Self-sustaining, low-Z wall coatings for reactors

We use a simple plasma-wall model based on sputtering and plasma impurity deposition to calculate equilibrium impurity densities in a plasma as a function of plasma edge temperature and wall material. We show how these impurity equilibria are unstable for all heavy elements, leading to the conclusion that low-Z coatings are desirable. Self-sustaining wall coatings can be produced if impurity densities are held above the equilibrium density by impurity injection because impurity deposition should then dominate erosion. We discuss metallurgical segregation effects which would maintain these coatings with the correct choice of materials. (orig.)