Making Superconducting Magnets

CICC Magnets: We've Got Cable

Today's low-temperature superconducting magnets are made from niobium-tin (Nb₃Sn) and/or niobium-titanium (NbTi). Niobium-titanium is cheaper and less fragile, but you can use niobium-tin in a higher magnetic field because it has a higher critical field. Often, both types of superconductors are combined in the same magnet. Below we explain how to make a magnet coil of niobium-tin.

The cables that are the end product of the cable-in-conduit process are made from several hundred wires. Each is about as thick as a paper clip wire and contains micrometer-thin filaments no wider than a human hair. Why use so many tiny component parts, rather than one thick tube of the stuff? Using lots of tiny wires increases the surface area. And as we'll see, the surfaces of all wires are exposed to coolant. This design allows heat generated by the magnet to be removed quickly, which in turn enables the magnet to be extremely stable.

In a multi-stage process, these filaments of niobium and tin are embedded in copper to make a wire less than 1 mm in diameter. These wires are then twisted together in a carefully designed pattern to create a cable. Cabling patterns vary by magnet. But in every case thousands of filaments are fashioned into wires, and hundreds of wires are combined to create a cable about as wide as a good-sized thumb, as this video demonstrates. When you do the math, that turns out to be quite a lot of wire. The Series Connected Hybrid magnet now under construction at the MagLab will contain about 1.8 km (1.1 miles) of cable, inside of which will be twisted 282 km (175 miles) of wire. That's more than enough to stretch across North Florida from Tallahassee to Jacksonville (or, for a West Coast reference, from Seattle, Wash., to Portland, Ore.).

The copper matrix that holds the superconducting material lends mechanical stability. Also, if an accidental rise in temperature causes the superconductor to stop carrying current, the copper will take over the job. This can prevent sudden overheating that would otherwise damage or destroy the expensive instrument.

Although the wires look very tightly packed, there is actually plenty of room in there: In fact, almost a third of it is empty. That space will not stay empty, however; in the operational magnet, liquid helium will flow right through it, providing the low-temperature environment the wires need to superconduct. This is one key difference between cable-in-conduit and wire-wound superconducting magnets; the latter are cooled from the outside in, from only the inner or outer surface of the coil, while in cable-in-conduit magnets the wires are cooled directly (and more efficiently), from the inside out.

As intricate as all this cabling is, we haven't even gotten to the really tricky part yet: welding the lengths of cable together to make one continuous conductor. The current will encounter some resistance (and generate heat) when it meets those joints: The trick is to minimize that.

"You've got this length of superconductor that you need to hook to another one," explains research engineer Lee Marks. "And in that joint, where the two come together, you have to minimize resistance. You have a heat budget, basically, so we want to know just how much heat energy is getting put into the cryogenic system, because you design for that. And if you have high-resistance joints, the refrigerator will take more time to maintain the required temperatures."

Next, the cables are jacketed in a round stainless steel or other alloy tube, then formed into a rectangular cross-section by a shaping mill. This allows the coil windings to be more compact. The jacketed cable then passes through a machine that bombards it with sound waves to clean off every last spec of dirt. Another specialized machine then wraps the cable with two layers of fiberglass tape to insulate it and give it structure.

The jacketed and insulated cable is then wound onto a form, like thread on a spool, then baked in a customized furnace at 600 to 700 degrees Celsius (about 1100 to 1300 degrees Fahrenheit) for about 10 days. It is this critical step that actually creates the superconducting niobium-tin. Prior to heating, the wire contains niobium and it contains tin – two kinds of filaments, made from two separate elements. But in the heat of the furnace, solid state diffusion takes place: The tin melts and diffuses into the niobium. In the process, two elements combine to make one superconducting intermetallic compound. At least, it will become superconducting after its temperature is lowered way down from that searing 700 degrees Celsius to close to absolute zero. To withstand such extremes of temperature and the accompanying expansion and contraction, niobium-tin must be some pretty tough stuff.

"It has to withstand fire and ice," says Painter.

Still, the heat treatment makes it very brittle and fragile. But it has one more process to endure before it's ready to perform as a magnet. Epoxy is injected into the coil, then sets for several days in a warm impregnation chamber to harden. This hardening process forms a compound around the coil much like the fiberglass epoxy composites found in boats or Corvettes. Finally, after the excess epoxy is painstakingly removed, the magnet is ready to be tested.

Next Page Wire-wound Magnets: Uniform Fields

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