Helium Sorption Fridge
Sorption fridges operate like rechargeable batteries -- there is an "on" time when the temperature is at a lower temperature (say 0.8K) and an "off" time while it is recharging and not at 0.8K. The sorption fridge relies on evaporative cooling and has three components: the pump, condenser, and evaporator.
The pump contains an absorber, which can be used to trap helium gas or expel it into the condenser. Helium gas is adsorbed if the absorber is cooled down, and is expelled if the absorber is warmed up. Cooling is performed by using a heat switch connecting the pump and condenser, which is connected to the 4K stage and is actuated by applying a small current to the heat switch. Warming up the absorber and expelling the helium into the condenser is performed by using a heating element connected to the absorber.
The condenser's purpose is to condense helium gas into liquid helium into the evaporator. The condenser is directly connected to the 4K stage of the cryocooler and requires that the 4K stage have a temperature that is below 4K.
The evaporator, also known as a 1K pot or still, consists of a volume that can hold a reservoir of liquid helium. If the pump is configured to adsorb helium, the helium in the evaporator will evaporate and drop the temperature to below 1K.
The benefits of a helium sorption fridge are:
- No superconducting magnets
- Simple: only active parts involved are low current heaters
- Fast charging: get an on-time of 12+ hours (or many days) and recharge time of 1-2 hours
- Can be easily scheduled via software so that the system is kept at 0.8K during work hours on a daily basis
- Does not require any special external hardware or continuous cryogenic gas supply to operate
- Can enable superconducting devices that require lower temperatures to function
- Lower cost and more reliable than other sub-Kelvin technologies, especially if you only need to go to ~0.8K
- Reliably provide <0.8K, even if the col-head temperature is near 4K
- Increases time between periodic maintenance on cold-head by 3-4x