SOFT-X-RAY MEASUREMENT OF INTERNAL TEARING MODE STRUCTURE IN A REVERSED-FIELD PINCH

The structure of internally resonant tearing modes has been studied in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol 19, 131 (1991)] with a soft x-ray detector system consisting of an imaging array at one toroidal location and several detectors at different toroidal locations The toroidal mode numbers of dominant m = 1 structures are in the range n = 5,6,7. The modes propagate with phase velocity v = 1-6 X 10(6) cm/sec, larger than the diamagnetic drift velocity v(d) approximately 5 X 10(5) cm/sec. Phase locking between modes with different n is manifested as a beating of soft x-ray signals which is strongest near the resonant surfaces of the modes (r/a = 0.3-0.5) and exhibits characteristics indicative of nonlinear mode coupling.