Spectral and Electric Diagnostics of Low-current Arc Plasmas in CO2 with N2 and H2O admixtures

Abstract Plasma diagnostics is a key tool to support the further development of plasma-induced chemical conversion of greenhouse gases (such as CO 2 ) into high-value chemicals. For this reason, spectroscopic and electric measurements of low current (below 1.7 A), stationary arc plasmas in CO 2 at atmospheric pressure with addition of N 2 or H 2 O are reported. High-speed photography, imaging emission spectroscopy and time-resolved electrical measurements are used to obtain time-space resolved gas temperatures as well as the electric-field current characteristics of the discharge. It is found that the lowest average electric field in a CO 2 arc plasma at atmospheric pressure is ∼20 kV mm −1 at a current between 0.8 and 1 A. If the current decreases below this level, the arc remains in vibrational–translational (VT) equilibrium by increasing the electric field. However, VT equilibrium conditions can be only maintained until a threshold minimum current of 0.33 ± 0.05 A, at which the arc transitions into a non-equilibrium condition with further increasing electric fields (reaching 68 ± 15 V mm −1 at 0.03 A). The addition of N 2 or H 2 O did not influence the electrical characteristics of the CO 2 arc within to the tested mixtures. However, there is only a significant decrease in the electric field of the formed transition arcs and the threshold minimum current in the presence of N 2 . The spectra of the low-current CO 2 arc is found to be dominated by emission from the C 2 Swan band system and the O I 777 nm triplet peak. However, the CN band dominates the spectra even when small amounts (0.5 wt%) of N 2 is present in the plasma. The gas temperature at the axis of the CO 2 arc plasma decreased slightly with decreasing current, from an estimated 7000 K at 1 A down to 6300 K at 0.4 A. The thermal radius of the arc is estimated to be larger than 1.2 mm, more than two times larger than the optical radius obtained from the emitted radiation. The addition of N 2 and H 2 O (up to 7 and 9 wt% respectively) lead to only to a 500 K decrease in the axial arc temperature.