The stochastic nature of ion-cyclotron-resonance wave–particle interaction in tokamaks

The threshold in wave amplitude for the onset of stochastic motion of ion-cyclotron-resonance-heated minority ions in tokamaks is calculated. As a separate result, the solutions to the guiding center equations of motion are presented. Full account is taken to the effects of the finite orbit width of high-energy particles and the ellipticity of the flux surfaces. The orbits of trapped particles are shown to become stochastic more easily than those of circulating particles, and near the trapped/passing boundary an arbitrarily weak wave field is needed for stochastization. Finite orbit width effects have significant impact on the results, decreasing the stochastization amplitude for highly energetic particles considerably, as compared with earlier theories. Nevertheless, for realistic JET parameters [Plasma Phys. Controlled Fusion 30, 1467 (1988)], only a small fraction of the minority ions move stochastically in a monochromatic ion-cyclotron range of frequencies (ICRF) wave field.