Influences of gas flow on gas temperature and discharge mode in dielectric barrier discharge of nitrogen at atmospheric pressure

Dielectric barrier discharge in nitrogen at atmospheric pressure is studied with the spectroscopy and the fast photography of the discharge. By the introduction of a nitrogen flow into the discharge gap, the homogeneous discharge in a 2 mm gap can be maintained. Based on the waveform of the discharge current characterized by a current pulse per half cycle of the applied voltage and the 1 mu s exposure discharge photograph showing a luminous layer covering the entire surface of the anode, the homogeneous discharge is identified with a Townsend discharge. The instrumental broadening of the spectrometer used in the experiment is calibrated with a helium-neon laser. The data relevant to the instrumental broadening are input into a code called Specair for calculating the spectrum profiles of 0-2 band in the second positive system of nitrogen molecules at different gas temperatures. By fitting the calculated spectrum profiles to the experimental one, the rotational temperature of the nitrogen molecules is determined. The results show that the dielectric barrier Townsend discharge in nitrogen at atmospheric pressure cannot heat the nitrogen to a high temperature (Delta Tg <= 50 K) and the small rising in temperature does not induce the thermal instability that leads to the transition of the Townsend discharge to a filamentary discharge. By the addition of a gas flow into the discharge gap, the nitrogen is indeed cooled down to a lower temperature. However, it is not the reason for the Townsend discharge to be maintained. By comparing the discharge spectra with and without the gas flow, it could be concluded that the gas flow much reduces the density of the impurity oxygen desorbed from the dielectric by the discharge and makes it possible for more nitrogen metastables to survive to the beginning time of the next discharge and to provide sufficient seed electrons which are necessary for Townsend discharge.