Physicists unveil unexpected properties in superconducting material

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But a key characteristic that explains the material's unusual properties remained tantalizingly out of reach in spite of the scientists' rigorous battery of experiments and exacting measurements. So members of that team from the University of Tokyo reached out to theoretical physicists at Rutgers University to help uncover the material's secrets.

In a paper published Jan. 21 in thejournal Science, the Tokyo and Rutgers researchers now report that the material can reach a point where seemingly contradictory electrical andmagnetic propertiescoexist, without being subject to massive changes in pressure, magnetic fields, or chemical impurities.

This point, which physicists call"quantum critical,"often defines whether and how a material can become superconducting– a valued property where all resistance to electrical flow vanishes. Superconductivity, discovered 100 years ago, has since been put to work in a variety of applications, from physics research to medical MRI scanners.

Scientists have long been able to"tune"materials toward quantum criticality by altering the materials' properties. This is done by exposing them to high magnetic fields and pressures, or by adding certain atomic impurities to the materials. The material studied by the Tokyo and Rutgers researchers, however, appears to be the first to exhibit quantum criticality in its natural state, without tuning.

"This is a completely unexpected result,"said Piers Coleman, professor of physics and astronomy, School of Arts and Sciences, at Rutgers."It could be the first example of what physicists describe as a 'strange' metallic phase of matter, manifesting itself intrinsically, without any tuning of the material's properties."

The material synthesized and studied by the Japanese experimental physicists is an exotic crystal made up of the elements ytterbium, boron, and aluminum. It has the chemical formula YbAlB4 but the physicists gave it the nickname"YBAL"(pronounced"why-ball"). Superconductivity had earlier been observed in YBAL, in a particular crystalline form called the"beta"structure. The Tokyo physicists suspected they could find a quantum critical point in the material; however, its superconducting behavior that kicks in slightly above absolute zero masked their ability to pinpoint it.

Coleman and postdoctoral researcher Andriy Nevidomskyy examined the data from the Tokyo experiments at a wide range of temperatures and magnetic field strengths. All the data, they found, collapsed onto a curve that pointed to the unobservable quantum critical point (QCP) hidden by the superconducting phase. The QCP was within hair's breadth of zeromagnetic field, with no externally applied tuning of pressure or other parameters.

"It's kind of a dream system,"said Coleman, also a member of the Rutgers Center for Materials Theory."We've found a material that is intrinsically quantum critical with very simple behavior. It's puzzling, because there's nothing simple about the material's structure. We're not sure why this happens."

Nevidomskyy, now an assistant professor of physics and astronomy at Rice University, likened the discovery of the QCP to finding a black hole in outer space.

"You can't see a black hole because light can't escape from its grip; however, you can observe the gravitational pull that a black hole has on nearby stars,"he said."Similarly, we couldn't see the quantum critical point directly, but we could see evidence of it in the material's magnetic properties and thereby deduce its position underneath the veil of superconductivity."

The discovery that most intrigues the physicists is that beta-YBAL could be revealing an exotic new phase of matter known as the"critical strange metal"phase. At the quantum critical point, the material can shift between conventional electrical behavior, which physicists call a Fermi liquid, to superconducting behavior, and to a condition that resembles neither, called"strange metal"behavior. This behavior has been observed in superconducting materials, but it's not known whether it occurs only in the vicinity of a QCP or whether it can exist over an extended range of physical conditions, which would essentially make it a phase of matter.

Proposed by Nobel laureate Philip Anderson, the idea of strange metal phases has been long debated by physicists."It is extremely controversial,"said Coleman."The experiments our Tokyo colleagues are doing right now might provide more evidence. It could change our basic understanding of materials going forward."

"We are very excited,"said Satoru Nakatsuji, professor and leader of the Tokyo research team."If true, this would be an amazing discovery, opening new horizons in our understanding of quantum criticality."

Coleman praised the working relationship that he and Nevidomskyy have with Nakatsuji's team, including the paper's primary author, Yosuke Matsumoto, and five other researchers: K. Kuga, Y. Karaki, N. Horie, Y. Shimura, and T. Sakakibara. The physicists are affiliated with the University of Tokyo's Institute for Solid State Physics in Kashiwa, Japan.

