Yeah, when doing a cold start, my uas are back to a small number. But after running an overnight scan, come back to find they had jumped again. 

The arcing event was a red herring -- it just heated up the tube, probably. 

When the tube is very warm, the filament can liberate more electrons, and easier to stay at that temperature.

So if I want to do faster scans, really important to have the x-rays actually on for a few hours. Running right now at 40 uas.