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Ripple-Induced Radial Diffusion of Fast Ions in Tokamaks

Radial diffusion of fast ions in tokamaks due to torodial field ripple is analysed using a Fokker-Planck equation. Neglecting pitch-angle scattering and parallel diffusion in velocity-space an approximate solution is obtained, from which criteria for the importance of this process is obtained. For a relative ripple strength δ given by δ(r) ∼ rλ where r is the minor radius it is shown that λ must be larger than 5 to avoid large losses of alpha particles. Thus, this process could pose relatively stringent design constraints on the torodial field coils.