Reaching 2K using Liquid Helium

Basically, there are two methods by which one can reach 2K or low using Liquid Helium. The first one is by pumping over the liquid bath. An evacuation pump of suitable pumping speed is used to reduce the pressure inside the container containing liquid helium. This method is also called “saturated temperature” type. In this method, if the pressure is constant, temperature of the liquid also remains constant due to latent heat of evaporation. Change in heat load only results in higher boil-off. In this method, the stability of temperature is directly linked with the stability in pressure over the bath. This method is normally used to meet the temperature requirements in case of Superconducting RF Cavities.

In the second method, a portion of liquid helium at 4.2 K is separately expanded through an J-T valve, and using heat exchangers the remaining liquid is cooled. In this method the liquid helium is in “Super Cooled state”. Special cooling properties of Liquid helium below 2.17K viz. Super Fluid properties – property to flow through fine capillaries and large thermal conductivity are utilized, at the same time the bulk of the liquid in the cryostat remains at pressure above atmospheric pressure. Normally, this method is used to cool Superconducting magnets operating in 2K region. This method relaxes conditions for system integrity of the peripheral systems, i.e. in case of very small leaks helium gas can leak to atmosphere. Limitations of this method are: in case of heat load variations, the temperature of the liquid will also vary or poor temperature stability. The first case has very good temperature stability due to involvement of latent heat, but small leaks in the system will result in air and moisture entering the system, thereby limiting the reliability or operation time of the system.