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

This paper presents 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 (PFN's) 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 (0.1% rms voltage ripple) 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, 2.5-1 μs pulses. By changing the transformer, we have also obtained 250-kV, 1.7-kA pulses for driving low-impedance relativistic magnetron diodes. 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 (FEL) and cyclotron auto-resonance masers (CARM). The modulator performance in our relativistic magnetron and CARM experiments is described