First results from the E302 efficiency$\unicode{x2013}$instability experiment at the FACET-II facility

TL;DR

This paper reports the first experimental detection of beam-breakup instability in plasma accelerators at FACET-II, combining novel diagnostics and simulations to understand its impact on beam quality and efficiency.

Contribution

It introduces a new experimental method to detect BBU instability and compares results with detailed 3D PIC simulations, advancing understanding of plasma accelerator limitations.

Findings

First experimental signatures of BBU instability detected.

Correlation between instability and beam quality degradation observed.

Simulations support experimental observations and provide insight into instability dynamics.

Abstract

The beam-breakup (BBU) instability in plasma accelerators is seeded by a transverse offset between the driver and a trailing bunch. The BBU instability induces oscillations in the trailing bunch, which are detrimental to its beam quality. When the instability is large, assuming little mitigation from ion motion and energy spread, the beam suffers emittance growth, and charge can be kicked transversely out of the plasma channel. The detrimental effect on beam quality is substantially worse at high efficiencies, which places constraints on the achievable power efficiency in applications such as linear colliders, where maintaining the beam quality is required. In this paper, we present the first experimental signatures of the BBU instability in data taken in the E302 experiment at the FACET-II facility at SLAC National Accelerator Laboratory. We use a specific beam-optical setup and a novel method to probe for transverse instabilities on diagnostic screens downstream of a magnetic dipole spectrometer. We complement the analysis with full 3D particle-in-cell (PIC) simulations of the plasma interaction using similar driver and trailing bunch parameters on a simulated FACET-II spectrometer.