Helium Free Magnets and Research Systems

Abstract

Cryocoolers operating at 4K and lower temperatures permit research systems and superconducting magnets to operate entirely without liquid helium. We describe the production of magnets that are directly conduction cooled and operate in vacuum as well as arrangements for cooling samples in a controlled environment from 300K to 1.6K using only one single cryocooler.

Many designs use the 4K cryocooler to recondense liquid helium but these are different from true Cryogen Free designs. The magnet or system cryostat is relatively conventional in design with a liquid helium reservoir and the cryocooler is used to recycle and recondense helium leaving the reservoir. The systems need liquid helium and nitrogen for precooling but then can be kept cold for long periods.

Our design for direct conduction cooling in vacuum has important advantages. First the design can be kept compact as there are no cold liquid vessels. All electrical feed throughs, including high current leads are at room temperature. The cooldown uses only the refrigerator. The internal design is simpler and there are fewer risks for vacuum leaks.

Magnets up to 750mm in diameter providing 0.5 Tesla have been built as well as 15 Tesla small bore (50mm) coils. Samples can be cooled to low temperatures and their properties measured in a controlled manner without imposing high heat loads on the cryocooler, which is also used to cool the magnet. This has been achieved over the 4K to 300K range using a gas convection cooled sample chamber, the design of which has been patented. Lower temperatures to 1.6K can be attained by pumping on a small volume of condensed liquid helium.