Click here to view “Work below 300mK”, a video about how the GL7 works.
The GL7 has a mainplate that interfaces to the cold head of a mechanical cryocooler. The GL4 has just one module, whereas the GL7 has two. The larger of the modules contains Helium-4, and the smaller module contains Helium-3. The GL7 exploits the different properties of these two isotopes; most importantly, Helium-3 liquifies at a lower temperature than Helium-4.
In the GL7, we use the Helium-4 module to cool the Helium-3 module. Both modules are connected together through thermal links. To get the Helium-3 module cold, we start by cooling the Helium-4 module. (For an explanation on cooling the 4-module, please view our work below 1K .) Once the 4-module is cold, the temperature of the head will be just below 1 Kelvin.
With the Helium-4 module is cold, we can start a similar process to cool the Helium-3 module. We heat the pump of the 3-module to release the Helium-3 gas adsorbed onto the charcoal. The gas is cooled by the thermal links to below the Helium-3’s liquefaction temperature. As the Helium-3 condenses into liquid, it collects in the head of the 3-module.
When we turn off the power to the 3-pump and turn the heat switch on, the pump cools down to the same temperature as the mainplate. Meanwhile the liquid Helium-3 in the head starts to evaporate, and this evaporation cools the 3-head down to around 300 millikelvin. The Helium-3 module will remain at this temperature until all the liquid Helium-3 has evaporated and returned to the charcoal in the pump. To re-cycle the GL7, one turns off both heat switches and warms up both pumps. The process will no be ready to start again.