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Electron-Injection Techniques in Plasma-Wakefield Accelerators for Driving Free-Electron Lasers

Principal Investigators: Jens Osterhoff and Alberto Martinez de la Ossa, Deutsches Elektronen-Synchrotron - DESY, Hamburg (Germany)
HPC Platform: JUQUEEN of JSC

Electron-Injection Techniques in Plasma-Wakefield Accelerators for Driving Free-Electron LasersFigure 1: OSIRIS 3D PIC simulation of a 1 GeV electron beam transversing a uniform plasma of ~10^18 electrons per cm^3. A short electron bunch is injected, by means of the wakefield-induced ionisation method, at the back of the accelerating plasma cavity. The injected bunch features high peak current and low normalised emittance, and it is being accelerated at a rate of more than 100 GV/m. After just 3 cm of acceleration, its electron energy becomes three times the initial electron energy of the driver.
Copyright: DESY, Hamburg

Plasma wakefield acceleration (PWA) is a quickly developing novel-acceleration technology, which may allow for a substantial increase of the average gradient in particle accelerators when compared to current state-of-the-art facilities. When focused into a plasma, an ultra short laser pulse (laser-wakefield acceleration, LWFA [1]) or a relativistic particle beam (beam-driven plasma acceleration, PWFA [2,3]) repels electrons out of its propagation path 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, significantly exceeding conventional radio-frequency accelerators by orders of magnitude in field strength. Such plasma accelerators offer a unique opportunity for the production of high-brightness beams for applications, such free-electron lasers (FEL). In the future, plasma accelerators may allow for miniaturised FELs [4,5] with order-of-magnitude smaller cost and footprint than available today.

The size of a plasma acceleration cavity for typical plasma densities (1017 to 1018 cm-3) is on the order of some tens of microns corresponding to a plasma oscillation period of few hundred femtoseconds. Owing to those small scales, ultra-short beams can be created and accelerated by these structures. The process of generating an electron beam at the back of the plasma cavity for subsequent acceleration is called injection.

The main challenge of this project is to study and design electron-injection techniques in plasma-wakefield accelerators for the production of high-quality beams, suitable for application as FELs. Since the physics involved in the process cannot be treated analytically in most of the cases of interest, particle-in-cell (PIC) simulations are required. PIC simulations allow to calculate the response of the plasma electrons to the pas-sage of charged beams and/or high-intensity lasers by numerically solving the Maxwell equations in a box that follows the driver. The electromagnetic fields of the system are discretised on a three-dimensional spatial grid inside the box (the cells), while the individual particles of the involved species (electrons, ions, etc.) are represented by the introduction of macro-particles. The computational load is distributed over a number of processors, which simultaneously solve the equations in different spatial regions of the system. This parallelisation and sharing of the work among few hundreds to ten thousands of processing units in supercomputing machines allows for a full numerical modelling of the relevant phenomena in plasma-based acceleration.

Figure 1 shows an example of a simulation done with the PIC code OSIRIS [6], that deals with the complete process of injection of a high-quality beam in a beam-driven plasma wake by means of the so-called wakefield-induced ionisation injection technique [7]. In this example, the wakefields at the rear end of the plasma cavity are so high that they are capable of ionising electrons from a well localised neutral helium gas region coexisting with a hydrogen plasma. Once the helium has passed, the wakefields have efficiently trapped a high-quality electron beam which continues being accelerated at a rate of more than 100 GV/m. At the end of the simulation of the injection plus acceleration process, the injected beam features three times higher energy per electron than the initial driver, and 10 times higher brightness. Typically, this kind of simulation utilises a JUQUEEN midplane for a few hours.

Control over the process of injection of electron beams in plasma wakefield accelerators is of utmost importance for the generation of high-quality electron beams. Therefore, this project explores a number of regimes and methods for high-brightness beam production from plasma-based accelerators with the intention to identify the most promising PWA design to drive X-ray FELs.

References:
[1] Tajima and Dawson, Phys. Rev. Lett. 43, 267 (1979)
[2] Veksler, Proceedings of CERN Symposium on High Energy Accelerators and Pion Physics 1, 80 (1956).
[3] Blumenfeld et al., Nature 445, 741 (2007).
[4] Fuchs et al., Nat. Physics 5, 826-829 (2009).
[5] Maier et al., Phys. Rev. X 2, 031019 (2012).
[6] Fonseca et al., Lecture Notes in Computer Science 2331, 342 (2002).
Fonseca et al., Plasma Phys. Control. Fusion 50, 124034 (2008).
[7] Martinez de la Ossa et al., Phys. Rev. Lett.111, 245003 (2013).
Martinez de la Ossa et al., Phys. Plasmas 22 093107 (2015).

Scientific Contact:
Alberto Martinez de la Ossa
Plasma Acceleration Group, FLA
Deutsches Elektronen-Synchrotron - DESY
Notkestr. 85, D-22607 Hamburg/Germany
e-mail: alberto.martinez.de.la.ossa [@] desy.de
plasma.desy.de
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January 2016