An 80 KW SCR VLF power source for drift-pump-coil excitation

The drift-pump antenna coils in the end regions of the TMX-U mirror machine at LLNL require kiloamperes of current excitation in the 10 to 30 kHz range during the 75 ms shots. The coil's self-inductance will be series-resonated at the operating frequency f/sub 0/ with a high-Q inductor and capacitor. The overall tank Q≥200. The resonator acts as a current multiplier and VLF energy reservoir whose time constant tau=Q/πf/sub 0/. The sources to be described are modular SCR pulse generators which are inductively coupled to the resonator, and deliver alternate-polarity, half-sine-wave current pulses which are 35μs wide. As in a Class C vacuum-tube harmonic generator, the coupled resonator selectively extracts the Fourier component of the pulse current nearest f/sub 0/, and allows other components to pass unimpeded. DC to VLF efficiencies >80% have been measured. Resonator fill-up at the start of a shot is a problem for the pulse generator, since the only circuit damping is provided by the gradual buildup of the coupled resonator voltage. A clamping circuit across the generator's capacitor bank limits its voltage to a level safe for the SCRs, with the clamped current being returned to the main dc supply. The clamped pulse generator behaves like a current source for any load resistance from zero up to a critical value where SCR commutation failure occurs