MICROINSTABILITY PROPERTIES OF NEGATIVE MAGNETIC SHEAR DISCHARGES IN THE TOKAMAK FUSION TEST REACTOR AND DIII-D

The microinstability properties of discharges with negative (reversed) magnetic shear in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawry luk et al., Plasma Physics and Controlled Nuclear Fusion Research, 199 4 (International Atomic Energy Agency, Vienna, 1995), Vol. I, p. 11] a nd DIII-D [R. D. Stambaugh for the DIII-D Team, Plasma Physics and Con trolled Nuclear Fusion Research, 1994 (International Atomic Energy-Age ncy, Vienna, 1995), Vol. 1, p. 83] experiments with and without confin ement transitions are investigated. A comprehensive kinetic linear eig enmode calculation employing the ballooning representation is employed with experimentally measured profile data, and using the correspondin g numerically computed magnetohydrodynamic (MHD) equilibria. The insta bility considered is the toroidal drift mode (trapped-electron-eta(i) mode). A variety of physical effects associated with differing q-profi les are explained. In addition, different negative magnetic shear disc harges at different times in the discharge for TFTR and DIII-D are ana lyzed. The effects of sheared toroidal rotation, using data from direc t spectroscopic measurements for carbon, are analyzed using comparison s with results from a two-dimensional calculation. Comparisons are als o made for nonlinear stabilization associated with shear in E-r/RBthet a. The relative importance of changes in different profiles (density, temperature, q, rotation, etc.) on the linear growth rates is consider ed. (C) 1997 American Institute of Physics.