Cryogenic Gases

A range of different compounds may be used as the working fluid in a cryocooler. The boiling point of the compound indicates the system operating temperature.

Nitrogen (N₂) and Helium (He) are most commonly used in superconducting applications. In order for a material to superconduct the cryocooler operating temperature must be below the superconducting transition temperature for the operating conditions (applied magnetic field / electrical current).

Figure 1: The upper critical flux density versus temperature of Nb-based conductors after L. T. Summers and J. R Miller, IEEE Transactions on Magnetics 23 (1987) 1552

Figure 2: Irreversibility curves for a range of different superconducting compounds.

As can be deduced from the figures, nitrogen is used with HTS conductors such as YBCO and helium is used with LTS conductors such as the Nb-based conductors and, currently, with MTS conductors such as MgB₂.

Hydrogen

The high cost of liquid helium cooling means that using hydrogen (boiling point = 20 K at atmospheric pressure) with MTS conductors such as MgB₂ would greatly reduce the application cost of these materials. Pressurised hydrogen is also suitable for use with some Nb-based conductors under low applied magnetic fields. Work on the preparation of hydrogen gas by photocatalysis is underway in the Applied Superconductivity and Cryoscience Group.