Development of filter for thyratron trigger circuit

In the XFEL facility SACLA, more than 70 thyratrons are used as a high voltage switch device for an oil-filled line-type modulator. Four years from the start of the SACLA's operation has elapsed, so that failure is recently increasing around the G1 and G2 trigger-circuit of an aged thyratron. The cause of the failure is a high-frequency surge generated at the G1 and G2. The huge surge with a voltage of several kV hits and breaks parts, such as a surge absorption diode in the circuit. To mitigate harmful effects form the surge to the parts, we focused on the frequency difference of 40 MHz between the G1 and G2 trigger-signal and the surge. They can be separated in the frequency domain, because of the frequency difference. A filter circuit is designed and added to the G1 and G2 circuit and successfully separates the trigger signal and the surge. The role of the filter circuit is to absorb the high-frequency surge from the thyratron and to conversely transmit the trigger signal form the circuit. In a test, the filter circuit effectively absorbs the surge and reduces its voltage from 4 kV to 1.4 kV. We finally confirm the TVS diode dose not suffer from any harmful effects by the surge. (author)