Beam extraction characteristics of a low-energy, high-current pulsed ion source of a versatile experiment spherical torus neutral beam injector

A neutral beam (NB) ion source aimed to inject a 0.5-MW NB has been developed for Versatile Experiment Spherical Torus (VEST) plasma heating. Beam energy is generally limited to less than similar to 20 keV to reduce particle orbit loss and beam shine-through. To meet this power demand, high-current ion beam extraction is important to achieve the required performance of the ion source. In this study, a 0.7-MW ion source was designed, fabricated, assembled, and optimized at the NB test stand in Korea Atomic Energy Research Institute (KAERI) considering the beam power losses in the beamline components. The ion source comprises a multicusped-bucket-type plasma generator and a multislot-triode-type ion accelerator with an extraction area of 12 cm x 46 cm. The characteristics of arc discharge and beam extraction were investigated using hydrogen gas to obtain the optimum operating parameters of the arc power, filament voltage, gas pressure, extraction voltage, and suppression voltage. The ion source was then conditioned and performance tests were primarily completed. Results of the experiments show that the ion source can extract sufficient ion beam power that is than 0.75 MW (50 A/15 keV) with 100-kW arc power. The arc efficiency and beam perveance were estimated to be 0.6 A/kW and 20-25 mu P, respectively. Moreover, the design goal of delivering a 0.5-MW NB into the VEST plasma will be achievable based on the optimum operating conditions. This ion source has been installed in the VEST NB injection system at Seoul National University and its commissioning is ongoing.