Interaction between plasma and neutrals near the divertor : the effect of particle and energy reflection

Abstract

In plasma physics, material walls are traditionally regarded as perfect sinks for charged particles and their energy. This work considers the special case that arises when the wall efficiently reflects the neutralized plasma particles (due to a large mass difference between incident particles and lattice atoms) and the upstream plasma is of sufficiently high density to stop these reflected neutrals. The kinetic energy of the reflected particles will thus be a considerable portion of the energy that their parent ions had gained in the acceleration over the plasma sheath that naturally interfaces the plasma and the wall. It may therefore be even larger than the local thermal energy and locally heat the ions in collisions. This effect was for the first time experimentally observed in the linear plasma generator Pilot-PSI at DIFFER and is relevant for the situation that is expected in next generation fusion devices.A single fluid 1D numerical model is developed to study the effect of energy reflection from the target and subsequent thermalization in the upstream plasma. Firstly, the effect on the upstream plasma flow is characterized and compared with new flow velocity measurements in Pilot-PSI. It demonstrates that the commonly encountered flow velocities of around half the sound speed in the upstream plasma can fully be explained by the interaction with the neutral source at the target. Subsequently, the effect on the power density transferred to the wall as well as the kinetic energy of the incident heavy particles is evaluated as a function of the energy reflection fraction, plasma conditions and plasma sheath voltage to evaluate the importance for plasma surface interaction in fusion devices.

Date of Award	30 Sept 2013
Original language	English
Supervisor	Gerard J. van Rooij (Supervisor 1), Roger J.E. Jaspers (Supervisor 2), W.J. Goedheer (Supervisor 2) & N. den Harder (External coach)