Self-consistent modeling of electric fields and microwave nitrogen plasmas: electromagnetic shielding as a limiting mechanism

Abstract

High-pressure microwave plasmas offer an energetic medium for nitrogen dissociation and chemical conversion. Optimizing experimental parameters such as reactor geometry, gas flow, and power input is crucial in maximizing the yield and energy efficiency of processes like NO x production. However, these parameters influence the plasma outputs through non-linear internal quantities, particularly the power deposition density. Accurate prediction of power deposition in microwave nitrogen plasmas is achieved by coupling a 1D fluid model with a novel 2D electromagnetic solver tailored for low-temperature plasmas. The results reveal that the gas temperature and the electron densities, the main governing factors of nitrogen dissociation, are critically sensitive to the power deposition profile. Results also show that, for a given plasma conductivity profile, the geometry of the waveguide system significantly impacts the spatial distribution of the deposited power for the same plasma conductivity profile. Parametric studies demonstrate that electromagnetic shielding at low pressures and high input powers redistributes the energy away from the core to the plasma periphery. At these operating conditions, the outer power deposition drives the electron density to the boundary of the plasma in a positive feedback loop. Electromagnetic shielding introduces a distinct saturation mechanism for the electron density, temperature and, consequently, dissociation yield. At 200 mbar and 10 slm, electron density and gas temperature do not increase locally beyond 2.5×1018 m −3 and 6350 K, respectively, limited by both the energy sink of the nitrogen dissociation and the electromagnetic shielding.