Beam realignment with quasi-adiabatic plasma density ramps in plasma wakefield accelerators*

Beam-driven plasma wakefield acceleration (PWFA) has shown great potential to be the basis for future linear

colliders (LCs). PWFA can acheieve high acceleration gradients with high energy transfer efficiency while main-

taining low energy spread. For linear collider applications and designs, the witness beam transverse spot sizes

and emittances are on the order of hundreds of nanometers with charges around a nanoCoulomb, and would be

accelerated over many stages. These beam parameters will cause the ions to collapse during the transit time of the

beam. Zhao et al. [1] recently showed quasi-adiabatic density ramps at the entrance and exit of the plasma can

match LC-class beams in the presence of ion motion with emittance preservation. Hildebrand et al. [2] showed

witness beams that are initially mislaigned with the drive beam axis can be realigned when the drive beam causes

ion motion. Plasma density ramps will suppress the oscillation amplitude of the witness beam in the upramp but

magnify it in the downramp. Therefore, we propose using drive beam induced ion motion to realign the witness

beam in the plateau such that it remains aligned in the downramp. We simulate a single stage of a PWFA-LC

using QPAD [3] where the witness beam is realigned with only ∼ 5% emittance growth, the energy spread is limited

to < 1%, and the energy transfer efficiency is ∼ 50%.

[1] Y. Zhao et al., “Emittance preservation in the presence of ion motion for low-to-high energy stages of a plasma

based accelerator”, Physics Review Accelerators and Beams 26, 121301 (2023)

[2] L. Hildebrand et al.,“Mitigation techniques of witness beam hosing in plasma-based acceleration”, Proceedings

of the Advanced Accelerator Conference, Breckenridge, CO (2018).

[3] F. Li et al., “A quasi-static particle-in-cell algorithm based on an azimuthal Fourier decomposition for highly

efficient simulations of plasma-based acceleration: QPAD”, Computer Physics Communications 261, 107784 (2021)