Speaker
Description
Inductively coupled plasma (ICP) wind tunnels such as the VKI Plasmatron and the Plasmatron X at the University of Illinois Urbana-Champaign reproduce the enthalpy and pressure of atmospheric entry for material testing. Characterizing the plasma they deliver is difficult: probes perturb the flow, spectroscopic and laser diagnostics are demanding, and each measurement returns one quantity at a time. Out of equilibrium, even a complete set of measurements gives only a partial picture, because temperatures, internal populations and composition are no longer tied together by equilibrium relations. Prediction must therefore combine modeling and experiment. This presentation gives an overview of our group’s activities in this direction.
The computational backbone is a modular multiphysics framework [1, 2]: HEGEL, a finite-volume solver for the non-equilibrium plasma flow; FLUX, a finite-element solver for the induction field of the coil; PLATO, a library providing thermodynamics, transport and kinetics at any level of fidelity; CHYPS and PATO, the material response solvers; and MURP, a radiative transfer solver. Coupling is managed through the preCICE library, and the quantities exchanged are shown in Fig. 1.
Figure 1. Coupled framework for ICP facility simulation: HEGEL (plasma), FLUX (electromagnetics), CHYPS and PATO (material response) and MURP (radiation), with the quantities exchanged between them.
Facility-scale simulations rely on simplified descriptions, local thermodynamic equilibrium or multi-temperature models such as Park’s two temperature model. For the more challenging conditions, where internal populations depart from Boltzmann distributions, the multi-group maximum entropy method coarse-grains the state-to-state master equations into a small number of groups solved fully coupled with the flow [3]. Gas-surface chemistry is described either by finite-rate surface reactions or by equilibrium ablation in the form of B′ tables. The talk addresses three questions. The first is the state of the gas in the discharge region,vwhere the coil heating drives the plasma out of equilibrium: Hegel applied to the VKI Plasmatron reveals ionization non-equilibrium in the core and recombination non-equilibrium near the walls. In the hot core the internal populations reach a quasi-steady state, so that global rates for reduced models can be derived from the state-specific kinetics; in the shell surrounding it the quasi-steady-state assumption breaks down, and the populations must be evolved in time by solving the master equations, an approach often referred to as a collisional-radiative model.
The second is the importance of radiative transfer. Murp computes line-by line spectra for nitrogen and air plasmas over a wide range of wavelengths with quasi-steady-state populations of the excited states, reduced by opacity binning and coupled back into the plasma energy equation. In Plasmatron X radiative losses are negligible below 5 kPa but reach 32 % of the coupled power in nitrogen and 22 % in air at atmospheric pressure and 350 kW [4]. The third is whether the operation of the entire facility can be simulated predictively, taking as inputs only the facility settings, such as power, mass flow rate, chamber pressure and coil geometry, with no parameter tuned to the experiment. Coupled plasma-material simulations of hemispherical graphite samples in Plasmatron X at 55 kW and 5.5 to 20 kPa, run in this way, reproduce measured surface temperatures within 12 % and recession rates within 10 % [5].
First steps in the validation of the gas-surface chemistry are also presented: laser-induced fluorescence imaging of O, N and N O above cooled copper samples in Plasmatron X resolves sub-millimetre gradients of temperature and species density, which the simulations reproduce only with the reaction-specific catalytic model [6].
References
[1] Munafò, A., Kumar, S., Jo, S. M., and Panesi, M., “Hegel: A High-Fidelity Flexible Software for Hypersonics and Plasma Simulations,” AIAA SciTech 2024 Forum, AIAA Paper 2024- 0449, 2024, doi:10.2514/6.2024-0449.
[2] Munafò, A., and Panesi, M., “Plato: A High-Fidelity Library for Multicomponent Gases and Plasmas,” Journal of Thermophysics and Heat Transfer, Vol. 39, No. 4, 2025, pp. 850-870, doi:10.2514/1.T7097.
[3] Kumar, S., Munafò, A., Jo, S. M., and Panesi, M., “Investigation of Non-Equilibrium Phenomena in Nitrogen RF Inductively Coupled Plasma Discharges: A State-to-State Approach,” Journal of Physics D: Applied Physics, Vol. 58, 2025, 025204, doi:10.1088/1361- 6463/ad80a4.
[4] Kumar, S., Jo, S. M., Munafò, A., and Panesi, M., “Impact of Non-Equilibrium Radiation in a High-Enthalpy Inductively Coupled Plasma Wind Tunnel,” Aerospace Science and Technology, Vol. 179, 2026, 113508, doi:10.1016/j.ast.2026.113508.
[5] Kumar, S., Munafò, A., Vollmer, B., Franco, M., Stephani, K. A., Bodony, D. J., and Panesi, M., “From Coils to Surface Recession: Multiphysics Simulation of Ablation in ICP Wind Tunnels,” arXiv preprint arXiv:2602.15500, 2026.
[6] Meyers, J. M., Singh, A., Munafò, A., and Panesi, M., “Gas Surface Interaction Reactant and Product Characterization in High Enthalpy Air Plasma Flows via nsTALIF/LIF Imaging in the UIUC 350 kW Plasmatron X Facility,” AIAA SciTech 2026 Forum, AIAA Paper 2026-2463, 2026, doi:10.2514/6.2026-2463.
Summary
An overview of the multiphysics framework, kinetic and radiative models, and validation campaigns developed to simulate inductively coupled plasma facilities such as the VKI Plasmatron and the UIUC Plasmatron X from the coil current to the material surface.