Design of a negative-ion-based NBI system for JT-60U

A negative-ion-based NBI (neutral-beam injection) system is planned as a key device in the JT-60U (JAERI Tokamak-60 upgrade) experimentation involving current drive and plasma core heating with high density plasmas. The NBI system will inject neutral beams of 500 keV, 10 MW for 10 s from a beamline with two ion sources. The neutral beam will be injected tangentially in the co-direction. Each ion source is a modified volume production-type negative-ion source with cesium vapor. The acceleration current is 22 A with a deuterium beam, and the current density is 13 mA/cm/sup 2/. Operational pressure in the negative-ion generator is less than 0.5 Pa. A three-stage electrostatic acceleration system is adopted as the accelerator. The beamline length between the ion source and the injection port is 24 m. The beamline consists of an ion source tank, neutralizer cells of 10 m in length, an ion dump tank, and a drift duct. Residual ions are deflected by the combined magnetic fields produced by the deflecting coils and the stray field from the tokamak. The two sources are connected to an acceleration power supply of 500 kV/64 A/10 s, while the negative-ion generator power, the extraction voltage, and electron-suppression voltage are fed individually.<>