Magnetic flux generation and sawteeth in the MST reversed-field pinch

Abstract

The Madison Symmetric Torus (MST) is a reversed-field pinch (RFP) that began operation in June 1988 (major radius of 1.50 m and minor radius of 0.52 m). Since its initial operation, discrete magnetic disturbances that result in an increase in the average toroidal magnetic flux and a decrease in the toroidal field at the wall have been observed. The plasma column also experiences a sudden inward shift as seen by magnetic pickup coils. These events suggest a flattening of the profile. Other RFPs have exhibited similar sawteeth, and they are believed to be related to the m=1 tearing mode. On MST, the sawteeth are observed for Theta as low as 1.2, whereas on other RFPs they are seen at larger Theta . Typically low frequency (10-25-kHz), global fluctuations are seen on edge magnetic pickup coils, as well as on soft X-ray signals, just prior to a sawtooth crash. On MST, these fluctuations have a poloidal mode number, m, of 1 and a toroidal mode number, n, of about -6. This suggests that n scales as aspects ratio, as would be expected for resonant modes. >