"In modern science, this interplay between theory and experiment is extremely important,"Coleman said."If you can get a powerful current of ideas going, you can take physics much further. A lot of our work has been done by video conference. Unfortunately with the time difference, it means one of our groups had to get up early while the other had to stay late at night."

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One-dimensional window on superconductivity, magnetism: Atoms are proxies for electrons in ultracold optical emulator

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The research appears this week in the journalNature.Using lithiumatomscooled to within a few billionths of a degree of absolute zero and loaded into optical tubes, the researchers created a precise analog of a one-dimensionalsuperconducting wire.

Because the atoms in the experiment are so cold, they behave according to the same quantum mechanical rules that dictate howelectronsbehave. That means the lithium atoms can serve as stand-ins for electrons, and by trapping and holding thelithiumatoms in beams of light, researchers can observe how electrons would behave in particular types ofsuperconductorsand other materials.

"We can tune the spacing and interactions among theseultracold atomswith great precision, so much so that using the atoms to emulate exotic materials like superconductors can teach us some things we couldn't learn by studying the superconductors themselves,"said study co-author Randy Hulet, a Rice physicist who's leading a team of physicists at Rice and six other universities under the Defense Advanced Research Projects Agency's (DARPA)Optical LatticeEmulator (OLE) program.

In the Nature study, Hulet, Cornell University physicist Erich Mueller, Rice graduate students and postdoctoral researchers Yean-an Liao, Sophie Rittner, Tobias Paprotta, Wenhui Li and Gutherie Partridge and Cornell graduate student Stefan Baur created an emulator that allowed them to simultaneously examine superconductivity and magnetism -- phenomena that do not generally coexist.

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Schematic showing an array of tubes containing lithium atoms. The system is probed by imaging the shadow cast by this ensemble. Image credit: Nature Supplementary Information

Superconductivity occurs when electrons flow in a material without the friction that causeselectrical resistance. Superconductivity usually happens at very low temperatures when pairs of electrons join together in a dance that lets them avoid the subatomic bumps that cause friction.

Magnetism derives from one of the basic properties of all electrons -- the fact that they rotate around their own axis. This property, which is called"spin,"is inherent; like the color of someone's eyes, it never changes. Electron spin also comes in only two orientations, up or down, and magnetic materials are those where the number of electrons with up spins differs from the number with down spins, leaving a"net magnetic moment."

"Generally, magnetism destroys superconductivity because changing the relative number of up and down spins disrupts the basic mechanism of superconductivity,"Hulet said."But in 1964, a group of physicists predicted that a magnetic superconductor could be formed under an exotic set of circumstances where a net magnetic moment arose out of a periodic pattern of excess spins and pairs."

Dubbed the"FFLO"state in honor of the theorists who proposed it -- Fulde, Ferrell, Larkin and Ovchinnikov -- this state of matter has defied conclusive experimental observation for 46 years. Hulet said the new study paves the way for direct observation of the FFLO state.

"The evidence that we've gathered meets the criteria of the FFLO state, but we can't say for certain that we have observed it. To do that, we need to precisely measure the distribution of velocities of the pairs to confirm that they follow the FFLO relationship. We're working on that now."

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Neighbor lends a hand: Spallation Neutron Source's tool to probe ITER's superconducting cable

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ITER is the international research facility in southeastern France whose mission is to demonstrate the feasibility of fusion as a practical long-term energy source. SNS, located a few miles down the road from the U.S. ITER Project Office, is the world's most powerful pulsed neutron source.
Ned Sauthoff, U.S. ITER project manager, said the VULCAN measurements have provided useful data and that VULCAN will have a role in the continuing investigation."Having seen the initial results, I think we have sufficient evidence to state that initial measurements have demonstrated the capability of VULCAN to measure important material properties, that meaningful results were achieved, and that the team is now focused on using the unique capability to gain important understanding aimed at solving the problem,"Sauthoff said.

