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=1−6×106 cm/sec, larger than the diamagnetic drift velocity vd∼5×105 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.