UAH and CFD Research Develop Computer Model to Predict Plasma Striations

Manufacturers that rely on plasma in semiconductor production and materials processing are getting a new way to avoid unwanted plasma stratification. The tool is a computer model of striations developed by The University of Alabama in Huntsville (UAH), part of the University of Alabama System, together with CFD Research Corp. (CFDRC).

Striations are alternating bright and dark layers that appear in gas discharges, and they can be seen in noble gases in some contexts. Dr. Vladimir Kolobov, a principal research scientist in UAH's Center for Space Plasma and Aeronomic Research (CSPAR), has studied them for 30 years. The noble gases include helium, neon, argon, krypton, xenon and mixtures of them. In the neon used inside fluorescent light tubes, he says, the striations move too fast for the naked eye but can be revealed with a stroboscopic technique.

Dr. Kolobov notes that people could see the phenomenon for 200 years, but only recently could it be reproduced in computer simulations. The layers were observed in gas discharges long before Dr. Irving Langmuir introduced the term plasma. "When you can develop a computer model and reproduce them, then you know that you understand this process," he says.

The team used a self-consistent hybrid model of collisional plasma to obtain moving striations in DC discharges of noble gases. According to Dr. Kolobov, the calculated properties of self-excited waves in neon and argon agreed with available experimental data, and the work clarified the origin of helium plasma stability due to stratification. The model relies on a simplified two-level excitation-ionization approach that neglects nonlinear effects from stepwise ionization, gas heating and Coulomb interactions among electrons.

The model offers insight into the conditions under which stratifications form, and Dr. Kolobov describes it as a first step toward a system that could help manufacturers set discharge conditions that discourage them in plasma-based production. Planned refinements would allow study of transitions between different wave types observed experimentally in noble gases. Expanding the plasma-induced chemistry would make it possible to study striations in molecular gases and gas mixtures. "The availability of a computer model for stratified plasma will help select appropriate discharge conditions for specific applications," he says.

The work also connects to space science at CSPAR. Solar wind plasma is wholly ionized, yet it shares many properties with weakly ionized gas discharge plasmas, so the model may be applicable to space weather forecasting. Dr. Kolobov says earlier work showed that electron kinetics in solar wind plasma has much in common with that in gas discharges, and that grid-based kinetic solvers can serve both collisional gas-discharge plasma and collision-less magnetized space plasmas. They have already been demonstrated for electron kinetics in the solar wind. The new solvers should also improve forecasting of space weather storms, the focus of an ongoing NASA SBIR Phase 2 project at CFD Research and UAH.

Dr. Gary Zank, the Aerojet Rocketdyne chair of UAH's Department of Space Science, director of CSPAR and principal investigator for the grants, said the dual use of such models shows the strength of the UAH and CFDRC partnership and the value of cross-disciplinary collaboration. That collaboration is a goal of the FTPP grant, which supports nine institutions across Alabama. He added that it is gratifying to see theory and modeling related to how energetic electrons from the sun's atmosphere carry energy to Earth succeed on practical manufacturing problems involving gas discharges.

The research was funded by the National Science Foundation's Established Program to Stimulate Competitive Research (EPSCoR) Future Technologies enabled by Plasma Processes (FTPP) initiative and its predecessor, Connecting the Plasma Universe to Plasma Technology in Alabama (CPU2AL). UAH leads both grants. The advancement of hybrid kinetic-fluid solver models is a foundational research focus of the new FTPP project.

The work was made possible by grid-based kinetic solvers for electrons in low-temperature collisional plasmas, which Dr. Kolobov says CFD Research and UAH have pursued for almost 25 years. He says the capability was not possible when he began studying plasma stratification 30 years ago in his PhD work, which is why he is so excited about it. The research used commercial COMSOL software and open-source software on UAH computer systems, and figures were produced with Tecplot software available at CSPAR.