The central solenoid, a joint Japan-U.S. ITER responsibility, is on a tight schedule. The superconducting cables, supplied by Japan, cost more than $3,000 per meter. Improving the cable performance by reducing the degradation of the superconducting strands is important to staying on schedule and on budget."We are working on an important problem that will have an immediate impact on science and technology on an international scale,"said Ian Anderson, head of ORNL's Neutron Sciences directorate, which operates SNS.

The team of Japanese, U.S. and ITER Organization engineers discovered in late 2010 in a sample test that the superconducting cables making up the central solenoid magnet at the core of the ITER design were losing their current-carrying capacity over time to an extent well beyond that experienced in an earlier ITER model coil test. The cables can generate a magnetic field as strong as 13 tesla, and the electromagnetic (Lorentz) force exerted on the wires by the high magnetic field and powerful current is known to cause some degradation over a period of constant magnetic cycling. The exact cause of the degradation in the conductor sample is unknown.

In addition to the Lorentz force, it may also be attributable to the sample manufacture or the particular sensitivity of the wires to the loads. The magnet team at the U.S. ITER Project Office in Oak Ridge consulted with scientists at SNS about using neutron scattering to examine the states of materials inside the cables. The samples examined at SNS are sections 1.65 inches in diameter and several inches in length cut from the much longer cables. The cables have a complex structure— copper wires interspersed with superconducting wires of a niobium-tin alloy— all contained in a stainless steel tube.

Neutrons are highly penetrating and nondestructive, so neutron scattering can return detailed data about the structure of the cable sections without destroying or altering them. SNS is the ideal facility for studying the thick cables because it has the most intense neutron beams of any pulsed neutron source in the world, said VULCAN instrument scientist Xun-Li Wang. And VULCAN is the ideal instrument because it is designed to handle large industrial-sized specimens rather than small lab samples.

The multi-phase material making up the cable is perfect for characterization by a time-of-flight diffractometer such as VULCAN, Wang noted.
"Neutron diffraction is a well-known technique for mapping strain or stress in engineering materials,"Wang said."With VULCAN we will be able to determine the deformation induced by the Lorentz force."On a fundamental level, we can also study in detail how the critical current in a superconducting wire responds to applied stress and develop a predictive model for the wires."

A plan for future study is being developed with the Japan Atomic Energy Agency, which is responsible for central solenoid conductor fabrication and theITEROrganization.

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Light touch transforms material into a superconductor

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One hundred years aftersuperconductivitywas first observed in 1911, the team from Oxford, Germany and Japan observed conclusive signatures of superconductivity after hitting a non-superconductor with a strong burst oflaser light.

‘We have used light to turn a normal insulator into a superconductor,’ says Professor Andrea Cavalleri of the Department of Physics at Oxford University and the Max Planck Department for Structural Dynamics, Hamburg.‘That’s already exciting in terms of what it tells us about this class of materials. But the question now is can we take a material to a much higher temperature and make it a superconductor?’

The material the researchers used is closely related to high-temperature copper oxidesuperconductors, but the arrangement of electrons and atoms normally act to frustrate any electronic current.

In the journalScience, they describe how a strong infrared laser pulse was used to perturb the positions of some of the atoms in the material. The compound, held at a temperature just 20 degrees above absolute zero, almost instantaneously became a superconductor for a fraction of a second, before relaxing back to its normal state.

Superconductivity describes the phenomenon where an electric current is able to travel through a material without any resistance– the material is a perfect electrical conductor without any energy loss.

High-temperature superconductors can be found among a class of materials made up of layers of copper oxide, and typically superconduct up to a temperature of around–170°C. They are complex materials where the right interplay of the atoms and electrons is thought to‘line up’ the electrons in a state where they collectively move through the material with no resistance.

‘We have shown that the non-superconducting state and the superconducting one are not that different in these materials, in that it takes only a millionth of a millionth of a second to make the electrons“synch up” and superconduct,’ says Professor Cavalleri.‘This must mean that they were essentially already synched in the non-superconductor, but something was preventing them from sliding around with zero resistance. The precisely tuned laser light removes the frustration, unlocking the superconductivity.’

