Cryogenic Gases

Related Films
The following related films are available in our free Lectures on Superconductivity

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.


The Future of Helium as a Natural Resource

Liquid helium is very widely used as the coolant for low temperature superconductors such as NbTi and Nb3Sn. In recent years, diminishing reserves and rising demand have resulting in increasing prices and supply shortages. There is therefore a growing motivation to use alternative cryogenics solutions, potentially with alternative superconductors. For MRI and other LTS magnet applications, zero boil-off systems (with a cryocooler for helium recovery) are increasingly common. Liquid hydrogen is also under consideration for cooling MgB2 and HTS in some 20-25K applications, either directly or using helium in a closed heat transfer system.

Work has been performed in collaboration with the Judge Business School and the Open University to quantitatively analyse helium costs and reserves. Our work in this area has been published as a book and a Nature comment:

William Nuttall, now at the Open University, has also released the video below:


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.