Observation and analysis of a resistive mode with interchange parity in negative central shear plasmas in the DIII-D Tokamak.

A magnetohydrodynamic (MHD) instability which has the characteristics that the displacement phase is in the same direction at all the affected flux surfaces (an interchange-like structure with no phase reversal across a flux surface), has been observed during the L mode in several negative central shear discharges of DIII-D tokamak [J. L. Luxon, P. Anderson, F. Baity et al., Plasma Physics and Controlled Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. The instability occurs when the minimum safety factor is around 2.0 and the profile of the safety factor q is deeply reversed in the center. Detailed stability analyses were carried out using standard numerical codes and the high quality magnetic probe, motional Stark effect and electron cyclotron emission (ECE) data. Analysis of the data after the onset shows that the instability has an n = 1 mode number and a growth time of about 400 µs. The electron temperature fluctuations obtained from ECE measurements indicate a localized interchange-like structure early in time, the resistive interchange criterion indicates marginal stability, and ideal mode analyses indicate robust stability with an ideal beta limit of about a factor of 2 higher than the β[sub N] value at the time of onset. Therefore this interchange-parity mode is not an ideal MHD mode. The marginal value of the resistive interchange criterion observed only in discharges with the instability, indicates that this is probably a resistive interchange mode. However, some observed characteristics of the instability may require models beyond the linear resistive interchange theory. [ABSTRACT FROM AUTHOR]