Saturation of the Two-Plasmon Decay Instability in Long-Scale-Length Plasmas Relevant to Direct-Drive Inertial Confinement Fusion

Measurements of the hot-electron generation by the two-plasmon-decay instability are made in plasmas relevant to direct-drive inertial confinement fusion. Density-scale lengths of $400\mu \mathrm{m}$ at $n_{\mathrm{cr}}/4$ in planar CH targets allowed the two-plasmon-decay instability to be driven to saturation for vacuum intensities above $\sim 3.5\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$. In the saturated regime, $\sim 1\%$ of the laser energy is converted to hot electrons. The hot-electron temperature is measured to increase rapidly from 25 to $90\mathrm{keV}$ as the laser beam intensity is increased from 2 to $7\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$. This increase in the hot-electron temperature is compared with predictions from nonlinear Zakharov models.

Figure 1

(a) The calculated plasma parameters as a function of the diameter of the laser beams, at constant intensity ($7\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$), demonstrates that the high intensities over large laser spots available in these experiments allow the TPD parameter ($I_q{L}_n/T_e$) to be extended to direct-drive ignition conditions. (b) At the conditions for this experiment ($1000\mu \mathrm{m}$ diameter laser spot), the density-scale length (left axis) is calculated to reach nearly $400\mu \mathrm{m}$ for the highest intensities tested (right axis).

Figure 2

(a) Thomson scattered light from near the quarter critical density for $3\omega$ light is spectrally and temporally resolved to measure the ion-acoustic features. The electron and ion temperatures as a function of time are obtained by fitting the standard dynamic form factor [31] to the measured spectra that are averaged over 50 ps. A best fit (white curve) to the measured spectrum at 0.8 ns (red curve) is obtained for $T_e=1.6\mathrm{keV}$, $T_i=1.0\mathrm{keV}$. (b) The electron (solid line) and ion (dashed line) temperatures calculated by DRACO compare well to the measurements. The data are obtained for an intensity of $3\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$.

Figure 3

The measured time history of the hard x-ray emission ($>40\mathrm{keV}$) is shown for (solid line) $7\times {10}^{14}$ (right axis) and (dots) $2.4\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$.

Figure 4

(a) The fraction of total laser energy deposited into the hot electrons is plotted as a function of the vacuum intensity (bottom axis) and $I_q{L}_n/T_e$ (top axis). (b) The hot-electron temperature inferred from the HXRD hard x-ray measurements (circles) is shown. The hot-electron temperatures calculated by ZAK (open squares) and QZAK (solid squares) are included for reference.