Boundary Conditions at the Vessel Walls being in Contact with a Hot Plasma

Abstract

The main atomic processes occuring at the solid wall during plasma wall interaction are presented. The data are based on experimental results obtained under laboratory and plasma conditions and/or by computer simulations.The particle and energy confinement for a plasma in a magnetic field is always limited due to drift-, diffusion-and radiation losses. For a burning fusion plasma the limited confinement is necessary for exhausting the energy from the fusion reactions and the helium ash. This is the source for plasma wall interactions, which cannot be avoided in a fusion reactor [I]. We rather have to learn to understand and to control them. Major processes in PSI are those determining the sheath potential, those determining the hydrogen particle balance and those causing erosion at the vessel walls and the introduction of wall atoms representing impuriiies in the plasma.The sheath potential between a plasma and the surface of a solid adjusts itself so as to reduce the electron flux to the surface until it is equal to the ion flux. The potential is reduced by electron emission from the surface caused by the electron and ion bombardment as well as by thermal emission [2].The average particle confinement time in today's plasma experiments is in the range of 1 ms to 1 s and thus short compared to the discharge time. Particles leaving the plasma impinge onto the vessel walls and during the discharge they are continuously introduced back into the plasma from the vessel walls. These mechanisms of particle recycling are a criiical issue for plasma density control. The plasma ions can be backscattered or they may be implanted and stopped in the wall material. If the surface layers of the vessel walls get saturated, hydrogen may be released as thermal molecules. At very high flux densities dynamical trapping was observed, i.e. hydrogen may be retained in the wall areas for a time of the order of the particle confinement time.