Magnetic field error measurements and effects on plasma in the MST reversed field pinch

The MST (Madison Symmetric Torus) has been in operation since June 1988. The vacuum vessel is 5-cm-thick aluminum, 1.5 m in major radius and 0.52 m in minor radius. The vessel serves as the toroidal field winding and conducting shell, with a single poloidal gap and a single toroidal gap. These gaps are a potential source of error fields if great care is not used in designing the winding system. The toroidal field system produces a vacuum error field with a dominant n = 4, m = 0 Fourier component of magnitude on the order of 0.2% of the toroidal field on axis, as expected. With the present temporary ohmic winding the maximum radial magnetic field at the poloidal gap for a typical plasma is about 60% of the poloidal gap for a typical plasma is about 60% of the poloidal field at the wall. Correction coils have been added to the poloidal gap to cancel the error field, which has a large m = 1 component. With the correction coils the radial field is reduced to about 25%, and the plasma resistance is reduced. With correction a coherent precursor (m = 1, n = 6) on the SXR signals shows the rotation of these modes. Without correction the SXR precursors are not present and the magnetic coils do not show any coherent structure in most of the shots.