To investigate the influence of pulse frequency on microwave plasma and the underlying mechanism of nitrogen fixation

a high-speed camera was employed to capture the morphological evolution of the plasma under different pulse frequencies

while Fourier-transform infrared spectroscopy (FTIR) was applied to quantify the energy cost for NOx. Furthermore

local thermodynamic equilibrium (LTE) modelling was conducted to simulate plasma development and the steady-state temperature distribution under various pulse frequencies. The experimental results reveal that there is a markedly distinction in plasma morphologies across the tested frequency range. The spatio-temporal development of the plasma morphology is analyzed

and the “characteristic luminous area” (CLA) is defined. The temporal variation of CLA exhibits a clear periodicity that coincides with the pulse modulation cycle. A positive correlation is observed between the CLA and the energy cost for NOₓ synthesis at different pulse frequencies. The LTE simulations demonstrate a positive correlation between local temperature and CLA

indicating that the inhomogeneous temperature within the plasma is a key factor responsible for the consistent trend of change between energy cost and CLA. The newly proposed CLA offers an alternative analytical perspective for investigating the influence mechanism of pulse frequency on plasma behavior. Meanwhile

the proposed morphological parameters enable the discrimination between the energy-active luminescence zone and the chemically active luminescence zone

thereby providing new insights into the contribution and role of microwave plasma afterglow in NOₓ synthesis processes.