Design, construction, and first operational results of a 5 megawatt feedback controlled amplifier system for disruption control on the Columbia University HBT-EP tokamak

This paper presents the electrical design and first operational results of a 5 Megawatt feedback controlled amplifier system designed to drive a 300 uH saddle coil set on the “HBT-EP” tokamak. It will be used to develop various plasma feedback techniques to control and inhibit the onset of plasma disruptions that are observed in high “β” plasmas. To provide a well characterized system, a high fidelity, high power closed loop amplifier system has been refurbished from the Los Alamos “ZT-P” equilibrium feedback system. In its configuration developed for the Columbia HBT-EP tokamak, any desired waveform may be generated within a 1100 ampere and 16 kV peak to peak dynamic range. An energy storage capacitor bank presently limits the effective full power pulse width to 10 ms. The full power bandwidth driving the saddle coil set is ~12 kHz, with bandwidth at reduced powers exceeding 30 kHz. The system is designed similar to a grounded cathode, push-pull, transformer coupled, tube type amplifier system. The push pull amplifier consists of 6 Machlett ML8618 magnetically beamed triodes, 3 on each end of the (center tapped) coupling transformer. The transformer has 0.1 volt-seconds of core and a 1:1 turns ratio. The transformer is specially designed for high power, low leakage inductance, and high bandwidth. Each array of ML8618´s is (grid) driven with a fiber optic controlled hotdeck with a 3CX10,000A7 (triode) output. To linearize the ML8618 grid drive, a minor feedback loop in the hotdeck is utilized. Overall system response is controlled by active feedback of the saddle coil current, derived from a coaxial current viewing resistor. The detailed electrical design of the power amplifier, transformer, and feedback system will be provided in addition to recent HBT-EP operational results