Arc Discharge Characteristics of a Long Pulse Ion Source for the KSTAR Neutral Beam Injector

A long pulse ion source (LPIS) for the neutral beam injector (NBI) has been developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) superconducting tokamak. The arc discharge characteristics of the ion source were investigated on the first KSTAR neutral beam injector (NBI-1). The ion source consists of a magnetic bucket plasma generator with multi-pole cusp fields and a set of prototype tetrode accelerators with circular apertures. The inner volume of the plasma generator in the LPIS ion source is approximately 47.2 liters. Design requirements for the ion source were a 120 kV/65A deuterium beam and a 300 s pulse length. Arc discharges of the plasma generator with deuterium gases have been controlled by using the space charge-limited mode and have been operated by using a sufficient heating voltage on the cathode filaments. Stable and efficient arc plasmas up to an arc power of 70 kW have been produced by using a constant-power-mode operation of the arc power supply. The reliable operation region of the arc discharges has been investigated for various filament heating voltages, arc powers, and amounts of injected deuterium gas in the LPIS.