Helium Transport in Plasma Edge Regions.

The transport of neutral helium atoms near diverter or limiter target plates in fusion devices has been studied. Two simulation codes, based on Monte Carlo techniques, have been developed. The first treats the problem in one-dimensional geometry and the second considers two-dimensional effects. The atomic processes of ionization of helium atoms by electron impact and elastic scattering with plasma ions are included. The total and differential elastic scattering cross-sections have been calculated classically using an ab initio calculation of the interatomic potential. The thermal motion and the streaming of the ions along the magnetic field, which can be at an angle to the target plate, are included. Results obtained with the one-dimensional code show significant effects of elastic collisions below about 10 eV, causing a substantial fraction of the helium atoms to be reflected back to the target plate. This effect can be beneficial for the pumping of helium from the discharge chamber. The two-dimensional Monte Carlo code has been used to study helium recycling near a flat, vented target plate. A parametric study is performed to examine the dependence of the pumping efficiency on plasma parameters and geometric aspects. Results show that the pumping of neutral helium can be increased by shortening and widening the ports as well as by increasing the angle between the magnetic field and the target plate. Also, keeping the ion temperature below about 10 eV and the plasma density around a few 10^{14} cm ^{-3} near the target plate can be beneficial for the pumping of helium gas.