UC Irvine

We present evidence that the fast-ion diffusion coefficient changes across the TFTR plasma column. Two MHD quiescent discharges are analyzed: a high power D-T plasma heated with 21 MW of deuterium and tritium beams, and an ohmic plasma into which a 10 MW, 20 ms deuterium beam pulse was injected. The localized charge-exchange measurements in the ohmic plasma, and the neutron flux measurements in the D-T plasma are compared with predictions from the 1 1/2 D transport simulation code TRANSP. We have modified the code to allow modelling with arbitrary fast-ion diffusion profiles Df(r). Significant improvement in the agreement between measurement and simulation is obtained with a Df(r) profile that has low values in the inner half of the plasma column (Df<0.05 m2 s-1) and then rises rapidly towards the plasma periphery. This suggests a common underlying mechanism of enhanced beam ion transport in the two discharges, such as stochastic ripple diffusion.