Thermal instability and volume contraction in a pulsed microwave N₂ plasma at sub-atmospheric pressure

Abstract

We studied the evolution of an isolated pulsed plasma in a vortex flow stabilised microwave (MW) discharge in N2 at 25 mbar via the combination of 0D kinetics modelling, iCCD imaging and laser scattering diagnostics. Quenching of electronically excited N2 results in fast gas heating and the onset of a thermal-ionisation instability, contracting the discharge volume. The onset of a thermal-ionisation instability driven by vibrational excitation pathways is found to facilitate significantly higher N2 conversion (i.e. dissociation to atomic N2) compared to pre-instability conditions, emphasizing the potential utility of this dynamic in future fixation applications. The instability onset is found to be instigated by super-elastic heating of the electron energy distribution tail via vibrationally excited N2. Radial contraction of the discharge to the skin depth is found to occur post instability, while the axial elongation is found to be temporarily contracted during the thermal instability onset. An increase in power reflection during the thermal instability onset eventually limits the destabilising effects of exothermic electronically excited N2 quenching. Translational and vibrational temperature reach a quasi-non-equilibrium after the discharge contraction, with translational temperatures reaching ∼1200 K at the pulse end, while vibrational temperatures are found in near equilibrium with the electron energy (1 eV, or ∼11 600 K). This first description of the importance of electronically excited N2 quenching in thermal instabilities gives an additional fundamental understanding of N2 plasma behaviour in pulsed MW context, and thereby brings the eventual implementation of this novel N2 fixation method one step closer.

A Modified Fokker–Planck Approach for a Complete Description of Vibrational Kinetics in a N2 Plasma Chemistry Model

“…Note that, with respect to the state-of-the-art vibrational kinetics scheme proposed by Guerra et al, 29 collisions between molecules excited to any vibrational level v and molecules excited to the vibrational level i (V−V i processes) with i > 2 are not included. This is due to their progressively less relevant contribution to the VDF, at values of electron density commonly found in MW nitrogen discharges, 10,30 which will be shown in the Results.…”

“…The temporal evolution of the population of vibrationally excited states of any molecule is described by the Master Equation (ME), a system, in the investigated case, containing as many ordinary differential equations (ODEs) as the number of vibrational levels, including hundreds of rate coefficients describing relaxation and excitation processes. Insights into the role of vibrational excitation on gas heating and chemical kinetics in plasmas have been provided using the State-to-State (STS) approach, − in which relevant internal energy states are treated as individual pseudospecies, whose evolution is described by rate balance equations. However, the computational efficiency of the STS approach decreases quickly as the number of vibrational states increases.…”

Relaxation of electronic excitation in nitrogen/oxygen and fuel/air mixtures: fast gas heating in plasma-assisted ignition and flame stabilization

“…This scenario is called ionization-heating instability [27]; in this case, the discharge contraction can significantly affect the ignition of combustible mixtures. This instability is observed in direct current (DC) discharges [27], [29], microwave discharges [33], [34], [35] and some other plasma types. Gas heating can also cause different discharge modes: it was shown in [36] that transition from Trichel pulses to pulseless glow mode of negative DC corona discharge in atmospheric pressure air is controlled by fast gas heating.…”

Producing oxygen and fertilizer with the Martian atmosphere by using microwave plasma

“…Further details can be found in our earlier works. 36,71,101,102 The Martian air chemistry (i.e., CO 2 /N 2 /Ar) employed here is assembled from our earlier works. 71,74,103 The model includes 149 species, i.e., the electrons, CO 2 , Ar, N 2 , O 2 , CO, various N x O y molecules in the ground state, and various (vibrational and electronic) excited levels, various radicals, atoms, and ions (see Table 1).…”