Long-pulse modulator for the superconducting RF test facility at KEK

This paper presents two types of long-pulse 1.3 GHz klystron modulators that were recently developed for the Superconducting rf Test Facility (STF) at High Energy Accelerator Research Organization (KEK). The STF plan includes phases 1 and 2. In phase 1, two modulators are used to drive a klystron. One is a direct-switched-type design with a 1:6 step-up transformer and a bouncer circuit to compensate for the output pulse droop within ±0.5% during the rf drive period; it can drive a single-beam klystron with up to 5 MW peak power, 1.5 ms rf pulse width, and up to 5 pps repetition rate. This modulator has been converted from a short-pulse klystron modulator system obtained from Power Reactor and Nuclear Fuel Corp. (PNC). The other modulator is also a direct-switched type design with a 1:15 step-up transformer and a bouncer circuit; it can drive a multi-beam klystron with up to 10 MW peak power, 1.5 ms rf pulse width, and up to 5 pps repetition rate. The main features of this modulator are the use of four 50 kW switching power supplies in parallel to charge the storage capacitors to 10 kV, self-healing-type capacitor to realize a compact storage capacitor bank, and a highly reliable IGBT switch to eliminate a crowbar circuit.