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Trapped-particle scattering by electrostatic turbulence in toroidal plasmas

This work is concerned with a study of possible effects of turbulent electrostatic waves on trapped particle motions in low-β toroidal plasmas. A mechanism of resonance between the frequencies of the waves in the turbulent spectrum and the harmonics of the particle bounce frequency in the magnetic wells is found to provide stochastic trapped particle diffusion. The effectiveness of this diffusive process in scattering trapped particles out of the trapping cone in velocity space has been investigated in detail as functions of the wave features and of the particle parallel energy for the cases of ion sound waves and of different types of drift waves. The conclusions are that an electrostatic turbulence of a relatively low level, with typical frequencies higher than the bounce frequency and wavelengths shorter than the connection length, can be effective in scattering trapped electrons out of the trapping cone in typical times longer than the bouncing time but still shorter than collisional times. On the other hand, similar effects cannot be obtained easily for the ions, owing to their greater inertia.