By Kristen Coyne
All of the magnets at the National High Magnetic Field Laboratory are electromagnets: The electricity that runs through them generates the high magnetic fields into which scientists put their experiments. Most of our electromagnets are resistive magnets, instruments that use huge amounts of electricity to generate their high fields – as high as our world-record 35 tesla resistive magnet.
That makes for a daunting electricity bill of more than $7 million a year.
Like most people, we'd rather pay less for electricity. Which is one of the reasons (though not the only one) that we use, and build, another type of electromagnet: a superconducting magnet.
These instruments pull off quite a trick: They create very high fields without running up our utility bill.
Resistive magnets are made of metal Bitter plates (left) stacked into a coil; Cable-in-conduit-conductor magnets are made using hundreds of superconducting wires twisted into cables (right), inserted into tubes and wound into a coil.
Although we're famous for making resistive magnets here, we're also world-leaders in designing and building superconducting magnets. This article explains why, and how, we make superconducting magnets.
Superconductivity in a nutshell
First, a brief primer on superconductivity. If you want to learn more than is covered here, read our article Superconductivity 101. If you know this stuff already, skip ahead to the next section.
Electricity, of course, is awesome: We couldn't enjoy our microwaves, stereos or computers without it. But it has its downsides, one of which becomes immediately apparent when we accidentally touch a light bulb: Ouch! Turns out that the electrons moving through a wire zigzag very inefficiently, bumping into things and creating the friction we feel as heat. That heat is wasted electricity.
Superconductivity is electricity without the unwanted side effect of heat. It's elegant and efficient: Electrons zip through wires with no collisions or friction at all. And because there's nothing to slow them down, electrons in a superconducting state can go on virtually forever, with little to no help from the local utility company.
But there's a catch: Superconductivity occurs only at extremely low temperatures. For most known superconducting materials, those operating temperatures are so low you need liquid helium, the coldest liquid in the universe, to produce them. Scientists go to a lot of trouble and expense to make this stuff: Helium liquefies at -269 degrees Celsius, or -452 degrees Fahrenheit.
Superconducting magnets rock
The trouble, though, is worth it, because superconducting magnets boast some key advantages over resistive magnets.
For starters, they cost less money to run. Even factoring in cryogens such as liquid helium and liquid nitrogen, which aren't cheap, operating a superconducting magnet costs only about 1 percent of what it costs to operate a resistive magnet.
"Once you have it built and operating, you're good to go," says Tom Painter, an engineer with the MagLab's Magnet Science & Technology department. "You can just turn it on and operate it for years."
The magnetic fields produced by these magnets are also more steady than those produced by resistive magnets. Electricity that comes through power lines can fluctuate in strength, which results in fluctuations in the magnetic field. Most experiments benefit greatly from a steady, reliable field. "It may not seem like a big thing, but for some experiments, it is a big deal," said Painter.
Steady superconducting currents can also create a more uniform magnetic field, which is also prized by scientists. Throughout the experimental space inside the magnet, there is very little variation in field.
Superconducting magnets are also more compact. You can pack in more current per square inch of space than in resistive magnets because there's no heat to worry about. And a more compact magnet is more efficient.
Finally, superconducting magnets generally last longer. Resistive magnets suffer from a lot of heat-induced wear and tear; not a problem for their cool superconducting counterparts.
Superconducting magnets don't have all the advantages, though. They are more complicated than resistive magnets (which are basically made of metal Bitter discs stacked one on top of the other), and as a result cost more money and time to develop. Also, superconducting magnets can't reach the fields of resistive magnets. The lab's most powerful superconducting magnet, the 900 MHz Nuclear Magnetic Resonance magnet, reaches 21.1 tesla (a measure of magnetic field strength). That's close to the world record for superconducting research magnets, but pales compared to the 35 tesla produced by the MagLab's strongest resistive magnet. All superconducting materials are limited by a critical field – a magnetic field strength above which they can no longer carry current.
