Present status of the multi-megawatt long pulse ion source development

A multi-megawatt long pulse ion source has been developed for the neutral beam injection (NBI) system of the Korea Superconducting Tokamak Advanced Research (KSTAR). The beam extraction experiments of the ion source were carried out in the neutral beam test stand (NBTS). The ion source consists of a magnetic bucket plasma generator with multi-pole cusp fields and a set of tetrode accelerators with circular apertures. Design requirements for the ion source were 120 kV/65 A deuterium beam and 300 s pulse length, and initial test beam of 90 kV/45 A with hydrogen. Arc discharges of the plasma generator have been controlled by the emission-limited mode, controlled by the applied heating voltage of cathode filaments. Stable and efficient arc plasmas with a maximum arc power of 100 kW have been produced by constant power mode operation of arc power supply. A maximum ion density of 8.3 x 1011 cm3 was obtained by using electrostatic probes. An optimum arc efficiency of 0.46 A/kW was estimated. The decelerating voltage has been always applied prior to the accelerating voltage by the pre-programmable fast-switch control system. The time sequence of applied voltages is to suppress effectively the back-streaming electrons produced at the time of initial beam formation. A maximum beam power of 0.9 MW with hydrogen has been measured for a pulse duration of 0.8 s. Optimum beam perveance, deduced from the ratio of gradient grid current to total beam current, was 0.7 μperv. The experimental results of beam extraction at the accelerating voltage of ≤70 kV appear to be less than the expected ones