Electron-Injection Techniques in Plasma-Wakefield Accelerators for Driving Free-Electron Lasers
Alberto Martinez de la Ossa and Jens Osterhoff
Deutsches Elektronen-Synchrotron, Hamburg (Germany)
Local Project ID:
HPC Platform used:
JUQUEEN of JSC
Plasma wakefield accelerators (PWAs) can sustain electric fields on the order of 100 GV/m for the acceleration of electrons up to GeV energies in a cm-scale dis-tance. Harnessing such highly-intense accelerating gradients requires precise con-trol over the process of injection of the electron beams. By means of large-scale simulations, this project explored multiple novel solutions for the generation of high-quality electron beams from a PWA, as required for free-electron lasers (FELs). Using PWAs, it is envisaged that miniaturized and cost effective FELs may be constructed, dramatically increasing the proliferation of this technology with revolutionary consequences for applications in biology, medicine, material science and physics.
lasma wakefield acceleration is a quickly developing technology which allows for a dramatic increase of the average accelerating gradient when compared to current state-of-the-art radio-frequency (RF) accelerator facilities. When focused into a plas-ma, an ultra-short laser pulse driver (laser-wakefield acceleration, LWFA ) or a rela-tivistic particle beam driver (beam-driven plasma acceleration, PWFA [2,3]) repels electrons from its vicinity and forms waves in electron density which are following the driver with a phase velocity close to the speed of light. This allows to create a cavity with simultaneous accelerating and focusing properties for charged particle beams. Inside such a plasma-accelerator module, gradients on the order of 100 GV/m can be sustained without being limited by material breakdown, outperforming conven-tional radio-frequency schemes by orders of magnitude. The length of the accelerat-ing plasma cavity is approximately given by the plasma wavelength, which for plasma densities around 1017 cm3 is on the order of 100 microns. Therefore, only short beams can be created and accelerated inside these structures.
Plasma-based accelerators offer a unique opportunity for the production of high-brightness beams for applications, such as FELs. In the future, plasma accelerators may allow for miniaturized FELs [4,5] with order-of-magnitude smaller cost and foot-print than available today.
One of the challenging tasks for obtaining high-quality beams from PWAs is to study and design injection techniques for the controlled generation of high-brightness wit-ness beams from PWAs, suitable for application in FELs. Owed to the highly nonline-ar nature of the dynamics in a PWA, analytical treatment only allows for an approxi-mate description. Thus, efficient numerical modeling is required in order to obtain a full description of the relevant physics. The particle-in-cell (PIC) method allows for a precise rendering of the complex dynamics with available computational costs.
Thanks to the hhh23 project for supercomputing at the Jülich Supercomputering Centre (JSC), we used large-scale PIC simulations performed with the PIC code OSIRIS  to study and design novel concepts for injection and acceleration of FEL-quality electron beams in PWAs. Particular emphasis was put on offering theoretical support for three of the main PWA projects at DESY:
The FLASHForward project  aims to produce, in a few centimeters of plasma, beams with energy of order GeV that are of a quality sufficient to demonstrate FEL gain. To achieve this goal, FLASHForward will utilize the electron beams produced in the RF FLASH accelerator as drivers for the generation of strong wakefields in a novel hydrogen plasma cell at a density of around 1017 cm-3. Several internal injection mech-anisms have been proposed and developed for FLASHForward thanks to PIC simu-lations performed at JSC, leading to numerous scientific publications. The wakefield-induced ionization [8,9] and the density down-ramp injection mechanisms  are just two highlighted examples of the successful work done in this respect. In addi-tion, multiple simulations for the study and control of critical instabilities for FLASH-Forward in particular, and PWFAs in general, have also been carried out, demon-strating that the drive beams can be intrinsically stabilized under appropriate conditions [11,12].
ATHENA-e will use the ultra-short and high-quality electron beams produced at the RF-accellerator ARES as an external source of witness beams for an LWFA. The LWFA will be powered by a TW high-power laser, focused into a tens-of-microns size diameter spot for a strong wakefield excitation at plasma densities between 1017 and 1018 cm-3. After an energy boost of 1 GeV over few centimeters of propagation in plasma, the electron beams will be sent to a magnetic undulator for FEL gain demonstration. The external injection of an RF-beam into an LWFA benefits from the high-quality and control provided by RF technology and offers the critical advantage of decoupling the process of injection from the wakefield excitation. However, it also brings particular challenges, as a femtosecond level time synchronization and a mi-cron level transverse overlap between the laser and the witness beam are required.
During the course of our computing project, multiple simulation studies have been performed to test original solutions for dramatically improving the synchronization  and the quality of the witness beam [14,15] in LWFA scenarios with external injection [16,17].
Hybrid LWFA | PWFA staging
PWFAs are thought to offer improved control over the process of injection and accel-eration of a witness beam in LWFAs. The reasoning relies on the way the electron-beam drivers propagate through the plasma, which is immune to the dephasing and diffraction effects affecting the performance of LWFAs. However, a major limitation for the widespread use of PWFA technology is the requirement of a large-scale acceler-ator facility providing the drive beams. In contrast, the comparably compact high-power laser facilities for LWFAs have proliferated over the past decade and electron beams with several hundreds of picocoulomb charge are nowadays routinely gener-ated by LWFAs all over the world.
In a hybrid LWFA-driven PWFA (LPWFA), the LWFA-generated high-current electron beam is used to drive a PWFA stage by itself. In the PWFA stage, a new witness beam with largely improved quality is generated and accelerated to higher energies. In essence, the PWFA stage operates as a beam brightness and energy transformer of the LWFA output, aiming to reach the demanding beam quality requirements of an FEL, without sacrificing the small spatial footprint and the relatively low cost of-fered by LWFAs. A conceptual study for LPWFAs as energy and brightness trans-formers, supported by large-scale PIC simulations performed on JSC HPC system JUQUEEN, have been recently accepted for publication .
The project hhh23 has explored, through dedicated computing time in JSC, multiple solutions and promising novel concepts for the production of high-brightness beams from plasma-based accelerators which can power a next-generation of miniaturized X-ray FELs.
We thank the OSIRIS consortium (IST/UCLA) for access to the OSIRIS code. Special thanks for support go to J. Vieira and R. Fonseca. Furthermore, we acknowledge the grant of computing time by the Jülich Supercomputing Centre on JUQUEEN un-der Project No. hhh23 and the use of the High-Performance Cluster (Maxwell) at DESY. This work was funded by the Humboldt Professorship of B. Foster, the Helm-holtz Virtual Institute VH-VI-503, and the ARD program.
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Dr. Alberto Martinez de la Ossa,
Particle Physics Division
Notkestr. 85, D- 22607 Hamburg (Germany)
alberto.martinez.de.la.ossa [@] desy.de
JSC project ID: hhh23