Schematic of cryogenic ignition hohlraum.
Schematic of ignition target in (a) and symcap target in (b) with Ge-doped ablator. The cryogenic fuel of the ignition target and the D3He gas fill of the symcap are transported to the interior of the Ge-doped plastic ablator using a fill tube.
Bright spots observed in broadband, gated (dt ∼ 40 ps) x-ray implosion images for photon energies greater than 8 keV provide an indication that heterogeneous mixing of ablator material into the hot spot may be occurring in NIF implosions. A sequence of Fourier-analyzed gated broadband implosion images taken along the hohlraum axis around peak compression for (a) a symcap shot and (b) a DT implosion. A detailed description of Fourier-analyzed gated broadband NIF implosion images can be found elsewhere.19 The color scale is the fraction of the local envelope intensity that the bright spot represents. The white curve is the 17% intensity contour of the peak envelope intensity representing the extent of the hot spot, and the fill tube location is indicated by the yellow line. The bright spots persist in time and are sometimes observed to coalesce in time.
A portion of the time-integrated, 1-D spectral image of an ignition-scale implosion recorded with the HSXRS on the NIF. The spatial resolution is100 μm and the spectral resolution is 12 eV. The 1-D spatial profile of Ge Kα emission from the compressed shell is broader than the Ge Heα + satellite emission from the hot spot, as expected.
Measured x-ray spectrum for symcap implosion N110208 showing prominent features (black curve). The x-ray continuum from the hot spot transmitted through the compressed shell is modeled (red curve) assuming the x-ray continuum and the shell optical thickness scale with photon energy (hν) as e − h ν / kT and (hν)−3, respectively. I C, M L, M K + L, and kT are fitting parameters, and hν K is the Ge K–edge photon energy.
The Ge areal density versus the contrast in the measured x-ray continuum signal around the Ge K edge. Shell transmission is modeled assuming optical thickness of the shell is μ cold Ge ρR(Ge), where μ cold Ge is the mass-absorption coefficient of neutral Ge. The inferred ρR(Ge) = 0.0146 to 0.0159 g/cm2. The areal density of the CH in the compressed shell can be obtained by assuming an atomic fraction between 0.5% and 1% of Ge in the CH.
(a) Model showing the x-ray continuum from the hot spot photopumps the surrounding compressed shell containing Ge dopant and produces Ge Kα emission with spectral brightness which is related to the Ge K-edge intensity drop ΔI K edge. (b) Predicted Ge Kα emission versus measured Ge Kα emission.
Measured Ge K-shell line emission in the 10- to 10.5-keV range (black circle) for (a) symcap implosion N110208 and (b) DT implosion N110620. Hot-spot–mix mass of CH ablator doped with 1% atomic Ge is inferred from the modeled spectrum (best fit = black curve, 1σ spectral fits = red and green curves). Dotted lines are contributions from He-, Li-, Be-, and B-like Ge charge states for the best fit.
Picture of mix mass in the target at peak compression.
Hot-spot–mix mass inferred from x-ray spectroscopy versus predictions of a simple model of the hot-spot–mix mass that combines linear analysis of the perturbation growth with detailed 2-D hydrodynamic simulations.
Ignition target with Si-doped plastic ablator.
Ignition target with Si-doped plastic ablator for hot-spot–mix experiments. The innermost layer is doped with Cu (layer 1). Layers 2 and 3 are doped with Ge and Si. Layer 4 is doped with Si only and the outer ablator is pure CH. The origin of the hot-spot mix will be examined by comparing the Cu and Ge mix–mass results.
Hot-spot–mix analysis for symcap implosions.
Hot-spot–mix analysis for DT and THD implosions.
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