The negative-ion based NBI system (N-NBI) for JT-60U was designed to deliver deuterium beams of 10MW at 500keV for 10sec using one beamline with two negative-ion sources. The power supply for the N-NBI system is composed of negative-ion generation, extraction and acceleration power supply. The negative ion generation power supply consists of four power supplies of filament cathode, arc, bias and magnetic filter generation. The extraction power supply (10kV/80A/10sec) is for extracting negative charged particles of both ions and electrons, and injecting negative ions only into accelerator by taking away the electrons in the extractor. The negative ion generation power supply and extraction power supply are set for each ion source and are mounted on the high voltage table floated on 500kV potential. The acceleration power supply is for acceleration negative ions up to 500keV in maximum, and has capability of 500kV/64A for 10sec with a ripple of less than 10%p-p. The power supply is used in common with two ion sources. The cut off time of the power circuit at the moment of ion source breakdown is required less than 200 ms to protect the accelerator grid and power supply itself against surge current inflow.

A high voltage test has been executed to confirm the performance of the power supply before beam acceleration. The high voltage test contains the following items: a voltage holding test up to 550kV which corresponds to 1.1 times higher of the rated voltage of 500kV in maximum, a breakdown test on 500kV potential using a sphere gap to confirm a surge propagation on the circuit. After having finished these power supply verification tests, the beam acceleration and beam injection tests have been progressed. A negative ion beam of 18.4A/H-at 350keV and 13.5A/D- at 400keV have been reached, and a neutral beam power of 3.2MW at 350keV has injected into JT-60U for 1sec with one ion source. After having modified the acceleration voltage and current regulation using computer control system at the end of 1996, the rising time of the acceleration voltage up to 500kV could be decreased to about 50ms, which is one third roughly of original rising time with an analog control system.