High-efficiency wake excitation in meter-scale beam-ionized hydrogen plasmas at FACET-II

Plasma wakefield acceleration has witnessed rapid progress in the past decades and is considered a promising route for building future linear colliders that demand both high repetition rates and high energy efficiency. In this talk, I will present the results that start to address the issues of repetition rate and energy transfer efficiency. These results were obtained from the Plasma Wakefield Acceleration Experiments (E300 Collaboration) at the newly commissioned FACET-II facility at SLAC [1, 2]. By self-ionizing a continuous hydrogen gas flow integrated into the accelerator beamline via a differential pumping system using time-structured 10 GeV electron bunches, we have generated plasmas with meterscale lengths. This plasma source makes it possible to significantly increase the PWFA repetition rates by rapidly replenishing the gas between shots. Our experiments have also demonstrated high-gradient acceleration with high drive-bunch to wake energy transfer efficiency. At pressures $\geq 1.5$ Torr, we observed the onset of pump depletion, which is an important first step towards achieving high overall energy transfer efficiency from the drive to the witness electron bunch in future two-bunch experiments. We also observed that the back of the drive bunch gains multi-GeV energy. These experimental findings are supported by particle-in-cell simulations, revealing a beam-to-wake energy transfer efficiency of approximately 60% at $\sim 2$ Torr [3]. Our results not only demonstrate efficient wake excitation but also underscore the potential of continuous flow hydrogen plasmas in achieving the high energy transfer efficiencies required for PWFA-based linear colliders.