Planar Two-Plasmon--Decay Experiments at Polar-Direct-Drive Ignition-Relevant Scale Lengths at the National Ignition Facility

Results from the first experiments at the National Ignition Facility (NIF) to probe two-plasmon$-$decay (TPD) hot-electron production at scale lengths relevant to polar-direct-drive (PDD) ignition are reported. The irradiation on one side of a planar CH foil generated a plasma at the quarter-critical surface with a predicted density gradient scale length of $L_{\mbox{n}} \sim 600\mu m,$ a measured electron temperature of $T_{e} \sim 3.5$ to 4.0 keV, an overlapped laser intensity of $I\sim 6\times 10^{14}$ W/cm$^{2}$, and a predicted TPD threshold parameter of $\eta \sim 4.$ The hard x-ray spectrum and the K$_{\alpha }$ emission from a buried Mo layer were measured to infer the hot-electron temperature and the fraction of total laser energy converted to TPD hot electrons. Optical emission at $\omega $/2 correlated with the time-dependent hard x-ray signal confirms that TPD is responsible for the hot-electron generation. The effect of laser beam angle of incidence on TPD hot-electron generation was assessed, and the data show that the beam angle of incidence did not have a strong effect. These results will be used to benchmark simulations of TPD hot-electron production at conditions relevant to PDD ignition-scale implosions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.