Helium-3 Cryostats

Abstract

The temperature range accessible with a liquid ⁴He bath is typically 4.2 ÷ 1.3 K, but this temperature range can be extended to about 0.3 K if the rare isotope ³He is used instead of the common isotope ⁴He. The main reason is that ³He has a substantially larger vapour pressure than ⁴He at the same temperature; the ratio P₃/P₄ is 74 at 1 K but about 10⁴ at 0.5 K (Figs. 2.7, 8). A further advantage of using liquid ³He instead of liquid ⁴He at temperatures below their normal boiling point is due to the fact that the specific heat of liquid ³He varies much less between, for example, 2 and 0.5 K than the specific heat of liquid ⁴He does (Fig. 2.9). One therefore has to evaporate only about 20% of ³He to cool this liquid from 1.5 to 0.3 K by using its own heat of evaporation. Furthermore, the specific heat of liquid ³He is larger than the specific heat of liquid ⁴He below 1.5 K, resulting in a larger heat reservoir in this temperature range. Finally, liquid ³He is not super-fluid in the temperature range of concern in this chapter. One therefore does not have the heat transfer problems sometimes arising from the superfluid film flow of liquid ⁴He (Sect. 2.3.5).