Improvements for the third generation plasma wakefield experiment E-164 at SLAC

The E-164 experiment at the Stanford Linear Accelerator Center is the third in a series investigating plasma wakefield acceleration where the wake is driven by electron bunches. A collaboration between SLAC, UCLA and USC, E-164 has up to 2×10¹⁰ electrons at 28.5 GeV in 100 micron long bunches. These bunches enter a 30 cm long Lithium plasma with density of 6×10¹⁵ electrons/cm³, where the transfer of energy from the head of the bunch to the tail takes place. In addition to acceleration, strong focusing, refraction of the electron beam and "betatron X-ray" production are all investigated. E-164 builds on related prior experiments, and its apparatus has evolved considerably. A third optical transition radiator has been added for real time twiss parameter measurements which include the effects of scattering. The plasma cell is moved to the focus of the final focus test beam facility in order to increase bunch electron density. Spectrometry is extended with an upstream chicane in a dispersive region to produce synchrotron X-rays. Performance of these improvements and status of the experiment are discussed.