Molecular excitation and dissociation in RF and ns-pulsed plasmas

Non thermal plasmas have been used in medicine as well as for CO2 dry reforming. For both application fields, molecular excitation and dissociation mechanisms are fundamental. In our work, we study the effect of pulse parameters and RF-excitation on molecular plasma chemistry.

A capacitively coupled atmospheric pressure plasma jet composed of two rectangular stainless-steel electrodes of 100 mm length separated by a 1 mm gap is powered by a 13.56 MHz RF signal or a ns-pulser. Helium flows through the inlet of the jet at 5 standard liters per minute with a 0 to 2% molecular admixture. The effluent of the plasma jet is collected and directed by a glass tube to a glass cell, where FIR emission spectroscopy is conducted by an InGaAs photodiode mounted on a narrow bandpass filter centered at 1270 nm [2]. The acquired signal is amplified by an operational amplifier before the signal is measured with an oscilloscope. This signal is calibrated to absolute concentrations of ¹O₂ by a conversion factor determined through a previously conducted ray-tracing Monte Carlo simulation of the transmission and reflection properties of the glass cell.

The electric excitations are measured by a high voltage probe (Cal Test–CT4028). The shape of the pulse is simulated and compared with experimental results by VI-VIEW [3], a tool allowing users to simulate ns-pulse propagation in coaxial cables depending on the load circuit and probe location yielding a better understanding of the “truly seen pulse” by the plasma.

The He-O₂ and the He-CO₂ plasma chemistry is modelled with a reduced set of reactions in a plasma chemistry simulation with a Boltzmann solver and a 0D chemical reaction model. ¹O₂ concentration in the effluent of the plasma is calculated based on the validated reduced electric field measured at the electrode. It is shown that modifying the electric pulse shape parameters and oxygen admixture concentration in a helium atmospheric pressure discharge yields maximum production of ¹O₂ and CO₂ dissociation. A better understanding of ns-pulse propagation allows accurate and simple modelling of He-O₂ and He-CO₂ chemistry in the plasma effluent.