting a sheet-like structure in phase spacevia a two successive electron-laser interactions in two undula-tors. Experimental results were obtained on harmonic 7 [132],15 [133] and 75 [134] on the Next Linear Collider Test Accel-erator and on the Shanghai FEL Test Facility [135]. The trend4s also to use superconducting high repetition rate linear accel-erator for FEL line multiplexing and for preventing from spacecharge e ff ects for some user experiments. Another approach in-vestigates compactness besides seeding and up-frequency con-version, by using novel acceleration techniques, such as laserplasma acceleration [136].
3. Strategies towards LPA based FELs
The laser invention led thus to free electron laser and to laserplasma acceleration. On can wonder then whether these twodi ff erent paths could join again for developing a laser plasmaacceleration based free electron laser. The idea arose ten yearsago [137, 138]. Issues related to this prospects are discussed. Inspired by the laser invention, in parallel to the Free Elec-tron Laser invention by J. M. J. Madey, emerged the idea oflaser wavefield acceleration two years later. The concept hasbeen described as follows by Tajima and Dawson [136]: ”An in-tense electromagnetic pulse can create a wake of plasma oscilla-tions through the action of the nonlinear ponderomotive force.Electrons trapped in the wake can be accelerated to high en-ergy. Existing glass lasers of power density 10 W / cm shownon plasma densities of 10 cm − can yield GeV of electron en-ergy per centimeter of acceleration distance. This accelerationmechanism is demonstrated through computer simulation. Ap-plications to accelerators and pulsers are examined”. Indeed, anintense laser pulse drives plasma density wakes to produce, bycharge separation, strong longitudinal electric fields, with ac-celerating gradient than can reach a 100 GV / m [139, 140], inwhich the electron with a proper phase can be e ffi ciently ac-celerated. Following the high power laser development thanksto chirped pulse amplification [141], significant electron beamacceleration was achieved [142, 143, 144, 145, 146].LWFA can nowadays produce electron beams in the few hun-dreds of MeV to severals GeV with a typical current of a fewkiloamperes with reasonable beam characteristics (relative en-ergy spread of the order of 1%, and a normalized emittance ofthe order of 1 π mm mrad) [147, 148, 149, 150, 151, 152, 153,154, 155, 156, 157, 158, 159]. However, all these ”best” per-formance are usually not achieved simultaneously and dependon the chosen configuration (bubble [139], colliding scheme[160], optical transverse injection [161], shock front injection[162], ionization injection [163, 164], plasma channel [142],frequency chirp [165] ...) and on staging [166, 167]. LWFA based undulator radiation has been observed, even atshort wavelengths [168, 169, 170, 171] and recently at LUX at9 nm [172]. The quality of the spectra do not meet yet whatis currently achieved and used on synchrotron radiation basedfacilities in terms of spectral bandwidth, intensity and stability.
Several conditions are required for the FEL amplification tobe possible, and they set specifications on the electron beam.The Pierce parameter ρ FEL is expressed as [173, 174]: ρ FEL = (cid:20) K u [ JJ ] ω p ω u (cid:21) / = γ k u (cid:18) µ o e K u [ JJ ] k u n e m o (cid:19) / (1)with ω p and ω u the plasma and undulator pulsations, µ o themagnetic permeability, [ JJ ] the planar undulator Bessel func-tion di ff erence term [175]. For the energy modulation andbunching to be maintain for insuring su ffi cient gain, the electronbeam should be rather ”cold”, its energy spread σ γ should besmaller than the bandwidth, i. e.: σ γ < ρ FEL . There should be aproper transverse matching (size, divergence) between the elec-tron beam and the photon beam along the undulator for insuringthe interaction. In consequence, the emittance (cid:15) n should not betoo large at short wavelength. The FEL gain increases with thebeam current provided that: (cid:15) n γ < λ π . The radiation di ff ractionlosses should be smaller than the FEL gain, i.e. the Rayleighlength should be larger than the gain length ( Z r > L go ). Thecondition can be smoothened in case of gain guiding. For longundulators, intermediate focusing is then put between undulatorsegments. High power short wavelength FELs require thus lowemittance electron beams (much smaller than 100 π mm . mrad and peak currents of the order of 100 A . Following undulator spontaneous emission observation, thisnew accelerating concept could thus be qualified by a FEL ap-plication. But achieving it remains to be demonstrated: the dif-ficulty comes from the intrinsic properties of the electron beam.Indeed, for an energy of a few hundreds of MeV, while linacbeams exhibit typically 1 mm transverse size, 1 µ rad diver-gence with 1 mm longitudinal size and 0 .
