Sawteeth and energy confinement in the Madison symmetric torus reversed-field pinch

Most Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2-5 ms, corresponding to magnetobydrodynamic (MHD) instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent.