The University Record, March 19, 1996

Computer simulation lets scientists 'watch' magnetic field lines

By Sally Pobojewski
News and Information Services

Like a river delta meandering to the sea, magnetic field lines called vortices flow through superconductors in streams that pool and eddy behind obstacles and merge into broad channels in open areas, according to a new computer simulation devel oped by U-M physicists.

The ability to "see" what happens inside superconductors could help physicists solve fundamental mysteries about how vortices and the electrical currents that whirl around them pass through superconducting materials, according to Franco M. Nori, associate professor of physics.

"When vortices move they dissipate energy and destroy the material's superconductivity-the unique ability to transmit electrical currents without resistance," Nori says. "Understanding how vortices alternately become trapped and br eak free as they move through superconductors is crucial to minimizing energy loss and can help us develop improved practical applications for superconducting technologyespecially more powerful magnets for use in medical imaging systems and particle accel erators."

In a presentation at the American Physical Society meeting held this week, Nori discussed research by several U-M physicists who are using the simulation technique to study different aspects of vortex movement.

"The advantage of these computer simulations is that they allow us to systematically vary the many factors that affect vortex transport phenomena," Nori says. "We can vary the temperature, magnetic field strength, or the number and position of defects or pinning sites in the material and then see how the vortices react."

The simulations were developed based on laboratory measurements of voltage pulses and magnetic fields generated by lines of magnetic flux passing through superconducting materials. " We collaborate with experimentalists to realistical ly simulate how vortices move through superconductors," Nori says.

To visualize what happens inside a superconductor, Nori uses the analogy of objects moving through a rugged landscape of valleys and pits. "Imagine an area of rugged potential energy filled with defects or pits," he says. "As the magnetic field drives more and more magnetic field lines into the superconductor, they tend to move along the bottom of the potential energy valleys. Occasionally, some vortices become trapped in the pits."

If the pits are deep or "strong," the vortex cannot escape and the pit remains filled, according to Nori. With the pits permanently filled, vortices flow through the valleys in channels with heavy traffic. If the pits are shallow or &q uot;weak," vortices can be pushed out by the pressure of other vortices piling up behind them, producing sudden bursts of energy and a branching network of narrow, meandering trails as the vortices alternately dam up and break through the pit barrier s.

"The forces producing these avalanches or sudden bursts of energy are currently the subject of intense study not only in superconductors, but also in sand piles, water droplets, magnetic bubble arrays, earthquakes and other complex systems,&qu ot; Nori says. "All these apparently dissimilar systems have interacting moveable objects that repel each other and are pushed toward instability by an external driving force. During the unstable state, particle transport occurs in the form of aval anches or cascades which release accumulated strain in the system."

According to Nori, U-M researchers are currently studying superconductors with periodic arrays of pinning sites that produce very stable vortex configurations which are unaffected by increasing currents or magnetic fields. They are using vortex t ransport simulations to explore basic questions of what happens when an elastic lattice is forced onto a rigid substrate-the answers to which have applications in many other fields of physics.

Nori's principal collaborator is Stuart B. Field, assistant professor of physics. Their research is supported by the National Science Foundation and the U-M.