The interaction of a finite amplitude RF field with trapped electrons in tokamaks

The interaction between waves having frequencies much lower than the cyclotron frequency and trapped electrons is considered in the limit α ≡ ω2w/λω2bo >> 1 (here ωw is the particle oscillation frequency in the wave field, ωbo is the bounce frequency of trapped particle and λ=kparallel qR). In this case, stochasticity arises due to the separatrix crossings experienced by electrons when they pass through a wave-particle resonance on the bounce trajectory. According to general theory, for successive resonance passages, changes in the adiabatic invariant are uncorrelated. As a result, diffusion in velocity space should arise. The calculated diffusion coefficient scales as D ∝ α-1 ln2 α, while in the opposite limit, a << 1, D ∝ α2. This degradation of diffusion coefficient is favourable for current drive efficiency, since a smaller fraction of the total RF power is absorbed by trapped electrons. (author). Letter-to-the-editor. 6 refs, 1 fig