I tried to match the seed laser beam and the Prometheus ASE. The Prometheus ran in the good condition when Thyratron trigger and X-ray Anode trigger was set by 1.78 us and 1.22us earlier than Railgap trigger respectively. So the time delay between seed pulse and Prometheus could not be set less than 1.78us. I tried to adjust the time delay from 1.8us to 1.5 us (as shown in the pictures). I found when time delay was 1.5us, two pulses were overlapped, but the thyratron current wave forms indicated the Thyratron had no time to totally turn off.
The best solution is to add an extra time delay in the front end.