Uncrewed Systems Technology 049 - April/May 2023

12 Platformone April/May 2023 | Uncrewed Systems Technology Researchers at the University of Illinois Urbana-Champaign have developed a technique that will help determine the lifetime of electric propulsion systems for autonomous space vehicles (writes Nick Flaherty). The team used data from low-pressure chamber experiments and large-scale computations to develop a model on a supercomputer to better understand the effects of ion erosion on carbon surfaces as the first step in predicting its failure. Electric space propulsion systems use energised atoms to generate thrust. These high-speed beams of ions bump against the graphite surfaces of the thruster, eroding them a little with each hit, and are its primary lifetime- limiting factor. When ion thrusters are tested on the ground in an enclosed chamber, the ricocheting particles of carbon from the graphite chamber walls can also be deposited back onto the thruster surfaces. This changes the thruster’s measured performance characteristics. “We need an accurate assessment of the ion erosion rate on graphite to predict thruster life, but testing facilities have reported varying sputtering [ion erosion] rates, leading to large uncertainties in predictions,” saidHuy Tran, a PhD student in theDepartment of Aerospace Engineering at UIUC, whoworked on the project. The research is part of NASA’s Joint Advanced Propulsion Institute, which includes researchers at nine universities, including UIUC. The simulations were performed using the Delta supercomputer at Illinois. A particular difficulty is replicating the environment of space in a laboratory chamber, because it is hard to build a sufficiently large chamber to avoid ion- surface interactions at the chamber walls. Although graphite is typically used for the accelerator grid and pole covers in the thruster, there isn’t agreement on which type of graphite is the most resistant to erosion. “The fundamental problemwith testing an ion thruster in a chamber is that the thruster is continuously spitting out xenon ions that also impact with the chamber walls, which are made from graphite panels, but there are no chamber walls in space,” said Tran. “When these xenon ions hit the graphite panels, they also sputter out carbon atoms that are redeposited on the accelerator grids. So instead of the grid becoming thinner and thinner because of thruster erosion, some people have seen in experiments that the grids actually get thicker with time because the carbon is coming back from the chamber walls.” The simulations resolved the limitations and uncertainties in the experimental data. “Whether it is pyrolytic graphite on the gridded ion optics, isotropic graphite on the pole covers, or poco graphite or anisotropic graphite on the chamber walls, our molecular dynamics simulations show that the sputtering rates and mechanisms are identical across all these different referenced structures,” said Huck Beng Chew, associate professor at UIUC and Tran’s supervisor. The sputtering process creates a unique carbon structure during the bombardment process. “When the ions damage the surface, they are transformed into an amorphous-like structure regardless of the initial carbon structure,” said Prof Chew. “You end up with a sputtered surface with the same unique structural characteristics. That is one of the main findings we have observed from our simulations. “The model we developed bridges the molecular dynamics simulation results and the experimental data. The next thing we want to look at is the evolving surface morphology over time as you put more and more xenon ions into the system. This is relevant to ion thrusters for deep space exploration.” Space vehicles Thruster lifespan tester High-speed ion erosion of a thruster’s graphite surfaces are its primary life-limiting factor