Measurements and phenomenological modeling of magnetic flux buildup in spheromak plasmas

Abstract

Internal magnetic field measurements and high-speed imaging at the Sustained Spheromak Physics Experiment [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion 39, 863 (1999)] are used to study spheromak formation and field buildup. The measurements are analyzed in the context of a phenomenological model of magnetic helicity based on the topological constraint of minimum helicity in the open flux before reconnecting and linking closed flux. Two stages are analyzed: (i) the initial spheromak formation, i.e., when all flux surfaces are initially open and reconnect to form closed flux in the toroidal average sense, and (ii) the stepwise increase of closed flux when operating the gun on a new mode that can apply a train of high-current pulses to the plasma. In the first stage, large kinks in the open flux surfaces are observed in the high-speed images taken shortly after plasma breakdown, and coincide with large magnetic asymmetries recorded in a fixed insertable magnetic probe that spans the flux c...