UVA professor developing ‘freeze ray’ technology for the Air Force

The discovery – which, unexpectedly, relies on heat-generating plasma – is not meant for weaponry, however. Mechanical and aerospace engineering professor Patrick Hopkins wants to create on-demand surface cooling for electronics inside spacecraft and high-altitude jets.

“That’s the primary problem right now,” Hopkins said. “A lot of electronics on board heat up, but they have no way to cool down.”

The US Air Force likes the prospect of a freeze ray enough that it has granted the professor’s ExSiTE Lab (Experiments and Simulations in Thermal Engineering) $750,000 over three years to study how to maximise the technology.

From there, the lab will partner with Hopkins’ UVA spinout company, Laser Thermal, for the fabrication of a prototype device.

The professor explained that, on Earth – or in the air closer to it – the electronics in military craft can often be cooled by nature. 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: “With the Air Force and Space Force, you’re in space, which is a vacuum, or you’re in the upper atmosphere, where there’s very little air that can cool,” he said. “So, what happens is your electronics keep getting hotter and hotter and hotter. And you can’t bring a payload of coolant onboard because that’s going to increase the weight, and you lose efficiency.”

Hopkins believes he’s on track toward a lightweight solution. He and collaborators recently published a review article about this and other prospects for the technology in the journal ACS Nano.

The fourth state of matter

The matter we encounter every day exists in three states: solid, liquid and gas. But there’s a fourth state: plasma. While it may seem relatively rare to us on Earth, plasma is the most common form of matter in the universe. In fact, it’s the stuff that stars are made of.

Plasmas can occur when gas is energised, Hopkins said. That powers their unique properties, which vary based on the type of gas and other conditions. 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.

The eye-popping results can be witnessed in the sudden flash of a lightning strike, for example, or the warm glow of a neon sign.

Though plasma screen televisions were once a thing, then phased out, don’t let that fool you. Plasma is increasingly being used in technology. It’s already utilised in the engines of many of the Air Force’s speediest jets. The plasma assists combustion, improving speed and efficiency.

But Hopkins pictures plasma also being 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.

Hopkins thinks the revised setup may be something like a robotic arm that roves in response to temperature changes, with a short, close-up electrode that zaps hot spots.

“This plasma jet is like a laser beam; it’s like a lightning bolt,” Hopkins said. “It can be extremely localised.”

The plasma enigma

Cool fact: Plasma can reach temperatures as hot as the surface of the sun. But it also seems to have this weird characteristic – one that would appear to violate the second law of thermodynamics. When it strikes a surface, it actually chills before heating.

Hopkins and his collaborator, Scott Walton of the US Navy Research Laboratory, made the unexpected discovery several years ago, just before the pandemic hit.

“What I specialise in is doing really, really fast and really, really small measurements of temperature,” Hopkins said of his custom-made microscopic instruments, which can record specialised heat registries.

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.”

Credit: Tom Cogill

What they realised

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.

A similar process happens when cool water evaporates off of our skin after a swim.

“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 specialised 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.

After the pandemic delay, Hopkins and collaborators published their initial findings in Nature Communications last year.

Then the question became: Could they get a reaction to be colder and last longer?

Refining the freeze ray 

Previously working with the Navy’s borrowed equipment – so lightweight and safe it was often used for school demonstrations – the UVA lab now has its own setup, thanks to the Air Force grant.

The team is 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.

Hirt provided a lab update.

“We haven’t really explored the use of different gasses yet, as we’re still working with helium,” he said. “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.”

Hirt said working with Hopkins on a project with such significant implications has rejuvenated his interest in research, in large part due to the supportive lab environment the professor fosters.

“I feel like it’s night and day comparing not only where I was as a scientist, but also my enjoyment of science, to where I am today,” he said.