Influences of gas flow on atmospheric pressure glow discharge in helium

Atmospheric pressure glow discharge (APGD) was produced in a 5-mm helium gap between two plane-parallel electrodes of 50 mm in diameter, each covered by a 1-mm thick quartz plate. The influence of the helium gas flowing in parallel through the helium gap on APGD was studied. The helium flow rate varies up to 12 liter per minute, corresponding to helium at a speed of 67 cm/s flowing through a 5 mm x 60 mm cross section of the gap. The discharge current pulse appearing per half cycle of the applied voltage shifts forward as the helium flow speed increases. In accord with this phase shifting of the current pulse, the breakdown voltage of the helium gap that was deduced from the measured applied voltage and discharge current decreases from 1200 V to 950 V. Both amplitude, i_m, and pulse width (FWHM), t_w, of the current pulse vary non-monotonically with the helium flow speed and are with same an inflexion point at flow speed of 1.4 cm/s. While im decreases from 31 mA to 20 mA and then slowly increases to 26 mA, t_w increases from 750 ns to 1350 ns and then slowly decreases to about 900 ns. Although im varies with the flow speed in a way contrary to that of t_w, the transferred charge calculated by integrating current over the time of one current pulse keeps almost a constant for different flow speed, which is consistent with the concept that the dielectric barrier acting as a capacitor. The side-view photographs of the discharge gap were taken by an ICCD camera with an exposure time of 20 ns. Compared with that in the case without helium flow, the discharge light with helium flow is relatively weaker over entire gap due to smaller discharge current. A distinct change in the discharge pattern with helium flow is that positive column gets shorter and Faraday dark space gets wider as the flow speed increases. It is important to find that spectrum line of 391.4 nm from the first negative system of nitrogen molecular ions gets weaker a- - nd weaker with the increase of helium flow speed. As is well known that the spectrum line of 391.4 nm is an indicative of Penning ionization between helium metastables and nitrogen impurity. The lower intensity of the spectrum line may be attributed to less impurity in the discharge gap with helium flow. As for the reason why the breakdown voltage of the helium gap decreases with helium flow, it was assumed that longer lifetimes of helium metastables result from the less impurity, quenchers of helium metastables, in the gap. For confirmation of this assumption, a monochrometer coupled with a photomultiplier is being prepared for measuring the time-resolved spectrum line of 391.4 nm.