The magnetic structure of a Rotamak discharge produced in a metal-walled vessel

An experimental investigation of the magnetic field structure of a Rotamak discharge, generated and maintained in a metal vessel by means of a rotating magnetic field, is described. The rotating magnetic field is produced by feeding r.f. (1 MHz) current pulses, of 40 ms duration and 90 degrees phase difference, through two orthogonal coils located inside the metal chamber. With a total r.f. power input of 30 kW, a plasma/field configuration is generated which consists, firstly, of a compact torus configuration lying within the r.f. coil structure and well away from the vessel wall. The (positive) current ring associated with this inner structure is approximately 250 A. Secondly, an extra two positive current rings, each carrying approximately 150 A, are observed in the outer region between the coil structure and the vessel wall. Finally, a fourth, and unexpected, ring of negative current ( approximately 15 A) is detected which lies between the three positive current rings. It is speculated that this negative current ring arises from a continuous flow of plasma.