01 % energy spread,plasma beams more likely provide 1 µ m transverse size, 1 mraddivergence with 1 µ m longitudinal size and 1 % energy spread.Combined to the initial divergence, the energy spread can leadto significant emittance growth [176, 177, 178]. Collective ef-fects and coherent synchrotron radiation can also play a role[179]. The present LWFA electron beam properties are not di-rectly suited for enabling FEL amplification, and electron beammanipulation is required. The beam divergence requires a strong focusing.With conventional accelerator techniques, the usually re-quired quadrupole strength often excludes the use of electro-magnetic quadruoles. Permanent magnet quadrupoles, locatedclose to the electron source are more widely used. For example,to so-called developed QUAPEVA [180, 181, 182], made of twoquadrupoles superimposed are capable of generating a gradientof 200 T / m. The first quadrupole consists of magnets shaped asa ring and attaining a constant gradient of 155 T / m, and the sec-ond one made of four cylindrical magnets surrounding the ringand capable of rotating around their axis to achieve a gradienttunability of ± T / m . Each tuning magnet is connected to a5otor and controlled independently, enabling the gradient to betuned with a rather good magnetic center stability ( ± µ m ) andwithout any field asymmetry. They are installed on translationstages, allowing the magnetic center to be adjusted.The focusing can also be done with a plasma itself, with aplasma lens [183, 184], active plasma lensing [185] or a tran-sient magnetised plasma [186]. Plasma lens provides a radiallysymetrical focusing. A first approach consists in passing the electron beamthrough a demixing chicane, which sorts them in energy andreduces typically the slice energy spread by a factor of 10[187, 188, 189]. Taking advantage of the introduced correlationbetween the energy and the position, the slices can be focused insynchronization with the optical wave advance, in the so-calledsupermatching scheme [190]. The chicane scheme also enablesto lengthen the electron bunch, for it not to escape the electronbunch because of the slippage.A second approach to handle the large energy spread consistsin using a Transverse Gradient Undulator (TGU) [191, 192] asconsidered in the early FEL days. The concept has been ap-plied to the case of LPA [193, 194, 195, 196]. The transversegradient undulator presents usually canted magnetic poles, thatgenerates a linear transverse dependence of the vertical undula-tor field in the form of K ( x ) = K o (1 + α x ) with α the gradientcoe ffi cient. Associated to an optics with dispersion introduc-ing a transverse displacement x with the energy according to x = η ∆ γ/γ , the resonant condition can be fulfilled provided η = (2 + K o ) /α K o . This technique reduces the sensitivity of theFEL gain length dependence on the energy spread. Jena / KIT -10-5 0 5 10-10 -5 0 5 10 x [ mm ] y [mm] N y [ a . u .] PC N x [a.u.] β [ m ] β x β y -50 500.0 0.5 1.0 1.5 k [ m - ] s [m] a -10-5 0 5 10-10 -5 0 5 10 x [ mm ] y [mm] P y [ a . u .] PC P x [a.u.] β [ m ] β x β y -50 500.0 0.5 1.0 1.5 2.0 2.5 k [ m - ] s [m] b -10-5 0 5 10-10 -5 0 5 10 x [ mm ] y [mm] P y [ a . u .] P P x [a.u.] β [ m ] β x β y -50 500.0 0.5 1.0 1.5 2.0 2.5 k [ m - ] s [m] c Figure3:Measuredbeamprofileswithaprojection P onandacut C alongeachaxis.Thequadrupolestrength k thepositionofthedipolesandthebetafuncionsaredepictedforeachsetupwith β =0.005mand α =0.