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We present a parametric scaling of hot electron (HE) generation at quarter critical density from the two-plasmon decay process. The study is conducted with the laser plasma simulation environment code, considering Langmuir decay instabilities (LDI) and laser pump depletion in 2D. The parameter scan is conducted as a function of electron temperature, ion–electron temperature ratio, drive strength, and density scale length. The scaling shows an hot electron (HE) conversion fraction up to 40%, HE fluxes up to 6 × 10 14 W / cm 2 , and average temperatures in the range of 30 to 100 keV. The electron angular distributions exhibit two main regions: the plasma "bulk," characterized by homogeneous emission, up to energies of 30 − 60 keV depending on the individual laser–plasma conditions, and a HE tail after ≃ 50 − 60 keV. The mid-energy electrons are homogeneously emitted toward the end of the plasma bulk and acquire energy through electron plasma wave (EPW) Landau damping from Langmuir wave collapse and LDI cascade. The HE tail has electrons emitted in the forward direction and at low divergence, due to turbulence and EPW Landau damping from multi-staged acceleration. Finally, the laser power transmitted through the quarter critical region reaches values from ∼ 80 % down to ∼ 35 % for increasing HE generation, with absorption due to EPW collisional damping in the range of ∼ 10 % − 35 %. [ABSTRACT FROM AUTHOR]

The stimulated Raman scattering (SRS) instability can inhibit the performance of laser-driven inertial confinement fusion (ICF) implosions by scattering light into unwanted directions or by generating hot electrons that preheat the target fuel. In principle, ICF target designs can avoid parameter regimes conducive to large, linear SRS gains. In practice, kinetic inflation—the nonlinear enhancement of SRS due to electron trapping in the excited plasma wave—makes this difficult. Here, we show that laser bandwidth in the form of frequency modulation can either decrease or increase the inflationary SRS (iSRS) threshold in inhomogeneous plasmas depending on the maximum chirp of the laser pulse. The threshold, mapped out by a series of particle-in-cell simulations, exhibits a minimum when the frequency change within the pulse cancels the spatial detuning due to density inhomogeneities along the trajectory of the scattered light. By tuning the pump laser parameters away from this minimum, the iSRS threshold can be larger than at zero bandwidth, providing a path to mitigating kinetic inflation in ignition relevant plasmas. [ABSTRACT FROM AUTHOR]

Higher levels of hot electrons from the two-plasmon–decay instability are observed when smoothing by spectral dispersion (SSD) is turned off in directly driven inertial confinement fusion experiments at the Omega Laser Facility. This finding is explained using a hot-spot model based on speckle statistics and simulation results from the laser–plasma simulation environment. The model accurately reproduces the relative increase in hot-electron activity at two different drive intensities although it slightly overestimates the absolute number of hot electrons in all cases. Extrapolating from the current ≈ 360 -GHz system while adhering to the logic of the hot-spot model suggests that a larger SSD bandwidth should significantly mitigate hot-electron generation, and legacy 1-THz OMEGA experiments appear to support this conclusion. These results demonstrate that it is essential to account for laser speckles and spatiotemporal smoothing to obtain quantitative agreement with experiments. A compilation of hot-electron data from the past two decades reveals several other important points: (1) many prior experiments are more easily understood using recent results from multibeam absolute instability theory and (2) experiments with ignition-scale conditions produce less hot electrons compared to OMEGA spherical experiments for a given vacuum overlapped intensity, which is a promising result for validating performance predictions based on hydrodynamic scaling relations. [ABSTRACT FROM AUTHOR]

Laser–plasma interaction instabilities can be detrimental for direct-drive inertial confinement fusion by generating high-energy electrons that preheat the target. An experimental platform has been developed and fielded on the National Ignition Facility to investigate hot-electron production from laser–plasma instabilities at direct-drive ignition-relevant conditions. The radiation-hydrodynamic code DRACO has been used to design planar-target experiments that generate plasma and interaction conditions comparable to direct-drive ignition designs: IL ∼ 1015 W/cm2, Te > 3 keV, and density-gradient scale lengths of Ln ∼ 600 μm in the quarter-critical density region. The hot-electron properties were inferred by comparing the experimentally observed hard x-ray spectra to Monte Carlo simulations of hard x-ray emission from hot electrons depositing energy in the target. Hot-electron temperatures of ∼40 keV to 60 keV and the fraction of laser energy converted to hot electrons of ∼0.5% to 5% were inferred in plastic targets for laser intensities at the quarter-critical density surface of (∼4 to 14) × 1014 W/cm2. The use of silicon ablators was found to mitigate the hot-electron preheat by increasing the threshold laser intensity for hot-electron generation from ∼3.5 × 1014 W/cm2 in plastic to ∼6 × 1014 W/cm2 in silicon. The overall hot-electron production is also reduced in silicon ablators when the intensity threshold is exceeded. [ABSTRACT FROM AUTHOR]

Stimulated Raman scattering (SRS) has been explored comprehensively in planar-geometry experiments at the National Ignition Facility in conditions relevant to the corona of inertial confinement fusion ignition-scale direct-drive targets. These experiments at measured electron temperatures of 4 to 5 keV simulated density scale lengths L n of 400 to 700 μm, and laser intensities at the quarter-critical density of up to 1.5 × 1015 W/cm2 have determined SRS thresholds and the scaling behavior of SRS for various beam geometries. Several SRS mechanisms, including saturated absolute SRS near the quarter-critical density and additional SRS, including near-backscatter or sidescatter at lower densities, have been identified. Correlation of time-dependent SRS at densities ∼0.15 to 0.21 of the critical density with hot-electron signatures as well as the magnitudes of these signatures across different experiments, is observed. Further modeling work is needed to definitively identify the density region in which hot electrons are generated and will guide SRS and hot-electron preheat mitigation strategies for direct-drive-ignition designs. [ABSTRACT FROM AUTHOR]

Multibeam experiments relevant to direct-drive inertial confinement fusion show the importance of nonlinear saturation mechanisms in the common-wave two-plasmon-decay (TPD) instability. Planar-target experiments on the OMEGA laser used hard x-ray measurements to study the influence of the linear common-wave growth rate on TPD-driven hot-electron production in two drive-beam configurations and over a range of overlapped laser intensities from 3.6 to 15.2 x 1014 W/cm². The beam configuration with the larger linear common-wave growth rate had a lower intensity threshold for the onset of hot-electron production, but the linear growth rate made no significant impact on hot-electron production at high intensities. The experiments were modeled in 3-D using a hybrid code LPSE (laser plasma simulation environment) that combines a wave solver with a particle tracker to self-consistently calculate the electron velocity distribution and evolve electron Landau damping. Good quantitative agreement was obtained between the simulated and measured hot-electron distributions using a novel technique to account for macroscopic spatial and temporal variations that were present in the experiments. [ABSTRACT FROM AUTHOR]

Three-dimensional laser-plasma interaction simulations show that laser frequency detuning by an amount achievable with current laser technology can be used to suppress the two-plasmon decay (TPD) instability and the corresponding hot-electron generation. For the plasma conditions and laser configuration in a direct-drive inertial confinement fusion implosion on the OMEGA laser, the simulations show that ~0.7% laser frequency detuning is sufficient to eliminate TPD-driven hot-electron generation in current experiments. This allows for higher ablation pressures in future implosion designs by using higher laser intensities. [ABSTRACT FROM AUTHOR]