How to Catch a Plasma Wave: Using Lasers to Map the Invisible

We present a linear optical interferometer for diagnosis of the density profile of a laser-ionized plasma filament. This plasma filament will serve as the plasma source in plasma wakefield accelerator (PWFA) experiments to be carried out at SLAC National Accelerator Laboratory’s FACET-II facility. The diagnostic is based on a design by colleagues at UCLA [1] and compares the relative phase delay of a femtosecond probe laser pulse to that of a twin reference pulse, both which travel along the same path. The twin pulses are produced in a birefringent crystal, which imposes a picosecond-scale delay between the two pulses. The plasma-producing high-intensity laser pulse is fired between the leading reference pulse and the trailing probe pulse. The reference and probe pulses are then recombined in a second birefringent crystal, allowing for a measurement of their mutual interference and thereby an inference of the plasma density profile. By optimizing the detection hardware and electronics, we expect to have sufficient sensitivity to measure plasma filaments with a thickness of order 100 µm and densities as low as 10^16 cm-3.

[1] Z. Nie, et al., “Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements”, Optics Express 30 25696 (2022)