Comparison of rotamak plasmas in FRC and ST configurations

The results of experiments in a newly rebuilt rotamak are presented. A comparison is made for two types of operations: the common field-reversed configuration (FRC) produced by driving plasma current with a rotating magnetic field (RMF), and the spherical tokamak (ST) configuration when a steady toroidal magnetic field is added. In both cases the driven plasma current develops two current rings, but in the ST configuration the inner ring current density is about three times larger than that in the FRC case. The addition of the toroidal field enables the ST to overcome the current limit for a given radio-frequency power. The total plasma current is found to have a peak at an optimum value of the applied toroidal field; the optimum toroidal field depends on the value of the equilibrium magnetic field. With proper choice of the toroidal field magnitude, the plasma current can be enhanced two- or three-fold in comparison with the limits in the cases of too a small or too large toroidal field. In the rotamak–ST, the temperature of electrons is increased by at least 50%. The density profile is triangular-shaped in the ST case and almost uniform in the FRC. The measurements of the oscillating fields indicate that the penetration of RMF into plasma is greatly enhanced in the ST case. The analysis of RMF profiles in the plasma supports the hypothesis that the improved penetration is due to the excitation of a whistler wave mode.