(a)Focusof60MeVonthesecondscreen;(b)focusof40MeVonthethirdscreenwithdeflectionbythedipoles;(c)parametersnecessaryfortheoperationoftheundulatorat40MeV.symmetricalalongthe x -axis(see.Fig.2a)thisasymmetryseemstoinfluenceallobservedprofilesofFig.3.Increasingtheenergyspreadfrom σ E =0.1%to3%,thesimulatedprofileinFig4cshowsthesamecharacteristicsasthemeasuredprofile:theverticalline,thediamondshapedmaximumandthehorizontallyslightlystretchedcenter.Thediscrepancymightstillbecausedbymisalignedmagnets,butalsotheasymmetricbeamprofileofthesource.ThesimulatedprofileinFig.4dforthesecondsetup(seeFig.3b)againshowsthesamecharacteristicasthemeasuredprofileapartfromtheweakersmearingalongtheverticalaxistohighervalues.Fornegativevaluesof x thesmearingis -2-1 0 1 2-3 -2 -1 0 1 2 3 x [ mm ] y [mm]c-1 0 1 x [ mm ] b-1 0 1 x [ mm ] a -4-2 0 2 4-4 -2 0 2 4 x [ mm ] y [mm]e-4-2 0 2 4 x [ mm ] d Figure4:left:Simulatedbeamprofilesatthepositionofscreen2with(a) σ x ′ , y ′ =1mrad,(b)themeasuredsourcedivergenceand(c)additionallyanenergyspreadof σ E =5%;right:simulatedbeamprofilesonscreen3withthemeasureddivergenceand5%energyspreadwith(d)setupofFig.3band(e)setupofFig3c. causedbythedeflectionoftheelectronsintheenergyrangebelowthecentralenergybytheoppositelypoleddipoles.ThelastprofileinFig.3cnowshowsastrongerdiscrep-ancyfromthesimulatedprofileinFig.4e.Ahorizontallinewithaslightmaximuminthecenterwouldbeexpected,butthereisasecondmaximumabovethebeamaxis,whichisprobablycausedbyalignmenterrors.Themeasurementshaveshownthatingeneralitispos-sibletotransportandshapethebeamoriginatingfromaLWFA.However,thedivergenceofsomemilliradiantandthesignificantenergyspreaddeterioratetheprofileconsider-ably.Toimprovethebeamtransportsystemamoreaccuratealignmentprocedure,anaperturetoreducethesourcedi-vergenceandanenergeticfiltershouldbeincludedinthesetup. SUMMARYANDOUTLOOK
InthiscontributionfirsttestsofthelinearbeamtransportsystemattheLWFAinJenawerepresented.Thesizeofthefociandtheshapeofthebeamprofiledidnotmatchtheexpectedvalues,astheparametersassumedforthede-signofthetransportsystemweredifferentfromthesourceparametersmeasured.Withanadaptionoftheseparame-tersinthesimulationsthemeasuredbeamprofilescanbereproduced.Apartfromthatitcanbeconcludedthatthealignmentprocedurehastobeimproved.Thetestofthedif-ferentcomponents,especiallythemagnets,wassuccessfulanditwasshownthatingeneralbeambasedalignmentofthequadrupolesispossible.ForsubsequentmeasurementsitisplannedtoimprovetheLWFAintermsofstabilityfrombunchtobunchandhigherelectronenergies.Furthermore,anaperturetolimitthedivergenceandanenergyfiltertonarrowtheenergyrangecouldbeapplied.
