Francesco Velotti

Acceleration of electrons in the plasma wakefield of a proton bunch

High-energy particle accelerators have been crucial in providing a deeper understanding of fundamental particles and the forces that govern their interactions. To increase the energy of the particles or to reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma wakefield acceleration1–5, in which the electrons in a plasma are excited, leading to strong electric fields (so called ‘wakefields’), is one such promising acceleration technique. Experiments have shown that an intense laser pulse6–9 or electron bunch10,11 traversing a plasma can drive electric fields of tens of gigavolts per metre and above—well beyond those achieved in conventional radio-frequency accelerators (about 0.1 gigavolt per metre). However, the low stored energy of laser pulses and electron bunches means that multiple acceleration stages are needed to reach very high particle energies5,12. The use of proton bunches is compelling because they have the potential to drive wakefields and to accelerate electrons to high energy in a single acceleration stage13. Long, thin proton bunches can be used because they undergo a process called self-modulation14–16, a particle–plasma interaction that splits the bunch longitudinally into a series of high-density microbunches, which then act resonantly to create large wakefields. The Advanced Wakefield (AWAKE) experiment at CERN17–19 uses high-intensity proton bunches—in which each proton has an energy of 400 gigaelectronvolts, resulting in a total bunch energy of 19 kilojoules—to drive a wakefield in a ten-metre-long plasma. Electron bunches are then injected into this wakefield. Here we present measurements of electrons accelerated up to two gigaelectronvolts at the AWAKE experiment, in a demonstration of proton-driven plasma wakefield acceleration. Measurements were conducted under various plasma conditions and the acceleration was found to be consistent and reliable. The potential for this scheme to produce very high-energy electron bunches in a single accelerating stage20 means that our results are an important step towards the development of future high-energy particle accelerators21,22.Electron acceleration to very high energies is achieved in a single step by injecting electrons into a ‘wake’ of charge created in a 10-metre-long plasma by speeding long proton bunches.

Beam Simulation Studies for the Upgrade of the SPS Beam Dumping System

The SPS at CERN currently uses a beam dumping system that is installed in the long straight section 1 (LSS1) of the SPS. This system consists of two beam stopper blocks for low and high energy beams, as well as two vertical and three horizontal kicker magnets, which deflect and dilute the beam on the dumps. Within the frame of the LHC injector upgrade project (LIU) the beam dumping system will be relocated to long straight section 5 (LSS5) and upgraded with an additional vertical kicker, new main switches and a single new beam dump, which covers the full energy range. The impact of a possible increase of the vertical kicker rise time on the beam has been studied in simulations with MAD-X for the different optics in the SPS. Furthermore, the impact on the beam in failure scenarios such as the non-firing of one kicker has been investigated. The results of these studies will be presented and discussed in this paper. Operational mitigation methods to deal with an increased rise time will also be discussed. INTRODUCTION The SPS presently uses an internal beam dumping system, which consists of two separate beam dump blocks for low and high energy beams installed in the long straight section 1 (LSS1, Fig. 1) [1]:  TIDH, energy range 14 – 28.9 GeV  TIDVG, energy range 102.2 – 450 GeV There exists a forbidden zone for beam energies between 28.9 GeV and 102.2 GeV, where no programmed beam dump is possible. The deflection onto the dump blocks is performed with two vertical kicker magnets (MKDV). Three horizontal kicker magnets (MKDH) dilute the beam on the dump blocks to reduce the beam density on their front faces. Figure 1: Layout of the present SPS beam dumping system (Figure courtesy of F. M. Velotti). The present SPS beam dumping system (SBDS) has several limitations and issues [1, 2]:  Incompatibility of upgrading present TIDVG for High-Luminosity LHC (HL-LHC) beam parameters;  Production of high air activation;  High activation of the narrow area around the TIDVG without the possibility of full shielding;  Interference with SPS injection system, which is also located in LSS1;  The above mentioned forbidden energy zone;  MKDV magnet reliability issues at high energy. These limitations will be addressed in an upgrade of the SBDS [2, 3] in the frame of the LHC injector upgrade project (LIU) [4], which aims for upgrading the LHC injector chain to enable the production of high-brightness beams required for the HL-LHC era. For the LIU SBDS upgrade, the following changes are planned [2]:  The relocation of the SBDS from LSS1 to LSS5. This will solve the interferences with the injection system;  Replacing the two separate beam dump blocks by one newly designed for the entire energy range;  Installation of an additional MKDV magnet with the full system operated with reduced voltage to decrease the risk of high-voltage breakdowns;  Upgrade of MKDV main generators with new solid state switches;  A new dump external shielding to reduce the surrounding dose. The upgrade of the generators is currently under development and it is not clear if the present MKDV rise time of 1.1 μs can be preserved with the new system. A slight increase of the rise time to 1.3 μs might be necessary. To study the impact of the increased rise time on the beam, tracking simulations were performed using MAD-X [5]. MKDV RISE TIME The studies were performed using the present kicker waveforms, shown in Fig. 2, since the new waveforms are not yet available, scaled to the expected operational voltage. For the 3rd MKDV, which will be installed during the Figure 2: Waveforms of the vertical (MKDV) and horizontal kickers (MKDH) used for the simulation. ________________________________________ * Email: christoph.hessler@cern.ch 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-TUPAF031 04 Hadron Accelerators T12 Beam Injection/Extraction and Transport TUPAF031 747 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

