HYDROGEN RETENTION IN THE FIRST WALL

A general model for hydrogen retention in tokamak walls is presented which assumes that a discharge results in a hydrogen saturated layer over a large fraction of the first wall, well beyond the main limiter or divertor. The outgassing of this layer between discharges is modelled using mechanisms previously applied to ion beam experiments. This short-term outgassing rate is determined by only two parameters: (1) the number of absorption sites and (2) a parameter which describes the overall hydrogen release rate given that a quasi-equilibrium exists between atoms in solution and in traps. To explain the long term build-up of hydrogen isotopes in the first wall, it is necessary to extend the simple two-parameter model to include the effects of diffusion of hydrogen into the bulk of the material and codeposition.