Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length ($\sim 500$ to $700\mu \mathrm{m}$), electron temperature ($\sim 3$ to 5 keV), and laser intensity (6 to $16\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is $\sim 0.7\%$ to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from $\sim {4\times 10}^{14}$ to $\sim 6\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

Figure 1

Time-resolved scattered-light spectra at collection angles of (a) 0°, (b) 23.5°, and (c) 50° relative to the target normal. These images were obtained in two CH target experiments. The image in (a) corresponds to an experiment (d) with the target oriented toward a streaked spectrometer and (e) irradiated by a ramp-flat pulse. The images in (b) and (c) correspond to an experiment (f) with the target oriented toward the south pole of NIF and (g) irradiated first by beams at incidence angles of 45° and 50°, followed by beams at 23° and 30°. The streaked spectrum from a spherical-geometry experiment on OMEGA [inset in (a), same wavelength and time axes] is contrasted to the image in (a).

Figure 2

Ratio of the absolute, single-beam intensity threshold for TPD [10] to the absolute, single-beam threshold for SRS [9] as a function of electron temperature and density scale length. Conditions corresponding to the NIF (OMEGA) spectra in Fig. 1 are represented by the black (white) star. With increasing scale length and temperature, SRS becomes increasingly prevalent with respect to TPD.

Figure 3

Fraction of laser energy converted to hot electrons as a function of simulated quarter-critical laser intensity, for laser drive using outer beams (diamonds) and inner beams (triangles), for CH ablators (blue, green) and Si ablators (orange). Uncertainty in $f_{\text{hot}}$ is based on statistical uncertainty in the hard-x-ray data, propagated through egsnrc modeling.