An experimental investigation of the toroidal magnetic field structure of a rotamak discharge

The self-generated bi-directional toroidal field of a spherical rotamak discharge is experimentally investigated. Detailed measurements made in an argon discharge have allowed us to decompose the toroidal field into its vector spherical harmonic. The radial dependence of this field harmonic is derived and it compares favourably with that theoretically predicted. The steady poloidal current which generates the toroidal field is derived from the measured vector harmonic of the field. The vector plot of the poloidal current density shows that the current has a quadrupole loop structure and is positively directed in the upper, and negative in the lower, halves of the poloidal cross sections. The toroidal field is found to be only about 8% of the poloidal field. Finally, the effect of the applied vertical field upon the bi-directional toroidal field is studied. (Author)