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

Abstract

The alpha particle power coupling efficiency to the bulk plasma, ηalpha identical to Pα(coupled)/Pα(birth), is reduced when the energetic alpha particles are subject to anomalous radial diffusion, Dαan, caused by alpha particle instabilities. If ηα is appreciably smaller than 1, this has a strong effect on the ignition margin MI [≡ Palpha (coupled)/Ploss] of a tokamak plasma. First, a simple analytical form of ηα is found to be ηα = 1 - (τSD/τL), where τL ≃ α2/4Dα is the spatial loss time and τSD is the slowing down time. The power coupling efficiency ηα decreases with increasing Te and increases with ne. Second, a reduced time dependent alpha particle kinetic equation containing the term ∇.Dα ∇nα, where Dα can be a non-linear function of δnα/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 Dα from first principles. When Dαan ≈ 3 m2/s, ηα is reduced to 0.95. The consequence for the accessible operating regimes of an ITER-like fusion device are also discussed