Direct Measurements of DT Fuel Preheat from Hot Electrons in Direct-Drive Inertial Confinement Fusion

Hot electrons generated by laser-plasma instabilities degrade the performance of laser-fusion implosions by preheating the DT fuel and reducing core compression. The hot-electron energy deposition in the DT fuel has been directly measured for the first time by comparing the hard x-ray signals between DT-layered and mass-equivalent ablator-only implosions. The electron energy deposition profile in the fuel is inferred through dedicated experiments using Cu-doped payloads of varying thickness. The measured preheat energy accurately explains the areal-density degradation observed in many OMEGA implosions. This technique can be used to assess the viability of the direct-drive approach to laser fusion with respect to the scaling of hot-electron preheat with laser energy.

Figure 1

Simple illustration of the hard x-ray comparison between an all-CH target and a DT-layered target. When the hot electron source is the same, the difference in hard x-ray signal is a direct indicator of electron deposition into the inner DT which emits less x rays for the same amount of preheat energy compared to CH.

Figure 2

The laser pulse (black) and the hard x-ray signals for DT-layered shot 77 064 (red) and all-CH shot 77 062 (blue) are plotted as a function of time. The difference in hard x-ray signals is proportional to the hot-electron energy deposited in the DT layer.

Figure 3

(a) Typical targets used in preheat experiments and in the simulation ensemble. (b) The hot-electron energy deposited into a non-CH payload from Eq. (1) is compared to the results of LILAC simulations (bottom plot). Both the hot-electron source and the payload material are varied in the simulations. The good agreement validates the preheat formula [Eq. (1)].

Figure 4

(a) The half-harmonic spectrum for all-CH implosion 82 056. (b) The signal integrated over all frequencies is plotted as a function of time for DT-layered implosion 77 064, all-CH implosion 82056, and $\mathrm{CH}+\mathrm{CH}(\mathrm{Cu})$ implosion 82058.

Figure 5

The hard x-ray predictions from a uniform deposition model [Eq. (4)] are compared to the measured hard x-ray signals from the CH(Cu) experiments. The good agreement indicates the electrons deposit their energy uniformly in the unablated mass.

Figure 6

Areal-density degradation versus preheat energy into the stagnated shell $E_{\mathrm{stag}}^{\text{preheat}}$ normalized to the shell internal energy at peak velocity ${\mathrm{IE}}_{\text{shell}}$ for a large ensemble of LILAC simulations with preheat energies ranging from 0 to 100 J and design adiabats between 2 and 5.5. The black curve is the best fit in Eq. (6).