Maggie Rivers (The University of Texas at Austin)

"Laser-Plasma Instabilities of Frequency Doubled Pulses at the ELI’s L4 Beamline"

Abstract: Laser Plasma Instabilities (LPI) are one of the principal problems in laser- driven inertial confinement fusion (ICF) schemes, scattering laser energy and producing hot electrons that can pre-heat a compressing ICF target. ICF systems generally employ short wavelength lasers, as LPI effects scale strongly with wavelength. As such, instabilities of particular concern, such as Stimulated Raman Scattering (SRS), Stimulated Brillouin Scattering (SBS), and Two Plasmon Decay (TPD), have been well characterized in the ultraviolet (UV) regime. However, converting to the higher frequencies (3ω of Nd:glass lasers) loses laser energy that can otherwise be irradiated on target. Additionally, higher frequencies can inflict significant damage to the optics in the laser chain. Compared to optics for green light (2ω of Nd:glass lasers), developments in UV optics have been slow to achieve durability to the higher photon energies associated with UV light. New fusion schemes propose 2ω light as an ICF driver but these LPI effects have a lower threshold compared to 3ω. This calls for further study of LPI with green light. To this end, we have built a backscatter diagnostic to measure SRS, SBS, and TPD at the Extreme Light Infrastructure’s L4n beamline. This work details measurements made with this diagnostic of laser-plasma instabilities of a 527 nm laser (intensities ranging from 0.5 × 1013 W/cm2 to 1.6 × 1015 W/cm2) relevant to direct drive with a plane solid target during commissioning of the beamline.