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In order to protect a klystron against severe damage due to internal arcing. The crowbar interlock must short-circuiting the klystron power supply within <100 µs in case of an arc. Arcs are detected by fast rising klystron current.

Experiences, especially made with the HERA rf systems, have shown that the crowbar system for klystron protection produces a high percentage of the transmitter trips. Just few of them are really caused by klystron arcing. Most of them are generated by any electromagnetic distortion somewhere inside the high voltage cabinet. Due to the lack of diagnostics the origin could not be found in many cases. In order to overcome this problem a more reliable crowbar ignition system was developed. Instead of one single current sensor M4 inserted into the plus line between klystrons and capacitor-bank, four current monitors were installed as shown (M1- M4). Their signals are analysed by the Crowbar Ignition Logic which based on ALTERA FPGAs. At least 2 of 3 current sensors in a klystrons current path must measure an overcurrent within a 2 µs time interval in order to release the crowbar trigger. Sensor M5 is used to verify successful crowbar firing. If the crowbar ignition logic recognizes a failure of one of the Pearson current sensor signals the restart of klystron high voltage is blocked until troubleshooting has been manually confirmed.