Characterisation of CH4 nanosecond pulsed plasma across a wide pressure range (0.5–2.0 bar)

This study explores the electrical, optical, and thermal properties of nanosecond pulsed plasmas at pressures ranging from 0.5 to 2.0 bar in a pure CH4 atmosphere. The plasma pulse energy was assessed using various techniques to identify the most accurate method, while outlining the limitations of others. The most accurate method indicates a plasma pulse energy between 6 and 15 mJ that drops with increasing pressure at a constant applied voltage. Optical characterisation involved time-resolved discharge dynamics through high-resolution intensified charge-coupled device imaging with optical filters and optical emission spectroscopy. This approach allowed to assess the pressure effect on plasma volume and electron density, which were found to range from 0.66 to 1.00 mm3 and 7.0–10.5 × 1017 cm−3, respectively. The high electron density indicates that the plasma operates in spark mode, achieving ionisation levels up to 5.6%. Additionally, the gas heating kinetics were explored using CH(A) and C2 Swan band emissions, validated by Rayleigh scattering spectroscopy, revealing the gas heating dynamics up to 35 µs. The peak temperature decreases from 3082 K to 1070 K upon increasing pressure from 0.5 to 2.0 bar at constant plasma pulse energy of 12 mJ. Soot generation was also investigated, revealing a significant pressure dependence, with the onset of particle growth time increasing exponentially from 15 μs at 0.5 bar to 60 μs at 2.0 bar. These findings suggest that this nanosecond pulsed spark plasma operates in a highly non-equilibrium state, with high electron density. Altogether, this work offers a comprehensive characterisation of a CH4 nanosecond pulsed plasma, establishing a foundation for optimising its application in hydrocarbon conversion processes.