Enhanced Ion-acoustic Wave Fluctuations Driven by Speckled Heater Beams

Laser facilities investigating indirect and direct-drive fusion techniques including the National Ignition Facility, Laser Mégajoule, and the OMEGA 60 system at the Laboratory for Laser Energetics, employ distributed phase plates (DPPs) to improve on-target laser uniformity and to mitigate hydrodynamic and laser-plasma instabilities (LPI). DPPs provide nearfield phase aberrations to the beams that produce characteristic speckled intensity distributions on target. Here, we provide evidence that gas-jet plasmas heated by these speckled beams with no smoothing by spectral dispersion (SSD) are subject to increased ion-acoustic wave (IAW) fluctuations that can enhance LPI. Experiments performed on the OMEGA 60 laser system couple a 2-ns 2ω Thomson scattering probe into a gas-jet plasma ionized and heated by 0.5-ns 3ω heater beams utilizing SG5-850 DPPs and no SSD. Resulting time-resolved Thomson scattering measurements show enhanced power scattered by ion-acoustic wave (IAW) fluctuations while the speckled heater beams are on. However, once the heater beams have turned off, the IAW-scattered power returns to thermal levels. In addition to the long IAW wavevectors measured by Thomson scattering, indirect evidence of enhanced short IAW wavevectors is observed in joint OMEGA 60 and OMEGA EP beam spray experiments. These experiments couple a short, 1-100 ps, 1ω beam apodized to a Gaussian f/50 into the same gas-jet plasma and show increased 1ω beam spray when propagating through the plasma while heater beams are on. In this case, the high-intensity speckles of the heater beams ponderomotively and thermally drive density perturbations above thermal levels that seed filamentation leading to large-angle beam spray.