Nonadiabatic effects of field ripple on energetic ions in Tokamaks

Abstract

Some effects of toroidal field ripple on energetic ion orbits in Tokamaks are considered. A finite gyroradius generalization of the adiabatic orbit theory condition for ripple trapping is derived. Resonances between the cyclotron motion and the motion through the field ripple are discussed. Conditions on the magnitude of the field ripple and the gyroradius are derived, such that the phase space islands corresponding to the cyclotron resonances overlap the ripple-trapping region. The intrinsic orbit stochasticity resulting from island overlap may be described as pitch-angle scattering. The diffusion coefficient is obtained from a mapping which is derived from the equations of motion. After scattering into the ripple-trapping region of phase space, particles are lost due to guiding drifts in the toroidal field. Applications to ions in reactor-size Tokamaks are discussed.