Characteristics of a Large Diameter Radio-Frequency Negative Hydrogen Ion Source

The characteristics of a 230-mm-diameter radio-frequency (rf) negative hydrogen ion source are investigated by the measurements of electron density and temperature. A hydrogen plasma is produced by an inductively coupled discharge operated with an rf frequency of 300 kHz and a power of a few tens of kW, where a field-effect-transistor-based inverter power supply is used as an rf generator. The ion source has a magnetic filter for reduction of the electron temperature near the plasma grid used for the extraction of a negative ion beam. A high electron density greater than 1018 m−3 is successfully obtained for an operating gas pressure of 0.3 Pa. The electron temperature near the plasma grid is observed to decrease to about 1 eV with increasing magnetic field strength of the magnetic filter.