Hohlraum Reheating from Burning NIF Implosions

As fusion experiments at the National Ignition Facility (NIF) approach and exceed breakeven, energy from the burning capsule is predicted to couple to the gold walls and reheat the hohlraum. On December 5, 2022, experiment N221204 exceeded target breakeven, historically achieving 3.15 MJ of fusion energy from 2.05 MJ of laser drive; for the first time, energy from the igniting capsule reheated the hohlraum beyond the peak laser-driven radiation temperature of 313 eV to a peak of 350 eV, in less than half a nanosecond. This reheating effect has now been unambiguously observed by the two independent Dante calorimeter systems across multiple experiments, and is shown to result from reheating of the remnant tungsten-doped ablator by the exploding core, which is heated by alpha deposition.

Figure 1

(a) Schematic of a typical ICF experiment at NIF, where 192 beams heat the interior of a gold hohlraum to $T_R\sim 300\mathrm{eV}$ in order to compress a 2 mm DT capsule to the conditions required for fusion. (b) Representative hohlraum emission spectrum observed by the Dante calorimeter showing thermal region (blue) and gold $m$-band emission (red).

Figure 2

(a) Dante 1 flux histories from the Hybrid-E shots in 2021 and 2022 showing clear fusion signature at approximately 10 ns. (b) A closer look at the reheating signatures illustrating the background subtraction and residuals for three of the higher yield experiments. (c) Integrated hohlraum reheating energy $E_H$ calculated using Eq. (1) as a function of fusion yield.

Figure 3

(a) Dante 2 flux histories from the Hybrid-E shots in 2021 and 2022 showing clear fusion signature at approximately 10 ns. (b) A closer look at the reheating signatures illustrating the background subtraction and residuals for three of the higher yield experiments. (c) Integrated hohlraum reheating energy $E_H$ calculated using Eq. (1) as a function of fusion yield.

Figure 4

(a),(b) Dante 1 and 2 reheating peaks compared with nuclear bang times from the GRH and QCD diagnostics respectively. (c) Cross plot showing the observed reheating peaks for Dante 1 and 2 with a correlation of $R^2=0.97$ and scatter $<50\mathrm{ps}$, well within the absolute uncertainty of the gold ball timing measurement.

Figure 5

Spectra from Dante 1 calculated using UNSPEC at peak laser drive for all shots; for comparison, the spectra from N221204 as the hohlraum is redriven by fusion energy is shown in dashed red.

Figure 6

(a) LASNEX calculations based on experiment N210808-001, alongside residual reheating signatures. (b) Radial electron temperature $T_e$ snapshots between 9.2 and 9.9 ns from the LASNEX simulations showing the blast-wave heated detritus remains hot several 100 ps after nuclear bang time (9.25 ns), as observed by Dante. (c) Radial density $\rho$ snapshots of the capsule between 9.2 and 9.9 ns, showing expansion after heating from the alpha heated core’s expanding blast wave.