Stratclyde aretheelectronchargeandmass,respectively.Thenormalizedlaservectorpotential,initially, a ¼ eA / m e c " A isthevectorpotential,growsto a > whichresultsinatrailingevacuatedplasmabubbleintowhichelectronsareinjectedfromthebackgroundplasma.Electronbeamsexitingtheacceleratorareinitiallycolli-matedusingatripletofminiaturepermanentmagnetquadru-poles(PMQs). Thefieldgradientofeachquadrupoleis andelsewhere showthatthedurationoftheelectronbeamwithin1moftheacceleratoris Theundulator(length1.5m, N u ¼ k u ¼ B u ¼ K ¼ Thedistancefromacceleratorexittoundulatorentranceis3.52m.Undulatoroutputradiationisdetectedusingavacuumscanning monochromator (with platinum-coated toroidalmirrorand300lines/mmgrating)and16-bitCCDcamera.Thegratingispositionedfora344nmdetectionbandwidthcentredon220nmwitharesolutionofabout5nm.Threeelementsattenuatetheradiationsignal:thetoroidalmirror(peakreflectivityof65%),thegrating(peakefficiencyof25%at150nm),andfinallythequantumefficiencyofthecamera(25%acrosstherelevantspectralrange).Laserlightandplasmaemissionhasbeenblockedbyanaluminiumfoil(thickness800nm)positionedbeforetheundulatoratLanexscreenL3.RemovalofthePMQsenablestheintrinsicdivergenceandprofileoftheelectronbeamtobeobservedonLanexscreen L1. The mean r.m.s. divergence is 3.5 mrad(Fig.2(b)),whichisreducedto1mrad(Fig.2(c))uponinsertionofthePMQs,i.e.,nearcollimationofthecentralpartofthebeam.ThePMQsactasanenergybandpassfilter,impartinglargeangletrajectoriesonelectronsoutsideoftheiracceptancerange.Hence,outlyingswirlsthatareevi-dentintheLaneximagearerelatedtothelowenergy“tail”orpedestaloftheelectronbeam.Themaincentralpartofthebeam,comprisingthehigherenergyquasi-monoenergetic“mainpeak”electronbunch,isthesolepartofthebeamthatispreferentiallytransportedthroughtheundulator.ElectronenergyspectraobtainedwithES1(Fig.2(d))illustratethe
FIG.2.(a)PlanviewoftheALPHA-XLWFAbeamline,falsecolorimagesoftheelectronbeamprofileat(b)L1withoutPMQs,(c)L1withPMQsin-line,(e)L3and(f)L4and(d)threeexamplesofES1spectrawithmainpeakcentralenergyandchargeof115,109,95MeVand0.4,0.8,1.3pC,respectively. etal.
Appl.Phys.Lett. ,264102(2014)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.159.68.155 On: Wed, 02 Jul 2014 07:30:13
COXINEL / X-Five
LOASIS, Berkeley Maier, A. R. , Laser-Plasma Acceleration in Hamburg
N.Delbos ,S.Jolly , ,V.Hanus , ,P.Messner ,M.Kirchen ,V.Leroux , ,M.Schnepp ,D.Trosien ,M.Trunk ,P.A.Walker ,C.Werle ,P.Winkler CenterforFree-ElectronLaserScienceandDepartmentofPhysics,HamburgUniversity,Hamburg,Germany ELIBeamlines,Praha,CzechRepublic
Plasma-basedacceleratorspromiseultra-compactsourcesofhighlyrelativisticelectronbeams,espe-cially suited for driving novel x-ray light sources. The stability and reproducability of laser-plasmageneratedbeamsis,however,stillnotcomparabletoconventionalmachines. WithintheLAOLACol-laboration,theUniversityofHamburgandDESYworkcloselytogethertocombineuniversityresearchwiththeexpertiseofalargeandwell-establishedacceleratorfacility. Wewilldiscusstheexperimentalprogramm and plasma-related activities in Hamburg, with a special focus on the recently commis-sioned200TWlaserANGUS.Itdrivestwobeamlines,REGAEandLUX,tostudyexternalinjectionofelectronsfromaconventionalgunintoaplasmastage,aswellasplasma-drivenundulatorradiation.WepresentourprogressinintegratingthelaserintotheacceleratorinfrastructureatDESY,progresstowardsstablelaseroperation,aswellasthecommissioningoftheLUXandREAGEbeamlines. Asanoutlook,wewilldiscusstheexperimentalstrategiesinHamburgtowardsafirstproof-of-principleFELexperimentusingplasma-drivenelectronbeamsavailabletoday.
