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Short Circuits in Plasma Devices Prevented by Fractals and Reduces Electron Emission

27 December 2017

Physicist Charles Swanson. Source: Elle Starkman/PPPL Office of CommunicationsPhysicist Charles Swanson. Source: Elle Starkman/PPPL Office of Communications

Physicists at the U.S. Department of Energy (DOE) Princeton Plasma Physics Laboratory (PPPL) have found a method to prevent plasma from causing short circuits in machines, like spacecraft thrusters, radar amplifiers and particle accelerators.

Charles Swanson and Igor Kaganovich, leaders of this research, report that applying microscopic structures that resemble feathers and whiskers to the surfaces of the machines will keep them at peak operating performance.

The team calculated that tiny fibers, called ‘fractals’ can trap electrons dislodged from the interior surfaces by other electrons zooming in from the plasma. Researchers call the dislodged surface electrons ‘secondary electron emissions’ (SEE). Trapping these emissions prevents particles from causing an electric current that interferes with the functions of important machines.

This research builds on previous experiments showing that surfaces with fibered textures can reduce the amount of secondary electron emission. Other research in the past has indicated that surfaces with plain fibers, called ‘velvet’ that lack feather-like branches, can prevent around 50 percent of the secondary electrons from escaping into the plasma. The velvet traps only half of these electrons because the electrons from the plasma strike the fibers at a shallow angle and the secondary electrons bounce away without obstruction.

"When we looked at velvet, we observed that it didn't suppress SEE from shallowly incident electrons well," Swanson said. "So we added another set of fibers to suppress the remaining secondary electrons and the fractal approach does appear to work nicely."

This new research shows that feathered fibers can capture the secondary electrons produced by that approach from a shallow angle. Because of this, the fractal fibers can reduce secondary electron emissions by up to 80 percent.

Swanson and Kaganovich confirmed these findings by performing computer calculations that compared the velvet and fractal featured textures.

"We numerically simulated the emission of secondary electrons, initializing many particles and allowing them to follow ballistic, straight-line trajectories until they interacted with the surface," Swanson said. "It was apparent that adding whiskers to the sides of the primary whiskers reduced the secondary electron yield dramatically."

The paper on this research was published in the Journal of Applied Physics.

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