If a laser- or particle beam-driven plasma wakefield accelerator operates in the linear or moderately nonlinear regime, injecting an externally produced particle bunch (witness) to be accelerated may encounter an alignment problem. If the witness and wakefield axes do not coincide, the witness wriggles around the wakefield axis, the quality of the witness deteriorates, its radius and emittance increase. We propose a method that can relax requirements on a witness-to-driver alignment [1]. The method works for linear or moderately nonlinear wakefields only, but this is exactly where it is most needed. Namely, the witness must be preceded by an additional bunch of particles of the same sort, which we call a damper. It is assumed that the damper is produced by the same injector as the witness. If the witness is misaligned with the driver axis, the damper is misaligned by the same amount. The damper is in approximately the same focusing field as the witness. It oscillates transversely with a frequency close to the witness oscillation frequency, and the wakefield perturbation induced by the damper dampens the witness oscillations. As a result, the witness aligns to the axis without quality loss. This method relies on that in a moderately nonlinear axially symmetric plasma wave, small transverse oscillations of beam particles are harmonic. The oscillation frequency does not depend on the magnitude and direction of the offset if the magnitude is much smaller than the characteristic transverse scale of the plasma wave. Therefore, for a certain damper charge and witness-to-damper distance, the witness comes onto the axis for any transverse offset in any direction, if this offset is not too large. For typical parameters of plasma wakefield experiments, the proposed method can relax the tolerances on witness alignment by more than an order of magnitude. The simulations illustrating the method are made with the three-dimensional quasistatic code LCODE [2].

[1] K.V. Lotov, I.Yu. Kargapolov, P.V. Tuev, Oscillation damper for misaligned witness in plasma wakefield accelerator. Phys. Plasmas 31, 123103 (2024).
[2] https://lcode.info/

plasma,plasma wakefield acceleration