Description
The focus of the inertial confinement fusion (ICF) community post-ignition is shifting towards practical schemes for inertial fusion energy production (IFE). Direct drive, where the intense lasers irradiate the fuel pellet exterior directly, and related variants such as shock ignition [1] and fast ignition is a primary candidate for IFE. A critical limitation facing direct drive is the excitation of laser-plasma instabilities (LPI) which act to couple laser waves into scattered light and hot electrons, reducing the efficiency of implosions. The growth of LPI is non-linear with laser intensity, making it a particular issue for shock ignition. Additionally LPI growth is highly dependent on the plasma parameters and kinetic effects, making predictive modelling challenging. Experiments at high power laser facilities with planar targets therefore offer a valuable tool for understanding the fundamental growth of these instabilities.
In this paper we present results from an experimental study performed at ELI-Beamlines measuring the Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS) instabilities. A full-aperture backscatter (FABS) diagnostic station [2] was used to measure LPI driven under irradiation of thick and thin “exploding foil” targets by the L4n laser in 2⍵ (λ = 526.5 nm) up to intensities ~3 x 1015 W/cm2. In addition, a Distributed Phase Plate (DPP) was used to spatially smooth the focal spot in conditions more relevant to ICF. Filamentation and self-focusing are found to play a significant role in driving LPI considering the larger f-number (f/10.8) of the laser focusing optic. Our measurements highlight the critical role of focal spot statistics and self-focusing in governing LPI growth and demonstrate the utility of exploding foils for expanding the parameter space accessible at high-power laser facilities [3].
[1] R. Betti et al., Phys. Rev. Lett. 98, 155001 (2007)
[2] F. Wasser et al., Rev. Sci. Instrum. 94, 093503 (2023)
[3] E. Hume et al., HPLSE (submitted)