Measuring the Density Profile of a Plasma Wakefield Accelerator using Femtosecond Interferometry

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. PWFA is a method of accelerating particles which provides multiple orders of magnitude higher accelerating fields than conventional accelerators. Future applications of PWFA could enable a lepton collider at the energy frontier and the miniaturization of high brightness x-ray light sources. The performance of these plasma accelerators is largely dependent on the density profile of the plasma source to optimize the energy gain and beam quality of the accelerated particles. The experiments at FACET-II use a plasma source which is on the order of 100s of micrometers thick and one meter long, but the density profile is challenging to diagnose.

Our density profile diagnostic is based on a design by colleagues at UCLA [Z. Nie, et al., Opt. Express 30 25696 (2022)] 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.