Contribution of the Different Erosion Processes to Material Release from the Vessel Walls of Fusion Devices during Plasma Operation

Abstract

In high temperature plasma experiments several processes contribute to erosion and loss of material from the vessel walls. This material may enter the plasma edge and the central plasma where it acts as impurities. It will finally be re‐deposited at other wall areas. These erosion processes are: • evaporation due to heating of wall areas. At very high power deposition evaporation may become very large, which has been named “blooming”. Large evaporation and melting at some areas of the vessel wall surface may occur during heat pulses, as observed in plasma devices during plasma disruptions. • At tips on the vessel walls and/or hot spots on the plasma exposed solid surfaces electrical arcs between the plasma and the vessel wall may ignite. They cause the release of ions, atoms and small metal droplets, or of carbon dust particles. • Finally, atoms from the vessel walls are removed by physical and chemical sputtering caused by the bombardment of the vessel walls with ions as well as energetic neutral hydrogen atoms from the boundary plasma. All these processes have been, and are, observed in today's plasma experiments. Evaporation can in principle be controlled by very effective cooling of the wall tiles, arcing is reduced by very stable plasma operation, and sputtering by ions can be reduced by operating with a cold plasma in front of the vessel walls. However, sputtering by energetic neutrals, which impinge on all areas of the vessel walls, is likely to be the most critical process because ions lost from the plasma recycle as neutrals or have to be refuelled by neutrals leading to the charge exchange processes in the plasma. In order to quantify the wall erosion, “Materials Factors” (MF) have been introduced in the following for the different erosion processes.

Subroutines for some plasma surface interaction processes: physical sputtering, chemical erosion, radiation enhanced sublimation, backscattering and thermal evaporation

“…(13). Thermal conductivity is assumed to vary as 1/(aT + b) and we use the expression for the temperature at the target which is same as in [29]. The inputs to this subroutine are:…”

“…The heat diffusion into the plasmafacing surface is assumed to be 1-dimensional. The thermal conductivity dependence on temperature can be described by 1/(aT + b) in most cases [29]. It is also assumed that there are no bulk sources or sinks of heat.…”

“…(10) with the boundary conditions specified by Eq. (11), (12) results in the temperature T p at the surface (as in [29]) given by:…”

Erosion Behaviour of Metal‐Doped Carbon Layers in Deuterium Low Pressure Plasmas and the Determination by Optical Emission Spectroscopy

“…This mechanism is critical considering the necessity of keeping the tritium inventory as low as possible in future fusion devices. The erosion of carbon surfaces in hydrogen plasmas depends on a number of parameters like energy and flux of the incident ions and atoms, the mass ratio of the incident particles to the surface atoms, as well as the surface temperature and morphology [4][5][6][7]. In a hydrogen plasma, the presence of ions and atomic hydrogen leads to a strong net erosion of carbon surfaces due to chemical sputtering [8], whereas the erosion yield Y (which is defined as the ratio of the carbon flux from the surface Γ C to the incident ion flux Γ ion ) is higher than the sum of the yields of chemical erosion and physical sputtering.…”