Great news today: FLUORESCENCE! I’m finally starting to figure out this vapor cell. After yesterday I was left with no other choice but to assume that we didn’t finish evaporating the oxide layer before we melted the lead shields. To avoid coating the windows while having to bring the Sr cup up to such hot temperatures I didn’t put any heat whatsoever on the valve and I didn’t insulate it outside of the band heater either. Plus the pump was still on the cell. Still with the windows at 380 C and the cup at 730 C the valve came up to ~176 C. I left the laser scanning through the cell and after a while I started to see fluorescence just above the cup. The scattering moved back in forth in space over the cup as the laser scanned in frequency.
I cooled the cup and when I came back from lunch I brought the valve up to 315 C and the cup up to 320 C with the cell sealed off from the pump. I couldn’t see any signal so I brought the cup up above 700 C again. The longer I left the cell the better the scattering got until I was finally totally absorbing the laser on good scans. I suspect that previously I had only exposed a very small surface area of Sr and that I was finally getting a large enough surface area to have a good vapor. As the laser scanned the absorption would first appear as a line across the cell then become a diffuse cloud of blue through the cell and finish as a line again.
Tomorrow I’ll see if I can get a decent vapor without having to get the cup so hot. Despite running the cell at these temperatures for the better part of the afternoon I noticed no degradation in visibility on the windows. Finally, some progress.
In other news I am experiencing major problems with the vacuum on the MOT chamber. When I came in this morning the pressure was at 5.86x10^-6 which wasn’t great but I figured it might be good enough given the junk I’d added to the chamber. So I turned on the heaters on the ion pump and packed the blank facing the oven with dry ice. The pressure dropped with the dry ice but started to come up with heat on the pump. So I started heating the nozzle and the oven back, but not the oven. Before the nozzle hit 200 C (oven back at 240 C I think) the pressure jumped to 10^-4 Torr. That scared me so I turned off all the heaters and let the dry ice run out. Back at room temperature the pressure was still greater than 10^-4. I figured it must have been the blank on the back of the oven, vented the chamber and changed out the gasket again. The old gasket looked like it had just barely sat off a little as it bore marks on one side from the sniffer port. This time I was extremely careful in tightening down the flange and was able to get it to seat evenly with the smallest gap we’ve seen on that flange yet (although still just bigger than you might normally see for some reason). I don’t know what’s up with that flange because without a gasket I can push the 2 flanges together with almost no effort at all.
When I pumped down the chamber it never got below 2x10^-4 T. Frustrated I took a heat gun and started testing copper gasketed flanges. I noticed no noticeable pressure raise on the oven back but when I put it on the blank facing the oven I got an immediate increase in pressure. At the 10^-4 T level I figured that was pretty significant so I vented the chamber again and changed out that gasket. When I pumped down again it only came down to the 10^-4 level. Argh! I was out of time so we’ll have to investigate the chamber more tomorrow. The gasket from this flange gave no hints to any problems. If we have to do leak detection it will have to wait until Monday unfortunately.
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