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Measurement of areal density in the ablators of inertial-confinement-fusion capsules via detection of ablator (n, n′γ) gamma-ray emission
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Image of FIG. 1.
FIG. 1.

Normalized 5-MeV-threshold GRH signal S′ ≡ S/QGYnDT for ten OMEGA capsule implosions, plotted vs. neutron yield. As expected, there is no apparent trend with neutron yield. Error bars show total random uncertainty resulting from several contributions to random uncertainty in S, as well as random uncertainty in Q, G, and YnDT , as discussed in the text. Uncertainty-weighted mean value ⟨S′⟩ HTS  = (8.87 ± 0.54) × 10−18 V ns/unit quantum efficiency/unit gain/neutron is shown as the solid horizontal line, with dashed lines indicating ±1σ variation.

Image of FIG. 2.
FIG. 2.

Comparison of observed and calculated MIPS ablator areal densities from Table III for the thirteen OMEGA plastic-shell implosions observed with GRH at 3-MeV threshold. Calculated ⟨ρR⟩s come from applying the MIPS model to the results of 1D spherically symmetric radiation-hydrodynamic simulations, as described in the text. Horizontal error bars show the uncertainty bounds on observed ⟨ρR⟩ determined as described in Appendix C . Vertical error bars show the variation in calculated ⟨ρR⟩ as described in Sec. IV . The blue triangles indicate capsules with ablators composed mostly of 13CH (shots #59495 and #59496), for which we can determine only lower bounds to the areal density. In ten of the thirteen cases, the observed values agree with simulations within the uncertainty bounds. The three outliers with large 1D calculated ablator areal density had thick shells, or low fill pressure and an equatorial perturbation.

Image of FIG. 3.
FIG. 3.

QE-, gain-, and yield-normalized high-threshold GRH signal for OMEGA shot #59494 (DT neutron yield = 2.25 × 1013), a DT-filled CH capsule imploded in September 2010 (black curve). Normalized signal from null shot #59497 (DT neutron yield = 1.99 × 1013), with Ethr  = ∞, is shown for comparison as the blue dash-dot curve. The timing fiducials for the two signals are aligned, but the time scale is arbitrary. The peak centered at about 217 ns is the capsule DT burn pulse. The straight dotted line shows one estimate of the background to be subtracted from the #59494 signal. It is defined by the mean value of the #59494 signal in two time windows before and after the DT peak, indicated by horizontal tick marks centered at about 216.2 ns and 217.9 ns. Another estimate of the background comes from a least-squares straight-line fit to the #59497 signal over the interval 216 ns–222 ns (straight dashed line).


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Table I.

High-threshold shots (GRH fill: 86.8–87.4 psia SF6, giving threshold = 5 MeV).

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Table II.

Low-threshold shots (GRH fill: 198.8–200 psia SF6, giving threshold = 3 MeV).

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Table III.

Observed and calculated ablator areal density for low-threshold shots.

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Table IV.

Contributors to total uncertainty in low-threshold signal for shot #58164 (with fDT γ ,low  = 0.6263 and fabl γ ,high  = 0.0244).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Measurement of areal density in the ablators of inertial-confinement-fusion capsules via detection of ablator (n, n′γ) gamma-ray emission