Effect of Magnetic Field Configuration on n = 1 Instabilities in Rotamak

The n = 1 tilt/radial shift modes are observed in rotating magnetic field (RMF) driven FRC plasmas. Experiments studying on the response of n = 1 instabilities to the changes of magnetic field structure have been conducted in 40 ms Rotamak discharges. In one series of experiments the axial current $I_{z}$ (which produces toroidal field) ramps linearly in time from 0 to 2 kA, leading to transition from FRC to ST configuration. The amplitude of the tilt mode is suddenly doubled when $I_{z}$ reaches 0.5 kA (compare to 2 kA plasma current); the amplitude remains at this level when $I_{z}$ is in the range of 0.5-1 kA. The tilt instability disappears when $I_{z}$ exceeds 1 kA. In other series of experiments, by using a middle shaping coil with a moderate current of 0.25-0.5 kA, the doublet-FRCs are formed which are completely free from both the tilt and radial shift modes.