SENSITIVITY OF ITER MHD GLOBAL STABILITY TO EDGE PRESSURE-GRADIENTS

In view of the preliminary nature of boundary models for reactor tokam aks, the sensitivity to edge gradients of the global mode MHD stabilit y of the ITER EDA configuration has been examined. The POLAR-2D equili brium and TORUS stability codes developed by the Keldysh Institute hav e been used. Transport-related profiles from the PRETOR transport code (developed by the ITER Joint Central Team) and axisymmetric equilibri a for these profiles from the TEQ code (L.D. Pearlstein, LLNL) were ta ken as a starting point for the study. These baseline profiles are fou nd to have quite high global stability limits, in the range g(Troyon) = 4 - 5. The major focus of this study is to examine global mode stabi lity assuming small variations about the baseline profiles, changing t he pressure gradients near the boundary. Such changes can be expected with an improved boundary model. Reduced stability limits are found in such cases, and unstable cases with g=2-3 are found. Thus, the assump tion of ITER stability limits higher than g=2 must be treated with cau tion.