A constant power capacitor charging structure for flicker mitigation in high power long pulse klystron modulators

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 controlled power charging structures 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, the increased demand on both accelerator peak power and pulse length (translating into higher average power), in conjunction with stricter regulations and standards represent additional challenges also in modulators' design. An alternative method for capacitor bank charging, implying use of a combination of a grid connected Active Front End (AFE) and a DC/DC buck converter is proposed. The AFE controls the AC line current to be sinusoidal (reducing harmonic content) and in phase with the AC line voltage (minimizing reactive power). The DC/DC converter is regulated in current mode for instantaneous constant power charging by measuring capacitor bank voltage and adjusting the current reference to match the exact average power consumed by the load over a pulse repetition cycle, allowing in steady state for complete reduction of the grid flicker despite the heavily pulsed loads. This paper explains in detail the working principle behind the proposed power electronic structure and associated control methodology, and provides successful power quality results obtained both in simulation and from experiments carried out on a klystron modulator prototype delivering long pulses (3.5 ms), high voltage (115 kV), and high pulsed power (peak power > 2 MW)