Improved kinetic modeling of Knudsen layer reduction of fusion reactivity

Recent work by Molvig et al. [1] has led to renascent interest in the premise that fuel ions in tails of distribution functions in finite assemblies of thermonuclear fuel can be depleted by proximity to a boundary. This can lead to decreased fusion reactivity and yield in ICF capsules. In this presentation, the theory of Molvig et al. is reviewed, extended, and compared to kinetic particle-in-cell simulations employing a binary collision model between plasma particles and lossy walls that absorb tail ions, reinjecting them as a ``core'' Maxwellian. Inferred reaction rates as a function of temperature and distance from a boundary are obtained from the simulations and compared with those found from various theoretical models. It is found that the Molvig et al. ``Knudsen distribution function'' provides an upper bound to the modification to reactivity, but that a more complete theory is required to accurately obtain these loss effects. \\[4pt] [1] Kim Molvig et al., submitted to Physical Review Letters.