I pulled apart the cell to clean it and create a better vacuum system. When I pulled the strontium out of the cell several pieces were dark red, most pieces were black, and a small surface area was clean Sr. Also, it was easy to notice that the along with the pad of strontium that had been sprayed on the cold tee there was a ring of black and red as well. I immediately submerged the Sr from the cell in our remaining dodecane. I cleaned the cell out with nitric acid. For an interesting story regarding that check this post. We have been concerned that our “glove sink” doesn’t do a good enough job evacuating oxygen from environment so this time I used a bag over the top of the sink. I fit the bag so that it could line the sink then I filled it with argon and deflated it again twice to get a good Ar environment. Then with positive Ar pressure I cut holes in the bag for my hands and vision. I took only the cleanest pieces of strontium and tried to scrape them off even more before depositing them in the chamber.
I ended up only using a fraction of the total Sr I initially had in the cell. This time I put the sapphire viewports on the cell. The idea is to run in a true vapor cell configuration but be able to evaporate the black colored (still don’t know what it is) and oxide layers from the Sr without coating the windows. I put the cell back on the pump with the chamber evacuating opposite the valve as opposed to through it. I put a blank on the valve end and heated the the valve assembly as well as part of the bakeout.
In the first phase I heated the viewports to 430 C and the rest of the CF flanges to ~330 C. Once the pressure had reached ~2x10^-7 T I cooled the cell so that I could unwrap the tee area between the viewports. I disconnected the heater on the flange joining the tees as well and brought the viewports back up to 430 C.
I then turned on the cartridge heater to ~715-720 C while using freeze spray and ice cubes to try to cool the chamber wall facing the cup. Eventually I realized that this was probably not necessary and the chamber is now sitting there in this state.
The pressure jumped to ~4.6x10^-6 T when I turned up the cartridge heater. Since there is not a lot of strontium to fill the cup most of the vapor will be directed down the tee and deposited in the vapor cell while only a small amount will be lost to areas outside the chamber. This is also true of the oxide layers, however, the cell will never again reach temperatures high enough to make them active. Right now the pressure in the cell is only down to 3.8x10^-6 T so there is a way to go before I’m going to turn it off.
A couple of outcomes are possible: 1) despite there being no direct line of sight to the viewports they still end up coated. 2) the pressure never comes all the way back down in a reasonable amount of time because the oxide layers are so thick. 3) all of the Sr and oxide layers are transferred from the cup to other parts of the cell and vacuum system. 4) the oxide layer is evaporated and a significant amount of Sr still remains in the cup.
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