Commissioning of beam instrumentation at the CERN AWAKE facility after integration of the electron beam line

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) is a project at CERN aiming to accelerate an electron bunch in a plasma wakefield driven by a proton bunch. The plasma is induced in a 10 m long rubidium vapor cell using a pulsed Ti:Sapphire laser, with the wakefield formed by a proton bunch from the CERN Super Proton Synchrotron (SPS). A 16 MeV electron bunch is simultaneously injected into the plasma cell to be accelerated by the wakefield to energies in the GeV range over this short distance. After successful runs with the proton and laser beams, the electron beam line was installed and commissioned at the end of 2017 to produce and inject a suitable electron bunch into the plasma cell. To achieve the goals of the experiment, it is important to have reliable beam instrumentation measuring the various parameters of the proton, electron and laser beams. This contribution presents the status of the beam instrumentation in AWAKE and reports on the performance achieved during the AWAKE runs in 2017.

JACoW : Phase Space Folding Studies for Beam Loss Reduction During Resonant Slow Extraction at the CERN SPS

The requested number of protons slow-extracted from the CERN Super Proton Synchrotron (SPS) for Fixed Target (FT) physics is expected to continue increasing in the coming years, especially if the proposed SPS Beam Dump Facility is realised. Limits on the extracted intensity are already being considered to mitigate the dose to personnel during interventions required to maintain the extraction equipment, especially the electrostatic extraction septum. In addition to other on-going studies and technical developments, a reduction of the beam loss per extracted proton will play a crucial role in the future performance reach of the FT experimental programme at the SPS. In this paper a concept is investigated to reduce the fraction of beam impacting the extraction septum by folding the arm of the phase space separatrix. Beam dynamics simulations for the concept are presented and compared to the phase space acceptance of the extraction channel. The performance potential of the concept at SPS is evaluated and discussed alongside the necessary changes to the non-linear optical elements in the machine.

Analysis and Operational Feedback on the New Design of the High Energy Beam Dump in the CERN SPS

The CERN’s Super Proton Synchrotron (SPS) high energy internal dump (Target Internal Dump Vertical Graphite, known as -TIDVG) is required to intercept beam dumps from 102.2 to 450 GeV. The equipment installed in 2014 (TIDVG#3) featured an absorbing core composed of different materials surrounded by a water-cooled copper jacket, which hold the UHV of the machine. An inspection of a previous equipment (TIDVG#2) performed in 2013 revealed significant beam induced damage to the aluminium section of the dump, which required imposing operational limitations to minimise the risk of reproducing this phenomenon. Additionally, in 2016 a vacuum leak was detected in the dump assembly, which imposed further limitations, i.e. a reduction of the beam intensity that could be dumped per SPS supercycle. This paper presents a new design (TIDVG#4), which focuses on improving the operational robustness of the device. Moreover, thanks to the added instrumentation, a careful analysis of its performance (both experimentally and during operation) will be possible. These studies will help validating technical solutions for the design of the future SPS dump to be installed during CERN’s Long Shutdown 2 in 2020 (TIDVG#5).

Analysis of the SPS Long Term Orbit Drifts

The Super Proton Synchrotron (SPS) is the last accelerator in the Large Hadron Collider (LHC) injector chain, and has to deliver the two high-intensity 450 GeV proton beams to the LHC. The transport from SPS to LHC is done through the two Transfer Lines (TL), TI2 and TI8, for Beam 1 (B1) and Beam 2 (B2) respectively. During the first LHC operation period Run 1, a long term drift of the SPS orbit was observed, causing changes in the LHC injection due to the resulting changes in the TL trajectories. This translated into longer LHC turnaround because of the necessity to periodically correct the TL trajectories in order to preserve the beam quality at injection into the LHC. Different sources for the SPS orbit drifts have been investigated: each of them can account only partially for the total orbit drift observed. In this paper, the possible sources of such drift are described, together with the simulated and measured effect they cause. Possible solutions and countermeasures are also discussed.