Anomalous Absorption by the Two-Plasmon-Decay Instability in Directly Driven Inertial Confinement Fusion Experiments*

Simulations of directly driven inertial confinement fusion experiments on the OMEGA Laser System were significantly improved with the inclusion of an inline model for crossed-beam energy transfer along with a nonlocal model for heat transport. Absorption and shell-velocity time histories are accurately predicted when experiments are driven at relatively low overlapped laser intensity. Discrepancies appear at higher intensity, however, with higher-than-expected laser absorption on target. Strong correlation between those discrepancies and signatures of the two-plasmon-decay instability (TPDI)---including time-dependent half-harmonic emission and hard x-ray signals---indicate that TPDI is responsible for this anomalous absorption. The data suggest that up to $\sim30\%$ of the laser right reaching $n_c/4$ can be absorbed locally when the TPDI threshold is exceeded, which is consistent with LPSE simulations.