Margarone, D. ELIMAIA: ELI Multidisciplinary Applications oflaser-Ion Acceleration
P.Cirrone ,G.Cuttone ,G.Korn IoP-ASCR,ELI-Beamlines,Prague,CzechRepublic LNS-INFN,Catania,Italy
Themaindirectionproposedbythecommunityofexpertsinthefieldoflaserdrivenionaccelerationistoimprovetheparticlebeamfeatures(maximumenergy,charge,emittance,divergence,monochro-maticity,shot-to-shotstability)inordertodemonstratereliableandcompactapproachestobeusedfor multidisciplinary applications, thus, in principle, reducing the overall cost of a laser-based facil-ity compared to a conventional accelerator one. The mission of the laser driven ion target area atELI-Beamlines,calledELIMAIA(ELIMultidisciplinaryApplicationsoflaser-IonAcceleration),istoprovide stable, fully characterized and tuneable beams of particles accelerated by PW-class lasers,andtoo erthemtotheusercommunityformultidisciplinaryapplications. TheELIMAIAbeamlineis currently being designed and developed at the Institute of Physics of the Academy of Science oftheCzechRepublic(IoP-ASCR)inPragueandattheNationalLaboratoriesofSouthernItalyoftheNational Institute for Nuclear Physics (LNS-INFN) in Catania. An international scientific networkparticularly interested in future applications of laser driven ions for hadrontherapy, ELIMED (ELIMEDicalapplications),hasbeenestablishedaroundtheimplementationoftheELIMAIAexperimen-tal system. Nevertheless, this is only one of the potential applications of the ELIMAIA beamlinewhichwillbeopentoseveralproposalsfromamultidisciplinaryusercommunitysuchasradiobiology,timeresolvedradiographyofdi erentmaterials,beam-targetnuclearreactionsgeneratingisotopesforpositronemissiontomographyorproducinghighbrilliancesecondaryradiationsources(e.g. neutronsand alpha-particles), etc. The two research groups currently working on the implementation of theELIMAIA beamline have been performing numerical simulations and experimental tests at interna-tional high power laser facilities aimed at the optimization of the laser driven ion source on targetaswellastheionbeamtransportanddosimetricsystems. Preliminaryresultswillbepresentedanddiscussed. [1] 2nd ELIMED Workshop And Panel, Catania, Italy,18-19 October 2012, AIP Conf.Proc.1546(2013)
Andreas R. Maier | [email protected] | lux.cfel.de | LPAW 2015 | May 12, 2015 |
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Laser-Driven Plasma Acceleration REGAE external injection K. Flöttmann see B. Zeitler et al., Proceedings SPIE 8779 (2012)
ANGUS new 200 TW laser A. R. Maier group
LUX undulator radiation A. R. Maier group
Andreas R. Maier | [email protected] | lux.cfel.de | LPAW 2015 | May 12, 2015 |
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Laser-Driven Plasma Acceleration REGAE external injection K. Flöttmann see B. Zeitler et al., Proceedings SPIE 8779 (2012)
ANGUS new 200 TW laser A. R. Maier group
LUX undulator radiation A. R. Maier group
Andreas R. Maier | [email protected] | lux.cfel.de | LPAW 2015 | May 12, 2015 |
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ANGUS Laser & LUX Beamline dedicated beamline for undulator radiation re-built 4-nm experiment, but with accelerator equipment Andreas R. Maier | [email protected] | lux.cfel.de | LPAW 2015 | May 12, 2015 |
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Currently cleaning chambers… dedicated beamline for undulator radiation re-built 4-nm experiment, but with accelerator equipment Andreas R. Maier | [email protected] | lux.cfel.de | LPAW 2015 | May 12, 2015 |
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Matthias Schnepp | [email protected] | lux.cfel.de | Laser-meeting | January 22, 2015 |
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ANGUS laser lab Mittwoch, 21. Januar 15 lab conditions ‣ better 0.1°C rms temperature stability ‣ stable by few % relative humidity ‣ consequently remove all heat sources ‣ get everything in water-cooled racks ‣ challenge is to keep the standards up high in day-to-day operation ANGUS Laser Lab end 2012 2015
Andreas R. Maier | [email protected] | lux.cfel.de | LPAW 2015 | May 12, 2015 |
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In focus ! Out of focus ! spectrum w/ referencenear- and farfield at the output get centroids of NF/FF and display trend chart on laser position and directiononline energy / power w/ trend chartbuilt with TINE/DOOCS/jddd Andreas R. Maier | [email protected] | lux.cfel.de | LPAW 2015 | May 12, 2015 |
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SCIENCE Control System end 2012 C Dominik C. Trosien, Matthias Schnepp, Vincent A. G. Leroux, Spencer W. Jolly, Byunghoon Kim, Andreas R. Maierlux.cfel.de | DPG Frühjahrstagung, Wuppertal | March 9th, 2015
BMBF
FSP302
Monitoring Tools
LAOLA.
