Witness emittance growth caused by driver density fluctuations in plasma wakefield accelerators

TL;DR

This paper identifies a new mechanism causing emittance growth in plasma wakefield accelerators due to driver density fluctuations, affecting beam quality over time.

Contribution

It reveals a novel effect in plasma wakefield accelerators where driver density fluctuations lead to witness emittance growth, especially in linear or moderately nonlinear regimes.

Findings

Witness experiences time-varying focusing forces.

Higher witness charge reduces emittance growth rate.

Degradation propagates backward along the witness bunch.

Abstract

We discovered a novel effect that can cause witness emittance growth in plasma wakefield accelerators. The effect appears in linear or moderately nonlinear plasma waves. The witness experiences a time-varying focusing force and loses quality during the time required for the drive beam to reach transverse equilibrium with the plasma wave. The higher the witness charge, the lower the emittance growth rate because of additional focusing of the witness by its own wakefield. However, the witness head always degrades, and the boundary between degraded and intact parts gradually propagates backward along the witness bunch.