We investigate the simplest model of alpha channelling, in which high-frequency electrostatic wave interacts with fusion alpha particles and the effect of low-frequency waves is modeled by an outward flux of particles at the threshold energy Ew for wave-particle interaction. The associated Fokker–Planck equation is then solved analytically in the limit of large wave-induced diffusion with absorbing boundary conditions at the outer edge. The power flowing from alpha particles to the wave is explicitly computed and its dependence on the outward flux at Ew and on the spatial extent of the high-frequency wave is determined. The best option for alpha channelling is realized when the outward flux equals the flux of pure wave-induced diffusion, in which case the whole perpendicular energy of alpha particles can be extracted. On the contrary, if the outward flux is negligible, no more than ∼ of alpha particle energy can be channelled to the wave for an extended source, which raises up to ∼ for a localized source.