Nanosecond-resolved 266 nm Mach-Zehnder interferometry for electron-density measurements of dense plasmas generated in supercritical fluids

Abstract

We developed a nanosecond-resolved 266 nm Mach-Zehnder interferometer for electron-density measurements of dense laser-produced plasmas generated in 100-bar supercritical-fluid (SCF) helium. A 1064 nm pump pulse was focused into the SCF helium medium, and the plasma-induced phase shift of a 266 nm UV probe beam was recorded using an ICCD-based interferometric imaging system. Plasma-arm-only and reference-arm-only images were used to normalize raw interferograms and improve the effective fringe visibility. The corrected interferograms were analyzed using a two-dimensional Fourier-transform method to reconstruct phase-shift maps, which were converted into line-integrated electron-density distributions through the plasma dispersion relation. Assuming cylindrical symmetry, Abel inversion was applied to the plasma with the largest line-integrated electron density, yielding a maximum local electron density of approximately \(2.5\times10^{18}~\mathrm{cm^{-3}}\). The measurement fidelity was evaluated by considering free-free absorption of the probe beam, probe-beam refraction by plasma electron density gradients, and effect of finite-collision-frequency effects on the plasma dielectric response. These estimates indicate that the inferred electron density is not altered by more than an order of magnitude under the present experimental conditions. The present system demonstrates the applicability of 266 nm UV interferometry to nanosecond-resolved density diagnostics of dense plasmas in high-pressure supercritical fluids.