EVIDENCE FOR SPATIALLY-VARIABLE BEAM ION DIFFUSION IN TFTR

We present evidence that the fast-ion diffusion coefficient changes ac ross the TFTR plasma column. Two MHD quiescent discharges are analysed : a high power D-T plasma heated with 21 MW of deuterium and tritium b eams, and an ohmic plasma into which a 10 MW, 20 ms deuterium beam pul se was injected. The localized charge-exchange measurements in the ohm ic plasma, and the neutron Bur measurements in the D-T plasma are comp ared with predictions from the 1 1/2D transport simulation code TRANSP . We have modified the code to allow modelling with arbitrary fast-ion diffusion profiles D-f(r). Significant improvement in the agreement b etween measurement and simulation is obtained with a D-f(r) profile th at has low values in the inner half of the plasma column (D-f < 0.05 m (2) s(-1)) and then rises rapidly towards the plasma periphery. This s uggests a common underlying mechanism of enhanced beam ion transport i n the two discharges, such as stochastic ripple diffusion.