Ultraviolet Thomson Scattering from Two-Plasmon--Decay Driven Electron Plasma Waves at Quarter-Critical Densities

Thomson scattering (TS) was used to probe electron plasma waves (EPW's) driven by the two-plasmon--decay (TPD) instability near quarter-critical density. TPD-driven EPW's were observed at densities consistent with the common-wave TPD model. Five laser beams $\left( {\lambda {\kern 1pt}_{3\omega } =351\mbox{\thinspace nm}} \right)$produced 400-$\mu $m-diam (FWHM) laser spots with overlapped intensities up to $3\times 10^{14}\;\mbox{W/cm}^{2}.$ A 263-nm TS beam was used to probe densities ranging from 0.18 to 0.26 $n_{\mathrm{c}}$, where $n_{\mathrm{c}}$ is the critical density for 351-nm light. The experimental geometry was chosen to match the five-beam TPD common wave $k$ vector. The TS spectrum shows a large amplitude, narrow ($\sim $1.6-nm FHWM) feature centered around 423.4 nm. This wavelength corresponds to scattering from EPW's with a normalized wave vector $k \mathord{\left/ {\vphantom {k {k_{3\omega } }}} \right. \kern-\nulldelimiterspace} {k_{3\omega } }=1.3,$ a density of $n_{\mathrm{e}}$/$n_{\mathrm{c}} =$ 0.243, and a temperature of $T_{\mathrm{e}} =$ 2 keV. This is consistent with the predicted values given by the dispersion relations and TPD maximum growth hyperbola. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.