Fast ion non-adiabaticity in spherical tokamaks

Transport processes of fast ions in axisymmetric low-aspect-ratio spherical toms (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 ω_B - lω_b: 0, where ω_B and ω_b represent the bounce-averaged gyrofrequency and the bounce frequency, respectively, and l is an integer. The critical threshold of the adiabaticity parameter, ε_cr, 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 > ε_cr, we assess the transport coefficients describing intense collisionless pitch angle diffusion, whereas, in the case of weak non-adiabaticity, ε < ε_cr, the more substantial coefficients of enhanced collisional radial diffusion and convection of fast ions gyrating resonantly with the bounce oscillations are estimated.