29th Symposium on Fusion Technology (SOFT 2016)

P4.005 Issues of arc discharge for long pulse KSTAR NBI

8 Sep 2016, 14:20

1h 40m

Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Board: 5

Poster A. Experimental Fusion Devices and Supporting Facilities P4 Poster session

Speaker

Wook Cho (Heating and Current Drive Research Team)

Description

In 2015 KSTAR Campaign, the maximum injection power of the KSTAR tangential Neutral Beam Injector (KSTAR NBI-1) is 5.39MW with three ion sources. Issues in beam extraction found during the experiment were 1) a large oscillation of beam current, 2) frequent interrupts in beam extraction due to breakdown in grids, and 3) a distortion of waveform. To solve these issues, we focused on the unstable arc discharge. Depending on the control mode and plasma impedance, characteristics, current overshoot and oscillation, of Arc voltage/current waveforms are affected. Because of initial waveform distortion, continuous interruption on beam extraction prevents stable operation. Constant-Power control mode (CP mode) results in severe overshoot and oscillation having amplitude of 10V. Adjusting interval between pre-arc discharge and main-arc discharge, or the duration of pre-arc discharge was not effective. Applying slow rising time (20ms) of CP mode or the combination of initial Constant-Voltage control mode (CV mode) and CP mode also causes distortion and overshot. Optimized control gains with CP mode without intentional ramping applied to dummy load results 1) reduced oscillation amplitude of about 4V, 2) stable initial rising in 4ms, and 3) elimination of distortion with optimized notch resister. Performance of power supply with arc discharge is also discussed.