Control of collective FSBS and backscatter SRS through plasma composition

Nominal NIF parameters are near the collective forward SBS (FSBS) threshold (P. M. Lushnikov and H. A. Rose, Phys. Rev. Lett. \textbf{92}, 255003 (2004), ``L{\&}R''). It will be shown that being on this instability edge can be used as a control lever: a small amount of high Z dopant may lead to qualitative change in FSBS regime at fixed laser intensity, possibly reducing backscatter instability losses (Such results have already been observed, but absent SSD, a key aspect of our theory: R. M. Stevenson et al., Phys. Plasmas \textbf{11}, 2709 (2004); L. J. Suter et al., 2738, ib.). P\textit{onderomotive }FSBS regimes are determined by the parameter $\tilde {I}=F^2\left( {{\mbox{v}_{\mbox{osc}} } \mathord{\left/ {\vphantom {{\mbox{v}_{\mbox{osc}} } {\mbox{v}_\mbox{e} }}} \right. \kern-\nulldelimiterspace} {\mbox{v}_\mbox{e} }} \right)^2{\left( {{n_e } \mathord{\left/ {\vphantom {{n_e } {n_c }}} \right. \kern-\nulldelimiterspace} {n_c }} \right)} \mathord{\left/ {\vphantom {{\left( {{n_e } \mathord{\left/ {\vphantom {{n_e } {n_c }}} \right. \kern-\nulldelimiterspace} {n_c }} \right)} \nu }} \right. \kern-\nulldelimiterspace} \nu $, with \textit{$\nu $} the dimensionless ion acoustic damping coefficient and $F$ the optic f/{\#}. Analytical results will be presented which show a decrease of $\tilde {I}{\kern 1pt}\mbox{'s}$ threshold value through the addition of high Z dopant to low Z plasma, owing to increased \textbf{thermal contribution} to FSBS. Alternatively, one may raise the threshold by managing the value of \textit{$\nu $}$_{ }$by, $e.g.$, adding He to SiO$_{2}$. For nominal NIF parameters, a range of He fraction in SiO$_{2}$ plasma is predicted to suppress backscatter SRS while maintaining control of forward SBS.