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).
Highlights
Beam-driven plasma wakefield accelerators (PWFAs) [1,2] can generate and sustain accelerating gradients in excess of ∼10 GV=m over meter-scale distances
Besides the extreme accelerating fields demonstrated in PWFAs, improved control over the injection of beams is a necessary step towards the production of beams of sufficient quality for applications such as free-electron lasers or future compact linear colliders
We find that shorter ramps lead to an increase of the trapped charge, which in turn, contributes to substantially reduce the emittance and the energy spread of the injected beams
Summary
Beam-driven plasma wakefield accelerators (PWFAs) [1,2] can generate and sustain accelerating gradients in excess of ∼10 GV=m over meter-scale distances. Notwithstanding the promising capabilities of ionization-based injection techniques, their implementation is limited to high-current drivers (≳5 kA) [9], that generate plasma waves in a strong blowout regime [10,11] Another promising approach consists on injecting electrons from the background plasma by means of controlled wavebreaking during a plasma density down-ramp (DDR) transition [12,13]. Exploiting this fact, we demonstrate that it is possible to use the DDR technique for the production and acceleration of high-quality beams (high charge, low emittance, and low energy spread) in the wakefields created by 2.5 kA electron drivers, when employing DDRs shorter than the plasma wavelength at the plateau density
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