Optical characterization of beam-driven plasma wake field accelerators

We report initial results of optical probing experiments (E-224) at the FACET/SLAC user facility. These experiments are being carried out in collaboration with the ongoing e-beam driven wakefield experimental campaign (E-200) at FACET. Their aim is to optically study both the short term and long term plasma structure produced by the e-beam driver. The SLAC plasma wakefield experiments have demonstrated the highest energy gain to date [1] and continue work on further optimization. Direct visualization of the plasma wake structure would aid in the understanding of the dynamics of the beam plasma interaction and acceleration and its optimization. Currently, such understanding is derived only from simulations, which are very time consuming and rely on limited knowledge of initial conditions. We describe the optical probing geometry used in this initial run, a variation of a method previously developed in our group [6], as governed by the unique experimental challenges of the FACET beam driven experiments in their current configuration. We discuss the current results, the limitations of the current experimental configuration, and the changes planned for future experiments.