Development of a solid-state high-voltage switch device for an insulation oil-filled klystron modulator

Although thyratrons are widely used as high voltage switch devices for pulsed power supplies, their aged degradation and short lifetime has been headache for large accelerator facilities. Especially for X-ray free electron laser (XFEL) facilities, an instability caused by the aged thyratron and a high maintenance cost of the thyratron replacement are the serious problem. In order to overcome this problem, we have developed a solid-state high voltage switch having a long lifetime for thyratron replacement. The high-voltage switch was designed to install to the oil-filled compact modulator for XFEL facility SACLA (Spring-8 Angstrom Compact free electron LAser). It should run at a 60 pps (pulse per second) repetition rate and conducts a large current of 5 kA with a 5 us pulse width from the pulse forming network circuit charged at 50 kV in maximum. We employ a static-induction (SI) thyristor as the high-voltage switch, because it has suitable characteristics for the thyratron replacement. In total, 192 SI-thyristors (24 series, 8 parallel) are used for a 50 kV switch module. Since the modulator is filled with an insulation-oil, water cooling of the device is not suitable. Hence, we attach the SI-thyristors on aluminum heat sinks forcibly cooled by oil circulation. Performance check in high-voltage operation for the switch was carried out by connecting it to the actual klystron and modulator. The switch stably run at a 50 kV charging voltage, a 5 kA pulse current and a 60 pps repetition rate. A temperature rise of the SI-thyristor is about 7 degree, which is lower enough. Validity of employing the module for the high-voltage switching is well confirmed.