AEDC and UTSI collaborate on cryogenic challenge

  • Published
  • By Philip Lorenz III
  • AEDC/PA
ARNOLD AIR FORCE BASE, Tenn. --  A recently completed cryo-contamination experiment in AEDC's small ultra high vacuum (UHV) chamber, overseen by ATA's Dr. Heard Lowry III and conducted by three University of Tennessee Space Institute graduate students, is being heralded as a success.

Jim Burns, AEDC's space chambers lead, initiated the project with UTSI approximately a year and a half ago, bringing UTSI's Dr. Trevor Moeller in as the principal investigator.

"This project has proven to be a win-win collaborative effort with UTSI and AEDC," Burns said.

Dr. Lowry, an ATA technology and analysis branch technical fellow, has been working with UTSI graduate students for 10 years at AEDC.

"We originally got Jesse Labello as a graduate student and had him looking at how optical components change spectrally when they go cold," Dr. Lowry said. "[Then Labello] got his masters and we shifted over into trying to find out more about cryodeposits. I've had several students and I've enjoyed working with all of them."

Dr. Moeller, an assistant professor for UTSI's Department of Mechanical and Aerospace and Biomedical Engineering, said cryo-contamination inside space chambers in a lab or on satellites in space is not a new problem.

"Jim and his group had been experiencing some problems with the buildup of ice on test surfaces and optics inside of their space environment chambers, and that would cause problems with optics because if ice would build up enough it might crack a little bit so it would distort things," Dr. Moeller said. "He was looking for ways to [provide] an early warning of this ice buildup and to mitigate the problem."

If the ice buildup is thick enough and cracks, less light will pass through a lens or reflect from a mirror due to absorption and scattering.

"This effect is similar to frost on the windshield," Burns said.

Also, the problem of cryo-contamination goes beyond degradation of chamber optics.

"The other effect it has is on the cryo-vacuum chamber's mechanical components," Burns said. "You get ice buildup on those and then drives don't move and things freeze up."

Dr. Moeller spoke about how the team approached the project.

"We focused on development of an early warning system first," he explained. "Our concept was if we put a small mirror that we can bounce a laser beam off of and have detectors arranged in the correct positions that as the ice builds up it forms an interferometer.

"We could count interference fringes and use theory to determine how much, or how thick that layer of ice is and the rate of build-up."

He said the concept behind the experiment is fairly simple, but they had to overcome some environmental challenges.

"For one thing, one of the most challenging scenarios that we've had is that we had to find a way to introduce water molecules into the chamber in a controlled way," Dr. Moeller explained. "People will spend their entire careers trying to find ways to keep water molecules out of vacuum chambers. You look in the literature and you can find all kinds of information how to keep the stuff out, but no way to find a way to put it in."

The team spent approximately nine months to a year investigating sources of water molecules and techniques for introducing them.

"The test that was just performed in July was really a major success for us because we had done a lot of our calculations ahead of time," he said. "We had predicted a flux rate of water molecules into the system and calculated how fast the ice would build up and then predicted the characteristics of the interference fringes.

"The thing that was so impressive about it is that when the raw data started coming up on the computer, when we were running the experiment, the shape of those interference fringes that were coming up were exactly as we predicted. It was a big thing for us."

James Rogers, who is working on his master's degree in aerospace engineering, said he and the other two students who worked on this experiment did a lot of work to prepare for the experiment at AEDC.

"We've got a material that supplies water that is going into the chamber and growing a thin film," he said. "So I studied the material that holds the water and figured out if it was going to work for this experiment or not. [We] did lots of tests with a vacuum chamber at UTSI before we came over here and tested and compiled all that data. We all looked at it and said it looks good enough and it should work."

Jesse Labello, a physics graduate student working on his doctorate, has been working on the base for five and a half years.

Since completing his dissertation research at Arnold, he has been acting "as a liaison between the UTSI group and the personnel at AEDC to actually get this set up."

Labello said, "I've done a lot of the background work as far as the properties of ice [are concerned]. Cryodeposits are a big problem in cryogenic vacuum chambers and I've been studying them, finding the properties, finding how they grow, how fast they grow in different situations and trying to look at different ways to get rid of them. Our big focus has been trying to calculate exactly what thickness distribution they will form under different environmental conditions."

Dr. Moeller said what will follow the cryo-contamination experiment is clear.

"We proved that our interferometer is effective," he said. "The next step would be to reduce its component size so that these detectors could be positioned inside of the test chambers at strategic locations to monitor the buildup of ice that's occurring in the natural environment. But for our particular test, we needed to know how much water and ice was going into the system, and developed a very controlled approach to prove the concept."

The team is also looking at mitigation techniques and technologies to reduce or remove ice from the critical components inside the space chambers. These same methodologies would be applicable to satellites in space.

"We talked about a helium curtain because helium does not condense at the temperature that you typically would see in one of these chambers," Dr. Moeller said."So, if you put a thin layer of helium across these surfaces, they kind of block the water molecules from getting to those surfaces, like a barrier.

"Another thing we're investigating and we have hopes for as far as mitigation techniques goes, is that water molecules are polarizable. The oxygen molecule and the hydrogen molecule aren't all in line, so if you applied a low voltage electrical field on critical test surfaces and just cycle that voltage rapidly enough [it] will excite those water molecules, kind of like your microwave oven does."

Moeller said this technique has a lot of potential.

"The electric fields from that voltage would add enough energy to the water molecules as they're approaching the critical surfaces that they don't condense [there]," he said. "So, they would still float around in other places and could condense on cold surfaces elsewhere where it's not an issue. And if we can show that this works, and doesn't interfere with the test, it's a very simple thing to do."