Accelerator Update

About the Fermilab Accelerators

Fermilab's Tevatron relies on superconducting magnets to guide protons and antiprotons around a four-mile ring. Cooled to temperatures below -450 degrees Fahrenheit, the magnet coils lose electrical resistance: they become superconducting. Fermilab operates the world's largest helium liquefier plant to satisfy the cooling demands of the Tevatron.

The History of Low-Temperature Research

In the 1860's, research into the theories of cryogenics identified the concepts of phase separation and critical phenomena. These concepts were identified long before gas was ever successfully liquefied. This early research discovered that "common gasses such as O₂ and N₂ have similar behavior when cooled to temperatures near their critical points."

By 1877 Cailletet in France and Pictet in Switzerland were both liquefying oxygen. Their work proved that when permanent gases neared the gas's boiling point both liquid and vapor coexisted (see note on triple point in the last paragraph). Further, Pictet's process of cascade cooling and heat exchanging is today's preferred method of efficient refrigeration systems.

In 1892, Sir James Dewar developed a storage container that "consisted of a glass double-walled vacuum vessel with inner walls silvered to reduce radiation heat transfer." An early schematic of a helium storage vessel showed that he used liquid nitrogen to further block thermal radiation. (Fermilab insulates its liquid helium lines with both liquid nitrogen and vacuum.)

The practical theory behind mechanical refrigeration came from a 28-year-old professor by the name of Carl von Linde. In 1870 Linde published a paper entitled, "The use of mechanical methods for the extraction of heat at low temperature." An Austrian brewery put his refrigeration designs to use immediately. Over the next eleven years a company Linde headed produced 1000 commercial refrigeration units. He continued to study the principles of thermodynamics and the work of Thomson and Joule. In 1895 Linde invented the first continuous process for air liquefaction, which allowed for the generation of bulk quantities of liquid oxygen.

Linde's liquefaction process relied on high-pressure gas and the Joule-Thomson effect. Unfortunately this process used a lot of energy.

In 1902, the French scientist George Claude developed a piston expansion engine that liquefied air by exporting mechanical work. This process was much more energy efficient and worked at lower pressures. L'Air Liquide was founded to exploit this process. Later, the American branch of this company changed its name to Airco.

What's a triple point? First let's describe phase equilibrium: phase equilibrium is the one temperature where a material coexists in either its solid and liquid form (melting point) or its liquid and vapor form (boiling point) with no phase exchange. Therefore, the triple point is where the solid, liquid, and vapor coexist in equilibrium. There is only one temperature and pressure where the triple point of a particular substance is possible.