This post is my thoughts as I’m designing my graduate experiment setup. If you’ve never done anything with cryogenics, but are planning on doing it in the future, hopefully it will dissuade you from some of the pit falls that I will invariably fall into; and for those that have done cryogenic work in the past, you can see me as I walk head-long into some of those pit falls. This is by no means supposed to be a comprehensive list to follow!
Designing any scientific experiment is an acquired skill, requiring some experience before it really becomes second nature. You have to figure out what specific data is most important to you, whether resistivity of a metal, thermal capacity some new insulating material, or a myriad of other things. Then you have to start thinking of how you can best measure that, balancing time, cost, and ease of design. And once you come up with a good idea, you have to iterate, iterate, and then once again iterate until you come up with the most robust design that you can manage and gets the job done. Sometimes it’s not the prettiest thing to look at, but when it works, and moreover works WELL, then you know you’ve really done what you wanted. You can feel accomplished. I still feel that I’m at the beginning of the journey to really feeling like I know exactly what I’m doing when it comes to experimental design from scratch.
Now, designing a scientific experiment that will go down to cryogenic temperatures (below 123K) is an even harder task. Not only do you have to worry about all the items I mentioned above, but now you have the added difficulties of low temperatures, vacuum pumps and all the fittings, and long set-up times just to get it down to temperatures that you are looking for. Whether you are cooling your experiment with a closed cycle cryocooler, or using liquid cryogenics (ex. Helium, Argon, Nitrogen), you can’t directly touch your experiment anymore; one, because getting it down to those low temperatures is difficult enough, and your body outputs more than enough heat energy to really warm things up (and give yourself some serious frostbite in the process), but also because these systems often need to be in a vacuum environment to reduce convective heat transfer, so are sealed from the surrounding environment.
As someone that has had little experience with cryo-designs, those examples given above and many others must be taken into account to get a design that not only works, but works well. It’s no wonder that my adviser, Jake Leachman, told me that “cryogenics work makes very good and very thorough engineers.”
Let’s look over some of the most important details that I need to take into consideration on the cryogenics side of things:
- Test Chamber
- Cryocooler/cold head
- Components inside
- Connection to the cold head
- Radiation shielding
- Heat losses
- Supports for test cell
- Optical components
- Heat flow/Thermal Dead ends
- Integrated Average Thermal Conductivity
The list can go on and on! Over the next entries I hope to touch on many of these in more detail as I get to them in the design itself.