Ion Thermal Decoupling and Species Separation in Shock-Driven Implosions

Abstract

Anomalous reduction of the fusion yields by 50% and anomalous scaling of the burn-averaged ion temperatures with the ion-species fraction has been observed for the first time in D 3 He-filled shock-driven inertial confinement fusion implosions. Two ion kinetic mechanisms are used to explain the anomalous observations: thermal decoupling of the D and 3 He populations and diffusive species separation. The observed insensitivity of ion temperature to a varying deuterium fraction is shown to be a signature of ion thermal decoupling in shock-heated plasmas. The burn-averaged deuterium fraction calculated from the experimental data demonstrates a reduction in the average core deuterium density, as predicted by simulations that use a diffusion model. Accounting for each of these effects in simulations reproduces the observed yield trends. In inertial confinement fusion (ICF), targets are imploded to generate a high-density, high-temperature environment where fusion can occur [1,2]. In the current ignition design, four weak shocks compress the cryogenic deuterium-tritium (DT) fuel, then combine into a single strong shock with Mach number ∼10-50 in the central gas, a DT vapor with initial density 0.3 mg=cc [3]. Convergence of this shock at the implosion's center sets the initial entropy of the central plasma "hot spot" and generates a brief period of fusion production ("shock bang"). The rebounding shock strikes the imploding fuel, beginning the hot spot compression that generates the main period of nuclear production ("compression burn"). Understanding the evolution of the plasma during the shock transit phase is fundamentally important for achieving ICF ignition, as this sets the initial conditions for hot spot formation, compression, ignition, and burn [4].The simulations used to design ICF experiments generally assume a single average-ion hydrodynamic framework. The equations of motion for a single ion-species plasma are solved iteratively to model the implosion. Multiple ion species are not treated separately: the ion mass and charge are set as a weighted average of the individual species. Recent experimental and theoretical work has questioned the validity of the average-ion assumption [5][6][7][8][9][10][11][12][13][14][15]. Anomalous reduction of the compression-phase nuclear yield has been observed in implosions filled with multiple fuel species, such as deuteriumhelium-3 (D 3 He) [5], DT [6], and other combinations [7,8]. Anomalous reduction of the shock yield has been ambiguous in these studies. Diffusive ion species separation driven by gradients in pressure [9], electric potential [10,11], and temperature [12] is a potential cause of these observations [13]. Kinetic physics can impact the evolution and nuclear performance of multispecies plasmas in computational studies [14,15], although, to the best of our knowledge, no fully kinetic model is yet capable of simulating an entire ICF implosion.The experiments described in this Letter demonstrate, for the first time, signatures of two multiple-ion kinetic phys...

Refractive Index Seen by a Probe Beam Interacting with a Laser-Plasma System

“…However, assessing this quantitatively requires multi-ion-fluid simulations and is considered outside the scope of this study. Species separation is an increasingly active field of research in the ICF community [28][29][30][31][32].…”

Kinetic studies of ICF implosions

“…Their relative concentrations can evolve substantially over the course of implosion due to inter-ion-species diffusion [12,13]. Observation of the resulting fuel stratification in experiments [14,15,16,17] and fully kinetic simulations [3,4] have recently been reported. The targets with high-Z dopants show particularly pronounced yield anomalies [8,9,18], suggesting that even stronger fuel stratification may take place.…”

Observation of shock-front separation in multi-ion-species collisional plasma shocks

“…A plasma shock front with multiple ion species contains additional structure compared to a single ion plasma shock. Prior experiments, simulations, and theoretical work explored multiion-species effects in the context of inertial confinement fusion (ICF), for which species separation in the fusion fuel potentially leads to neutron yield degradation [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Interspecies ion separation and velocity separation were experimentally observed [19][20][21][22][23].…”