While I’m on here updating every day I thought I might take the time to keep notes on daily happens on the experiment so that I can keep track of things as they happen. This is really for me and I’m not going to try to write or explain things in a way that others would better understand. So if you’re one of the few people who have absolutely so little to do every day that you come to find out how fat I am and what I’ve eaten, feel free to skip these posts and don’t feel like you missed something if you decide to read them and things don’t make sense.
Today, I installed the 2” cartridge heater on the vapor cell. This time I put a thermocouple on the lead end and let the lead end protrude from the insulation ~1/16”. On top of that it required a fan in order to run the heater with the right temperature gradient across it. With the windows at 380 C and the valve at 400 C I was able to get the sheath to 640 C while the lead end was still 265 C (thanks to the fan). The termocouple gauge never read any pressure rise most likely because the differential pumping with the long tube on the valve was to large. However, the windows very suddenly coated in soot. I was worried that this might have permanently attacked the window, but as soon as I vented the cell with argon, most of the soot was swept away. I didn’t have a chance to clean the window yet, but I’m assuming whatever is left will wipe off. I will find out tomorrow.
Either way this poses a serious problem for the operation of the cell. Every time the cell is vented, even with dry Argon, some thin oxide layer will recover the Sr and the process will have to be repeated, recoating the window. I ordered a second CF tee today in order to operate the cell more along the lines of a short 6-shooter, with the tee containing the cup on one side with a blank and the valve and the windows on the opposing tee. We could have lots of cold spots for the soot to collect when evaporating the strontium oxide and then still heat the windows to run more like a vapor cell in the long run.
In the meantime, I’m going to attempt an idea I had based on an Sr ampule I came across. The ampoule contains clean Sr with an indium cap. Once the ampoule is in or connected to vacuum, the indium can be melted (~150 C) and the Sr can then be heated to whatever temperature produces the desired vapor pressure. I was thinking we could use a similar idea to protect the windows while evaporating the Sr oxide. By tacking a sheet of lead or indium onto a copper gasket, we could protect the windows from the evaporated oxide and then melt the shield when we’re ready.
Indium foil would have cost ~$100 per window, but I could buy purified lead from Chemstocks for ~$1/lb. Initially, I attempted to melt the lead inside the copper gasket. I finally succeeded at this by taking a band heater tightened on a stainless steel blank and placing the gasket and a piece of lead on the back side of the blank. Unfortunately, it was extremely difficult to get a thin sheet of the molten lead to spread across the inner area of the gasket due to the surface tension created by a constantly forming oxide layer on the molten lead. In the end I ended up filling the whole area with lead the thickness of the gasket. I didn’t wet very well to the gasket either because of oxidation.
However, I then had the idea of pounding out the lead into the thin foil I would want. Dallin suggested we could then simply tack the foil onto the gasket with a soldering iron, avoiding completely oxidizing the gasket while making sure the lead staid well enough for installing the window. This is the first thing to complete tomorrow.
Otherwise, the pressure in the MOT continued to come down (slowly). I’m worried about the operation of the ion pump and I wonder if what’s really wrong is that we accidentally got parts of the magnet above its Curie temperature during the initial bakeout. It’s hard to tell without buying a spare magnet. I now have the new band heater for the oven and might attempt to evaporate the strontium oxide in that chamber if I have time tomorrow. Frustratingly, the 922 nm laser was behaving very finicky when I turned it on today, and was having trouble running stably at different master diode currents despite good optical feedback. I also think that coupling to the cavity has decreased, most likely due to some bumping around with I insulated the ion pump to re-bake it. I’m getting nervous as I’m running very short on time for having a cold ion source in time for August graduation. Here’s hoping for significant progress tomorrow.
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