An experimental investigation into the development of ionization in a cylindrical crossed-field plasma source is described. The observations are based mainly on microwave diagnostic methods; 35-Gc/sec interferometry is supplemented by measurements of the reflection of 10-Gc/sec waves from the plasma surface, enabling observations far above the density range normally accessible to microwave methods. We find that there are two stages of ionization. For the first, or pre-ionization stage, measured values of the electron density under various preparation conditions are presented, and estimated values of collision frequency are used for the calculation of the conductivity of the pre-ionized gas. Observations are made of the second stage of ionization, wherein a supersonic ionization front propagates through the pre-ionized gas raising it to a highly ionized state. Front velocities are measured for various pressures, magnetic induction fields and currents. Microwave reflections are used to monitor the build-up of ionization following the arrival of the ionization front at the end of the plasma vessel.
Electron cyclotron/upper hybrid resonant pre-ionization in the ISX-B tokamak