Design, Development and Performance Evaluation of HV Energy Storage Capacitors for PFN-Based Klystron Modulator Applications

The high power klystron modulators and microwave systems are the workhorse for the linacs. Linacs find applications in industry, medical and research field. In industry, electron linacs are mainly used for irradiation of food products for improving the shelf life, medical sterilization and radiography. All these applications demand reliable subsystems having high Mean Time Between Failures (MTBF) and low mean time to repair (MTTR). Most commonly, the linacs are powered from a klystron-based high power pulsed microwave systems. The pulse forming network (PFN) based klystron modulators are the most common type of modulators used for biasing the klystron amplifier. The capacitors used in the PFN are required to be capable of simultaneously handling the high voltages, high peak currents, high RMS currents and high pulse repetition rates (PRR). The energy storage capacitors manufactured by the Indian industries have limited life compared to imported capacitors. Additionally, in the modulator system developed at RRCAT using these capacitors, high temperature rise and failures were observed during their operation at pulse repetition rates of 300 Hz at rated voltage and current stress. The cause of temperature rise and failure was analysed by thermal simulation, carried out using ANSYS. The temperature rise and failures were majorly due to improper selection of dielectric material, manufacturing process and packaging. For improvement, low loss polypropylene material has been chosen as a dielectric material. Improvements in manufacturing process and packaging were carried out after discussion with Indian manufacturers. A 40 nF, 40 kV, 400 Hz polypropylene dielectric-based capacitor has been designed and manufactured in Indian industry. The ESR and performance of the capacitor have been tested using indigenously developed test stand. The ESR value of similar to 100 m Omega has been achieved with the improved design. The temperature rise of similar to 4 degrees C was achieved when subjected to 21 kV, 600 A peak current at 400 Hz pulse repetition rate (PRR). The background, design, construction, measurement and performance qualification will be discussed in the paper.