uses 10 times less helium than earlier generations and can operate as long as 3 years between helium fills. Furthermore, while superconducting MRI systems are often the high-end performers, most of the growth in the current U.S. and world markets is for systems at lower magnetic field strengths, which use electromagnets or permanent magnets and require no cryogens.

Semiconductor Processing

Silicon wafer suppliers are the primary users of helium in the semiconductor industry. The process of pulling 12-in. (300-mm) crystals uses helium-cooled superconducting magnets to mechanically stabilize the hot boules of semiconductor material. Other activities in the industry that use helium are plasma etching and vacuum pumping. In the latter, liquid-helium-cooled surfaces cryopump gas contaminants. Recycling techniques could be introduced in each of these processes to reduce helium consumption.

The total volume of helium used by the semiconductor industry is unknown. Nevertheless, it is very important to this key industry. An increase in the price of helium could be accommodated, but if helium were to disappear altogether, semiconductor processing would be severely hampered.

Large-Scale Fundamental Research Requiring Helium Temperatures

The term "large scale" in the context of superconductor-based fundamental research refers to systems that are physically large (e.g., accelerators whose rings are many kilometers in circumference). Superconducting magnets and superconducting microwave and radio-frequency cavities are important components of charged-particle accelerators for nuclear and particle physics and the technology of choice for the highest energy colliding-beam accelerators. All such systems currently employ superconductors that must be cooled by liquid helium. In particular, significant amounts of helium are required to cool the magnets in large particle-storage rings (the circumferences of the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, the Tevatron at Fermilab, and the ring at the Large Hadron Collider at CERN are 5 mi (8 km), 3.7 mi (6 km), and 17 mi (27 km), respectively). The cryogenic plants used at accelerator facilities must be able to provide tens of kilowatts of refrigeration and require tens of megawatts of electric power.

In some systems, especially those using radio-frequency or microwave resonators of large size, the uniquely high thermal conductivity of liquid helium also assures that the superconducting system is kept at the uniform temperatures critical to operation. Since there is no alternative material with comparably high thermal-conductivity properties, helium is the only cryogenic fluid that can be reliably used to reach the low temperatures required for all superconducting electromagnets and radio-frequency and microwave resonators currently in use in large-scale systems. Another use of large superconducting magnets is in specialized particle detectors, and helium is also used to cool accelerator targets to low temperatures.

Like the MRI community, the large-scale fundamental research community is taking steps to increase the efficiency of its systems. The use of closed-cycle systems that reliquefy helium is now commonplace and growing. For example, the new accelerators that are coming on line are extremely efficient, with most of the gas being reused and only small amounts being lost through



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement