Development of a 50 KV solid-state switch for an oil-filled klystron modulator in SACLA

In the X-ray free electron laser facility SACLA (Spring-8 Angstrom Compact free electron LAser), 79 klystron modulators are operated as high-power rf sources for an electron linac. Since the typical lifetime of a thyratron is short for about 4 years, its high-maintenance cost is 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 device of our modulator should run at a 60 pps repetition rate and conducts a large current of 5 kA with a 5 μs pulse width from a pulse forming network circuit charged at 50 kV in maximum. We employ the static-induction (SI) thyristor as the high-voltage switch, because it has suitable characteristics for the thyratron replacement; a high off-state voltage, large pulse current capacity, a fast switching time, and a low conduction loss. In total, 192 SI-thyristors (24 series, 8 parallel) are used for a 50 kV switch module. Since our modulator is filled with an insulation-oil, water-cooling of the device is not so easy. Hence, we attach the SI-thyristors on aluminum heat sinks forcibly cooled by oil circulation. Performance check in high-voltage operation for the high-voltage switch was carried out by installing it in to the actual klystron 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 installed in the high-voltage switch is about 7 degree, which is low enough and corresponding to the acceptable power loss. Validity of employing the module for the high-voltage switching is well confirmed. (author)