2014
DOI: 10.1103/physrevlett.112.105003
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Novel Characterization of Capsule X-Ray Drive at the National Ignition Facility

Abstract: Indirect drive experiments at the National Ignition Facility are designed to achieve fusion by imploding a fuel capsule with x rays from a laser-driven hohlraum. Previous experiments have been unable to determine whether a deficit in measured ablator implosion velocity relative to simulations is due to inadequate models of the hohlraum or ablator physics. ViewFactor experiments allow for the first time a direct measure of the x-ray drive from the capsule point of view. The experiments show a 15%-25% deficit re… Show more

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Cited by 92 publications
(28 citation statements)
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“…Both data and model see a decrease in inner to outer beam ratio in going to the thicker capsule. This, plus the increase in laser backscatter [7], is an indication of the influence of capsule blow-off on plasma conditions. Figure 4 shows that the model underestimates the ratio for the two low foot experiments with CBET and the HiFoot experiment but agrees with the data for the LOW CBET case.…”
Section: Resultsmentioning
confidence: 99%
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“…Both data and model see a decrease in inner to outer beam ratio in going to the thicker capsule. This, plus the increase in laser backscatter [7], is an indication of the influence of capsule blow-off on plasma conditions. Figure 4 shows that the model underestimates the ratio for the two low foot experiments with CBET and the HiFoot experiment but agrees with the data for the LOW CBET case.…”
Section: Resultsmentioning
confidence: 99%
“…These experiments use an ignition hohlraum with one end truncated (see Figure 1a) to allow views of the hohlraum interior from the capsule point-ofview. [7,8] The drive irradiance measured through the open end, which needs no source size correction was 70-85% of that predicted by the model [7] and thus explained nearly all of the velocity deficit.…”
Section: Introductionmentioning
confidence: 98%
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“…NIF implosion experiments consistently show a nearly 15%-25% deficit in (inferred) absorbed capsule energy compared with the nominal level of ∼170 kJ [20]. However, dedicated "ViewFactor" hohlraum experiments have established that much of this deficit is due to an (as yet) unidentified mechanism for reduced laser-hohlraum coupling [6], allowing still for a possible role of separation energy losses in the implosion dynamics. By comparison, direct-drive implosions are less susceptible to this separation energy loss, owing to the relatively fewer number of ablator ions (due to the low ablation front speed) and the high laser coupling efficiency to the capsule (∼60%); i.e., fractionally lower separation losses are expected.…”
Section: Thermodynamics Of Species Separationmentioning
confidence: 96%
“…Indeed, a recent high power (520 TW) and energy (1.86 MJ) shot with a uranium hohlraum appears to have demonstrated the required peak implosion velocity and remaining ablator mass above the ignition goal [5]. Dedicated "ViewFactor" hohlraum experiments have since established that much of this inferred drive deficit is due to an (as yet) unidentified mechanism for reduced laserhohlraum coupling [6] and can "explain nearly all of the disagreement with the velocity data." However, multi-ion species capsules may still be subject to a modest degree of separation phenomena that degrade performance through an energy expense from transiently lower entropy, motivating the use of potentially more efficient ablator materials such as high-density carbon (HDC)-which is also being explored on the NIF [7].…”
Section: Introductionmentioning
confidence: 99%