The Nuclear Superconduction Cyclotron Laboratory (NSCL) at Michigan State University has a long tradition of accelerator expertise, starting in the early 1960s with the design and construction of a 50 MeV proton cyclotron. This cyclotron had a very precise beam used to do detailed measurements in nuclear physics. In the early 1970s, superconducting technology had developed to a point where it was feasible to build a more compact – and less expensive – accelerator using superconducting coils at liquid helium temperature. At these temperatures, some materials lose their resistivity, and it is possible to produce high magnetic fields, necessary to contain the accelerated beams, with no power consumption.
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. Liquid nitrogen is also used in the helium re-liquefaction process.
The superconducting magnets operated at NSCL are spread out all over the laboratory. A 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.
The helium refrigeration plant cools down the superconducting linear accelerator (SC LINAC) being installed by the NSCL. The cooling medium used in the cavities is supercritical helium.
Linde’s main scope of supply consists of a cycle compressor and two cold boxes: the first cold box uses compressed ambient-temperature stored helium and cools it down in a modified Brayton process to supercritical cold gas. The supercritical gas leaving the first cold box is throttled and delivered either as liquid helium in the second cold box, also called sub-cooler, or it will pass through a coil located in the sub-cooler and be throttled after the sub-cooler, to cool down the cavities of the SC LINAC. After cooling down the cavities, the helium stream returns to the sub-cooler. The vapor phase produced during the liquefaction in the sub-cooler will be reintroduced in the low-pressure section of the first cold box.
The plant design and the turbines are designed by Linde Kryotechnik in Switzerland. The cold boxes are manufactured by Linde Cryogenics in the USA.
The new refrigeration plant shares the liquid nitrogen supply system and the warm gas storage tank system with already existing cryo plants at the NSCL.