HORIZON 2020 EuPRAXIA Design Study

[Proceedings of the 8th International Particle Accelerator Conference : IPAC2017 14 - 19 May 2017, Bella Center, Copenhagen, Denmark]Editors: Volker RW Schaa (GSI, Darmstadt, Germany); Gianluigi Arduini (CERN, Geneva, Switzerland); Juliana Pranke (ESS, Lund, Sweden); Mike Seidel (PSI, Villigen, Switzerland); Mats Lindroos (ESS, Lund, Sweden) 8th International Particle Accelerator Conference, IPAC2017, Copenhagen, Denmark, 14 May 2017 - 19 May 2017 ; Geneva, Switzerland : JACoW,  1265-1268(2017). doi:10.18429/JACoW-IPAC2017-TUOBB3

section_content: The EuPRAXIA collaboration is the first plasma accelerator collaboration on this scale bringing together 16 European partner laboratories and additional 22 associated partners from the EU, Israel, China, Japan, Russia and the USA [1]. EuPRAXIA is structured into 14 working packages each headed by two work package leaders from different institutions. Eight work packages receive EU funding and their topics include: plasma and laser simulations (WP2), plasma accelerator structures (WP3), laser design (WP4), conventional beam physics (WP5), FEL radiation (WP6), and a table-top test beam for HEP and other applications (WP7). Two further EU work packages are concerned with the management of the collaboration (WP1) and the outreach to the public (WP8). In-kind work packages (WP9 -WP14) include additional approaches: beam driven plasma acceleration PWFA (WP9), hybrid acceleration schemes (WP14), alternative radiation generation (WP13) and laser sources such as fiber lasers (WP10). WP11 and WP12 connect to prototyping on plasma-based FEL's and facility access for experiments until 2019. Industry partners Amplitude Technologies, Thales and TRUMPF Scientific take part in the scientific advisory board and contribute their experience towards a successful completion of the design report.
section_name: INTRODUCTION

section_content: Scientists, medical doctors and engineers have used radio-frequency (RF) based particle accelerator beams for ninety years to probe nature, to produce new particles, to generate light of exquisite quality or to irradiate tumors. The accelerators are of outstanding quality, but have grown in size and cost due to the materials used for construction, which can only sustain accelerating fields of around 100 MV/m before electrical breakdown occurs. 

Plasma accelerators are not subject to these electrical breakdown limits and the accelerating field reaches 100 GV/m, three orders of magnitude larger than in an RF accelerator. As a consequence, the size of plasma accelerators can potentially be quite small, reducing kilometer scale machines to the meter scale. A new generation of cost-efficient and compact accelerators could open completely new usages of particle accelerators, for example in hospitals and universities. This requires suitable stability and repetition rates. 

The great potential of plasma waves for particle acceleration was first recognized by Veksler [2] and Tajima and Dawson [3]. The longitudinal plasma waves can be excited by both electron beams (PWFA) or intense laser pulses (LWFA) and are well suited for accelerating charged particles to relativistic energies [4]. Electron beams that are accelerated inside a plasma accelerator structure (PAS) can originate from the background plasma within the PAS itself ("internal injection") or from an accelerator that is situated in front of the PAS ("external injection"). Within the last two decades, the beam quality of LWFA accelerators has significantly improved [5] [6] [7] [8] [9] [10] [11] [12] [13] and the current peak energy lies at 4. 2 GeV [14]. Using these beams, various types of X-ray radiation such as betatron, synchrotron and undulator radiation down to the waterwindow wavelengths were produced [15] [16] [17] [18] [19] [20] [21]. While several tens of laboratories use laser systems to accelerate electrons, few laboratories have the electron beam needed for beam-driven plasma acceleration [22] [23] [24] [25] [26] [27]. FACET at SLAC achieved energy doubling within a single electron beam in 2007 [24] and energy was transferred successfully from a drive beam to a witness bunch in 2014 [25]. 

In the EuPRAXIA study, both laser driven and beam driven approaches as well as combined plasma acceleration schemes -using LWFA-produced beams as drivers of PWFA [28] [29] [30] -are taken into consideration. The final EuPRAXIA design report in 2019 will include various configurations of a possible EuPRAXIA facility. Depending on available budget and the targeted science case, one of these options, or a combination of options, might be  Configuration 5: PWFA with LWFA produced electron beam (hybrid schemes). In addition to the 5 GeV electron beam, the facility aims to provide a medical imaging X-ray source as well as FEL radiation ultimately concentrating in the range between 1 nm and 0. 1 nm. TW laser pulses synchronized to the electron and X-ray radiation will be available in the user areas. Parameter tables for medical imaging and a tabletop test beam for HEP and other applications are currently being finalized.
section_name: PLASMA ACCELERATION

