Topic/Type: 1.5 Low-temperature, dusty and nano-plasmas, Oral

### Kinetic modeling of cesium redistribution in negative ion sources for Neutral Beam Injection

P.Diomede1, S.Longo1, 2, M.Capitelli1, 2

1 Dipartimento di Chimica, Universita' di Bari, Bari, Italy
2 CNR-IMIP, Bari Section, Bari, Italy

Plasma-based sources of negative ions (D$\small ^{-}$) are under development as components of the Neutral Beam Injection facilities for plasma heating in thermonuclear fusion reactors. These devices are based on a low temperature deuterium plasma, but an additional component, cesium, is evaporated from an oven in order to increase the negative ion yield. Neutral Cs atoms and Cs$\small ^{+}$ ions diffuse in the deuterium plasma and produce Cs films on surfaces, which at the same time undergo evaporation and sputtering which redistribute cesium in the source. The modeling of this phenomenon, of great technological importance, is complicated by the effect on the particle transport of the rarefied deuterium flow. A Monte Carlo model based on a new methodology developed by the authors for seeded atomic beams is presented. The model calculates the rarefied deuterium flow in the device taking into account the geometry with boundary conditions appropriate for different surfaces by a nonlinear version of the Test Particle Monte Carlo method. The deuterium flow is then used as an input for the calculation of Cs and Cs$\small ^{+}$ particle transport based on an exact Monte Carlo solution of the related transport equation and including atom ionization, film sputtering and evaporation. Any complex feature of the source geometry is easily implemented.