Evidence for spatially variable beam ion diffusion in TFTR

We present evidence that the fast-ion diffusion coefficient changes across the TFTR plasma column. Two MHD quiescent discharges are analysed: 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 form 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 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. (Author)