Predicting the lifetime of an electric ion thruster is notoriously difficult. You have to account for the chamber wall affects which are not present in space environments. Researchers within several different aerospace disciplines in The Grainger College of Engineering, University of Illinois Urbana-Champaign worked together to simulate the ion activity, then validate it in a unique experiment that will help predict the lifespan of electric thrusters.
“We can simulate the damage to the engine caused by sputtering, but until now, we could not validate that our simulations were correct,” said Professor Huck Beng Chew. “Because both the engine and chamber walls are coated with impact-resistant carbon, we didn’t know whether the coating damage was from accelerated ions directly hitting the engine or whether the coating damage was artificially mitigated by the deposition of carbon from ion bombardment off the chamber walls.”
Chew said validation of his simulations came from a collaborative effort. Professor Joshua Rovey and his grad students conducted an experiment proving where the carbon fragments originated by testing two different carbon-coated plates in the vacuum chamber—one with 12C and the other with 13C—making the identification clear.
“It’s difficult to track the carbon particles being deposited back on the surfaces because they’re atomic-sized,” Chew said. “We can’t see them, but we can observe the damage they cause. But because we knew the location origins of the two types of carbon, we could identify them on the other surfaces.”
Another collaborator on the project was Professor Deborah Levin. “Debbie simulates how the carbon is transported. She developed a code that calculated the amount of carbon atoms that are displaced. I work on the plasma/material interface, and Josh is the experimentalist.”
The team was also able to examine how the angle of impact from the dislocated carbon particles affected the surfaces.
Huy Tran, Chew’s former Ph.D. student, created a scale bridging model. He used molecular dynamics, to run the simulations and combine Monte Carlo models--which rely on repeated random sampling to obtain numerical results--to look at the sputtered species surfaces.
Chew explained that there are many different species of carbon. It can be crystalline diamond or graphite, or amorphous carbon.
“We ran simulations and found that the specific type of carbon didn’t matter. When the Xenon or Krypton ions impact the surface, the result is a very random, amorphous arrangement of atoms. This means the substrate doesn’t matter. The regular crystalline structure becomes a mixed liquid. This finding simplifies the analysis.”
Another significant finding from the research concerned surface roughness.
“Because the set of simulations were done on a molecular level using molecular dynamics, we could not account for surface roughness. We performed a simulation that assumes initial surface roughness. After the surface is bombarded with a normal incident angle, the rougher surface became smoother as the surface is being chipped off.
“When the surface was impacted with a sharper, oblique ion incident angle, the material shaved off in more of a zigzag profile. The question was: Are the simulation results real? Josh Rovey’s experiment verified the answer to be yes.”
He said, what this means is that now they can expect the surface morphology will become flatter or rougher depending upon the incident angle.
“We can be confident that the sputter yield—the rate at which the carbon film wears off—is correct.”
In terms of informing predictions about the lifespan of a thruster in space, Chew said “that’s the ultimate goal, but because this is fundamental research, we’re still a long way from that. However, knowing the angle is an important piece. With that knowledge, you can design surface topology—surface structures—to mitigate sputtering. That's a grand challenge, and now there's a solution forward.
“Experiments are real. Simulations are based on a set of assumptions and involve a lot of simplification. What the experiments told us is that the simulations are on the right track. Confirming our assumptions brings more trust into the simulation data.”
This work is supported by NASA through the Joint AdvaNced PropUlsion InStitute, or JANUS.
The study, “Carbon Transport in Electric Propulsion Testing – I: Multiscale Computations for Carbon Sputtering by Low Energy Ion Bombardment,” was written by Huy Tran, Sean Clark, Reed Thompson, Deborah Levin, Joshua Rovey, and Huck Beng Chew. It is published in the American Institute of Aeronautics and Astronautics. DOI: 10.2514/6.2024-1135
The work received the 2024 AIAA Electric Propulsion Best Paper from the 2024 AIAA SciTech Forum.