The breakdown of a hydrogen plasma using RF heating has been studied in a modular stellarator. The time delay between the beginning of an RF pulse and the production of a measurable density is examined as a function of puff pressure, RF power and magnetic field strength. The confinement time of the ionizing electrons is found to be independent of the collision frequency and their scaling is inversely proportional to the square root of the applied power. The ionization coefficient peaks when the electron cyclotron resonant layer coincides with a saddle point in the magnetic field on the outside of the torus, in agreement with a simple model for the heating rate. Under this condition, the electron cyclotron emission during the discharge is greater by an order of magnitude than when the resonant layer coincides with the magnetic axis. Application of approximately 100 W of power in the ion cyclotron range of frequencies in conjunction with electron cyclotron heating is shown to improve the confinement of the ionizing electrons by about 70%.
Ion cyclotron heating and RF propagation in the Proto-Cleo stellarator