Experimental identification and control of spheromak tilting instabilities

The properties of the n = 1 tilting instability in both Proto S-1/A and Proto S-1/C are examined. The plasma behavior is determined experimentally by mapping the evolution of the magnetic field configuration in the toroidal plane perpendicular to the tilt axis with small magnetic probes. The mode is found to be not quite rigid, with the outer portions of the plasma having a roughly constant growth rate which is faster than that of the region near the magnetic axis for most cases. Growth rates measured for the tilting instability are compared to the predictions of a simple current ring model, and to the results of numerical simulations. The growth rates scale approximately as predicted by a simple model. Two different configurations of passive coils are shown to be effective in stabilizing the tilting mode in both Proto S-1/A and Proto S-1/C. These are the figure eight coils and the saddle coils. The figure eight coils are effective against both tilting and shifting modes, but restrict access to the plasma. The saddle coils permit free access, but are effective only against the tilting. A peak value of approximately 4% of the toroidal plasma current induced in the coils appears sufficient to suppress the tilting instability for the lifetime of the plasma in these experiments. This level of current in the coils corresponds to an applied horizontal field in the plane of the tilt which is less than or equal to 4% of the toroidal field at the magnetic axis