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High hopes for freeze ray technology

An unusual discovery by a University of Virginia School of Engineering and Applied Science professor is being developed as an on-demand cooling solution for high-flying military electronics.

Mechanical and aerospace engineering professor Patrick Hopkins wants to create on-demand surface cooling for electronics inside spacecraft and high-altitude jets because “a lot of electronics on board heat up, but they have no way to cool down”, he says.

With a US$750,000 (AU$1,160,000) grant over three years from the US Air Force to study how to maximise the technology, Hopkins plans to build a prototype in partnership with UVA company Laser Thermal.

“On Earth, or in the air closer to it, the electronics in military craft can often be cooled by nature,” Hopkins explains. “The Navy, for example, uses ocean water as part of its liquid cooling systems. And closer to the ground, the air is dense enough to help keep aircraft components chilled.”

However, spacecraft operate in a vacuum or in the upper atmosphere, where there’s very little air that can cool, so the electronics keep getting hotter and hotter.

“And bringing coolant on board is not a solution because that will increase the weight,” Hopkins says. But there is another lightweight solution: plasma.

Plasmas can occur when gas is energised.

“That powers their unique properties, which vary based on the type of gas and other conditions,” says Hopkins. “But what unites all plasma is an initial chemical reaction that untethers electrons from their nuclear orbits and releases a flow of photons, ions and electrons, among other energetic species.”

Plasma is increasingly being used in technology, including in jet engines.

“It assists combustion, improving speed and efficiency,” he says, “but it can also be used in the interior of the craft.

“The typical solution for air and space electronics has been a ‘cold plate’, which conducts heat away from the electronics toward radiators, which release it. For advanced electronics, however, that may not always be sufficient.”

The plasma jet is like a laser beam. It can be extremely localised, and It can also reach temperatures as hot as the surface of the sun, but when it strikes a surface, it actually chills before heating.

Hopkins and his collaborator, Scott Walton of the US Navy Research Laboratory, discovered this when they experimented with firing a purple jet of plasma generated from helium through a hollow needle encased in ceramic. The target was a gold-plated surface.

“When we turned on the plasma, we could measure temperature immediately where [it] hit, then we could see how the surface changed,” Hopkins says. “We saw the surface cool first, then it would heat up.”

Prof. Hopkins says that with no prior literature on this, they were puzzled as to why this was happening. However, with the help of then-UVA doctoral researcher John Tomko and the Navy lab, they finally determined that the surface cooling was the result of blasting an ultrathin, hard-to-see surface layer, composed of carbon and water molecules.

Hopkins’ microscopes work by a process called “time-resolved optical thermometry” and measure something called “thermoreflectance”. He says when the surface material is hotter, it reflects light differently than when it’s colder. The specialised scope is needed because the plasma would otherwise obliterate any directly touching temperature gauges.

The researchers determined they were able to reduce the temperature by several degrees, and for a few microseconds; enough to make a difference in some electronic devices. The next question was, could they get a reaction to be colder and last longer?

The team is now looking at how variations on their original design might improve the apparatus. Doctoral candidates Sara Makarem Hoseini and Daniel Hirt are considering gases, metals and surface coatings that the plasma can target.

“We haven’t really explored the use of different gasses yet, as we’re still working with helium,” Hirt says. “We have experimented so far with different metals, such as gold and copper, and semiconductors, and each material offers its own playground for investigating how plasma interacts with their different properties.

“Since the plasma is composed of a variety of different particles, changing the type of gas used will allow us to see how each one of these particles impact material properties.”

More information is available at the UVAToday website.

Photo courtesy of University of Virginia.

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