ROTATIONAL AND MAGNETIC SHEAR STABILIZATION OF MAGNETOHYDRODYNAMIC MODES AND TURBULENCE IN DIII-D HIGH-PERFORMANCE DISCHARGES

The confinement and the stability properties of the DIII-D tokamak [Pl asma Physics and Controlled Nuclear Fusion Research 1986 (Internationa l Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high-performanc e discharges are evaluated in terms of rotational and magnetic shear, with an emphasis on the recent experimental results obtained from the negative central magnetic shear (NCS) experiments. In NCS discharges, a core transport barrier is often observed to form inside the NCS regi on accompanied by a reduction in core fluctuation amplitudes. Increasi ng negative magnetic shear contributes to the formation of this core t ransport barrier, but by itself is not sufficient to fully stabilize t he toroidal drift mode (trapped-electron-eta(i) mode) to explain this formation. Comparison of the Doppler shift shear rate to the growth ra te of the eta(i) mode suggests that the large core E X B flow shear ca n stabilize this mode and broaden the region of reduced core transport . Ideal and resistive stability analysis indicates the performance of NCS discharges with strongly peaked pressure profiles is limited by th e resistive interchange mode to low beta(N) less than or equal to 2.3. This mode is insensitive to the details of the rotational and the mag netic shear profiles. A new class of discharges, which has a broad reg ion of weak or slightly negative magnetic shear (WNS), is described. T he WNS discharges have broader pressure profiles and higher beta value s than the NCS discharges, together with high confinement and high fus ion reactivity. (C) 1996 American Institute of Physics.