Fast ion non-adiabaticity in spherical tokamaks

Abstract

Transport processes of fast ions in axisymmetric low-aspect-ratio spherical torus (ST) plasmas are investigated, which are induced by non-conservation of the magnetic moment μ. The reason for non-conservation of μ of fast ions in STs is the relatively large adiabaticity parameter ε typically exceeding the value 0.1 (ε = ratio of ion gyroradius to the gradient scale length of the magnetic field). Both analytical and numerical evaluations of the magnitude of non-adiabatic variations of μ are performed. Non-adiabaticity 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 ωB−lωb = 0, where ωB and ωb represent the bounce-averaged gyrofrequency and the bounce frequency, respectively, and l is an integer. The critical threshold of the adiabaticity parameter, εcr, to be exceeded for the transition to stochastic behaviour of fast ions in axisymmetric STs is inspected. Non-adiabatic variations of μ are shown to lead to collisionless transformation of trapped orbits into circulating ones and vice versa. For the case of strong non-adiabaticity, ε>εcr we assess the transport coefficients describing intense collisionless pitch angle diffusion, whereas, in the case of weak non-adiabaticity, ε<εcr, the more substantial coefficients of enhanced collisional radial diffusion and convection of fast ions gyrating resonantly with the bounce oscillations are estimated.