Numerical Analysis of Ripple Loss in Reversed Shear Tokamak Operation.

Ripple loss of energetic alpha particles and neutral beam ions was calculated for reversed shear discharges in ITER-FDR(Final Design Report). The result indicates that, compared with normal operation with positive shear, reversed shear operation dramatically enhances the ripple loss. Ripple loss of alpha particles can reach 25 % with the maximum heat load of 3.7 MW/m2 on the outboard wall, while that of a 1 MeV negative ion source beam can be as high as 20 % as well. The heat load due to alpha particle loss is marginal in the light of a wall tolerance. The calculation suggests that the Toroidal Field (TF) ripple in a fusion reactor should be designed to be less than 0.6 % at the plasma surface so that ripple loss of alpha particles or beam ions can be acceptably low. Ferritic steel insert to the vacuum vessel, is a probable solution to reduce the ripple loss to an allowable level in the ITER-FDR design with the TF ripple reduction of by a factor of 0.4. With TF ripple reduction, ripple loss for alpha particles and beam ions is expected to decrease to 10 % and 4 %, respectively.