Plasma ‘freeze ray’ Technology for Cooling Components
Plasma can reach temperatures as hot as the surface of the sun. But it also seems to have this weird characteristic – when it strikes a surface, it actually chills before heating
This unexpected discovery was made by Mechanical and aerospace engineering professor Patrick Hopkins and his collaborator, Scott Walton of the U.S. Navy Research Laboratory.
In their experiment, they fired a purple jet of plasma generated from helium through a hollow needle encased in ceramic. The target was a gold-plated surface. The researchers chose gold because it’s inert, and as much as possible, they wanted to avoid surface etching by the focused beam, which could skew the results.
“So when we turned on the plasma,” Hopkins said, “we could measure temperature immediately where the plasma hit, then we could see how the surface changed. We saw the surface cool first, then it would heat up.
“We were just puzzled at some level about why this was happening, because it kept happening over and over. And there was no information for us to pull from because no prior literature has been able to measure the temperature change with the precision that we have. No one’s been able to do it so quickly.”
What they finally determined, in association with then-UVA doctoral researcher John Tomko and continued testing with the Navy lab, was that the surface cooling must have been the result of blasting an ultrathin, hard-to-see surface layer, composed of carbon and water molecules – just like cool water evaporates off our skin after a swim.
Doctoral candidates Sara Makarem Hoseini and Daniel Hirt observe the plasma ray setup. Though Hirt wears a knit cap and puffy jacket for effect, the cooling is localized and doesn’t have much influence on the surrounding room temperature.
“Evaporation of water molecules on the body requires energy; it takes energy from body, and that’s why you feel cold,” the professor said. “In this case, the plasma rips off the absorbed species, energy is released, and that’s what cools.”
Hopkins’ microscopes work by a process called “time-resolved optical thermometry” and measure something called “thermoreflectance.”
Basically, when the surface material is hotter, it reflects light differently than when it’s colder. The specialized scope is needed because the plasma would otherwise obliterate any directly touching temperature gauges.
So how cold is cold? They determined they were able to reduce the temperature by several degrees, and for a few microseconds. While that may not seem dramatic, it’s enough to make a difference in some electronic devices.
Find out more about the research
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