The largest current use of helium is for cryogenics. The amount of helium used for cryogenics was about 620 million scf (17 million scm) in 1996. The main cryogenic applications to be discussed in this section are magnetic resonance imaging, semiconductor processing, and large-scale and small-scale fundamental research that requires helium temperatures. Unfortunately, no specific data are available on the quantities used for each of these applications.

It should be noted that the availability of safe, inexpensive helium resulted in the abandonment of liquid hydrogen as a coolant. In a situation of extreme shortage, hydrogen could be used as a refrigerant, with appropriate safety precautions. The lowest temperature attainable with hydrogen, however, is 9 K, which would require a change in magnet wire technology to a niobium-tin alloy or some other high-temperature material. Systems cooled only by liquid hydrogen would also not be able to achieve low enough temperatures to permit a second stage of refrigeration based on 3He. This problem might be resolved by using some of type of magnetic refrigeration as an intermediate stage. Any changes would require a large capital investment in infrastructure, however.

Magnetic Resonance Imaging

The main medical use of liquid helium is for magnetic resonance imaging (MRI) systems. Liquid helium is needed as a refrigerant for the superconducting magnets that are critical components in many of these devices. The availability of helium at a favorable price and the stability of the helium market have contributed to the rapid growth of MRI as a diagnostic tool and have allowed it to make a significant contribution to health care in the United States. There is a large, stable base of superconducting machines in the United States and abroad (>4,000), and this base is expected to grow at 15 percent per year. At least one reason for the abundance of superconducting MRI systems in the United States (approximately 80 percent of the worldwide installed base) is the ease with which liquid helium can be obtained and its relatively low cost.

There is no substitute for helium in this application. Use of high-temperature superconducting wire, while under investigation by several manufacturers, is not likely to be a viable alternative to conventional superconductors in MRI machines. High-Tc superconductors cost 500 times as much as conventional niobium-titanium superconducing wire. Although this cost might decrease in the future, both the mechanical and electrical properties of high-Tc would have to be improved before they could be used economically for this medical application.

However, even with the increasing medical importance of MRI systems, this application's demand for helium is expected to be flat or to grow only slowly. Because it is difficult for them to obtain reliable supplies of liquid helium in other parts of the world, MRI manufacturers have introduced new technologies that dramatically decrease the quantity of liquid helium required. These technologies include improved cryostats with superior thermal efficiency, cryocoolers that recondense the helium, and improved magnet wire and junctions that reduce the size and weight of materials that need to be cooled and lower the small heat load that the magnet itself generates. These systems, which often trade the low cost of electricity for the high price and relative inconvenience of cryogens, have dramatically decreased helium consumption in the superconducting MRI systems that are manufactured today. The newest generation of machines

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