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

MST (Madison Symmetric Torus) has been in operation since June 1988. The vacuum vessel is 5-cm-thick aluminum, 1.5 meters in major radius and 0.52 meters 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 of order 0.2% of the toroidal field on axis as was 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 field at the wall. The radial and poloidal fields at the poloidal gap are measured. Correction coils are added to the poloidal gap to cancel the error field. With the correction coils the radial field is reduced to about 25%. Detailed structure of these fields as well as the radial fields at the toroidal gap are presented