Investigation of Oxygen-Induced-Arcing in Cs-Seeded Negative Ion Source

High-power and long-pulse arc discharges of 150 kW for 100 s have been achieved for the first time by suppressing oxygen-induced-arcing which was one of the critical issues on JT-60SA negative ion source. Toward the realization of such arc discharge, the suppression of recycling of oxygen from the arc chamber wall was found to be a key issue in the large-size arc-driven negative ion source. Because the experimental results showed that this oxygen was derived from release of water due to high temperature chamber wall of heated by an arc discharge, the baking of the arc chamber was tried to reduce water adsorbing to the chamber before the long-pulse discharge. After the baking at 80 ◦C for 8 hours, since the oxygen level was reduced to half of that before baking, the available pulse duration became twice longer. By combining this baking and long-pulse conditioning shots, the stable long-pulse arc discharge has been obtained. This result contributes to the improvement of availability of the large-size negative ion sources which require long-conditioning time.