Long pulse production of high current $D−$ ion beams in the JT-60 negative ion source^a)

The first long pulse production of high power $D−$ ion beams has been demonstrated in the JT-60 U negative ion sources, each of which was designed to produce $22A$⁠, $500keV$ $D−$ ion beams. Voltage holding capability and the grid power loading were examined for long pulse production of high power $D−$ ion beams. From the correlation between voltage holding and the light intensity of cathodoluminescence from the Fiber Reinforced Plastic insulators, the acceleration voltage for stable voltage holding capability was found to be less than $320–340kV$ where the light was sufficiently suppressed. By tuning the extraction voltage, the grid power loadings in the ion sources were decreased to the allowable levels for long pulse injection without a significant reduction of the beam power. After tuning the acceleration and extraction voltages, $D−$ ion beams of 12.5 and $9.8A$ were produced at $340keV$ with cesium seeding at a rate of $∼14μg∕s$ into the ion sources. The pulse duration of these $D−$ ion beams was extended step by step, and then was successfully extended up to $18s$ without degradation of the negative ion production. The $D−$ ion beams were neutralized to yield $3.6MW$ $D0$ beams by a gas cell, and then injected into the JT-60 U plasma. Further, a slight reduction of $D−$ ion beam power allowed the longer injection duration of $21s$ at a $D0$ beam power of $3.2MW$⁠. The success in the long pulse production of a high power $D−$ ion beam shows that negative ion beams can be produced during a few tens of seconds without degradations of negative ion production and the voltage holding in a large Cs-seeded negative ion source.