Conduction and Specific Heat capacity
Conduction:
K is the thermal conductivity
Specific heat:
A lot of our iterations had plays on these equations. But ultimately we wanted to maximize the left of the conduction equation for cooling the sample. And minimize the left side when trying to keep heat out. And keep the mass small so there is not as much heat to cool.
- Max
- Copper puck smaller (less mass to cool)
- The braids shorter and thicker (increase in A, decrease delta x)
- Min
- Printed with Peek holder and many iterations (lower k)
- Arrest wires to cryo head
Convection
Heat can transfer through convection. Air becomes less dense as it is heated up and therefore has more buoyancy. In a Vacuum this does not happen. It is often times a cause for concern because things that typically don't get hot in air can get hot in a vacuum. Nothing to carry away the heat.
But for this chamber there is nothing to carry the heat so that is good for keeping the cold. The Rayleigh number can help predict the convection process. There is turbulence involved so it can be a very messy and sometimes a purely analytical analysis.
Radiation and Black Body Radiation
Radiation is the transfer of heat through photons, beta decay, neutron emission, alpha decay, etc. With this chamber we are mostly worried about IR wavelengths as the chamber walls are cool to our touch but hot in relation to the cryo cooled sample.
Black body radiation is radiation that primarily caused by most things. Blackbody radiation is from an object or system which absorbs all radiation that shines upon it and re-radiates energy which is characteristic of this radiating system only, not the stuff it is absorbing. The sun is a black body, it emits light because of the fusion inside but will absorb all light that shines on it. The moon is not.
We want our system to be around 10K with 4.9K on the cryo finger. The chamber is at roughly 300k. P absorbed by the sample would be ~-(300^4 - 10^4). This is a huuuuge number! We have a trick though and can wrap our system in stuff that is highly reflective of these wavelengths.
Adiabatic Cooling (Closed loop cryostat)
Adiabatic cooling is where no heat is gained by the system, the helium. The helium is heavily compressed, work is done on it by driving a piston. The liquid is then let into a low pressure environment in with the liquid expands and cools the cold finger. Then rinse and repeat.
PV=constant there is no heat gained.
More quick info here: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/adiab.html