Optimizing density down-ramp injection for beam-driven plasma wakefield accelerators

Abstract

Density down-ramp (DDR) injection is a promising concept in beam-driven plasma wakefield accelerators for the generation of high-quality witness beams. We review and complement the theoretical principles of the method and employ particle-in-cell (PIC) simulations in order to determine constrains on the geometry of the density ramp and the current of the drive beam, regarding the applicability of DDR injection. Furthermore, PIC simulations are utilized to find optimized conditions for the production of high-quality beams. We find and explain the intriguing result that the injection of an increased charge by means of a steepened ramp favors the generation of beams with lower emittance. Exploiting this fact enables the production of beams with high charge (∼140 pC), low normalized emittance (∼200 nm) and low uncorrelated energy spread (0.3%) in sufficiently steep ramps even for drive beams with moderate peak current (∼2.5 kA).

FLASHForward: plasma wakefield accelerator science for high-average-power applications

“…In recent years a number of novel beam-driven injection techniques have been proposed [15][16][17][18]. One promising approach is based on the concept of injecting electrons from the background plasma by means of controlled wave-breaking during a plasma density down-ramp (DDR) transition [19]. In the case of FLASHForward this particular injection method has the benefit of requiring lower peak currents than other methods, e.g.…”

FLASHForward X-2: Towards beam quality preservation in a plasma booster

“…The linac works at a repetition rate of 10 Hz with a maximum of 800 bunches per bunch train. 1 After the linac the beam is distributed between the FLASH1 and FLASH2 undulator beamlines: a fraction of the bunches within each bunch train can be sent to FLASH2 using a combination of a fast kicker and a Lambertson septum, while the remaining bunches propagate to FLASH1 (see Fig. 1).…”

“…In the beam-driven scheme, an intense charged-particle beam (drive beam) excites a plasma-wakefield which accelerates a trailing or witness beam. The witness beam can either be generated internally, from plasma background electrons [1] or by controlled ionisation of a dopant gas species [2], or injected externally. The latter process is crucial for staging of multiple plasma-accelerating modules, as required to achieve high beam energies.…”

Generating high quality ultrarelativistic electron beams using an evolving electron beam driver

“…While the generation of ultra-relativistic electron beams through selfinjection in an evolving plasma wake has been observed in LWFA experiments [5][6][7][8] and demonstrated in simulations [13][14][15][16], the beams produced to date do not exhibit the sufficiently low energy spreads σ γ and high normalized brightnesses B n = 2I/ǫ 2 n required to drive XFEL devices [17] where I and ǫ n represent the current and normalized emittance, respectively. In recent years, electron injection schemes involving field ionization [18][19][20][21][22][23] or the use of a plasma density down ramp (DDR) [24][25][26][27][28] have shown tremendous potential for high quality beam generation for XFEL applications.In this Letter, we propose and demonstrate a new method of controllable injection using an electron beam driver whose spot size is decreasing in the nonlinear blowout regime to control the wake phase velocity and hence induce electron trapping. As when using a DDR, this proposed method relies on gradually elongating the ion column or cavity length as the drive beam propagates.…”