Nonuniformly Driven Two-Plasmon-Decay Instability in Direct-Drive Implosions

Half-harmonic emission spectra and images taken during directly driven implosions show that the two-plasmon decay (TPD) instability is driven nonuniformly over the target surface and that multibeam effects dominate this instability. The images show a spatially limited extent of the TPD instability. A prominent spectral feature is used to determine the electron temperature in the corona. Near threshold the temperatures agree with one-dimensional hydrodynamic predictions but exceed them by $\ensuremath{\sim}10%$ above the TPD threshold. Two-dimensional hydrodynamic simulations indicate that a significant part ($\ensuremath{\sim}20%$) of the laser intensity must be locally absorbed by the TPD instability (i.e., by collisional damping of the electron plasma waves) to maintain these temperature islands.