Preionization and start-up in the ISX-B tokamak using electron cyclotron heating at 28 GHz

A 28-GHz gyrotron is used to produce a plasma at the electron cyclotron resonance in the Impurity Study Experiment (ISX-B) tokamak. The influence of the toroidal magnetic field magnitude, error fields, gas pressure, microwave power, microwave pulse length, and microwave timing is studied for experiments with magnetic field and gas only. Also, experiments with preionization followed by capacitor discharges are carried out in which these quantities are varied, as are the capacitor bank voltages. Optimum conditions of preionization for some of the parameters are determined. A theoretical model that adequately reproduces the data is given. Calculations based on this model show the temporal evolution of the electron temperature and density, the neutral density, and the plasma current. The model adequately accounts for present and previous experimental results and can be used to make predictions for future experiments. The decay of the discharge with no input power is not correctly predicted by the model.