Saturday, July 16, 2005
Today the magnet reached the lambda point. For lower frequency systems, the magnet coils are cooled to superconducting temperature by liquid helium, which is about 4 degrees Kelvin, or -269 degrees celcius. For larger systems like the 900 however, the magnet is cooled by superfluid helium. This is accomplished by cooling the liquid helium with a fridge system. Essentially this is just a big coil of tubing inside the magnet can that you can flow a small amount of liquid helium into, and is pumped with a vacuum. Under vacuum, the evaporation of the liquid helium cools the coil and the surrounding helium just like the evaporation of sweat from your skin cools your body. Using this fridge system the temperature of the helium surrounding the magnet can be cooled gradually (over about 30 hours) to 2.17 degrees Kelvin. At this point, called the lambda point, the helium becomes a superfluid and behaves completely unlike a normal liquid. In the case of the magnet, the lower temperature of the helium lends stability to the superconductivity of the wire, and allows the magnet to be brought to 900 MHz. Right now the magnet is energized to about 675 MHz, and will be left to stabilize at 2.17 K for the next week before being brought to full field.