Physics of particle exhaust in pump limiters

Abstract

Monte Carlo calculations are presented of neutral particle flows in hollow collection probes and large area limiters used in recent experiments. The collected neutral hydrogen flux scales linearly with plasma density for n i < 2 × 10 11 cm −3 where the plasma is essentially transparent to the neutrals. For 2 × 10 11 < n i < 2 ×10 12 cm −3 the neutral flux varies mildly nonlinearly as n 1.26 i . The transition between regimes is driven by the mean free path for molecular hydrogen dissociation being of the order of the probe dimensions. Calculations of the neutral energy spectra emerging from the pump ducts of various lengths indicates that within the accuracy of avialable low energy surface data, energy transfer to the duct walls is very inefficient. Furthermore, the results indicate that “energy freezing” occurs at epithermal energies as a consequence of particle and energy reflection coefficients approaching unity at low energies.