Thermal instability and volume contraction in a pulsed microwave N-2 plasma at sub-atmospheric pressure

We studied the evolution of an isolated pulsed plasma in a vortex flow stabilised microwave (MW) discharge in N-2 at 25 mbar via the combination of 0D kinetics modelling, iCCD imaging and laser scattering diagnostics. Quenching of electronically excited N-2 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 N-2 conversion (i.e. dissociation to atomic N-2) 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 N-2. 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 N-2 quenching. Translational and vibrational temperature reach a quasi-non-equilibrium after the discharge contraction, with translational temperatures reaching similar to 1200 K at the pulse end, while vibrational temperatures are found in near equilibrium with the electron energy (1 eV, or similar to 11 600 K). This first description of the importance of electronically excited N-2 quenching in thermal instabilities gives an additional fundamental understanding of N-2 plasma behaviour in pulsed MW context, and thereby brings the eventual implementation of this novel N-2 fixation method one step closer.