Effects of anomalous alpha particle diffusion on fusion power coupling into tokamak plasma

The alpha particle power coupling efficiency to the bulk plasma, η alpha identical to P α (coupled)/P α (birth), is reduced when the energetic alpha particles are subject to anomalous radial diffusion, D α an , caused by alpha particle instabilities. If η α is appreciably smaller than 1, this has a strong effect on the ignition margin M I [≡ P alpha (coupled)/P loss ] of a tokamak plasma. First, a simple analytical form of η α is found to be η α = 1 - (τ SD /τ L ), where τ L ≃ α 2 /4D α is the spatial loss time and τ SD is the slowing down time. The power coupling efficiency η α decreases with increasing T e and increases with n e . Second, a reduced time dependent alpha particle kinetic equation containing the term ∇.D α ∇n α , where D α can be a non-linear function of δn α /δτ, is derived and solved using a multiple energy group method. An Alfven wave turbulence theory by Gang (Phys. Fluids B 4 (1992) 3152) is used to parametrize D α from first principles. When D α an ≈ 3 m 2 /s, η α is reduced to 0.95. The consequence for the accessible operating regimes of an ITER-like fusion device are also discussed