1. Helium-4 or Helium-3 evaporation

1.1 Physical principle

Evaporative cooling is undoubtedly the most widespread technique: it's a universal method, practiced by all human beings and many animal species. In fact, it's the same physical mechanism that enables us to regulate our body temperature: perspiration. In this case, the passage of water from the liquid to the vapor state requires energy; in the absence of an external source, this energy is drawn directly from the thermal agitation of atoms and, consequently, this process induces cooling. This property extends to virtually all fluids and, when used with helium, enables temperatures below 1 K to be achieved. Obviously, to improve process efficiency, the evaporation rate is increased by pumping. The physics involved are linked to the states of the fluid, which in...