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LTLT
A cylinder made of glass reinforced nylon (GRP). ID=40, OD=50, IL=80mm. I did a few tests at high pressure: first cell broke at 13 bar, second one at about 2.5 bar. Then I realised that this happens because of too fast cooling. The third cell survived 35 bar, the forth cell is the final make.
The cell #4 is made of a tube and two cups with threaded connections (assembled with Stycast-2850FT). I also wrapped it with fishing line to make walls more ridgid (not sure how useful it was).
Heat switch is a piece of aluminium inside a magnet. When magnetic field is off it's superconductive (low heat conductivity) when field is on it's normal (high heat conductivity).
An important parameter of a heat switch is resistance in normal state - small resistance means good thermal conductance. We use pure aluminium and pure annealed silver. It's not trivial to make a good contact between aluminium and silver: one should avoid both aluminium oxide and mutial diffusion of metals at the interface. There are many methods how to make the contact. In Lancaster spot-welding is used. Silver wire is heated by graphite electrode and pushed into aluminium. The process is fast (~1s) and mutual diffusion of metals is small. Best heat switches have resistance about 10 nOhm, bad heat switches - above 10 uOhm. This one is about 2 uOhm/wire, the bottleneck is thin silver wires.
The heat switch is done as a pair of discs connected with a cylindircal leg. This is done to avoid direct path between silver wires and reduce possible heat transfer along vortices in superconductive state. The final version has 6 cuts on the top disk and 8 cuts on the buttom one, to separate all wires (we want to disconnect cells from each other) and reduce eddy current heating when changing magnetic field.
To transfer heat from silver wires to the mixing chamber of dilution refrigerator we use heat exchangers made of sintered silver powder. Silver powder is pressed in a form and then quickly heated to ~150C to sinter silver particles. Higer temperature or longer heating results in degrading of surface area. One heat exchanger contains 6g of powder and has 25-30 m^2 area.
1mm thick annealed copper plates covered with 0.1mm layer of silver sinter.
Two paper+Stycast boxes with 16 nuclear stage plates in each. Aegogel sample in the cell #1. Each cell contains three vibrating wires: 127um tantalum wire, 4.5um NbTi wire, 0.45um NbTi wire.
In this cell I wanted to play with copper NMR thermometers (and replace copper with platinum later, if everything works). I made four copper probes, RF coils, and NMR solenoid. The solenoid was located in a gap between two niobium shields. Unfortunately it's very hard to get homogeneous field in this configuration (it's too sensitive to exact dimensions of the coil system). I found CW NMR line from copper, but it was too wide. NMR solenoid was used later to protect heat switch Nb shield from strong field of the main magnet.
Probes are made of high-purity annealed copper to avoid quadrupole intrinsic field (annealing is not that important for Pt) and to increase thermal conductivity. Thin foils are used to reduce skin effect and to decrease eddy current heating by RF field.
Between spring 2021 and spring 2022 we found a fixed a few leaks in the dilution refigerator and the cell:
After sucsessful experiment in April-May 2022 the cell has exploded because of a fault of a power supply.
