Arc discharge efficiency of a multi-megawatt long pulse ion source for the KSTAR neutral beam injector

A multi-megawatt long pulse ion source (LPIS) was developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) neutral beam (NB) injector. Arc discharge characteristics of the ion source were investigated on the NB test stand. The ion source consists of a magnetic bucket plasma generator with multi-pole cusp fields and a set of tetrode accelerators with circular apertures. The inner volume of the ion source, including the accelerator column, is approximately 150 litres. Design requirements for the ion source were a 120 kV/65 A deuterium beam and a 300 s pulse length and initially an 80 kV/48 A hydrogen beam for a 20 s pulse length. Arc discharges of the plasma generator with hydrogen gases have been controlled by the emission-limited mode, operated by the applied heating voltage of the cathode filaments. Stable and efficient arc plasmas, with a maximum arc power of 110 kW, have been produced by a constant voltage (CV) mode operation of an arc power supply. The CV mode operation of the arc powers was more efficient than the constant power (CP) mode operation in the LPIS. A maximum ion density of 9.1 x 1011 cm-3 was measured by using electrostatic probes. The plasma non-uniformity of the ion source was less than 8% and was under the design limit. An optimum arc efficiency, defined by the ratio of extractible ion beam current to arc input power, of 0.46 A kW-1 was estimated for the CV mode operation and 0.44 A kW-1 for the CP mode operation. This arc efficiency is enough to extract the expected hydrogen beam of 48 A