EXPERIMENTAL INVESTIGATION ON TILT STABILIZING EFFECT OF EXTERNAL TOROIDAL FIELD IN LOW-ASPECT-RATIO TOKAMAK

This paper describes experimental investigations on the equilibrium an d global stability of low aspect ratio tokamaks with different aspect ratios ranging from 1.1 to 1.9. The Z-theta pinch spheromak formation technique is used to produce low aspect ratio tokamaks in an external toroidal field generated by the center conductor. Using this operation , the plasma stability has been investigated in the transition regime from tokamaks to spheromaks. It has been found that there exists a low er critical value of the center conductor current to surpress the glob al plasma instability of the n = 1 tilt and/or shift modes. The ratio of this critical current to plasma current is experimentally measured for the first time as a function of the aspect ratio. Glass-tube cylin drical limiters with different radii are installed along the symmetric center axis of the Spherical Torus-3 device [Y. Ono et al., Phys. Flu ids B 5, 3691 (1993)]. As the aspect ratio is decreased from 1.9 to 1. 1, it is observed that the critical ratio of the center conductor curr ent to plasma current decreases from 1.2 to 0.2. The safety factor q a t the plasma edge corresponding to this critical current is roughly 1. 5 to 3.0. Similar experiments are also carried out with a thin metal c over surrounding the surface of the glass tube limiter. The thin metal cover permits the decrease in the critical current and the correspond ing edge q value of q similar to 1. These experimental results of the critical current ratio are found to be comparable to that predicted fr om theoretical models where the restoring force against the tilt motio n is considered to be generated by the interaction of the external tor oidal field with the n = 1 induced surface currents in the tilt motion . (C) 1997 American Institute of Physics.