This study compares dry plasma and plasma/liquid systems in terms of their energy efficiencies for guiding researchers and engineers in selecting reactors and intensifying the plasma-based “zero CO2” nitrogen fixation process toward NO _x production.

Experimental data on the evolution of a helium atmospheric pressure plasma jet driven by two different voltage waveforms are presented. The characteristics of directed ionization waves (guided streamers) were compared for a sinusoidal voltage waveform with a frequency of 52 kHz and a voltage waveform that was formed via the superposition of 41.6 kHz bipolar square pulses and 300 kHz oscillations. With the sinusoidal voltage, two consecutive ionization waves were observed. With a special tailoring voltage, control of the guided streamer propagation in a stepwise mode was achieved. The observed second streamer and the second step of propagation could be regarded as a secondary ionization wave for both voltages. A change in the voltage waveform led to significant variations in the secondary ionization wave formation and streamer parameters. The voltage waveform enabled the number of ionization waves and their propagation to change, which provided the possibility of controlling the plasma parameters of the jet.

Plasma-based nitrogen fixation, leveraging non-thermal plasma technologies, offers a promising alternative to conventional processes, with the potential to decentralize fertilizer production, reduce CO₂ emissions, and employ renewable energy sources.

In this work, the kinetics of nitrogen fixation via plasma-induced N ₂ oxidation in a 10 ns pulsed atmospheric pressure water-contacting discharge sustained in air is investigated. Two pulse regimes, a single pulse and a three-pulse burst of 100 kHz, are considered. The densities of relevant radicals (NO, O) are studied by time- and space-resolved laser-induced fluorescence spectroscopy. It is concluded that in a single pulse mode, O atoms are mainly generated by O ₂ reacting with electronically excited states of N ₂ ( A 3 Σ u + , B 3 Π g , C 3 Π u ) and are primarily reduced as a result of O ₃ formation. The O density shows a maximum at ∼100 ns after the plasma pulse with number density of ∼10 ²³ m ⁻³ . NO radicals, on the other hand, are primarily formed by reacting with the N 2 A 3 Σ u + state (up to ∼1 μ s after the pulse) and with OH radicals (up to ∼10’s of μ s), peaking at approximately 60 μ s with a peak density of ∼10 ²¹ m ⁻³ . The NO loss pathway is represented by the reversed Zeldovich mechanism at short time delays (∼10’s μ s), whereas at longer delays (>100’s of μ s) HNO ₂ and NO ₂ formation causing NO loss start to be dominant. In the burst mode, the energy efficiency of NO formation decreases despite higher N ₂ conversion, for which three reasons are suggested: (1) NO removal by the generated O ( D 1 ) after the discharge pulse through the reverse Zeldovich mechanism, (2) NO oxidation via the accumulated O ₃ , (3) pre-ionization induced by high pulse repetition rate (100 kHz) leading to shrinkage of the plasma bulk.