TINE Control System
Quality and stability monitoring software for a 200TW laser ! Power/energymeasurement: • After everyamplificationstage• Photodiodesfor MHz andkHz range(power)• Thermoelectricsensorsfor 5Hz Pulses (energy) ! Temperaturemeasurement: • Temperaturemeasurementofthedifferent lasercrystals• Measurement ofair/tabletemperatureofeachlaserbox ! Spectrometers • togetthelaserspectrumtothecorrespondingenergy/ power values ! Cameraserver: • tomonitorthebeam-pointingin crucialpartsofthelaserchain ! Future plan: humiditysensors " Big numberofmonitoredimportantlaserparameters ! Integration intotheDESY Tine Control System " tobeabletoaccessitfromanyDESY pcandtostorethedatapermanently ! TINE: Three-fold Integrated Networking Environment • Multi-Platform (Win, UNIX, MACOS,…)• Multi-Protocol (UDP, TCP, IPX, and PIPEs) • Multi-ArchitectureClient-serverPublisher-SubscriberProducer-Consumer " Plug andPlay ! LocalStorage (for 90 days) • Short-term: 10min with1/s, long-term (>10min) with1/15min whenchanges<10% ! Central Archive (for years) • Adjustablefor all kindsofneeds " Professional andapprovedsolutionfor dataarchiving
Long-term Analysis / Benefits ! Wearenowabletoseehowthesystemreactson: • Perturbationslike vibration(peopleworkingin thelab)• Temperaturechanges• Influence/qualityofthecoolingcircuit• Degradation overtime • Warm uptime togetstable• Energycorrelationsbetweentheamplificationstages• Correlationsbetweenenergy, temperature, beam pointingandspectrum ! Thereforewecanlocateandsolveproblemsfaster " Importantfor a reliableand stablelaser
Power/Energystabilitymeasurementsoftheoscillatorandthewholelaserchain:A.Measurement oftheML power lossoftheoscillatorduringnormal workingconditionsB.Fullchainstabilitymeasurement, rightafter startingthelaserC.Observation oftheoscillatorbehaviorwhilekeepingitin ML B Control Panels and Analysis Software ! Control panels showing all important laser parameters for all amplification stages ! Data access for analysis: • MatLab• Different in-house java applications ! Monitored values : • Picture of the beam spot,• beam centroid, • energy trend and latest value, • temperature, • current spectrum and reference spectrum
Angus Laser in Concept !" @')+'
B(CD)+""()
E/= @')+' A UHH, CFEL, Hamburg
EuPRAXIA
SIOM, China
ELI, Czech Republic
ImPACT, Japan
Figure 7: Test experiments around the world
Several experiments (see Fig. 7) are under way.The COXINEL (SOLEIL, LOA, PhLAM, France) [197, 198,199] project, part of the LUNEX5 one [200, 201] aims at FELamplification at 200 nm at typically 180 MeV, before increas-ing the energy up to 400 MeV for radiation down to 40 nm.Electrons are generated by the ”salle Jaune” 2x60 TW laser inionization configuration (see Fig. 8) . Strong focusing variablestrength permanent magnet quadrupoles located very close to the electron generation source handles the large electron beamdivergence. A energy de-mixing chicane then deviates the elec-trons by 32 mm in horizontal, sorting them out in energy. Theelectron bunch duration is lengthened, for the photons not toescape from the electron beam distribution because of the slip-page (delay between photons and electrons). A second set ofquadrupoles located in front of the undulator (2 m long in-vacuum U20 or U18, then 3 m long cryo-ready undulator [202])permits to perform a chromatic matching in the FEL interactionregion, with a proper setting of the chicane. Each slice can befocused in synchronisation with the optical wave progress alongthe undulator. Simulations show an increase of FEL power inthe supermatching condition [190]. The electron beam has beenproperly transported along the line thanks to a specific beampointing alignment compensation enabling the separate com-pensation of position an dispersion. Undulator radiation hasbeen observed.
