The Cooldown Cycle of a Dilution Refrigerator
Bringing a dilution refrigerator from room temperature to base takes a day or more, proceeding through precooling, condensing, and circulation phases.
A staged descent
A dilution refrigerator does not reach base temperature instantly. Cooling a large cold mass through a factor of thirty thousand in temperature, from about 300 kelvin to ten millikelvin, is a staged process that typically takes one to two days. Understanding the phases helps operators diagnose problems and plan experiments around the long turnaround.
Precooling to 4 kelvin
First the cryostat is evacuated to remove air, which would freeze and conduct heat. Then the pulse-tube cooler pulls the outer stages down over many hours from room temperature to roughly 50 kelvin and then to about 4 kelvin. Because the cold mass has large heat capacity at higher temperatures, this first stage takes the longest. Some systems add exchange gas temporarily to speed heat transfer, then pump it out before going colder.
Condensing the mixture
Once the system is at 4 kelvin, the helium-3 / helium-4 mixture is admitted and condensed at the condensing stage. As liquid forms and fills the mixing chamber and still, the system cools below 1 kelvin. The mixture must be condensed carefully to avoid excessive pressure and to ensure the phases form correctly.
Circulation and base temperature
With liquid in place, the still heater is turned on and circulation begins. Helium-3 starts crossing the phase boundary in the mixing chamber, and the temperature falls through the still range toward tens of millikelvin. Over the following hours the heat exchangers reach their steady profile and the system settles at base temperature, limited by residual heat leaks. Warming up reverses the process, and the mixture is captured into storage for the next run.
- Full cooldown typically takes one to two days
- Evacuate, then pulse-tube precool to about 4 K
- Condense the helium mixture at the condensing stage
- Start circulation to descend to base temperature