Still, the benefits of these instruments far outweigh the shortcomings, particularly for applications that demand very uniform fields. That's why we spend so much time and money developing different types of superconducting magnets:
- We build wire-wound superconducting magnets, such as our 900 MHz NMR magnet.
- We build superconducting magnets using cable-in-conduit conductors (CICC). One of our CICC magnets was paired with a powerful resistive magnet to make our world-record 45 tesla magnet.
- We experiment with non-conventional, high-temperature superconductors (HTS), such as YBCO (yttrium barium copper oxide) and BSCCO (bismuth strontium calcium copper oxide). Unlike the "low-temperature" superconductors currently used in superconducting magnets, HTS don't need liquid helium to operate because they work at (relatively) higher temperatures. Lab scientists and engineers have already built a prototype high-temperature superconducting magnet that holds the world record (33.8 tesla) for a magnetic field created by a superconductor.
CICC and wire-wound magnets have different advantages, shortcomings and applications. Below we take a closer look at how we make these 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 the below video demonstrates.
When you do the math, that turns out to be quite a lot of wire. The Series Connected Hybrid magnet being built at the MagLab will contain about 1.8 km (1.1 miles) of cable, inside of which are twisted 384 km (238 miles) of wire. That's more than enough to stretch from New York City to Washington, DC.
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.
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.
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.
What are these two types of superconducting magnets used for?
The main advantage to CICC magnets: They can be turned off and on relatively easily. They can go from no magnetic field to, say, 15 tesla in a matter of minutes. This is called "ramping up" or "ramping down" a magnet. When magnetic fields increase or decrease, they actually create electrical currents in the conductors around them; with that current comes heat. Excess heat can endanger a magnet, but CICC magnets can handle it because the superconducting wires are bathed in liquid helium. Wire-wound magnets, however, can't tolerate rapid changes in field and the consequent heat because the coolant is further from the superconducting wires. In fact, when wire-wound magnets are first turned on, engineers ramp them up very gradually, over several hours, days or sometimes even weeks; anything faster would create heat that the system is not designed to tolerate.
In other words, CICC magnets allow scientists to put their experiments in a changing magnetic field, to see how that affects their sample. And because they're more heat tolerant, CICC magnets can be combined with resistive magnets to build hybrid magnets, together creating a magnetic field that neither the superconducting magnet nor the resistive magnet alone could achieve.
Wire-wound magnets, however, have their own advantage over CICC magnets: They create a field that is far more uniform. That's because they are not powered from an outside source. After they are first ramped up, they are unplugged from their power supply. The superconducting current continues to run on its own: Our 900 MHz has been conducting electricity continuously, uplugged, since 2004. That smooth current makes a smooth, homogenous magnetic field that travels a precisely controlled path.
CICC magnets, on the other hand, are always plugged in; they use only a little electricity, but enough to cause irregularities in the magnetic field caused by the directions of the current paths. This results in variations in magnetic field, both over time and across the space inside the magnet where the experimental samples are placed. In CICC magnets, this "field homogeneity" is measured in parts per thousand. In wire wound magnets, it is so high it is measured in parts per million, and in some cases billion.
A uniform field is unimportant for many experiments, but for scientists doing nuclear magnetic resonance (NMR), it is critical; Even the tiniest fluctuation in magnetic field creates "noise" in their experiments, which means their data will be less clear.
"They're looking at complex molecular structures, doing very precise measurements," explains Iain Dixon, MS&T research associate and project leader for the 900 MHz magnet. "For that, very high-quality instrumentation, high-quality probes and high-quality magnetic fields are needed. If you have variation in the magnetic fields, then the scientists are unsure of what they're getting; it's got to be more or less exact."
These powerful magnets are a tough act to follow. But MagLab scientists and engineers are working to do just that. New high-temperature superconductors with a higher upper critical field are on the horizon. These new materials, some of which are being studied here at the lab, may soon allow engineers to create even higher field superconducting magnets.
"You could call it a revolution in superconducting magnets," says MagLab Director Greg Boebinger. "These new materials may free superconducting magnets from the 'niobium jailhouse.'"