The advance immediately offers a new way to probe with great control how superconductivity arises in this class of materials, a puzzle ever since high-temperature superconductors were first discovered in 1986.

But the researchers are hopeful it could also offer a new route to obtaining superconductivity at higher temperatures. If superconductors that work at room temperature could be achieved, it would open up many more technological applications.

‘There is a school of thought that it should be possible to achieve superconductivity at much higher temperatures, but that some competing type of order in the material gets in the way,’ says Professor Cavalleri.‘We should be able to explore this idea and see if we can disrupt the competing order to reveal superconductivity at higher temperatures. It’s certainly worth trying!’

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Physicists observe exotic state in an unconventional superconductor

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"We've been on the trail of astate of mattercalled a half-quantumvortexfor more than three years,"said Budakian."First proposed in the 1970s to exist in superfluid helium-3, a half-quantum vortex can be thought of as a 'texture' that arises from the spin phase of the superconducting order parameter."

Budukian's group investigated strontium ruthenium oxide (SRO), an unconventional superconductor that has been proposed as the solid-state analog of the A-phase of superfluid helium-3. Using state-of-the-art nanofabrication methods and exquisitely sensitive cantilever-based magnetometry techniques developed by the group, the researchers observed minute fluctuations in the magnetism of tiny rings of SRO.

"Strontium ruthenium oxide is a unique and fascinating material, and the half-quantum vortices that have been conjectured to exist in it are particularly interesting,"said Anthony J. Leggett, the John D. and Catherine T. MacArthur Professor and Center for Advanced Study Professor of Physics, who shared the 2003 Nobel Prize in Physics for his work on superfluid helium-3."It is believed that these half-quantum vortices in SRO may provide the basis for topologicalquantum computing. If this novel form of computing is eventually realized, this experiment will certainly be seen as a major milestone along the road there."

Budakian is an assistant professor of physics and a principal investigator in the Frederick Seitz Materials Research Laboratory at Illinois. Five years ago, he was instrumental in pioneering a technique, magnetic resonance force microscopy, to measure the force exerted on a micrometer-scale silicon cantilever by the spin of a single electron in a bulk material. He and his group have now adapted their ultrasensitive cantilever measurements to observe the magnetic behavior of SRO.

In the experiment, the researchers first fabricated a micron-sized ring of SRO and glued it to the tip of the silicon cantilever. How small are these rings? Fifty of them would fit across the width of a human hair. And the tips of the cantilevers are less than 2μm wide.

"We take the high-energy physics approach to making these rings. First we smash the SRO, and then we sift through what's left,"said Budakian.

The researchers first pulverize the large crystals of SRO into fragments, choose a likely micron-sized flake, and drill a hole in it using a focused beam of gallium ions. The resulting structure, which looks like a microscopic donut, is glued onto the sensitive silicon cantilever and then cooled to 0.4 degrees above absolute zero.

"Positioning the SRO ring on the cantilever is a bit like dropping one grain of sand precisely atop a slightly larger grain of sand,"said Budakian,"only our 'grains of sand' are much smaller."

Budakian added that this technique is the first time such tiny superconducting rings have been fabricated in SRO.

Being able to make these rings is crucial to the experiment, according to Budakian, because the half-quantum vortex state is not expected to be stable in larger structures.

"Once we have the ring attached to the cantilever, we can apply static magnetic fields to change the 'fluxoid' state of the ring and detect the corresponding changes in the circulating current. In addition, we apply time-dependent magnetic fields to generate a dynamic torque on the cantilever. By measuring the frequency change of the cantilever, we can determine themagneticmoment produced by the currents circulating the ring,"said Budakian.

"We've observed transitions between integer fluxoid states, as well as a regime characterized by 'half-integer' transitions,"Budakian noted,"which could be explained by the existence of half-quantum vortices in SRO."

In addition to the advance in fundamental scientific understanding that Budakian's work provides, the experiment may be an important step toward the realization of a so-called"topological"quantum computer, as Leggett alluded.

