Observation of the Stability Window in the Fast-Toroidal-Pinch Experiment

A fast toroidal pinch with very small aspect ratio (2.6) is operated to test tokamak configuration. A narrow stability window appears between $q=1.3\mathrm{} \mathrm{and} 1.5$. In the stable region the energy loss from the plasma is mainly governed by the decay of the plasma current, which, in the present case, is controlled by the external circuit. In the unstable region, the energy-loss rate observed under clean vacuum conditions is found to agree approximately with the calculated growth rate of the kink mode.