Making Superconducting Magnets

Wire-wound Magnets: Uniform Fields

Wire-wound magnets start out with the same main ingredient as cable-in-conduit magnets: niobium-tin and niobium-titanium. They, too, are basically wires wound into coils. But there are key differences in how they are made and how they are used.

Workers install 100 layers of insulation around the 900 MHz magnet to help keep it cold.

To make a niobium-tin coil for a wire-wound magnet, you use a bronze-processed conductor. First, tens of thousands of filaments of niobium are embedded in bronze (which is a mixture of tin and copper). This wire core is wrapped in either niobium or tantalum, then jacketed in copper (the niobium/tantalum layer prevents the copper from interacting with the tin inside). The finished wire, squared off, typically measures between 1 mm to 3 mm in width.

Glass insulation is braided around the wire, forming a sleeve to electrically isolate each turn of conductor. The wire is then wound as tightly as possible on a stainless steel spool with hundreds or thousands of turns, usually in several layers. When the winding is done, the coil is placed in a very hot furnace. There, the tin in the bronze matrix reacts with the niobium filaments in the wire to create the superconducting niobium-tin. As in CICC fabrication, epoxy is injected into the winding pack, excess is scraped off, and the magnet coil is finished.

There is a lot more work to be done, and more parts to be fabricated, before the entire magnet system is finished. As with CICC magnets, several more coils of varying diameter will be made and stacked one inside the next. Then the whole shebang is inserted into a cryostat, a kind of gigantic Thermos that will keep the magnet cold enough to superconduct. This cryostat keeps the magnet cold from the outside; there is no liquid helium running inside the cables, as with CICC magnets. Using a combination of vacuum, cryogenics (liquid helium and liquid nitrogen) and insulation, the cryostat will keep the magnet inside it at operating temperature.

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