Kind of a frustrating day today. I finished alignment of the 461 nm beam throughout the entire lock setup. I always forget how long alignment can take the first time it’s done. That took me the first part of the day. While I was doing that I brought the slower window up to 375 C, the nozzle and back up to 360 C, and the oven up to 250 C.
The pressure in the MOT peaked at ~2x10^-5 T and started coming back down. It only came down to about 2x10^-6 T which I thought was pretty good but not good enough as will soon be seen.
After the aligning the laser, I put the saturated absorption signal on a photodiode to look at on the o-scope. However, there was so much noise on the laser that it was impossible to see any sort of absorption signal happening. While I was doing the laser alignment I played with the rf power for the lock a little bit. I took a 2 dBm attenuator from the amplifier input and put it on the Vmod output of the box that goes to the phase detector. This made the signal going to the AOM at 6.9 V rms and just under 1 watt (1 watt is 7.07 V rms), and the input to the mixer is now 544 mV rms and just under 8 dBm where the mixer is spec’d at 7 dBm and has a damage threshold of 15 dBm.
I looked more closely at the laser and could see that the beam was visibly bouncing around a little bit. It was also easy to see this on the farthest beam point in the MOT alignment. I tracked it back to the doubling cavity box. I’m pretty impressed with my lock circuit that it could keep the laser locked at all with that kind of mechanical noise affecting the cavity. However, it’s not going to fly for running the experiment.
I think the problem is the pump on the MOT chamber at the moment. I put my hand on the top of the doubling cavity box and the throbbing of the pump was noticeably moving the whole enclosure. I’m pretty sure the piezo mounted mirror is shaking up and down with all the vibration.
Still, I noticed that with the mot at the temperatures above and the laser scanning across the resonance there was strontium fluorescence in the mot chamber. This was exciting for at least one reason: we don’t have to worry about evaporating another thin oxide layer off the strontium. However, because of the temperatures it probably means that when we did evaporate the oxide we managed to turn out nozzle into a source itself, which is bad because it has direct line of sight to the new mirror I just put in.
This is also a bad sign in terms of the vapor cell because if I could see fluorescence with those modest temperatures on the MOT, why then is it barely noticeable in the vapor cell? Argh. So I think that maybe there is a major cold point at the valve. The valve comes right out across the flange top into the air and by the time it gets to the knob its cold enough to touch for opening and closing when the band heater is at 400 C. I’m going to slip a thermocouple in there to see what the temperature is when the valve is hot. Hopefully, we don’t have to put the cartridge heater back in the cell now that I’ve got it in the setup. I put the valve up to 380 C at the heater and still there was no significant change in the absorption. I’m really confused by this, maybe we have to run it with the valve cold and the cup really hot.
Anyways, until I can turn off the pump on the MOT I’m stymied on the laser system. When I cooled down everything but the ion pump the pressure only came down to 1x10^-6 T. Very depressing. I’m going to have to bake out the MOT chamber entire again. I started preparing the chamber to heat but couldn’t get it done to bake out over night. I did manage to get all the heaters installed but couldn’t get the setup insulated or wired. Instead of trying to force the plate heaters on the top and bottom of the chamber again where they don’t really sit right and can unevenly heat the viton sealed windows. Instead I machined plates that could fit over the top and bottom window ports. I then used the double sided high temperature tape to coil heat tapes on the top and bottom of the chamber main body. The aluminum should help prevent hot spots from occuring and burning out the tape.
First thing tomorrow is insulate and get the chamber under heat. It’ll probably take way too long to get all that down. I can also go ahead and align and tune the ionization laser and maybe even the ion fluorescence laser. I had better figure out if I’m going to want the cartridge heater on the vapor cell before I do that last though.
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