Field error and its effect on the plasma performance in TPE-RX, reversed-field pinch device

The effects of field errors at the poloidal gap of the thick shell on the plasma performance and the wall-locking are intensively studied. The coil system of TPE-RX (major/minor radii, R/a= 1.72/0.45 m) has several features for cancelling the field errors at the thick shell gap and for controlling the equilibrium position of the plasma column. It is confirmed that these field error cancellation schemes work well for the normal operating conditions in TPE-RX. It is shown quantitatively that the global performance of the revised-field pinch operation is affected if the shell gap field error exceeds about 5% of the poloidal magnetic field. As expected, the probability of the mode to lock at the shell gap increases as the field error increases.