Monday, July 02, 2018

What Happens When You Plug a Wire into a Lightning Bolt? Electrical Instability Caught on Camera

“It’s easy to forget, looking at a lightbulb filament, that electricity is still untamed and dynamic,” says Trevor Hutchinson, a graduate student in the physics department at the University of Nevada, Reno (UNR).

A two-nanosecond picture of the surface on an aluminum wire carrying 910 kiloAmps (a typical lightning bolt is ~30 kiloAmps). The electric current flows up in the picture, and the rippling is into and out of the page, producing alternating bands of hot (and cold) aluminum—although cold is still 9000 Kelvin. The whole picture is only 0.6 mm2.
Image credit: T. M. Hutchinson, T. J. Awe, B. S. Bauer, K. C. Yates, E. P. Yu, W. G. Yelton, and S. Fuelling.
Toasters, phone chargers, and twinkling streets lights can give us the sense that electricity is always well-behaved, waiting patiently to be used. But that’s not entirely true. Our attempts to harness electricity have come a long way, but electricity is still a wild, unpredictable thing at times. For example, if you were to plug the wire from your toaster into the equivalent of 20 lightning bolts, you would get huge, complicated electrical structures that we don’t fully understand and whose behavior we can’t predict very well.

There are much safer and less expensive ways to burn toast, but there are situations when it’s important to know what happens to a wire carrying such intense current. Situations like in magneto-inertial fusion experiments—focused on igniting a virtually limitless supply of clean energy—or in shock wave experiments exploring planetary formation and the properties of extremely compressed materials.

Hutchinson is part of a team of scientists from UNR, Sandia National Laboratories, and the University of New Mexico, Albuquerque that recently captured pictures of the “untamed dynamics” of electricity in this kind of extreme situation, under the direction of Dr. Tom Awe (Sandia) and Dr. Bruno Bauer (UNR). The pictures reveal behavior that has never been seen directly before and demonstrates a new way to investigate this behavior moving forward. The team recently published its work in the American Physical Society’s journal Physical Review E.

When such intense current meets even a tiny asymmetry in a metal wire, like a rough spot or imperfection, the asymmetry feeds an electrical instability. As Hutchinson explains, “A ripple forms and grows in the current and the wire’s surface. This runaway growth is an electrical instability.” One of the results of the instability, at least according to theoretical predictions, is the formation of alternating bands of high and low temperatures in the wire.

Although predicted by theory, researchers have never been able to see this instability before—at least not in the kind of wires used in the situations described above. The problem is that a tiny fraction of a second after the instability forms, a plasma forms around the wire that outshines the metal surface.

The researchers spent a year designing a way to get around this problem, building on the sustained efforts of many people over many years. They machined 11 aluminum wires with diameters just under 1 mm. They covered five of the wires with a thin transparent coating that didn’t conduct electricity and left the other six uncoated. Then, over a few weeks, they conducted an experiment—they sent large, quick electrical pulses through each wire and took ultra-fast, high-resolution pictures of the metal surfaces.

“It only took three weeks to collect the raw data, but more than two years to understand it,” says Hutchinson. The analysis included checking (and triple checking) details like when precisely the pictures were taken, writing a computer program to convert the images into temperature maps of the metal surfaces, and identifying patterns, he says.

The pictures showed rippling along the surface of both the coated and uncoated rods, a telltale sign of the alternating bands of hot and cold temperatures. These bands were visible for about 5 nanoseconds (ns) on the uncoated rods before being outshone by plasma. However, the bands were visible for much longer, more than 30ns, on the coated rods and the plasma never made an appearance. This suggests that the coating actually stops the plasma from forming, giving scientists more time to directly observe the electrical instability.

Currently, the team is studying how the instability is influenced by the roughness of the metal, its composition, and the thickness of the coating. The researchers are also trying to input what they’ve learned into simulations for fusion experiments. “Computer simulations for fusion energy experiments rely on recreating electrical explosions of metal vapors hotter than the surface of the sun. Glimpsing this instability brings us closer to taming its effect on fusion devices,” explains Hutchinson. “It is especially intriguing how real-world applications depend on mastering the rich microscopic underlayer that is ordinarily invisible to us,” he says.

Kendra Redmond

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