Two-Plasmon–Decay Suppression Using Laser Wavelength Detuning

March 2018

Color maps

Color maps of the electrostatic potential from 2-D LPSE simulations late in time (t = 10 ps)
when the field structure is dominated by the absolutely unstable modes. (a) Zero frequency
detuning and (b) 1% frequency detuning (two color). The frequency detuning results in
a spatial separation of the absolutely unstable modes and doubles the instability
threshold. The higher threshold limits the production of the
two-plasmon decay in the experiment.

For direct-drive inertial confinement fusion (ICF) experiments, the two-plasmon–decay (TPD) instability poses significant problems given its low threshold and high-energy electron generation. Results of 3-D laser–plasma interaction simulations, recently performed by Russ Follett and five other LLE scientists, indicate that only modest amounts of laser frequency detuning can suppress the TPD instability and the corresponding hot-electron generation. The conventional metric for hot-electron mitigation suggests a requirement of >1% bandwidth, which reaches beyond the current capabilities of existing ICF lasers. However, Follett's recent work shows that less than 0.5% frequency shifts between laser beams can suppress hot-electron generation.

The laser–plasma simulation environment (LPSE) computer code designed to solve problems in high-energy-density physics (HEDP) has successfully reproduced experiments including the measured plasma-wave amplitudes and hot-electron generation. The addition of a laser bandwidth model to LPSE has enabled the exploration of new strategies for mitigating laser–plasma instabilities. Coupling LPSE with the recent upgrade to the OMEGA EP Laser System to support ultraviolet detuning up to 0.7% makes it possible to investigate these results in upcoming experiments.

Color maps

Scientist Russ Follett

Three-dimensional LPSE simulations of an OMEGA implosion have been run, varying amounts of frequency detuning to determine the fraction of the incident laser energy converted into hot electrons. The simulations used two types of detuning, including (1) "multicolor," where each beam was split into three frequencies, and (2) "tiled," where each beam is monochromatic, but the different beams have three different frequencies (alternating between beams). The simulations show that, in either case, as little as 0.5% frequency detuning is sufficient to suppress hot electrons in current OMEGA implosions.