The CF cryostat is a lovely piece of kit for quickly measuring samples down to 4.2K. Here is a picture:
The dewar containing the liquid Helium can be seen on the left. The setup is rather simple, you have a gas pump, a thin transfer tube (the silver tube joining the dewar to the cryostat) and the sample space (inside the brass-coloured cryostat body). When the pump is turned on, it draws helium from the dewar through the thin tube and the sample space and returns it to a pipe on the wall (part of the overall in-house helium gas system installed in all the labs). Because the transfer tube is thin, the liquid Helium comes through quite slowly (you can adjust the flow rate) and a single dewar of liquid Helium (which holds ~45L) can therefore go a long way.
The electronics to control the experiment can be seen on the rack of equipment to the right in the photograph. This particular setup allows current-voltage (IV) characteristics and differential conductance (dI/dV) measurements to be performed on Josephson Junctions, with a PC controlled DAQ (data acquisition) system. A patch panel on top of the experimental insert allows up to 20 DC lines to be chosen for the measurement, so that many junctions to be tested in the same run.
Controlling the temperature on this type of cryostat is notoriously tricky. The generally accepted method is to run the pump to cool the system, whilst simultaneously heating the sample until the temperature stabilises at the required value. Which is a bit wasteful, it’s like running the heater and the air-con in your car at the same time. It can be done automatically with a PID temperature controller. Another way to obtain a measurement as a function of temperature is just to record data whilst the system cools down to 4.2K whilst the pump is running, or warms back up to 300K after the pump is switched off. Doing this on the warming cycle is slightly more stable. Additionally you don’t have the additional electrical noise of the pump if you are trying to conduct a low-noise experiment. This is a good way of obtaining Resistance-versus-Temperature (RT) measurements, which allow you to see the point at which your sample/junction goes superconducting. From this data you can also obtain the Residual Resistivity Ratio (RRR) which gives a measure of the quality/purity of the material being used.
A couple of thermometers are usually placed near the sample to give a good idea of the temperature gradient inside the cryostat. If there is a high temperature gradient near the sample, the temperature you read on your thermometer (which is generally a few cm away) might not be the exact sample temperature.
One of the best features of the CF is that if you run the pump for long enough, once the temperature inside the cryostat gets down to 4.2K, liquid Helium starts to collect at the bottom. Once you have collected some, you can turn off the pump and the system will stay at 4.2K until the collected liquid has all boiled off. This is a nice way to ensure that your measurement stays at a definite temperature for about an hour. Of course we don’t turn the heater on when we have liquid Helium in the bottom of the cryostat, or else the top of your experiment blows out and hits the ceiling as the Helium gas tries to occupy 700 times its liquified volume 🙂
With Niobium based Josephson junctions (Tc~9K), 4.2K is a good temperature to take measurements of their superconducting IV curves.
Why would you want to measure IV curves? Well, that’s another post 🙂