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

摘要

The interaction between waves having frequencies much lower than the cyclotron frequency and trapped electrons is considered in the limitw2/bo2>>1 (herewis the particle oscillation frequency in the wave field,bois the bounce frequency of trapped particles and= kqR). 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-1ln2while in the opposite limit,<<1, D2. 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