Unlike a classical computer, which encodes information as bits whose values are either 0 or 1, a quantum computer would rely on the interaction among two-level quantum systems (e.g., the spins of electrons, trapped ions, or currents in superconducting circuits) to encode and process information. The massive parallelism inherent in quantal time evolution would provide rapid solutions to problems that are currently intractable, requiring vast amounts of time in conventional, classical machines.

For a functional quantum computer, the quantum bits or"qubits"must be strongly coupled to each other but remain sufficiently isolated from random environmental fluctuations, which cause the information stored in the quantum computer to decay—a phenomenon known as decoherence. Currently, large-scale, international projects are underway to construct quantum computers, but decoherence remains the central problem for real-world quantum computation.

According to Leggett,"A rather radical solution to the decoherence problem is to encode thequantum informationnonlocally; that is, in the global topological properties of the states in question. Only a very restricted class of physical systems is appropriate for such topologicalquantum computing, and SRO may be one of them, provided that certain conditions are fulfilled in it. One very important such condition is precisely the existence of half-quantum vortices, as suggested by the Budakian experiment."

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Hot booze turns material into a superconductor

The scientist, Dr. Yoshihiko Takano of the National Institute for Materials Science (NIMS) in Tsukuba, Japan, made the discovery after a party, soaking samples of a potential superconductor in hot alcoholic drinks before testing them next day for superconductivity. The commercialalcoholic beverages, especially wine, were much more effective than either water or pure alcohol.

Superconductorsare metallic substances that allow electricity to flow through them with zero resistance below a certain temperature. Those found so far only work at very low temperatures (often as low as nearabsolute zero), and so finding one that works at room temperature could have important applications, such as power lines with superconducting cables, and perhaps inlevitationof large objects like trains, since superconductors can repel magnetic fields. The phenomenon is still not completely understood even though superconductors have been known since their discovery in 1911 by a Dutch scientist Heike Kamerlingh Onnes.

The researchers created the samples of FeTe_0.8S_0.2by sealingiron(Fe), tellurium (Te) and tellurium sulfide (TeS) powders into an evacuate quartz tube and heating the mixture at 600°C for 10 hours. This material is not normally a superconductor but can become one if exposed to oxygen or if soaked in water.

After a party for a visiting researcher Takano wondered if the drinks they were consuming would work as well as pure water. To find out, they tested the FeTe_0.8S_0.2samples with beer, red and white wine, Japanese sake, Shochu (a clear distilled liquor) and whisky, and with various concentrations of ethanol and water. The samples were all heated and kept at 70°C for 24 hours.

The results were that the ethanol-water samples showed increased superconductivity that was not dependant on the ethanol concentration. The samples heated inalcoholic drinksall showed greater superconductivity, but again not dependant on the alcohol content. Red wine was the most effective. The research team calculated the superconducting volume fraction of the samples and found they ranged from 23.1% for Sochu up to 62.4% for red wine, but none of the ethanol samples were over 15%.

The authors speculate that because wine and beer oxidize easily and since oxygen induces superconductivity in the material, the beverages could be playing an important role in supplying oxygen into the sample as a catalyst. Further research is needed to confirm the exact mechanism.

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Superconductors face the future

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But superconductors, especially superconducting electromagnets, have been around for a long time. Indeed the first large-scale application of superconductivity was in particle-physics accelerators, where strong magnetic fields steer beams of charged particles toward high-energy collision points.

Accelerators created the superconductor industry, andsuperconducting magnetshave become the natural choice for any application where strong magnetic fields are needed - for magnetic resonance imaging (MRI) in hospitals, for example, or for magnetic separation of minerals in industry. Other scientific uses are numerous, from nuclear magnetic resonance to ion sources for cyclotrons.

Some of the strongest and most complex superconducting magnets are still built for particle accelerators like CERN’sLarge Hadron Collider(LHC). The LHC uses over 1,200 dipole magnets, whose two adjacent coils of superconducting cable create magnetic fields that bend proton beams traveling in opposite directions around a tunnel 27 kilometers in circumference; the LHC also has almost 400 quadrupole magnets, whose coils create a field with fourmagnetic polesto focus the proton beams within thevacuum chamberand guide them into the experiments.

