Multi-Angled, Multi-Pulse Time-Resolved Thomson Scattering on Laboratory Plasma Jets

Abstract

Streaked Thomson scattering measurements have been performed on plasma jets created from a 15 $\mu\mathrm{m}$ thick radial Al, Ti, or Cu foil load on COBRA, a 1 MA pulsed power machine. The streaked system enables collecting scattered light from two separate laser pulses separated in time by between 3 and 14 ns from two different scattering angles. The time separation is created by splitting the initial 2.2 ns full width half max duration, 10 J, 526.5 nm laser beam into two separate pulses, each with 2.5 J, and delaying one beam relative to the other. To collect streaked spectra from two angles simultaneously a split fiber bundle was used to couple light from the plasma to the spectrometer, and the size of the fibers, 100 $\mu \mathrm{m}$ diameter, was used as the effective spectral slit width. The 2.5 J laser pulse is shown to heat the plasma jet by inverse bremsstrahlung radiation, as measured by the streaked Thomson scattering system. Analysis of the streak camera image showed that the electron temperature of the Al jet was increased from 20 eV up to 50 eV within about 2 ns for both laser pulses when they were separated by 12 ns in time. Initial results from the Cu jets showed more heating then the Al jets while Ti jets showed the most heating of these three materials. In addition to this increased heating, the scattering profiles from Ti and Cu jets had ion-acoustic features that were more difficult to interpret then those from Al jets. Results from the two scattering angles showed reasonable agreement of the measured electron temperature, and implied that the electron density was at least $2\times 10^{18}\text{cm}^{-3}$ . Results will also be presented with the laser pulses separated by only 4 ns, creating a more continuous laser pulse. In addition, the effects of the current polarity in the radial foil on the measured electron temperature will be shown.