I ran a 3-minute degas cycle of the RGA filament for the first time yesterday (in this process, the filament gets very hot in order to "boil off" settled particulate). I was surprised to see the pressure (of not just the RGA volume, but also the main volume) rise from 2e-8 to 5e-6 torr.
This observation, plus the rise in pressure seen every time the filament is activated for an RGA scan, indicates we have significant particulate settling on the RGA filament. This may be systematically biasing RGA scans taken within a short time (~15 minutes) of turning on the filament.
I have two ideas to reduce particulate on the RGA. However, before implementing these, I first want to consult with the vacuum experts at LIGO Lab.
- Run a second RGA degas cycle, but next time with the main volume valved off with only the RGA volume being pumped (through the bypass line). This will prevent "boiled off" particulate from entering the main chamber and will also increase the pumping rate for the RGA volume, reducing the amount of particulate that resettles on the RGA filament.
- I also noticed that the SRS manual states that the filament is designed to be long-lived and it is recommended to leave it on any time the RGA is on. By leaving the filament on all the time (i.e., hot), we could reduce the amount of particulate that is evidently settling on it between scans. I am checking whether LIGO Lab does this in their own chambers.
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