Streaked Thomson Scattering To Measure Heating Of Laboratory Plasma Jets By The Probe Laser

Abstract

Thomson scattering measurements have been performed on laboratory plasma jets in order to measure the flow velocities and electron temperatures inside and near the edge of the jet. These jets were created using $\mathrm {a}15 \mu \mathrm {m}$ thick radial Al foil load on COBRA, a 1 MA pulsed power machine. The laser used for these measurements had a maximum energy of 10 J at 526.5 nm. Using this 10 J laser, however, significantly heats the $5\mathrm {x} 10 ^{18}$ cm $^{-3}$ jet by inverse bremsstrahlung, creating a density bubble in the jet. This was demonstrated by reducing the probe laser energy to 1 J, which reduced the measured electron temperature in the jet from about 40 to 20 eV and nearly eliminated the bubble. These measurements were made using a time gated spectrometer, with at least a 5 ns gate window, in order to encompass the entire 3 ns laser pulse. Therefore, no time dependent electron temperature measurements could be made from the Thomson scattering spectra. We are now developing an experimental arrangement to send the collected scattered spectra into a streak camera, in order to determine the plasma temperature as a function of time throughout the 3 ns laser pulse. With the streak camera data, we hope to track the changing plasma parameters during the laser heating of the plasma jet. In the future we hope to extend this diagnostic arrangement to other configurations, enabling time resolved measurements of the plasma parameters of various plasmas.