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article imageMan-made St. Elmo's fire created in open air for the first time

By Karen Graham     Nov 14, 2017 in Science
For the first time, engineers at Caltech have created a stable ring of plasma in the open air—essentially capturing lightning in a bottle, but without the bottle.
There are four states of matter that exist on Earth — solid, liquid, gas,and plasma. Of the four, only plasma does not naturally exist on the Earth's surface under normal conditions and must be artificially generated from neutral gases.
Plasmas are made up of charged particles, ions and electrons, and when they do occur on Earth, it is in the form of lightning during storms, or in the weather phenomenon called St. Elmo's fire, in which glowing balls of light sometimes appear on pointed objects during storms. We can also find plasma in neon lights and in plasma cutting torches.
Lightning striking
Courtesy Hot Docs Festival
While lightning follows the path of least resistance through the air, shooting down from the clouds in jagged forks, man-made plasmas have only been confined to vacuum chambers or electromagnetic fields. Generally, though, plasmas don't have a clearly defined shape or size of their own.
However, in 2013, University of Missouri researchers managed to create rings of plasma that can hold their shape without the use of outside electromagnetic fields. The plasma rings lasted just 10 milliseconds but reached temperatures greater than the sun's fusion core surface at around 6600 to 7700 degrees Kelvin.
Hall effect thruster. The electric field in a plasma double layer is so effective at accelerating io...
Hall effect thruster. The electric field in a plasma double layer is so effective at accelerating ions that electric fields are used in ion drives. July 24, 2004
Plasma rings that last indefinitely
All of the above has changed. Engineers at CalTech have created a stable ring of plasma in open air using just a stream of water and a crystal plate. Their findings are set to be published in the Proceedings of the National Academy of Sciences on November 13.
It may sound rather simple, but there is more involved than just squirting water from a hose at a plate. "We were told by some colleagues this wasn't even possible. But we can create a stable ring and maintain it for as long as we want, no vacuum or magnetic field or anything," says co-author Francisco Pereira of the Marine Technology Research Institute in Italy, a visiting scholar at Caltech.
A diagram showing in profile how the plasma rings are formed.
A diagram showing in profile how the plasma rings are formed.
Mory Gharib/Caltech
The stream of water is an 85-micron-diameter jet blasting from a specially designed nozzle at 9,000 pounds per square inch. The jet of water strikes the crystal plate with an impact velocity of around 1,000 feet per second. Plain and simple? — that's a stream of water narrower than a human hair moving about as fast as a bullet fired from a handgun.
The researchers experimented with different types of crystal plates, including quartz and lithium niobate, both of which can induce the triboelectric effect. The Triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into frictional contact with a different material.
Testing different textures of the various crystals, the team found the smoother the surface, the clearer the structure of the plasma ring. The ring is stable, and as long as the water continues to flow, the ring maintains its shape and size.
A glowing plasma lamp [Image credit: Luc Viatour /]
A glowing plasma lamp [Image credit: Luc Viatour /]
Luc Viatour via Wikimedia Commons
A new form of energy storage may be possible
Engineers noticed something strange was happening to their cell phones when they were in the room while the experiment was going on — the cell phones encountered high levels of radio frequency noise or static. They discovered that the plasma ring emits distinct radio frequencies.
"That's never been seen before. We think it's because of the piezo properties of the materials that we used in our experiments," Pereira says, referring to the materials' ability to be electrically polarized through mechanical stress — in this case, the flowing of water. The ability to generate a stable ring of plasma without powerful electromagnetic fields or vacuums suggests the possible use of plasma structures to store energy.
The paper is titled "Toroidal plasmoid generation via extreme hydrodynamic shear." Gharib and Pereira's coauthors include Caltech undergraduate student Sean Mendoza; Caltech consultant Masoud Beizai; and Moshe Rosenfeld of Tel Aviv University in Israel. This research was supported by Caltech and the Charyk Foundation.
More about Plasma, charged particles, triboelectric effect, radio frequencies, Energy storage
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