Ion separation effects in mixed-species ablators for inertial-confinement-fusion implosions

Abstract

Recent efforts to demonstrate significant self-heating of the fuel and eventual ignition at the National Ignition Facility make use of plastic (CH) ablators [O. A. Hurricane et al., Phys. Plasmas 21, 056314 (2014)]. Mainline simulation techniques for modeling CH capsule implosions treat the ablator as an average-atom fluid and neglect potential species separation phenomena. The mass-ablation process for a mixture is shown to lead to the potential for species separation, parasitic energy loss according to thermodynamic arguments, and reduced rocket efficiency. A generalized plasma barometric formula for a multispecies concentration gradient that includes collisionality and steady flows in spherical geometry is presented. A model based on plasma expansion into a vacuum is used to interpret reported experimental evidence for ablator species separation in an inertial-confinement-fusion target [J. S. Ross et al., Rev. Sci. Instrum. 83, 10E323 (2012)]. The possibility of "runaway" hydrogen ions in the thermoelectric field of the ablation front is conjectured.

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

“…In principle, hydrogen -being lighter and having a higher charge to mass ratio -is expected to lead the plasma channel expansion, leaving a higher concentration of carbon in the interaction region. This effect has been observed previously using simultaneous electron and ion feature Thomson scattering in an expanding CH plasma [26,27]. However, assessing this quantitatively requires multi-ion-fluid simulations and is considered outside the scope of this study.…”

Simulation and assessment of ion kinetic effects in a direct-drive capsule implosion experiment

“…Ion kinetic effects outside the scope of single-component hydrodynamics have been proposed to explain fusion yield anomalies in recent experiments using glass capsules filled with a mix of deuterium, tritium, and helium-3 fuel at the OMEGA laser facility of the Laboratory for Laser Energetics (LLE) and at the National Ignition Facility (NIF) [1][2][3][4][5]. While theoretical investigations [6][7][8] and multi-fluid numerical studies [9,10] may guide our understanding of kinetic effects in fusion experiments, they fall short of offering a self-consistent model applicable over a broad range of regimes.…”

Collisional and collisionless shocks

“…Shock waves are a ubiquitous feature of plasma flows, both in the laboratory and in natural environment. They appear in fusion research, both inertial [1,2] and magnetic [3,4] shocks may play significant role in particle acceleration [7,8] and magnetic field generation [9,10], and in geophysics [11,12]. Sometimes the shock structure is determined by collisions between plasma particles; sometimes collisions are rare (see an additional discussion later in this section), and the shock is formed by particle interactions with highly non-equilibrium random electromagnetic fields excited by plasma instabilities in the shock transition.…”