Tent-induced perturbations on areal density of implosions at the National Ignition Facilitya)

Tommasini, R.; Field, J. E.; Hammel, B. A.; Landen, O. L.; Haan, S. W.; Aracne‐Ruddle, Chantel; Benedetti, L. R.; Bradley, D. K.; Callahan, D. A.; Dewald, E. L.; Doeppner, T.; Edwards, M.; Hurricane, O. A.; Izumi, N.; Jones, O.; Ma, T.; Meezan, N. B.; Nagel, S. R.; Rygg, J. R.; Segraves, K.; Stadermann, Michael; Strauser, R. J.; Town, R. P. J.

doi:10.1063/1.4921218

Cited by 90 publications

(36 citation statements)

References 23 publications

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“…[5][6][7] In addition, the measured in-flight shell thickness at peak velocity (typically $0.5 ns before BT) was $20% thinner for sub-ns coasttimes. 6,7 However, there was no increase in yield with shorter coast-times, 37 later understood as due to large ablation-front growth amplifying capsule imperfections and engineering features [8][9][10][11] leading to variable hot-spot mix 12 being a dominant factor.…”

Section: Introductionmentioning

confidence: 99%

On the importance of minimizing “coast-time” in x-ray driven inertially confined fusion implosions

et al. 2017

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By the time an inertially confined fusion (ICF) implosion has converged a factor of 20, its surface area has shrunk 400×, making it an inefficient x-ray energy absorber. So, ICF implosions are traditionally designed to have the laser drive shut off at a time, toff, well before bang-time, tBT, for a coast-time of tcoast=tBT−toff>1 ns. High-foot implosions on NIF showed a strong dependence of many key ICF performance quantities on reduced coast-time (by extending the duration of laser power after the peak power is first reached), most notably stagnation pressure and fusion yield. Herein we show that the ablation pressure, pabl, which drives high-foot implosions, is essentially triangular in temporal shape, and that reducing tcoast boosts pabl by as much as ∼2× prior to stagnation thus increasing fuel and hot-spot compression and implosion speed. One-dimensional simulations are used to track hydrodynamic characteristics for implosions with various coast-times and various assumed rates of hohlraum cooling after toff to illustrate how the late-time conditions exterior to the implosion can impact the fusion performance. A simple rocket model-like analytic theory demonstrates that reducing coast-time can lead to a ∼15% higher implosion velocity because the reduction in x-ray absorption efficiency at late-time is somewhat compensated by small (∼5%−10%) ablator mass remaining. Together with the increased ablation pressure, the additional implosion speed for short coast-time implosions can boost the stagnation pressure by ∼2× as compared to a longer coast-time version of the same implosion. Four key dimensionless parameters are identified and we find that reducing coast-time to as little as 500 ps still provides some benefit. Finally, we show how the high-foot implosion data is consistent with the above mentioned picture.

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Section: Introductionmentioning

confidence: 99%

On the importance of minimizing “coast-time” in x-ray driven inertially confined fusion implosions

et al. 2017

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“…The correlation of peak growth factor with velocity is quite linear, and the doubling of the peak growth factor with increasing velocity is again apparent. As discussed below, this increase in instability with increasing velocity likely accounts for the apparent plateau in high foot performance at neutron yields of ∼10 16 . In this context, it is worth bearing in mind that even the most unstable of these high foot implosions is still a factor of 2-3 times more stable in terms of growth factor than the low foot implosions tested during the NIC.…”

Section: Linear Stability Simulationsmentioning

confidence: 99%

“…More recent modeling of low foot implosions [7,14], guided by experimental data from NIF, indicates that these low foot implosions were unacceptably unstable given the perturbation sources present. In particular, for the low foot, the perturbation seeded by the plastic membrane or 'tent' used to support the capsule in the hohlraum [15][16][17] appears to have had a devastating impact on the integrity of these implosions. Simulations suggest that, by itself, the tent accounted for more than an order of magnitude reduction in the neutron yield in these implosions.…”

Section: Introductionmentioning

confidence: 99%

Capsule modeling of high foot implosion experiments on the National Ignition Facility

Clark

Kritcher

Milovich

et al. 2017

Plasma Phys. Control. Fusion

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This paper summarizes the results of detailed, capsule-only simulations of a set of high foot implosion experiments conducted on the National Ignition Facility (NIF). These experiments span a range of ablator thicknesses, laser powers, and laser energies, and modeling these experiments as a set is important to assess whether the simulation model can reproduce the trends seen experimentally as the implosion parameters were varied. Two-dimensional (2D) simulations have been run including a number of effects-both nominal and off-nominal-such as hohlraum radiation asymmetries, surface roughness, the capsule support tent, and hot electron pre-heat. Selected three-dimensional simulations have also been run to assess the validity of the 2D axisymmetric approximation. As a composite, these simulations represent the current state of understanding of NIF high foot implosion performance using the best and most detailed computational model available. While the most detailed simulations show approximate agreement with the experimental data, it is evident that the model remains incomplete and further refinements are needed. Nevertheless, avenues for improved performance are clearly indicated.

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“…2) has been measured with radiography and shows good agreement with simulations. 10,13,14 At bangtime, the latest time shown in Fig. 2, two damaging effects are occurring.…”

Section: Introductionmentioning

confidence: 99%

Improving ICF implosion performance with alternative capsule supports

et al. 2017

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The thin membrane that holds the capsule in-place in the hohlraum is recognized as one of the most significant contributors to reduced performance in indirect drive inertial confinement fusion (ICF) experiments on the National Ignition Facility. This membrane, known as the “tent,” seeds a perturbation that is amplified by Rayleigh-Taylor and can rupture the capsule. A less damaging capsule support mechanism is under development. Possible alternatives include the micron-scale rods spanning the hohlraum width and supporting either the capsule or stiffening the fill-tube, a larger fill-tube to both fill and support the capsule, or a low-density foam layer that protects the capsule from the tent impact. Experiments are testing these support features to measure their imprint on the capsule. These experiments are revealing unexpected aspects about perturbation development in indirect drive ICF, such as the importance of shadows coming from bright spots in the hohlraum. Two dimensional and 3D models are used to explain these features and assess the impact on implosion performance. Experiments and modeling suggest that the fill-tube supported by a perpendicular rod can mount the capsule without any additional perturbation beyond that of the fill tube.

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Tent-induced perturbations on areal density of implosions at the National Ignition Facilitya)

Cited by 90 publications

References 23 publications

On the importance of minimizing “coast-time” in x-ray driven inertially confined fusion implosions

On the importance of minimizing “coast-time” in x-ray driven inertially confined fusion implosions

Capsule modeling of high foot implosion experiments on the National Ignition Facility

Improving ICF implosion performance with alternative capsule supports

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