William Tsubasa Taitano

Shock-driven implosions with 100% deuterium (D2) gas fill compared to implosions with 50:50 nitrogendeuterium (N2D2) gas fill have been performed at the OMEGA laser facility to test the impact of the added mid-Z fill gas on implosion performance. Ion temperature (Tion) as inferred from the width of measured DD-neutron spectra is seen to be 34% +/- 6% higher for the N2D2 implosions than for the D2-only case, while the DD-neutron yield from the D2-only implosion is 7.2 +/- 0.5 times higher than from the N2D2 gas fill. The Tion enhancement for N2D2 is observed in spite of the higher Z, which might be expected to lead to higher radiative loss, and higher shock strength for the D2-only versus N2D2 implosions due to lower mass, and is understood in terms of increased shock heating of N compared to D, heat transfer from N to D prior to burn, and limited amount of ion-electron-equilibration-mediated additional radiative loss due to the added higher-Z material. This picture is supported by interspecies equilibration timescales for these implosions, constrained by experimental observables. The one-dimensional (1D) kinetic Vlasov-Fokker-Planck code iFP and the radiation hydrodynamic simulation codes HYADES (1D) and xRAGE [1D, two-dimensional (2D)] are brought to bear to understand the observed yield ratio. Comparing measurements and simulations, the yield loss in the N2D2 implosions relative to the pure D2-fill implosion is determined to result from the reduced amount of D2 in the fill (fourfold effect on yield) combined with a lower fraction of the D2 fuel being hot enough to burn in the N2D2 case. The experimental yield and Tion ratio observations are relatively well matched by the kinetic simulations, which suggest interspecies diffusion is responsible for the lower fraction of hot D2 in the N2D2 relative to the D2-only case. The simulated absolute yields are higher than measured; a comparison of 1D versus 2D xRAGE simulations suggest that this can be explained by dimensional effects. The hydrodynamic simulations suggest that radiative losses primarily impact the implosion edges, with ion-electron equilibration times being too long in the implosion cores. The observations of increased Tion and limited additional yield loss (on top of the fourfold expected from the difference in D content) for the N2D2 versus D2-only fill suggest it is feasible to develop the platform for studying CNO-cycle-relevant nuclear reactions in a plasma environment.

The ion velocity distribution functions of thermonuclear plasmas generated by spherical laser direct drive implosions are studied using deuterium-tritium (DT) and deuterium-deuterium (DD) fusion neutron energy spectrum measurements. A hydrodynamic Maxwellian plasma model accurately describes measurements made from lower temperature (< 10 keV), hydrodynamiclike plasmas, but is insufficient to describe measurements made from higher temperature more kineticlike plasmas. The high temperature measurements are more consistent with Vlasov-Fokker-Planck (VFP) simulation results which predict the presence of a bimodal plasma ion velocity distribution near peak neutron production. Furthermore, these measurements provide direct experimental evidence of non-Maxwellian ion velocity distributions in spherical shock driven implosions and provide useful data for benchmarking kinetic VFP simulations.

Recent inertial confinement fusion experiments have shown primary fusion spectral moments which are incompatible with a Maxwellian velocity distribution description. These results show that an ion kinetic description of the reacting ions is necessary. We develop a theoretical classification of non-Maxwellian ion velocity distributions using the spectral moments. At the mesoscopic level, a monoenergetic decomposition of the velocity distribution reveals there are constraints on the space of spectral moments accessible by isotropic distributions. General expressions for the directionally dependent spectral moments of anisotropic distributions are derived. At the macroscopic level, a distribution of fluid element velocities modifies the spectral moments in a constrained manner. Experimental observations can be compared to these constraints to identify the character and isotropy of the underlying reactant ion velocity distribution and determine if the plasma is hydrodynamic or kinetic.

Mixing at interfaces in dense plasma media is a problem central to inertial confinement fusion and high energy density laboratory experiments. In this work, collisional particle-in-cell simulations are used to explore kinetic effects arising during the mixing of unmagnetized plasma media. Comparisons are made to the results of recent analytical theory in the small Knudsen number limit and while the bulk mixing properties of interfaces are in general agreement, some differences arise. In particular, "super-diffusive" behavior, large diffusion velocity, and large Knudsen number are observed in the low density regions of the species mixing fronts during the early evolution of a sharp interface prior to the transition to a slow diffusive process in the small-Knudsen-number limit predicted by analytical theory. A center-of-mass velocity profile develops as a result of the diffusion process and conservation of momentum. Published by AIP Publishing.

In two recent publications, it was demonstrated that the nonlinear diffusion acceleration (NDA) algorithm, a moment-based accelerator, could be modified to accelerate the solution to neutron transport calculations with anisotropic scattering. It was demonstrated, however, that as the scattering became less isotropic, the performance of the algorithm degraded. Furthermore, it has been shown that Anderson acceleration (AA) could be used to speed up neutron transport and plasma physics calculations. In this paper, we combine these ideas to demonstrate that AA can be used to remedy the degraded performance of NDA when scattering is anisotropic. We describe each of the methods in detail and demonstrate the results on a series of fixed-source calculations and a pair of k-eigenvalue calculations.

In this study, we demonstrate a fully implicit algorithm for the multi-species, multidimensional Rosenbluth-Fokker-Planck equation which is exactly mass-, momentum-, and energy-conserving, and which preserves positivity. Unlike most earlier studies, we base our development on the Rosenbluth (rather than Landau) form of the Fokker-Planck collision operator, which reduces complexity while allowing for an optimal fully implicit treatment. Our discrete conservation strategy employs nonlinear constraints that force the continuum symmetries of the collision operator to be satisfied upon discretization. We converge the resulting nonlinear system iteratively using Jacobian-free Newton-Krylov methods, effectively preconditioned with multigrid methods for efficiency. Single- and multi-species numerical examples demonstrate the advertised accuracy properties of the scheme, and the superior algorithmic performance of our approach. In particular, the discretization approach is numerically shown to be second-order accurate in time and velocity space and to exhibit manifestly positiveentropy production. That is, H-theorem behavior is indicated for all the examples we have tested. The solution approach is demonstrated to scale optimally with respect to grid refinement (with CPU time growing linearly with the number of mesh points), and timestep (showing very weak dependence of CPU time with time-step size). As a result, the proposed algorithm delivers several orders-of-magnitude speedup vs. explicit algorithms. Published by Elsevier Inc.

In this study, we propose a charge, momentum, and energy conserving discretization for the 1D-1V Vlasov-Ampere system of equations on an Eulerian grid. The new conservative discretization is nonlinear in nature, but can be efficiently converged with a moment-based nonlinear accelerator algorithm. We demonstrate the conservation and convergence properties of the scheme with various numerical examples, including a multi-scale ion-acoustic shockwave problem. (C) 2015 Elsevier Inc. All rights reserved.