EFFECTS OF ANOMALOUS ALPHA-PARTICLE DIFFUSION ON FUSION POWER COUPLING INTO TOKAMAK PLASMA

The a particle power coupling efficiency to the bulk plasma, eta(alpha ) = P-alpha(coupled)/P-alpha(birth), is reduced when the energetic a p articles are subject to anomalous radial diffusion, D-alpha(an), cause d by alpha particle instabilities. If eta(alpha) is appreciably smalle r than 1, this has a strong effect on the ignition margin M(I) [= P-al pha(coupled)/P-loss] of a tokamak plasma. First, a simple analytical f orm of eta(alpha) is found to be eta(alpha) = 1 - (tau(SD)/tau(L))/(2 + tau(SD)/tau(L)), where tau(L) similar or equal to alpha(2)/4D(alpha) is the spatial loss time and tau(SD) is the slowing down time. The po wer coupling efficiency eta(alpha) decreases with increasing T-e and i ncreases with n(e). Second, a reduced time dependent a particle kineti c equation containing the term del.D(alpha)del n(alpha), where D-alpha can be a non-linear function of partial derivative n(alpha)/partial d erivative r, is derived and solved using a multiple energy group metho d. An Alfven wave turbulence theory by Gang (Phys. Fluids B 4 (1992) 3 152) is used to parametrize D-alpha from first principles. When D-alph a(an) similar or equal to 3 m(2)/s, eta(alpha) is reduced to 0.95. The consequence for the accessible operating regimes of an ITER-like fusi on device are also discussed.