The Still
The still selectively evaporates helium-3 from the dilute phase, driving the osmotic circulation that keeps a dilution refrigerator running.
The pump that moves helium-3
The still is the stage that makes a dilution refrigerator a continuous machine rather than a one-shot cooler. It operates near 0.6 to 0.7 kelvin and is connected to the dilute phase of the mixing-chamber circuit. Its job is to remove helium-3 from the dilute phase so that fresh helium-3 must flow in to replace it, sustaining the flow across the phase boundary in the mixing chamber.
Selective evaporation
At still temperatures the vapor pressure of helium-3 is far higher than that of helium-4, even though helium-3 is the minority component of the dilute liquid. A room-temperature pump connected to the still therefore removes a vapor that is roughly 90 percent or more helium-3. This selective distillation is the mechanism that concentrates helium-3 in the returning stream.
The osmotic drive
Removing helium-3 from the dilute phase at the still lowers its concentration there relative to the mixing chamber. The resulting osmotic-pressure gradient drives dilute-phase helium-3 up from the mixing chamber toward the still, while concentrated helium-3 flows down to cross the phase boundary. The still is thus the engine; the mixing chamber is where the useful cold appears.
Film burners and heaters
Superfluid helium-4 creeps as a film along walls and can carry unwanted helium-4 to the still, wasting pumping capacity. A restriction and a small film-burner heater near the still outlet suppress this creep. A controlled still heater also sets the helium-3 circulation rate, which directly determines the cooling power available downstream.
- Operates near 0.6-0.7 K
- Preferentially evaporates helium-3 due to its higher vapor pressure
- Sets the circulation rate via a controlled heater
- Film burner suppresses superfluid helium-4 creep