Helium Transport in Advanced Tokamak Scenarios with Internal Transport Barrier

The results are presented of investigations on advanced scenarios for plasmas of next-generation tokamaks by means of a 1 1/2-dimensional transport code. The role of thermonuclear alpha particles and helium ash is analyzed by a two-group model and by introducing experimentally validated mixed Bohm/gyroBohm models on the assumption that the diffusion of helium ash can be treated like the diffusion of bulk plasma ions. Recycling of helium ash is modeled by introducing a wall source. Results are presented of parameter studies presenting the equilibrium helium fraction as a function of the recycling factor. It is shown that for a given scenario, the fraction of effective helium confinement time and energy confinement time is a time-dependent quantity and not a constant, as was assumed in earlier research.