Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: I. Measurement procedure and results

Abstract

The non-equilibrium ro-vibrational distribution functions of molecules in a plasma can heavily influence the discharge operation and the plasma-chemistry. A convenient method for measuring the distribution function is coherent anti-Stokes Raman scattering (CARS). CARS spectra are measured in a ns-pulsed plasma between two parallel, 1 mm spaced molybdenum electrodes in nitrogen at 200 mbar with pulse durations of 200 ns/250 ns and a repetition rate of 1 kHz. The CARS spectra are analyzed by a fitting routine to extract information about the vibrational excitation of the nitrogen molecules in the plasma. It is found that during the discharge the vibrational distribution for v ≲ 7 can be described by a vibrational two-temperature distribution function. Additionally, the electric field is measured by the electric field induced second harmonic generation method during the discharge pulse. It is found to be constant in time after the initial ionization wave with values close to 81 Td for the investigated conditions. During the afterglow between two discharge pulses a more general fitting approach is used to obtain the population differences of two neighboring vibrational states. This allows to capture the more complex vibrational dynamics in that time period. The measurement results are discussed in more detail and compared to simple plasma models in a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305205).

Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: II. Analysis

“…This works reasonably well for their measurements as they measure only up to the third vibrational state to the limited spectral range. In our measurements in [1] and previous CARS measurements in different plasma sources it can be seen that usually the higher vibrational states are overpopulated compared to a Boltzmann distribution determined by the first two or three vibrational states. For this reason a distribution function is chosen which includes two vibrational temperatures and one rotational, either 1: Voltages (solid) and currents (dashed) at the cathode used for the measurements in [1].…”

“…In our measurements in [1] and previous CARS measurements in different plasma sources it can be seen that usually the higher vibrational states are overpopulated compared to a Boltzmann distribution determined by the first two or three vibrational states. For this reason a distribution function is chosen which includes two vibrational temperatures and one rotational, either 1: Voltages (solid) and currents (dashed) at the cathode used for the measurements in [1]. called vibrational two-temperature distribution or simply two-temperature distribution function (TTDF) in the following.…”

“…T vib = 5000 K where N v+1 Nv ≈ 0.5. For this reason in our companion paper [1] a different approach is used. Similar to CARS measurements in thermal equilibrium a distribution function is assumed for fitting theoretical spectra to measured ones, like it was done already by Messina et al [13] in a plasma burner.…”

“…called vibrational two-temperature distribution or simply two-temperature distribution function (TTDF) in the following. As the TTDF is motivated by the underlying plasma physics in the discharge, a detailed derivation was omitted in [1] where the focus was on the diagnostic method. In the present paper we motivate the use of the TTDF based on the excitation processes in the plasma in section 3.1.…”

“…Additionally, simple models are derived for the parameters in the TTDF connecting them to the plasma parameters. Finally, the population difference results from [1] in the afterglow where the TTDF is not valid anymore are compared to a simple kinetic model for the vibrational subsystem in section 3.2.…”