Electron plasma wave Thomson scattering on laboratory plasma jets

Plasma jets created from a 15 μm thick Al foil on a 1 MA pulsed power machine were studied using a new electron plasma wave (EPW) Thomson scattering system in conjunction with previously developed ion acoustic wave (IAW) Thomson scattering and interferometry. These diagnostics give multiple ways of measuring the electron temperature and density of the jet. Analysis of the EPW feature found the on-axis density of the jet to be between $5\times {10}^{18}$ and $1.4\times {10}^{19}$ cm⁻³, which either matched or was higher than interferometry measurements. Outside of the jet, both of these diagnostics measured a density of $7\times {10}^{17}$ cm⁻³. On one shot, the EPW spectral feature showed two pairs of peaks within a 250 μm scattering length on the edge of the jet, which shows that the boundary of the jet ∼1 mm radius jet is $\lesssim$ 0.1 mm. While electron temperature measurements of the plasma jet are complicated by the probe beam producing inverse bremsstrahlung heating of the jet, comparison of the electron temperature measured between IAW and EPW Thomson scattering showed the EPW feature to imply significantly higher electron temperatures than the IAW feature (e.g., 160 eV vs 70 eV in one case). Various sources of this discrepancy (for example, density gradients, collisions, and a lag in ionization) and their impact on the plasma are discussed.