Figure 8: Picture of the COXINEL experiment
The set-up at LBNL (USA) consists of electrons producedby a 100 TW laser in a gas jet, an active plasma lens [185], achicane, a triplet, the THUNDER undulator and the magneticbeam dump. A stable jet-blade has been developed [203, 204].At LUX (DESY / MPG / Univ. Hamburg), a 200 TW laserproduces the electrons since 2016, and 9 nm undulator radia-tion has been measured in 2017 [172]. The scheme for the FELconsiders a demixing chicane. In the frame of the ImPACT col-laboration in Japan, e ff orts are conducted to reduce the emit-tance and the energy spread, the pointing stability, with a veryshort undulator period (4 mm) for 0.4 T peak field.LPA based FEL experimented using the transverse gradi-ent undulator (TGU) are implemented in a F. Shiller Univ.,Jena / KIT collaboration using the JETI-40 laser, focused in a3 mm gas cell, an achromatic transport line, a superconductingTGU [205] and in Shanghai [206] with a 200 TW laser.
4. Conclusion
Among the panorama of light sources [207, 208, 209], theadvent of X-ray Free Electron Laser implemented on conven-tional linear accelerator took place nearly 40 years after theFEL invention, thanks to the technological developments madefor colliders and step by step progresses in the FEL domain. Inparallel, the spectacular development of laser plasma accelera-tion (LPA) with several GeV beam acceleration in an extremely6hort distance appears very promising. As a first step, the qual-ification of the LPA with a FEL application sets a first chal-lenge. Still, energy spread and beam divergence do not meetthe stat-of-the-art performance of the conventional accelera-tors and have to be manipulated to fulfill the FEL requirement.Several intermediate results are very encouraging in the pathtowards LPA based FEL. Indeed, undulator radiation (sponta-neous emission) has been seen with a simple first focusing andafter transport, at 5 Hz, down to 9 nm (very short photon du-ration, not very intense) at LUX. The electron beam proper-ties through a transport line, including alignment are controlledat COXINEL. On paper solutions for FEL amplification withtypical LPA beam parameters exist with 1 π mm.mrad, 1 µ m ,1 mrad, 1 % energy spread beam properties. In parallel, im-provements of LPA performance are under way, with for ex-ample, further LPA characterization and control leading to 3.5pC / MeV, few percent energy spread electron beams. A designstudy is carried out at an European level with the EuPRAXIAcollaboration [210]. FEL amplification remains very challeng-ing and constitutes an real full scale example of a demandingLPA application. Besides, sensitivity to parameters has also tobe studied in depth: deviations from the optimum parameterscan make the amplification no more possible; shot to shot vari-ations on the electron parameters and day to day reproducibil-ity could be very critical for setting an optimum situation forattempting amplification.
5. Acknowledgments
This work was supported by the European Research Coun-cil under Grant COXINEL (number 340015, PI M. E. Cou-prie); the EuPRAXIA European Design study (653782), and theFoundation de la Coop´eration Scientifique for the QUAPEVAcontract (2012-058T).
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