Filament power supplies (AC-AC converters) and their design for long pulse Neutral Beam Injector of SST-1

The Neutral Beam Injector of SST-1 (Steady-state Superconducting Tokamak-1) uses Filamented Ion Sources with AC heating. A regulated filament power supply is essential to maintain constant filament temperature, beam current and plasma. This paper presents the design and performance of AC-to-AC converters for the filament power supply system. A 24 phase heater supply is used to minimize the ripple on plasma density. The front-end converter circuit at the input maintains the total current harmonic distortion of line within 5% and power factor at unity with 10kVA power. A PWM inverter along with output filters generates the three-phase output. Three phase AC output is controllable from 0-415V (rms), 400Hz, sinusoidal. The secondary from a single-phase step down transformer (ratio 12:1) placed at the output of the converter is connected to one filament. The stability at the output is 0.1 % for line or load fluctuations with 20 mSec transient response. Overall efficiency is approximately 90%. Eight power supplies, all capable of being synchronized with an external phase reference trigger pulse are used to generate a 24-phase filament heating system for the ion source. The PWM generation carrier waveform is generated by a digital scheme, with a start trigger for each cycle. The control is matched to meet the filament temperature stability with compensation for actual current in the filament.