Three-dimensional simulations of low foot and high foot implosion experiments on the National Ignition Facility

Abstract

In order to achieve the several hundred Gbar stagnation pressures necessary for inertial confinement fusion ignition, implosion experiments on the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)] require the compression of deuterium-tritium fuel layers by a convergence ratio as high as forty. Such high convergence implosions are subject to degradation by a range of perturbations, including the growth of small-scale defects due to hydrodynamic instabilities, as well as longer scale modulations due to radiation flux asymmetries in the enclosing hohlraum. Due to the broad range of scales involved, and also the genuinely three-dimensional (3D) character of the flow, accurately modeling NIF implosions remains at the edge of current simulation capabilities. This paper describes the current state of progress of 3D capsule-only simulations of NIF implosions aimed at accurately describing the performance of specific NIF experiments. Current simulations include the effects of hohlraum radiation asymmetries, capsule surface defects, the capsule support tent and fill tube, and use a grid resolution shown to be converged in companion two-dimensional simulations. The results of detailed simulations of low foot implosions from the National Ignition Campaign are contrasted against results for more recent high foot implosions. While the simulations suggest that low foot performance was dominated by ablation front instability growth, especially the defect seeded by the capsule support tent, high foot implosions appear to be dominated by hohlraum flux asymmetries, although the support tent still plays a significant role. For both implosion types, the simulations show reasonable, though not perfect, agreement with the data and suggest that a reliable predictive capability is developing to guide future implosions toward ignition.

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

“…Previous lower resolution two-dimensional (2D) simulations have been described in [6], and selected highresolution, three-dimensional (3D) results have been reported in [7]. This paper compliments [6,7] by describing more completely the 2D and 3D higher resolution simulations run to model the high foot database. Assessing a larger dataset of simulations against experiment is important to determine whether the trends observed in high foot experiments (as laser and capsule parameters were varied) can be reproduced by the simulations.…”

“…Finally, the DSR values are over predicted in simulation by ∼20% for all of the implosions except for the lowest velocity case. The 2D simulations summarized in this figure follow a similar methodology to that described in [7,14]. Further details of the simulation methodology are given in the appendix.…”

“…In each rendering, the outer surface shows the ablation front colored by the local temperature, while the cutaways show ion temperature on the left and density on the right. Note that the 3D simulation of N140819 is the same as that described in [7] and is included for completeness. The 3D simulation of N130927 is slightly updated from that discussed in [7] with a more accurate model of the hohlraum asymmetries.…”

“…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. Specifically, the low foot implosions modeled in [7] had peak growth factors of 1000-1500, even though these implosions had much more modest implosion velocities of 320-340 km s −1 , well below the 390 km s −1 of the most unstable high foot. This disparity underscores both the sensitivity of implosions stability to the x-ray drive characteristics and its importance to overall implosion performance.…”

“…For this reason, high foot-type designs were not pursued during the NIC, and the low foot design was chosen instead [13]. 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.…”