EOS-BPM: A new diagnostic for PWFA experiments.

A beam driven plasma wakefield accelerator (PWFA) utilizes two electron bunches. A "drive" beam creates a wake in the PWFA plasma source, blowing out plasma electrons and leaving behind a stationary ion column. A “witness” beam is injected behind the drive bunch and feels a high accelerating gradient throughout the PWFA. The energy gain in a PWFA depends on the longitudinal displacement of the witness beam from the drive bunch. Additionally, to avert hosing instability the beams must be well aligned transversely. Diagnostics providing information on relative beam positions will be an integral tool in future PWFA research. One design for such a diagnostic is the electro-optic sampling beam position monitor (EOS-BPM). The setup consists of two EO crystals positioned on either side of the beamline. The EO signal from each crystal measures same beam current profile, and the relative strength of the signal is correlated with the nearness of each bunch to either crystal. From the combined temporally resolved signals of the crystals, the transverse position of each bunch can be independently determined. Simulations of an EOS-BPM using beam parameters for upcoming experiments at the SLAC linac are presented.