These LHC magnets use cables made of superconducting niobium titanium (NbTi), and for five years during its construction the LHC contracted for more than 28 percent of the world’s niobium titanium wire production, with significant quantities of NbTi also used in the magnets for the LHC’s giant experiments.

What’s more, although the LHC is still working to reach the energy for which it was designed, the program to improve its future performance is already well underway.

Designing the future

“Enabling the accelerators of the future depends on developing magnets with much greater field strengths than are now possible,” says GianLuca Sabbi of Berkeley Lab’s Accelerator and Fusion Research Division (AFRD). “To do that, we’ll have to use different materials.”

Field strength is limited by the amount of current a magnet coil can carry, which in turn depends on physical properties of the superconducting material such as its critical temperature and critical field. Most superconducting magnets built to date are based on NbTi, which is a ductile alloy; the LHC dipoles are designed to operate at magnetic fields of about eight tesla, or 8 T. (Earth’s puny magnetic field is measured in mere millionths of a tesla.)

The LHC Accelerator Research Program (LARP) is a collaboration among DOE laboratories that’s an important part of U.S. participation in the LHC. Sabbi heads both the Magnet Systems component of LARP and Berkeley Lab’s Superconducting Magnet Program. These programs are currently developing accelerator magnets built with niobium tin (Nb3Sn), a brittle material requiring special fabrication processes but able to generate about twice the field of niobium titanium. Yet the goal for magnets of the future is already set much higher.

“Among the most promising new materials for future magnets are some of the high-temperature superconductors,” says Sabbi. “Unfortunately they’re very difficult to work with.” One of the most promising of all is the high-temperature superconductor Bi 2212 (bismuth strontium calcium copper oxide).

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In the process called“wind and react,” Bi-2212 wire - shown in cross section, upper right, with the powdered superconductor in a matrix of silver - is woven into flat cables, the cables are wrapped into coils, and the coils are gradually heated in a special oven (bottom).

“High temperature” is a relative term. It commonly refers to materials that become superconducting above the boiling point of liquid nitrogen, a toasty 77 kelvin (95 K, or -321 degrees Fahrenheit). But in high-field magnets even high-temperature superconductors will be used at low temperatures. Bi-2212 shows why: although it becomes superconducting at 95 K, its ability to carry high currents and thus generate a high magnetic field increases as the temperature is lowered, typically down to 4.2 K, the boiling point of liquid helium at atmospheric pressure.

In experimental situations Bi-2212 has generated fields of 25 T and could go much higher. But like many high-temperature superconductors Bi 2212 is not a metal alloy but a ceramic, virtually as brittle as a china plate.

As part of the Very High Field Superconducting Magnet Collaboration, which brings together several national laboratories, universities, and industry partners, Berkeley Lab’s program to develop new superconducting materials for high-field magnets recently gained support from the American Recovery and Reinvestment Act (ARRA).

Under the direction of Daniel Dietderich and Arno Godeke, AFRD’s Superconducting Magnet Program is investigating Bi-2212 and other candidate materials. One of the things that makes Bi-2212 promising is that it is now available in the form of round wires.

“The wires are essentially tubes filled with tiny particles of ground-up B-2212 in a silver matrix,” Godeke explains. “While the individual particles are superconducting, the wires aren’t - and can’t be, until they’ve been heat treated so the individual particles melt and grow new textured crystals upon cooling - thus welding all of the material together in the right orientation.”

Orientation is important because Bi-2212 has a layered crystalline structure in which current flows only through two-dimensional planes of copper and oxygen atoms. Out of the plane, current can’t penetrate the intervening layers of other atoms, so the copper-oxygen planes must line up if current is to move without resistance from one Bi-2212 particle to the next.

In a coil fabrication process called“wind and react,” the wires are first assembled into flat cables and the cables are wound into coils. The entire coil is then heated to 888 degrees Celsius (888 C) in a pure oxygen environment. During the “partial melt” stage of the reaction, the temperature of the coil has to be controlled to within a single degree. It’s held at 888 C for one hour and then slowly cooled.

