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Description
In order to generate high voltage high pulsed power, klystron modulators necessarily contain at least one capacitor bank charging structure supplying the energy to be released during the pulse. Conventional charging structures are based on AC/DC front-end units typically based on diode rectifiers combined with on/off power charging as a second stage, producing prohibitive levels of grid flicker and harmonic contents on the AC grid side while operating at suboptimal power factor; problems usually corrected by both costly and spacious external grid compensators.
Today, increased demand on both accelerator peak power and pulse length (translating into average power), in conjunction with stricter regulations and standards on flicker as well as generated harmonics augment these problems, representing additional challenges in modulator design.
To cope with this, an alternative method for capacitor bank charging implying use of a combination of a grid connected active rectifier and a dc/dc buck converter is proposed. This combination allows, first, the active rectifier to control the AC line current to be sinusoidal (reducing harmonic content) and in phase with the AC line voltage (minimizing reactive power). Second, the dc/dc buck converter is regulated in current mode for instantaneous constant power charging by measuring capacitor bank voltage droop, adjusting the current reference to match the exact average power consumed by the load, i.e. complete reduction of grid flicker is possible despite heavily pulsed loading.
This paper explains in detail the working principle behind the proposed control methodology, and provides successful power quality results detailing modulator AC grid side quantities obtained both in simulation and from experiments carried out on a klystron modulator prototype delivering long (3.5 ms), high voltage (115 kV), and high power pulses (peak power > 2 MW).
