Thomson-Scattering Measurements of Ion-Acoustic Wave Amplitudes Driven by the Two-Plasmon-Decay Instability

Thomson scattering was used to measure enhanced ion-acoustic waves (IAW's) driven by the two-plasmon-decay (TPD) instability. The IAW amplitude scales with the 3/2\textit{$\omega $} emission (a TPD signature). Up to 20 beams with 860-\textit{$\mu $}m-diam laser spots generated by 2-ns-long pulses of 3\textit{$\omega $} (0.351-\textit{$\mu $}m) light with overlapped intensities up to 4 $\times $ 10$^{14}$ W/cm$^{2}$ were used to produce $\sim $300-\textit{$\mu $}m density-scale lengths. The IAW amplitudes were measured using 4\textit{$\omega $} Thomson scattering near 3\textit{$\omega $} quarter-critical densities. Time-resolved 3/2\textit{$\omega $} spectroscopy was used to compare the amplitude of 3/2\textit{$\omega $} emission to the IAW amplitude. \textit{QZAK}\footnote{ K. Y. Sanbonmatsu\textit{ et al.}, Phys. Rev. Lett. \textbf{82}, 932 (1999).}$^{,}$\footnote{ K. Y. Sanbonmatsu\textit{ et al.}, Phys. Plasmas \textbf{7}, 2824 (2000).} modeling shows a similar onset threshold and wave amplitude as the experiments. The model suggests that the source of the IAW growth is from the beating of electron-plasma waves, which drive density perturbations through the ponderomotive force. This conclusion is supported by the experimental geometry. This process is shown to be a saturation mechanism for TPD from simulations.\footnote{ R. Yan\textit{ et al.}, Phys. Rev. Lett. \textbf{103}, 175002 (2009).} This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.