A Compact Oil-Filled Modulator for C-Band Klystrons

We have been developed a compact, insulation oil filled modulator for the X-ray free electron laser (X-FEL) project at SPring-8. The modulator supplies 114 MW of peak power at a 60 pps maximum repetition rate. The first prototype was constructed in March 2003, and a second was constructed with minor-improvements in January 2005. We obtained the expected design performance: PFN charging voltage up to 50 kV using an inverter-type high voltage power supply; a -22 kV pulsed (4.5 musec FWHM) output voltage at a 60 pps repetition rate. The modulator drives a 1:16 step-up transformer to deliver -350 kV to a 50 MW C-band (5712 MHz) klystron. Over the output 2.5-musec flattop, repeatability and variations in flatness and of the pulsed voltage is less than 0.5% (p-p).