Accelerated procedure to solve kinetic equation for neutral atoms in a hot plasma

Abstract

By reaching the first wall of a fusion reactor, charged plasma particles, electrons and ions are recombined into neutral molecules and atoms of hydrogen isotopes. These species recycle back into the plasma volume and participate, in particular, in charge–exchange (cx) collisions with ions. As a result, hot atoms with chaotically directed velocities are generated and some of them hit the wall. Statistical Monte Carlo methods often used to model the behavior of cx atoms are too time-consuming for comprehensive parameter studies. Recently1 an alternative iteration approach to solve one-dimensional kinetic equation2 has been significantly accelerated, by a factor of 30–50, by applying a pass method to evaluate the arising integrals from functions, involving the ion velocity distribution. Here, this approach is used by solving a two-dimensional kinetic equation, describing the transport of cx atoms in the vicinity of an opening in the wall, e.g., the entrance of a duct guiding to a diagnostic installation. To assess the erosion rate and lifetime of the installation, one need to know the energy spectrum of hot cx atoms escaping from the plasma into the duct. Calculations are done for a first mirror of molybdenum under plasma conditions expected in a fusion reactor like DEMO.3,4 The results of kinetic modeling are compared with those found by using a diffusion approximation5 relevant for cx atoms if the time between cx collisions with ions is much smaller than the time till the ionization of atoms by electrons. The present more exact kinetic consideration predicts a mirror erosion rate by a factor of 2 larger than the approximate diffusion approach.