Inverse Marx modulators for self‐biasing klystron depressed collectors

Abstract

A novel pulsed depressed collector biasing scheme is proposed. This topology feeds forward energy recovered during one RF pulse for use on the following RF pulse. The presented 'inverse' Marx charges biasing capacitors in series, and discharges them in parallel. Simulations are shown along with experimental demonstration on a 62 kW klystron.Introduction: The SLAC National Accelerator Laboratory (SLAC) is actively developing next-generation high-power RF sources. In response to a US Federal mandate [1] and growing particle accelerator demands, RF power source efficiency is gaining importance. One method for increasing efficiency, a depressed collector, is studied for application in pulsed RF systems.In low duty cycle, high peak power (>1 MW), short-pulse (<1 µs) RF systems, traditional depressed collector stage biasing methods can introduce cathode oscillations, affecting the RF output phase and the output. Decoupling the collector biasing from the driving modulator avoids these deleterious cathode oscillations and enables recovering the wasted energy in the rise and fall time, critical in short-pulse applications.

A pulsed depressed collector for a 5 MW S-band klystron

“…Collector stages must be biased at potentials approaching the beam voltage (100's kV), while typical modulators store energy between 1 and 25kV. Therefore, the "inverse Marx" was invented to convert the potentials for this application [4]. This topology is effectively a pulsed step-down converter: energy is recovered at high voltage, and resonates back to the modulator at low voltage.…”

“…The bias potential varies over the length of the pulse as the inverse Marx capacitors charge up. This topology is detailed further elsewhere [4].…”