Non-destructive diagnostics able to resolve transverse offsets and longitudinal separation of ultra-relativistic, two-bunch electron beams are necessary for a variety of applications including the ion channel laser (ICL) and other plasma wakefield (PWFA) experiments. A prototype electro-optic beam positioning monitor (EOS-BPM) utilizing two independent laser pulses traveling through a pair of EO crystals has been installed at the SLAC National Accelerator Laboratory FACET-II facility. This system is capable of order 10 fs temporal resolution and order 100 µm transverse position resolution. To achieve better transverse resolution we introduce a new design using an axicon lens to create a donut beam and a multi-crystal structure placed around the axis of propagation of the electron beam. Experimental results of the prototype EOS-BPM along with the simulated response of the new EOS-BPM design to the ultra-relativistic, two-bunch electron beam used for PWFA experiments at FACET-II will be presented.
Ros et al. (2026) studied this question.