Study of a Fast Gas Heating in a Capillary Nanosecond Discharge. Discharge parameters and temperature increase in the afterglow.

Abstract

Experimental studies and numerical calculations of parameters of a spatially uniform nanosecond discharge in air in a pressure range 3 − 9 mbar have been made in our recently published paper. Temporal behavior of the discharge current, reduced electric field E/N and specific deposited energy were measured. The data was used for development and verification of a numerical model of kinetics in the discharge. Due to the relatively small diameter of the discharge tube, the current density was high enough (160-200 A/cm). This provided high specific deposited energy (0.1−0.13 eV/molecule) at reduced electric fields of 200−400 Td. The evolution of gas temperature has been studied in the discharge and in the afterglow. The temperature was measured from the rotational structure of the second positive system of molecular nitrogen. The data in the afterglow was obtained with the help of additional nanosecond pulses of relatively low intensity. Thus, fast gas heating in an N2:O2=4:1 mixture has been measured with well–controlled electric fields and specific energy input during the discharge stage for E/N = 200 − 400 Td. The results prove that the main energy input to gas heating takes place at time typical for quenching of electronically excited states of molecular nitrogen. The main energy release in the model takes place in reactions of quenching of electronically excited nitrogen molecules, such as N2(AΣu , BΠg, CΠu, a’Σu ) by oxygen, quenching of excited O(D) atoms by N2, and in reactions of nitrogen and oxygen dissociation by electron impact. These processes provide more than 80% of total gas heating. An agreement between experimental data and results of calculations of gas temperature has been obtained for pressures 3, 6 and 9 mbar. The observed temperature increase in the afterglow is connected with the relaxation of electronic excitation, namely relaxation of O(D) and N(D) atoms and N2(AΣu , BΠg, CΠu, a’Σu ) molecules, so the suggestions concerning the mechanism of fast gas heating in were confirmed experimentally. Typical times of these reactions do not exceed a few microseconds, which is why under our experimental conditions the main energy relaxation takes place during 50− 100 microseconds. During this time, about 24% of the discharge energy goes to fast gas heating. A few mechanisms describing fast gas heating in nitrogen-oxygen mixtures have been recently suggested. Paper presents results of experiments in a surface nanosecond dielectric barrier discharge (SDBD) in air at atmospheric pressure together with numerical calculations. To explain high values of temperature increase at a given energy input, the authors suggest that at high electric fields and high pressures all the energy released in ion–molecular reactions and in ion–ion recombination is spent on gas heating and leads to