Time evolution of sputtered metal ions in high power impulse magnetron sputtering (HiPIMS) discharge with a positive voltage pulse applied after a negative one (regime called “bipolar pulse HiPIMS”—BPH) is studied using 2-D density mapping. It is demonstrated that the ion propagation dynamics is mainly affected by the amplitude and duration of the positive pulse. Such effects as ion repulsion from the cathode and the ionization zone shrinkage due to electron drift towards the cathode are clearly observed during the positive pulse. The BPH mode also alters the film crystallographic structure, as observed from X-ray diffraction analysis.

Significant improvement in the energy efficiency of plasma-assisted CO ₂ conversion is achieved with applied power modulation in a surfaguide microwave discharge. The obtained values of CO ₂ conversion and energy efficiency are, respectively, 0.23 and 0.33 for a 0.95 CO ₂ + 0.05 N ₂ gas mixture. Analysis of the energy relaxation mechanisms shows that power modulation can potentially affect the vibrational–translational energy exchange in plasma. In our case, however, this mechanism does not play a major role, likely due to the low degree of plasma non-equilibrium in the considered pressure range. Instead, the gas residence time in the discharge active zone together with plasma pulse duration are found to be the main factors affecting the CO ₂ conversion efficiency at low plasma pulse repetition rates. This effect is confirmed experimentally by the in situ time-resolved two-photon absorption laser-induced fluorescence measurements of CO molecular density produced in the discharge as a result of CO ₂ decomposition.

A pulsed microwave surfaguide discharge operating at 2.45 GHz was used for the conversion of molecular nitrogen into valuable compounds in several gas mixtures: N ₂ :O ₂ , N ₂ :O ₂ :CO ₂ and N ₂ :CO ₂ . The ro-vibrational absorption bands of the molecular species were monitored by a Fourier transform infrared apparatus in the post-discharge region in order to evaluate the relative number density of species, specifically NO production. The effects of specific energy input, pulse frequency, gas flow fraction, gas admixture and gas flow rate were studied for better understanding and optimization of the NO production yield and the corresponding energy cost (EC). By both the experiment and modelling, a highest NO yield is obtained at N ₂ :O ₂ (1:1) gas ratio in N ₂ :O ₂ mixture. The NO yield reveals a small growth followed by saturation when pulse repetition frequency increases. The energy efficiency start decreasing after the energy input reaches about 5 eV/molec, whereas the NO yield rises steadily at the same time. The lowest EC of about 8 MJ mol ⁻¹ corresponding to the yield and the energy efficiency of about 7% and 1% are found, respectively, in an optimum discharge condition in our case.

The reactive chemical species generated by non-equilibrium plasma under atmospheric pressure conditions are key enablers for many emerging applications spanning the fields of biomedicine, manufacturing and agriculture. Despite showing great application potential, insight in to the underpinning reactive species generation and transport mechanisms remains scarce. This contribution focuses on the spatiotemporal behaviour of reactive nitrogen species (RNS) created and transported by an atmospheric pressure air surface barrier discharge (SBD) using both laser induced fluorescence and particle imaging velocimetry measurements combined with experimentally validated numerical modelling. It was observed that highly reactive species such as N are confined to the discharge region while less reactive species such as NO, NO₂ and N₂O closely followed the induced flow. The concentration of key RNS was found to be in the 10–100 ppm range at a position of 25 mm downstream of the discharge region. A close agreement between the experimental and computational results was achieved and the findings provide a valuable insight in to the role of electrohydrodynamic forces in dictating the spatiotemporal distribution of reactive chemical species beyond the plasma generation region, which is ultimately a key contributor towards downstream treatment uniformity and application efficacy.

We report a very low energy cost (EC) for nitric monoxide (NO) radical formation found in atmospheric spark discharge. For this purpose non-equilibrium air discharges working in spark and glow regimes were studied showing the optimum performance in the vicinity of the spark-to-glow transition. The minimum EC for NO production of about 80 eV/molecule (calculated based on the total direct current power applied to the discharge generator) is achieved in the spark regime before the transition, whereas the maximum NO yield (of about 10 ⁴ part per million) corresponds to the glow regime. Based on the estimated power absorbed in plasma the EC below 4 eV/molecule is achievable in the spark regime, which is close to the Zeldovich reaction enthalpy of NO formation (about 3 eV/molecule). The result implies that the energetic efficiency of a single spark likely exceeds that of the modern Haber–Bosch cycle (with EC of about 5 eV/molecule in the case of ammonia, according to Patil et al (2015 Catal. Today 256 49). The found low EC is associated with the discharge non-equilibrium caused by short spark duration.