Good news: what I though was a leak wasn't actually, so I'm in good shape. Andrew Zwicker (the director of the lab and my mentor's boss) came by to take a look, along with a visiting physicist who works in exactly my field of dusty plasma, and they pretty much laughed at my attempts to find a leak, since the speed at which I was depressurizing the chamber now was better than great. The reason I was seeing a pressure rise when I turned off the pump, and the reason it got worse after I had opened up the main window, was because of outgassing: water vapor had condensed on the walls, and was slowly becoming water vapor again and increasing the pressure. When I opened up the window and kept it open for about an hour, water vapor had had lots of time to get into the chamber, and that's what I was seeing now. I'd read about outgassing when researching vacuum leaks, but didn't realize that water vapor would have such a large effect. It's called a "virtual leak"; it looks like the chamber is leaking, but it's really just a matter of gas forming on the inside.
So I don't have a leak, and I have a good vacuum, but I don't have a working probe. Yet.
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