Measuring the polarization-dependent X-ray flash from a QED plasma

During the development of laser facilities over the last 30 years, the advent of multi-PW lasers has opened the door to new regimes of physics within a laboratory setting. As higher intensities have become possible, quantum electrodynamic (QED) effects are becoming more relevant to the study of plasmas and their dynamics in the incredibly strong electromagnetic fields in the focal spots of these lasers. In these QED plasmas, strong-field processes such as nonlinear Compton scattering (NLCS) and nonlinear Breit-Wheeler electron-positron pair creation play a significant role in the system dynamics, so it's important to identify when such a plasma has been produced. Our experiment at the ZEUS high-power laser facility looked for evidence of the characteristic X-ray flash produced by NLCS during the formation of a QED plasma as accelerated electrons radiate much of their energy in a single burst. Particle-in-cell codes have shown that this radiation is polarized in the same direction as the laser, differentiating it from the unpolarized background bremsstrahlung emission that can conceal the signal of interest. Thus, by rotating the incident polarization and using a crystal polarimeter that preferentially reflects s-polarized light, we looked for polarization-dependent differences in the signal that could be attributed to the presence of the X-ray flash. In addition, we fielded novel gamma spectrometers at two different positions to measure angular differences in the emitted spectra along the laser axis and perpendicular to target normal. This talk will cover an overview of the experiment and our preliminary results.