A high-voltage modulator for high-power RF source research

The authors present the design, construction, and operating results of a high-voltage modulator system capable of generating 700 kV, 2.5 μs pulses at 5 pps into a load of 900 Ω. The modulator is used to energize a variety of high-power microwave devices requiring voltage stability and reproducibility. Voltage ripple is less than 0.2% during the 1.0 μs flat top, with a shot-to-shot voltage variation of less than 0.1%. The primary circuit consists of two seven-stage tunable Rayleigh-type pulse forming networks (PFNs) connected in parallel with a total impedance of 2.25 Ω, a total capacitance of 0.56 μF, and a total inductance of 2.8 μH. The PFN is charged by a highly stable 80 kV capacitor charging power supply at a rate of 10 kJ/s. The total energy stored (1.5 kJ) is released through an ITT F-187 thyratron into a 20:1 pulse transformer, which generates 700 kV, 2.5 μs pulses. By changing the transformer, 250 kV, 1.7 kA pulses for driving low impedance relativistic magnetron diodes were also obtained. The flat-top voltage generated by the modulator is highly desirable for driving RF sources requiring high-quality electron beams, such as free-electron lasers and cyclotron auto-resonance masers