Proton beam self-modulation seeded by electron bunch in plasma with density ramp

Abstract

Seeded self-modulation in a plasma can transform a long proton beam into a train of micro-bunches that can excite a strong wakefield over long distances, but this needs the plasma to have a certain density profile with a short-scale ramp up. For the parameters of the AWAKE experiment at CERN we numerically study which density profiles are optimal if the self-modulation is seeded by a short electron bunch. With the optimal profiles, it is possible to ‘freeze’ the wakefield at approximately half the wavebreaking level. High-energy electron bunches (160 MeV) are less efficient seeds than low-energy ones (18 MeV), because the wakefield of the former lasts longer than necessary for efficient seeding.

Injection tolerances and self-matching in a quasilinear wakefield accelerator

“…Figure 11 shows the result of a delay scan carried out when the charge of the drive beam is increased by a factor two, giving a peak wakefield of ∼ 950 MeV/m (slightly more than double due to increasing nonlinearity). This is similar to the wakefields achievable if the gap between the self-modulation and acceleration stages can be avoided [20,24]. In this case, the 400 pC beam gives best results.…”

“…Other parameters of the drive bunch are chosen to reproduce the wake from the full bunch train. In the experiment, the use of a plasma density step in the modulation stage will result in a plasma wake which is essentially constant during the acceleration stage [24]. The toy model therefore uses a non-evolving drive bunch with a Lorentz factor of 426, chosen to match the 400 GeV proton beam delivered by the SPS at CERN.…”

“…The modulated proton beam excites a quasilinear wake [21], below the threshold for wavebreaking and self-injection [22]. Simulations show that the use of a plasma density step during the self-modulation stage will result in near-constant wakefields during the acceleration process [23,24].…”

Witness electron beam injection using an active plasma lens for a proton beam-driven plasma wakefield accelerator

“…This is a seeding method known as ionization-front seeding used for AWAKE RUN 1 experiments [28]. Even though the electron-seeded self-modulation of the proton beam [29,30] is scheduled for the next phase of the AWAKE experiment, we first discuss the interaction among the APL plasma, witness electron beam, and proton bunches modulated by the ionization-front seeding method. We further assume that the plasma density is uniform along the first plasma source for the SSM without any density step or ramp.…”

Ultimate plasma wakefield acceleration with 400 GeV proton driver

“…The length and precise position of the witness beams relative to the wave are optimized to reduce the final energy spread while maintaining a high acceleration rate and conserving the injected charge (Table I). When simulating witness dynamics, we use an energy-dependent substepping [18].…”