Plasma Vessel Research Articles

Preionization and shock-wave ionization in a crossed-field plasma source

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.

A High Current Radio Frequency Proton Source with Radial Extraction

A theory outlining the mechanism of the low pressure hydrogen plasma used in radio frequency proton sources is given. Taking into account secondary emission from surfaces of plasma vessel it is shown that the electron density is maximum at the tube's center. This is confirmed experimentally, and a source is designed with its extraction electrode positioned radially near the plasma center. Compared with axial extraction used in conventional r.f. sources, radial extraction yields beam currents over 20% higher, and allows the application of higher extraction voltages. Beam currents up to 150 Ma. can be extracted, of which more than 85%, is proton current. Two methods are used for the measurement of the emittance of the accelerated beam.