Fast Imaging Reveals the Dynamic of Plasma Discharge sustained with Ultra-short Microwave Pulses at Atmospheric Pressure

Energy efficient conversion of stable CO₂ and CH₄ molecules is attractive for renewable energy storage as well as for mitigation of green-house emissions and closing the carbon cycle. Among the variety of Power-to-X technologies, conversion technologies using plasmas show high potential for the efficient use of intermittent renewable energies. To date, the highest efficiency (above 80 %) of the plasma assisted conversion of CO₂ into CO is reported in experiments with microwave-sustained plasmas [1].

Supplying microwave energy into the plasma discharge not continuously but rather in short pulses with a modulation time comparable with characteristic vibrational-to-translational energy transfer times, promotes the efficiency of the process. At atmospheric pressure, the demand on the fastness of energy pulsations is much higher than at vacuum conditions. Recently, we have reported on the beneficial effect of pulsation approach implemented for atmospheric CO₂ plasma where the optimum was found for the pulse time of several microseconds [2]. It was shown that the process is very likely favored by generation of non-equilibrium plasma states and by mitigation of gas temperature enabled through the control of pulse and inter-pulse times. Fast imaging employed in that study has provided a reliable monitor of the plasma discharge shape and intensity with a nanosecond time resolution. It correlates well with T_gas (t) history measured with ultra-fast optical emission spectroscopy (OES).   

In present paper and by employing fast imaging and OES diagnostics, we analyze plasma sustained with different duty cycles and pulse and inter-pulse times at atmospheric pressure. With a time resolution down to 20 ns we investigate the re-ignition phase of discharge and its relaxation after end of pulse and correlate it with a gas temperature and with spectral characteristics of the emitted light in the range of 250 to 950 nm as well as with a process efficiency. 

[1] Fridman, A. Plasma chemistry, Cambridge University Press, 2008

[2] Sergey Soldatov, Guido Link, Lucas Silberer, Clara Marie Schmedt, Emile Carbone, Federico D’Isa, John Jelonnek, Roland Dittmeyer, and Alexander Navarrete, ACS Energy Lett. 2021, 6, 124-130