Radiation-hydrodynamic simulations of directly driven fusion experiments at the Omega Laser Facility predict absorption accurately when targets are driven at low overlapped laser intensity. Discrepancies appear at increased intensity, however, with higher-than-expected laser absorption on target. Strong correlations with signatures of the two-plasmon decay (TPD) instability—including half-harmonic and hard-x-ray emission—indicate that TPD is responsible for this anomalous absorption. Scattered light data suggest that up to $\approx 30\%$ of the laser power reaching quarter-critical density can be absorbed locally when the TPD threshold is exceeded. A scaling of absorption versus TPD threshold parameter was empirically determined and validated using the laser–plasma simulation environment code.

• Received 18 October 2019
• Revised 11 March 2020
• Accepted 17 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.185001