hydrogen (LH₂) and liquid oxygen (LOx) fuel tanks or systems that may be at liquid air temperatures or colder. Although gaseous hydrogen might have the right physical properties for use in LOx systems, its reactivity with oxygen precludes its use. Nitrogen is not desirable because nitrogen might contaminate the LOx. In LH₂ environments, all gases other than helium and hydrogen would freeze, clogging fuel lines and systems and rendering the rocket engines nonfunctional.

• Superconductivity. One such use is in the superconducting magnets that all medical magnetic resonance imaging (MRI) machines employ. Current materials and technologies dictate that only helium can act as the crucial refrigerant.

• Basic research. Here, no other substance can be used as a refrigerant to achieve temperatures from 4.2 K above absolute zero down to millikelvins (thousands of a kelvin).

Because price plays such a fundamental role in demand and the 1996 Act has had a profound effect on the price of crude (and therefore refined) helium, this chapter catalogues, where information is available, the existence, usefulness, and cost of substitutes, as well as the potential for recovery, recycling, and other conservation strategies in different markets. Ideally, quantitative estimates of the elasticity of demand with respect to price would be provided for these various applications but since the data are not adequate the discussion remains mainly qualitative.

Turning to consumption in the United States, Figure 3.4 illustrates domestic helium usage, by general application categories, for 1975-2008. Figure 3.5 provides the latest public data on the share of total domestic usage of helium accounted for by each application in 2007. The remainder of this chapter discusses the applications for which data have been collected, with alternatives and conservation measures provided where possible.

The largest share of helium in the United States is accounted for by cryogenic applications, which take advantage of helium’s uniquely low boiling point. Cryogenic uses range from medical uses and high-technology manufacturing to science and technology investigations in academic laboratories.