Tolerances to driver-witness misalignment in a quasilinear plasma wakefield accelerator

TL;DR

This paper investigates how misalignments between driver and witness beams affect emittance preservation in quasilinear plasma wakefield accelerators, providing analytical models and simulations to guide alignment constraints.

Contribution

It develops analytical models for transverse witness dynamics under misalignment in the quasilinear regime and validates them with particle-in-cell simulations.

Findings

Analytical models accurately predict witness motion under misalignment.

Simulation results agree with the models, confirming their validity.

The work informs alignment constraints for plasma accelerator experiments.

Abstract

Plasma-based accelerators offer high accelerating gradients and scalability through staging or long plasma sources, which makes them good candidates for future accelerator and collider concepts. Proton-driven accelerators in particular have the potential to bring particles to high energy in a single stage. In the quasilinear regime - where the plasma wake is only partially evacuated - a witness bunch of electrons drives a cavitated wake, which acts to preserve the emittance of the portion of the witness inside this self-blowout. In the case of a misalignment between the driver and witness, this behaviour can persist, but its effectiveness is reduced. In this paper, we study transverse witness dynamics in this regime, and develop analytical models to describe the witness motion, and develop a metric to estimate emittance preservation based on a single parameter which estimates the density of the witness after phase mixing. Particle in cell simulations using the AWAKE Run 2c baseline parameters show excellent agreement with the predictive models developed. This work allows alignment constraints to be set both for the AWAKE experiment and other wakefield acceleration schemes operating in the quasilinear regime.