We withdrew the X-ray gun even it had been taken out last month. I opened the back plate and removed the back insulator. It looked very dirty, and the measured resistance was less than 6M Ohms. Undoubtedly the back bushing is the key to make the x-ray anode trigger failure. It's incredible that the back bushing bacame black dirt after running a month. I think it may be caused by the surface grease when we installed it? By the way, the Viton O-ring 348 to sealthe epoxy block and aluminium plate looked coarse, maybe this caused gas leaking? When I removed the x-ray cathode bar, I found one of the binding nuts used two o-rings for sealing, maybe this carelessness caused the leakage?
We polished the epoxy bushing using the scouring pad and cleaned it with Kimberly wipers and ethanol. After all the resistance became larger than 50G Ohms.
Otherwise, the resistance between cathode and ground became 1.6M Ohms, it's very low. So we removed the cathode bars to clean them again, make sure the cathode is open to the ground.