Analytical model of fast ion behavior in current hole tokamak plasmas

Abstract

A qualitative theoretical approach is proposed for the study of fast ion behavior in tokamak plasmas with a current hole. For the equilibrium magnetic configuration in a current hole tokamak we employ a simplified model based on an analytical approximation of the poloidal flux function shape. For that the radial extent of the plasma core area with zero rotational transform is used as a characteristic parameter. The modification of the fast ion orbit topology, as caused by the current hole, is studied. The model yields manageable analytical expressions describing completely the current hole effect on the confinement domains as well as on the boundaries separating the regions of different-type orbits in the constants-of-motion space. Subject of investigation is the current hole effect, both on the convective transport of fast ions induced by their slowing down as well as on the diffusive transport caused by pitch angle scattering. The influence of slowing down on the distribution functions of energetic particles (charged fusion products, NBI ions) in current hole tokamak plasmas is calculated and illustrated. The approach proposed is tested for a specific JET current hole scenario and the results are compared with numerical Fokker-Planck predictions.