Blocking bubbles leads to a breakthrough

Lance Cooley

Lance Cooley, head of the Superconducting Materials Department, wrote this column.

Accelerators use magnets to steer and focus particle beams. At the Energy Frontier, very high magnetic fields are required and can be created only by energizing magnets with superconducting wires.

Fermilab scientist Tengming Shen and colleagues at Brookhaven and Berkeley laboratories, Florida State University and Oxford Instruments &#150 Superconducting Technology have identified a new treatment for an emerging material called Bi-2212, which could lead to magnets twice as powerful as LHC magnets.

Bi-2212 has some nice qualities. It has a high critical temperature, which means it doesn’t have to be cooled as much as typical superconductors. What’s better is that, when it is cooled by the same liquid-helium refrigeration already used for accelerators, Bi-2212 can maintain superconductivity at huge magnetic fields, perhaps opening up new frontiers of energy.

Unlike other high-temperature superconductors, Bi-2212 can also be formed as round wires, making it relatively straightforward to wind cables and coils. Since we do this now with low-temperature superconducting wires, a new magnet technology using Bi-2212 might not be too far of a leap. However, its ability to carry high electrical current has been less than adequate to sufficiently energize the magnets we envision.

Recently, Shen and colleagues learned how to beef up the electrical capacity.

They found that Bi-2212’s low current density is due to gas bubbles that form during a melting process that is an integral part of wire and magnet fabrication. The voids naturally present in the wires coalesce into large gas bubbles, which partially obstruct the current path along the wire. The collaborators found evidence that carbon dioxide and water vapor were liberated when the wires were heated. The liberated gas may have come from moisture, solvents or other contaminants. OI-ST, the manufacturer of the wire, took measures to improve their handling techniques and reduce surface contamination.

This “cleaner is better” approach yielded a current density that was two times higher than before, but some bubbles seemed to remain, especially in long-length conductors for which gas can barely diffuse out through two ends. So Shen and Brookhaven collaborators used a very precise laser micrometer to detect subtle changes in the wire diameter. Wires bulged in the center, and where bulges occurred, bubbles also remained, still limiting the current density. Near the exposed ends of the wire, on the other hand, gas could escape and the bubble formation was reduced. The result was a much higher current density, enough to envision magnets with very high magnetic fields.

The understanding led to a technique that prevents bubble formation almost entirely. The team carried out the melting process under high external gas pressure, enough to balance the internal pressure of gas bubbles and prevent the wire from swelling. They observed five times higher current relative to a coil processed using the standard recipe. This discovery demonstrates a current-carrying capability well above what is necessary for future magnet technology, even for fields of 30 Tesla and beyond, three to four times higher than generated at present by existing accelerator magnet technology. It also signifies the birth of a new high-performance magnet conductor and could enable a new class of superconducting magnets.