Mapping charge capture and acceleration in a plasma wakefield of a proton bunch using variable emittance electron beam injection

TL;DR

This study investigates how varying electron beam injection parameters affects charge capture and reproducibility in proton-driven plasma wakefield acceleration, demonstrating over 15% charge capture efficiency with optimized conditions.

Contribution

It introduces a flexible UV laser system for producing electron bunches with variable charge and emittance, improving control over injection parameters in plasma wakefield experiments.

Findings

Charge capture rates exceeding 15% were achieved.

Variable emittance electron beams enable optimized injection.

Reproducibility of wakefield acceleration was analyzed.

Abstract

In the Phase 2 of the AWAKE first experimental run (from May to November 2018), an electron beam was used to probe and test proton-driven wakefield acceleration in a rubidium plasma column. In this work, we analyze the overall charge capture and shot-to-shot reproducibility of the proton-driven plasma wakefield accelerator with various electron bunch injection parameters. The witness electron bunches were produced using an RF-gun equipped with a Cs2Te photocathode illuminated by a tailorable ultrafast deep ultraviolet (UV) laser pulse. The construction of the UV beam optical system enabled appropriate transverse beam shaping and control of its pulse duration, size, and position on the photocathode, as well as time delay with respect to the ionizing laser pulse that seeds the plasma wakefields in the proton bunches. Variable photocathode illumination provided the required flexibility to produce electron bunches with variable charge, emittance, and injection trajectory into the plasma column. We demonstrate charge capture rates exceeding 15% (40 pC of GeV accelerated charge for a 385 pC injected electron bunch) under optimized electron injection conditions.