Hot Spot Evolution Measured by High-Resolution X-Ray Spectroscopy at the National Ignition Facility

Evolution of the hot spot plasma conditions was measured using high-resolution x-ray spectroscopy at the National Ignition Facility. The capsules were filled with DD gas with trace levels of Kr and had either a high-density-carbon (HDC) ablator or a tungsten (W)-doped HDC ablator. Time-resolved measurement of the Kr $\mathrm{He}\beta$ spectra, absolutely calibrated by a simultaneous time-integrated measurement, allows inference of the electron density and temperature through observing Stark broadening and the relative intensities of dielectronic satellites. By matching the calculated hot spot emission using a collisional-radiative code to experimental observations, the hot spot size and areal density are determined. These advanced spectroscopy techniques further reveal the effect of W dopant in the ablator on the hot spot parameters for their improved implosion performance.

Figure 1

(a) Hohlraum parameter. The rectangle indicates the viewing window for dHIRES. The yellow dot indicates the capsule. (b) Pie diagrams of the undoped HDC capsule and W-doped HDC capsule. (c) Laser pulses delivered to the capsules.

Figure 2

(a) The cross-calibrated streak camera data for the $\mathrm{He}\beta$ complexes from undoped HDC experiment N171102. (b) The time-integrated streak camera data are overlaid on top of the simultaneous image plate measurement.

Figure 3

History of the high-resolution Kr $\mathrm{He}\beta$ spectra for N171102. $E_1$ and $E_2$ denote the energy thresholds at which the widths of the Stark broadened Kr $\mathrm{He}\beta$ complexes were determined for the spectrum at 7.05 ns (green). The inset shows the derived $\mathrm{He}/\mathrm{Li}$ line ratios (blue) and the Stark widths (red).

Figure 4

(a) The experimentally measured $n_e$ (solid lines) for the undoped and W-doped HDC capsules compared with the spatially averaged, x-ray weighted calculations (dashed lines). (b) The experimentally measured $T_e$ (solid lines) for both capsules compared with the spatially averaged, x-ray weighted calculations (dashed lines).

Figure 5

Evolution of the hot spot radius $R_{\mathrm{hs}}$ (a) and areal density $\rho R_{\mathrm{hs}}$ (b) for the undoped and W-doped HDC shots.