Models of Plasma Wall Interactions

Impurity control is a major issue for controlled fusion research. The leading concept for obtaining the conditions for controlled fusion (using magnetic fields) is the tokamak. This paper will discuss models for impurity control in the context of applications for the tokamak. The fusion reaction (e.g. D + T → n + He + 17.6 MeV) requires plasma temperatures in the range of 10 KeV. When the condition for the minimum temperature is factored in with the requirement for beta (plasma pressure/magnetic field pressure), the plasma density is required to be in the 1014 – 1015/cm3 range. The heat and particle fluxes on the vacuum vessel components that result from the fusion power (one fifth of the fusion power heats the plasma) create a potentially severe impurity problem. Fluxes of highly energetic particles can sputter wall material into the plasma. High heat fluxes can melt and evaporate first wall components. All of these “Plasma Wall” processes can lead to erosion of the wall components and to the mixing of wall and surface materials with the plasma. The major processes are desorption of surface materials such as oxygen, water, and carbon monoxide by incident ions, electrons, and photons, chemical, physical sputtering of the wall material by incident ions and atoms, and chemical reactions with the wall and surface constituents including reactions such as 2H° + O → H2O.