I ran the system this morning, the x-ray cathode pulse still was unstable. Sometimes the cathode spark gap missed fire. The high voltage monitor showed the voltage was 17 kV when the spark gap fired, but it was dropped down to 15 kV after several shots, then spark gap missed fires. Please looked at the picture named voltages.jpg. Mike suggested we should cut the voltage cable input ends, he guessed that damaged cables caused the voltage decrease because of the breakdown. So we cut both cable input and output ends. We really found there were some tracks on the cable surface, please check the pictures named "input ends.jpg" and "output ends.jpg".
Unfortunately, the x-ray cathode spark gap still missed fires even I repaired the cable ends. We have not found the main point of the voltage decreasing yet. Mike guessed the 3M Ohm resistor does not work well, he asked me to check it tomorrow. But I think that the transformer may not work properly, since the voltage monitor is mearuing the transformer output. I want to measure the transformer output without connecting the thyratron firstly, make sure the voltage source is all right.