Design of a multistage 250 kJ capacitor bank for ohmic transformer of tokamak "ADITYA"

Abstract

Tokamaks require toroidal loop voltage for breakdown of the neutral gas, current rise, and the flat top phase. The temporal profile of the loop voltage established by the change of flux linked by the ohmic transformer has to be a noncosine waveform. In this paper a multistage capacitor bank is described which was used to energize the ohmic transformer in tokamak ADITYA with a major radius of 0.75 m, minor radius of 0.25 m, and a toroidal field of 1.5 T at the plasma center. A combination of capacitors charged to different voltages are switched in at appropriate times, to realize an experimental demand for initial high loop voltage followed by a lower sustaining loop voltage. Theoretical prediction for the duration of the secondary loop voltage as a function of circuit parameters, for a fast bank operation of 6 kV, slow bank, 4-4.5 kV, and slow bank, 2-2.5 kV yield t₀=1.25 mS, t₁=4.95 mS, and t₂=24.1 mS. These values are in close agreement to the measured values of t₀=1.39 mS, t₁=5.7 mS, and t₂=23.7 mS. The trigger delays to the various capacitor bank sections are parameter dependent. To avoid repetitive adjustments in the delays, a novel scheme for consistent triggering is also highlighted.