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

TL;DR

The study tackles the beam-breakup (BBU) instability in plasma accelerators and its constraint on power efficiency through the efficiency–instability relation $\eta_t \ge \frac{\eta_p^{2}}{4(1-\eta_p)}$, and reports the first experimental signatures of BBU using a novel magnetic-dipole spectrometer at FACET-II E302, complemented by end-to-end 3D PIC simulations with HiPACE++ and ABEL coupling. The experiments vary the drive-to-trailing efficiency by adjusting bunch separation, revealing a clear correlation between increasing $\eta_p$ (and hence separation) and larger maximum transverse kicks in the trailing bunch, with observed kicks up to about $2\,\mathrm{mrad}$ near $12\,\mathrm{GeV}$. End-to-end simulations including instability reproduce the experimental trends, while wake-only simulations underpredict the kicks, highlighting the role of BBU; damping mechanisms such as BNS damping and energy spread help explain onset delays. Collectively, the results support the instability–efficiency relation and motivate improved diagnostics and wake-field mapping to enable precise quantification and design guidance for plasma-based accelerators.

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.

Figures (10)

• Figure 1: Schematic of the relevant parts of the beamline at the FACET-II facility for the E302 experiment (not to scale). The beam trajectory is indicated by the black arrow.
• Figure 2: The correlation between bunch spacing found from the electro-sampling crystal and the readback value from the bunch length monitor in BC14 obtained from a L2 linac section phase scan dataset.
• Figure 3: The correlation between bunch separation, found from converting the bunch length monitor signal using the linear fit previously extracted, and the driver-to-trailing power transfer efficiency calculated from the spectrometer screen.
• Figure 4: Three spectrometer shots (a,c,e) with lines drawn to indicate the chromatic expansion of the beam due to imaging effects for bunch slices with initial divergences of -1 and 1 mrad. The x'-E distributions (b,d,f) are shown after conversion using the known lattice parameters. Bunch separation and efficiency are estimated to be around 71 microns and 0.17 (a,b), 95 microns and 0.36 (c,d) and 112 microns and 0.43 (e,f).
• Figure 5: Left and right threshold value lines drawn for each shot in the scan of the L2 phase. Mean charges and efficiency calculated from the spectrometer screen are annotated on each window of the figure for each estimated bunch separation interval, along with their standard deviation.