Silver is the only practical matrix material that allows the wires to“breathe” oxygen during the reaction and align their Bi-2212 grains. Unfortunately 888 C is near the melting point of silver, and during the process the silver may become too soft to resist high stress, which will come from the high magnetic fields themselves: the tremendous forces they generate will do their best to blow the coils apart. So far, attempts to process coils have often resulted in damage to the wires, with resultant Bi 2212 current leakage, local hot spots, and other problems.

“The goal of the program to develop Bi-2212 for high-field magnets is to improve the entire suite of wire, cable, coil making, and magnet construction technologies,” says Dietderich. “The magnet technologies are getting close, but the wires are still a challenge. For example, we need to improve current density by a factor of three or four.”

Once the processing steps have been optimized, the results will have to be tested under the most extreme conditions.“Instead of trying to predict coil performance from testing a few strands of wire and extrapolating the results, we need to test the whole cable at operating field strengths,” Dietderich says. “To do this we employ subscale technology: what we can learn from testing a one-third scale structure isreliable at full scale as well.”

Testing the results

Enter the second part of ARRA’s support for future magnets, directed at the Large Dipole Testing Facility.

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The LD1 test magnet design in cross section. The 100 by 150 millimeter rectangular aperture, center, is enclosed by the coils, then by iron pressure pads, and then by the iron yoke segments. The outer diameter of the magnet is 1.36 meters.

“The key element is a test magnet with a large bore, 100 millimeters high by 150 millimeters wide - enough to insert pieces of cable and even miniature coils, so that we can test wires and components without having to build an entire magnet every time,” says AFRD’s Paolo Ferracin, who heads the design of the Large Dipole test magnet.

Called LD1, the test magnet will be based on niobium-tin technology and will exert a field of up to 15 T across the height of the aperture. Inside the aperture, two cable samples will be arranged back to back, running current in opposite directions to minimize the forces generated by interaction between the sample and the external field applied by LD1.

The magnet itself will be about two meters long, mounted vertically in a cryostat underground. LD1’s coils will be cooled to 4.5 K, but a separate cryostat in the bore will allow samples to be tested at temperatures of 10 to 20 K.

“There are two aspects to the design of LD1,” says Ferracin. “The magnetic design deals with how to put the conductors around the aperture to get the field you want. Then you need a support structure to deal with the tremendous forces you create, which is a matter of mechanical design.” LD1 willgenerate horizontal forces equivalent to the weight of 10 fully loaded 747s; imagine hanging them all from a two-meter beam and requiring that the beam not move more than a tenth of a millimeter.

What’s more, Ferracin says, since one of the most important aspects of cables and model coils is their behavior under stress, “we need to add mechanical pressure up to 200 megapascals” - 30,000 pounds per square inch. “We have developed clamping structures that can provide the required force, but devising a mechanism that can apply the pressure during a test will be another major challenge.”

The cable samples and miniature coils will incorporate built-in voltage taps, strain gauges, and thermocouples so their behavior can be checked under a range of conditions, including quenches - sudden losses of superconductivity and the resultant rapid heating, as dense electric currents are dumped into conventional conductors like aluminum or copper.

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At top, two superconducting coils enclose a beam pipe. Field strength is indicated by color, with greatest strength in deep red. To test components of such an arrangement, subscale coils (bottom) will be assessed, starting with only half a dozen cable winds generating a modest two or three tesla, increasing to hybrid assemblies capable of generating up to 10 T.

The design of the LD1 is based on Berkeley Lab’s prior success building high-field dipole magnets, which hold the world’s record for high-energy physics uses. The new test facility will allow testing the advanced designs for conductors and magnets needed for future accelerators like the High-Energy LHC and the proposed Muon Collider.

“These magnets are being developed to make the highest-energy colliders possible,” says Sabbi. “But as we have seen in the past, the new technology will benefit many other fields as well, from undulators for next-generation light sources to more compact medical devices. ARRA’s support for LD1 isan investment in the nation’s science and energy future.”

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