section_content: The laser used in the LWFA cases is being studied in work package 4 (WP4) with colleagues from Thales and Amplitude industry. WP4 reviewed current laser systems in 2016 [32] and proposed preliminary specifications of the EuPRAXIA laser, the so-called "100 cube" laser challenge (energy of 100 Joule, a pulse length of 100 fs (FWHM), and a repetition rate of 100 Hz, with a contrast of 10 10 at 10 ps). The present work towards this challenging goal disfavors a complete Ti:Sa laser system and is considering a diode-pumped solid-state laser pumping scheme. A second laser system, used for the plasma injector [33], will operate at lower energy and shorter pulse length. Design work on the drive beam for the PWFA case is being performed in WP5. One option under discussion is that both configuration 2 (LWFA) and 4 (PWFA) use at low energy the same S-band injector and RF linac [34]. The simulated transverse phase space of a possible electron drive beam for a PWFA application is shown in Fig. 1. This electron beam has an energy of 548 MeV, a peak beam current of 1 kA, transverse normalized emittances of 1 μrad m and an energy spread of below 0. 07 %. After acceleration through S-band and X-band structures, the beam is focused by both conventional, electromagnets, and permanent quadrupole magnets before entering the plasma.
section_name: LASER AND BEAM DRIVERS

section_content: Plasma components necessary for the design of the PAS were reviewed by WP3 in 2017 [35] in which published experimental results were examined and compared not Figure 2 : PIC simulation [36] of a LWFA case (configuration 2). The laser pulse (red) propagates the plasma (electron density shown in blue) from left to right and excites a wakefield, which accelerates electrons (shown in green) from 0. 1 to 1 GeV in 2. 5 cm. only in terms of achieved electrons properties, but also regarding their reliability, stability, or scalability to larger electron energy, or repetition rate. The proposed criteria from [35] for selecting a specific PAS will be used to decide which types of PAS will ultimately be incorporated into the design report. 

Figure 2 shows a particle-in-cell (PIC) simulations [36, 37] performed with the OSIRIS code [38] in which a 1 PW laser traverses a plasma accelerator structure of 1. 2 10 17 cm -3 electron density. The externally injected electron beam (initially: energy E = 100 MeV; relative energy spread σ E /E = 0. 1 %; transverse emittance ε N,x = 1 μrad m) exits the plasma after 2. 5 cm with an energy of 1 GeV (σ E /E = 1. 5 %; ε N,x = 1 μrad m). While emittance is well preserved, the energy spread is significantly increased due to the sizable variation of the accelerating field along the injected bunch. Beam loading techniques will be used in order to compensate this gradient on the accelerating field and minimize the induced energy spread [39] [40] [41] [42]. After completion of 1 GeV simulations with conservation of all beam qualities, simulations of the 5 GeV beam will continue.
section_name: PLASMA ACCELERATOR STRUCTURE

section_content: The preliminary layout of the EuPRAXIA accelerator tunnel [43] is shown in Fig. 3, excluding user areas. Configurations 1, 2, and 4 are visualized. In the current layout, laser and RF infrastructure are situated on the level above the accelerator level floor (not shown). If individual configurations were built separately, the area for the accelerator tunnel for configuration 1, 2, 3, and 4 are 75 m 2, 175 m 2, 150 m 2, and 225 m 2, respectively and configuration 1 to 4 can incorporate configuration 5. Hence the footprint of the accelerator tunnel can be up to 5 times smaller than in conventional accelerator facilities. EuPRAXIA is a site-independent design study. Potential sites will be included in the design report and EuSPARC (Frascati, Italy), SINBAD (Hamburg, Germany), CILEX (Paris, France), CLF (Didcot, UK) and ELI (Prague, Czech Republic) have been discussed as potential sites.
section_name: LAYOUT CONSIDERATIONS

section_content: The EuPRAXIA collaboration is preparing a conceptual design report for a multi-GeV plasma-based accelerator with outstanding beam quality. The facility design aims to include FEL radiation in the soft (to hard) X-ray range, a table-top test beam for HEP detectors and industry, and a compact X-ray source for medical imaging. Synchronized TW laser beams will be available in the user areas. Both laser and electron beams are considered as power sources for the plasma accelerator. EuPRAXIA will prepare a proposal to be included on the ESFRI roadmap in 2020 as an innovative European research infrastructure. Ultimately, EuPRAXIA will: use the world-wide leading high power lasers from European industry, drive laser innovation in the connected companies, provide for the first time usable electron beam quality from a plasma accelerator, and serve pilot users from science, engineering, medicine and industry. PWFA. Using the same infrastructure of RF gun and S-band structure, the PWFA case uses additional X-band structures to accelerate the beam to several hundred MeV before using it inside a single plasma accelerator stage.
section_name: SUMMARY

section_content: * Work supported by the European Union 's Horizon 2020 research and innovation programme under grant agreement No. 653782.