Development of all power supplies with a new design utilized for NBI-2 system on KSTAR

Abstract The power supply for the 6 MW (2 MW × 3 sets) of the neutral beam injector (NBI-2) system is newly developed. The rated output of 100 kV/60 A DC power supply based on the pulse step modulation (PSM) technique is adopted for the main acceleration G1-grid power supply, which can be regulated from 20 to 100 kV by the series connection of 200 power-modules. Especially, the 16 modules have been allocated as the spare modules to maintain the G1-grid voltage at the desired value without an interrupt by a failed-module that stops working by an error during the operation. The voltage corresponds to 80∼90 % of G1-grid voltage by branched out among the power-module is applied to the gradient grid (G2-grid) without using the voltage-divider system used in the former NBI system. The arc power supply was modified by configuring four converters in parallel with the phase-shift operation to obtain the faster rising/falling time compared to the former NBI-1. The core snubber using FINEMET® core materials have been adopted to reduce the delivered energy from the power supply to the grid less than 10 J in case of breakdown at 100 kV to prevent damage of the beam source. And the passive RL snubber is applied at the end of transmission cables instead of a fast switch system to reduce the delivered energy. One of the main target features for the NBI-2 system is the variation of the beam energy/power into the plasma. Therefore, all of the power supplies are designed to realize real-time control during the operation. In this paper, the design and commissioning status of the new power supplies for the KSTAR NBI-2 will be described.