Transition from two-plasmon decay to stimulated Raman scattering under ignition conditions.

Theoretical and numerical studies on stimulated Raman scattering (SRS) and two-plasmon decay (TPD) with parameters ranging from a direct-drive scheme to shock ignition are presented. A transition from the TPD-dominant regime, through TPD with strong pump depletion, to the SRS-dominant regime shows that SRS is becoming increasingly significant under ignition conditions. Theoretically, we present an envelope model for both SRS and TPD in inhomogeneous plasmas with density not very close to quarter-critical density and analyze a highly nonlinear regime of TPD where the compression of Langmuir waves occurs. However, as the convective threshold for SRS is surpassed, low-density SRS itself controlled by filamentation can dramatically suppress high-density TPD. These physical changes are well described by two-dimensional particle-in-cell simulations. In addition, scalings of hot-electron fraction and reflectivities also indicate such a transition process, and qualitatively agree with recent experiments. [ABSTRACT FROM AUTHOR]