Fast Ion Non-adiabaticity in Spherical Tokamaks
Authors: V.A. Yavorskij, D. Darrow, V.Ya. Goloborod'ko, S.N. Reznik, U. Holzmueller-Steinacker, N. Gorelenkov, and K. Schoepf
Date of PPPL Report: August 2002
Published in: Nuclear Fusion 42 No 10 (October 2002) 1210-1215
Transport processes of fast ions in axisymmetric low-aspect-ratio spherical torus (ST) plasmas are investigated, which are induced by the non-conservation of the magnetic moment m . The reason for non-conservation of m of fast ions in ST's is the relatively large adiabaticity parameter e typically exceeding the value 0.1 (e = ratio of ion gyroradius to the gradient scale length of the magnetic field). Both analytical and numerical evaluations of the magnitude of nonadiabatic variations of m are performed. Nonadiabaticity effects are shown to be most significant for fast ions for which the bounce oscillations are in resonance with the gyromotion, i.e., for ions with wB - l wb = 0, where wB and wb represent the bounce-averaged gyrofrequency and the bounce frequency, respectively, and l is an integer. The critical threshold of the adiabaticity parameter, ecr, to be exceeded for the transition to stochastic behavior of fast ions in axisymmetric ST's is inspected. Nonadiabatic variations of m are shown to lead to collisionless transformation of trapped orbits into circulating ones and vice versa. For the case of strong nonadiabaticity, e > ecr, we assess the transport coefficients describing intense collisionless pitch-angle diffusion, whereas, in the case of weak nonadiabaticity, e > ecr, the more substantial coefficients of enhanced collisional radial diffusion and convection of fast ions gyrating resonantly with the bounce oscillations are estimated.