Things are moving along fast now. I spend the better part of yesterday working on the vapor cell while the MOT pumped down with the ion gauge hot at ~280 C. I removed the cartridge heater from the vapor cell and insulated the bottom as well as the valve. Now when the windows are heated to 380 C the valve comes to equilibrium at 217 C and the cup comes up to 304 C. I brought the valve up to 350 C which then raised the cup to 322 C. This should have resulted in a vapor dense enough to absorb almost all the light, but I could only notice the absorption by eye when looking at the edges of the beam profile. However, I did not have the opportunity to during the day to setup up a photodiode and find out what was really going on. Any noticeable absorption by eye should be enough to provide a signal to lock to however. I started lining up the lock but found that the cell was sitting slightly slanted and that the beam height on one side of the cell had to be lower than the fixed height of the pedestals allowed. I was worried about damage to the AOM, but found that it with our beam diameter of ~ 2mm going through a 200 mm lens it would take 7 mW of light to meet the damage threshold. Since at best we send 8 mW to the lock and then we split that for the saturated absorption setup, there should not be any need to worry.
Today I spend almost the entire day on the MOT, which was kind of slow since I was pretty much just babysitting it like a nervous wreck sure that something would go wrong. I turned off the ion gauge everytime the pressure hit 1X10^-4 T. This happened when I brought the oven above 150 C, and again when I brought the heaters up to 400 C. During the entire day I kept a dry ice pack on the blank facing the oven and the temperature at the blank never rose above –10 C and sat mostly at ~-20 to –40 C.
After baking for an hour at 400 C the pressure had come down to ~ 2.7x10^-5 T. I decided then to bring the oven up to ~ 700 and evaporate the oxide layer on the strontium. Unfortunately, I’m an idiot. When I bought the new heater for the oven the one that was in stock was a 240 V 400 W heater. I so I made the stupid mistake of thinking it would be 200 W at 120 V. So stupid, a factor of 2 in voltage is a factor of 4 in power. This actually works really well for normal operation because if that controller gets the switch stuck again it means that the temperature won’t run away like crazy. But for evaporating the oxide layer this is a problem. Anyway, I borrowed a 2-1 transformer from Scott Daniels and wired it to a variac so I would have some control over the heater. That did the trick. At about 670 C the pressure hit 10^-4 T and I turned off the ion gauge. The foreline pressure actually rose to about 3.6 mT before starting to come back down. I was going to wait until the forline pressure came all the back down to .4 mT but I turned the laser on for Mark and it just happened to lock on the transition. The MOT chamber BURST into light with the laser. After the laser hopped I started it scanning.
There was obviously A LOT of Sr floating around the chamber so I turned off the oven and cooled everything down. To keep the blank cold while I was at Ben’s baseball game I put a cardboard box around the flange filled with dry ice. I wasn’t sure how long it would last because the sits right next to the ion gauge which is effectively a heat source, but it stayed cold until I was prepared to vent the chamber.
There was one unintended consequence. The window just to the left of the blank has a direct line of site from the nozzle and was clouded by strontium during the run. Luckily, we had one extra window and I swapped it out while the chamber was vented. The window to the right did fine because it sits on a longer arm and doesn’t have line of site.
I can’t understand what happens, but just like with the vapor cell the black oxide gunk we burned off, of which there should have been a ton, disappeared when I vented the chamber with argon. I can’t figure out what happens to it. Before I vented the chamber the pressure had come down to 2x10^-7 T again which is a good sign. After putting the slower window back on and changing out the clouded window I started pumping down again. I don’t like the duniway gaskets because they down allow the the flanges to press as close together as it feels like they should when the bolts are to torque. Nevertheless, the seal must have been good because the pressure came right down into the 10^-6 T range. I turned the heater on the pump back on and we’ll see what things look like in the morning.
I didn’t heat the vapor cell today but just worked on the beam alignment. While I was working on the MOT Mark tried to straighten out the vapor cell tilt. He did okay, it still lilts to the one side but its workable. Anyway, I was able to align the laser to the AOM and finish out the saturated absorption alignment. Tomorrow if things go very well we’ll see exactly what kind of signal we get without a cartridge heater and maybe even finish out the lock. If things go EXTREMELY well we might even make our first ions, although I doubt I’ll get the MOT alignment and the lock done with enough time left to do that.
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