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.
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