Cryogenics at NSCL

Superconducting quadropole magnet is wrapped with aluminized mylar foil to reduce heat loss. more

The main reason for operating a helium liquefier is to provide the cooling power needed to operate magnets built with super-conducting wire.

The mechanism for cooling is the same effect used when boiling potatoes. At its boiling point at one atmosphere of pressure, water maintains its temperature at 212 °F or 373 Kelvin (K). Even though the flame or heating element under the pot is much hotter, the potatoes don't burn because the water boiling away maintains constant temperature.

In the case of super-conducting wire, we use liquid helium boiling in the pot holding the magnet. The boiling helium maintains the magnet temperature at
-451.6 °F (4.5 K). At this temperature the wire stays super-conducting, and a strong magnetic field can be maintained using little electric power.

This is the reason for using cryogenic technology; magnets can be more compact and cost less to operate.

To minimize the amount of liquid helium needed, boiling liquid nitrogen is used to shield the magnet pot from outside heat. We also use liquid nitrogen in the helium re-liquefaction process.

The superconducting magnets operated at NSCL are spread out all over the laboratory. The helium liquefier is at one end of the site, and the liquid helium must be distributed to all magnets. Some magnets are operated in a mode where most of the boil-off gas is returned cold to the liquefier (it takes less energy to liquefy cold gas). Some magnets are so efficient that they only need periodic refilling, and they only return gas at ambient (room) temperature for liquefaction. Transfer lines and distribution boxes distribute liquid helium and liquid nitrogen and their cold-return gases. Three helium liquefiers generate the liquid helium. The liquid nitrogen is delivered every other day by truck which pumps about 6,000 gallons (gal) per delivery into a 9,000 gal storage tank.