Helium transport in vented divertors and limiters

The transport of neutral helium atoms near divertor or limiter target plates in fusion devices has been studied using Monte Carlo simulation techniques. The atomic processes of ionization of helium atoms by electron impact and elastic scattering with plasma ions are included as well as the thermal motion and the streaming of the ions along the magnetic field. The results obtained 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. A two-dimensional Monte Carlo code has been used to study helium recycling near a vented target plate. It is found that a substantial pumping efficiency can be obtained if the transparency of the vented plate is high (≥30%) and the ports are aligned nearly parallel to the magnetic field.