On the optimal conditions for alpha channelling in tokamaks

We investigate for a generic tokamak equilibrium the performance of alpha channelling driven by a mode-converted ion Bernstein wave with the effect of Alfvénic turbulence accounted for through an outward particle flux at the threshold magnetic moment μmin for wave particle interaction. The formal solution of the associated Fokker-Planck equation is given in the limit of large wave-induced diffusion with absorbing boundary conditions at the outer edge. By choosing a specific ITER-like equilibrium, the power flowing from alpha particles to the wave is explicitly computed and its dependence on the relevant physical parameters is discussed. In particular, alpha channelling is maximized when the outward flux equals the flux of pure wave-induced diffusion, in which case up to ∼55% of the total alpha particle energy can be extracted. The spectra of particles hitting the outer edge and crossing μmin are also derived.