Application of Technological Gyrotrons for Plasma-chemical Decomposition of Carbon Dioxide

Modern technological gyrotrons, being one of the most powerful sources of electromagnetic radiation in the millimeter range, are increasingly used to solve various practical applications in plasma chemistry and microwave processing of materials. This paper presents recent results on the application of continuous microwave radiation of gyrotrons for the conversion of greenhouse gases in low-temperature plasma at atmospheric pressure. The efficiency of decomposition of carbon dioxide (CO2) molecules in the plasma, which was created and heated in a waveguide plasmatron by microwave radiation of a gyrotron was compared at different frequencies: 24 GHz (TE11 mode) ⧺ 40.5 GHz (TE03 mode). The power of microwave radiation at both frequencies in the studies did not exceed 600 W. The design of the waveguide plasmatron was calculated and optimized for each of the used frequencies to minimize the reflection of microwave radiation from the narrowing section of the waveguide. Microwave radiation with frequency of 24 GHz allows to maintain discharge in pure CO2 flow up to 3.4 l/min starting from power ~ 500 W, achieving a conversion rate (ratio of the number of dissociated CO2 molecules to their initial number) of ~ 10% and energy efficiency of 12%. The obtained values are close to the record values for atmospheric pressure discharges in pure CO2 without the use of catalysts. As the power increases, the degree of conversion is smoothly reduced, which is associated with an increase in the gas temperature and thermalization of the microwave plasma due to VT-relaxation processes. For operation at 40.5 GHz, the TE03 mode was excited in the gyrotron, which required an increase in the output aperture of the plasmatron, as a result of which the discharge in CO2 could be maintained only in a mixture with argon (Ar). The maximum degree of CO2 conversion was ~ 15% at the ratio Ar: CO2 = 10 : 1, which is twice lower than the values obtained earlier in the discharge in Ar-CO2 mixture with heating at 24 GHz. Thus, to maintain the discharge in pure CO2 and increase the CO2 conversion rate at higher frequency, it is necessary to use a radiation converter in a lower mode (e.g., TE11). Also, the calculations show that taking into account absorption in the plasma makes it possible to reduce the diameter of the exit aperture of the tapered waveguide section keeping the minimum reflection coefficient.