Noise-Free Rapid Approach to Solve Kinetic Equations for Hot Atoms in Fusion Plasmas

Abstract

At the first wall of a fusion reactor, charged plasma particles are recombined into neutral molecules and atoms recycling back into the plasma volume where charge exchange (cx) with ions. As a result hot atoms with chaotically directed velocities are generated which can strike and erode the wall. An approach to solve the kinetic equation in integral form for cx atoms, being alternative to statistical Monte Carlo methods, has been speeded up by a factor of 50, by applying an approximate pass method to evaluate integrals, involving the ion velocity distribution function. It is applied to two-dimensional transfer of cx atoms near the entrance of a duct, guiding to the first mirror for optical observations. The energy spectrum of hot cx atoms, escaping into the duct, is calculated and the mirror erosion rate is assessed. Computations are done for a molybdenum first mirror under plasma conditions expected in the fusion reactor DEMO. Kinetic modeling results are compared with those found with a diffusion approximation valid in very cold and dense plasmas. For ducts at the torus outboard a more rigorous kinetic consideration predicts an erosion rate by a factor up to 2 larger than the diffusion approximation.