Zhuo Liang

Observation of the transition from a Townsend discharge to a glow discharge in helium at atmospheric pressure

Dielectric barrier discharge in helium at atmospheric pressure was studied by taking fast images of the discharge during one current pulse using an intensified charge couple device. It was observed that there appears a weakly luminous layer close to the anode at the very beginning of the discharge, then the luminous area gradually expands into the entire gap as the anode layer moves toward the cathode, and finally a highly luminous layer forms close to the cathode at the time around the maximum of the current pulse. The evolution of the discharge pattern indicates a transition from Townsend discharge to glow discharge.

Influence of wire mesh electrodes on dielectric barrier discharge

A dielectric barrier discharge in a 2 mm air gap was studied. The experiments show that the mesh electrode and PET film really make the discharge looking homogeneous. The breakdown onset voltage in the case of a mesh electrode and PET film is 6.4 kV, considerably lower than 8 kV, the breakdown voltage in the usual case using two spherical electrodes. The field calculations show that even with this much lower voltage the gap field in the region very close to the PET film is enhanced to a value near the breakdown field in the usual case. It may be this field enhancement that initiates a corona discharge which provides seed electrons, leading to the breakdown of the whole air gap at a lower voltage. A lower voltage applied to the gap means a lower averaged field over the gap. Because the development of an electron avalanche is very sensitive to the electric field, a small decrease in the field will depress the avalanche significantly, preventing a homogeneous discharge from transiting to filament discharge. This may be the reason why a mesh electrode and PET film make the discharge look homogeneous.

Homogeneous dielectric barrier discharge in nitrogen at atmospheric pressure

A homogenous dielectric barrier discharge in nitrogen at atmospheric pressure was produced and identified with Townsend discharge. With the nitrogen flowing at a speed higher than 7 cm/s through a gap not longer than 3 mm, a stable Townsend discharge was always obtained. For a 2-mm nitrogen gap, the Townsend discharge was generated in a limited range of the applied voltage, from Vmin to Vmax. While Vmin keeps almost unchanged at a value of about 9.75 kV, Vmax decreases from 12.4 kV to 9.75 kV as the frequency of the applied voltage increases from 1.5 kHz to 7 kHz. When the flow rate increases from 0 to 140 cm/s, the discharge current decreases from 3 mA to 2.5 mA and the breakdown voltage of the nitrogen gap increases from 5.3 kV to 5.9 kV, which is attributed to the nitrogen pressure in the gap rising up with the flow rate. The release of the trapped electrons from the dielectric surface plays an important role not only in the initiation of the Townsend discharge but also in the extinction of the discharge. The metastables N2(A3Σu+) impacting on the dielectric surface release sufficient primary seed electrons for the nitrogen gap to be broken down at the Townsend breakdown voltage, a much lower voltage than the streamer breakdown voltage, which is necessary for getting a Townsend discharge rather than a filamentary streamer discharge. With the nitrogen flow, the density of the impurity oxygen is much reduced in the discharge gap, which allows much more N2(A3Σu+) to survive to the time of the succeeding discharge for initiating a Townsend breakdown. It was found that the discharge is extinguished while Fgas continues rising up. The extraordinary distinction of the Townsend discharge could be explained with the limited number of the trapped electrons that could not provide the long-time lasting Townsend discharge with sufficient secondary electrons.

Radial evolution of dielectric barrier glowlike discharge in helium at atmospheric pressure

The radial evolution of dielectric barrier discharge in helium at atmospheric pressure was studied by taking end-view photographs of the discharge during one current pulse using a fast-gated intensified charge coupled device. The photographs were transformed into three-dimensional images that make the radial evolution of the discharge more visualized. It was found that the discharge begins with a weak Townsend discharge covering the entire electrode surface, develops much more rapidly in the central part where the electric field is higher, and spreads radially outward.

Effect of gas flow in dielectric barrier discharge of atmospheric helium

Based on the diagnostic techniques of electrical measurement, emission spectroscopy and fast photography, the effects of the helium flow in dielectric barrier discharge of atmospheric helium were investigated. It was shown that the nitrogen molecules as impurity in the discharge gap are reduced by a helium flow through the gap. As nitrogen molecules are very efficient quenchers of helium metastables, the introduction of the helium flow may induce an increase in the density of helium metastables after discharge and thus an enhanced contribution of these metastables to the next breakdown of helium gap at a lower voltage. The area of the higher density of He metastables after discharge moves from upstream to downstream as the flow velocity increases, leading to the area of starting discharge moving from upstream to downstream. It seems that the nitrogen molecules as the impurity in helium play more important roles in influencing the behaviour of helium discharge than the products etched from the dielectric surface by discharge.

Modification of terylene fabric by homogeneous discharge in air at atmospheric pressure

Homogeneous discharge with no filaments is capable of operating at atmospheric pressure when the dielectric barriers were polyethylene terephthalate (PET) films with mesh wires. The nonthermal plasma produced by the homogeneous discharge has many applications in the textile or polymer surface modification. In this paper, the terylene fabric is treated by the atmospheric plasma and the surface hydrophilicity is studied. The discharge device is powered by a high voltage AC source with a frequency of 50 Hz. Two parallel-plane electrodes are designed as the shape of Rogowski electrodes to achieve the field uniformity. The change of wickability of the fabric surface is measured and its durability is tested. The study shows that after the treatment by the discharge with mesh between the electrodes, the wickability is improved. The surface modifying effect is sustained during a long time.

Homogenous dielectric barrier discharg E in nitrogen at atmospheric pressure

A homogenous dielectric barrier discharge in nitrogen at atmospheric pressure was produced and identified with Townsend discharge. With the nitrogen flowing at a speed higher than 7 cm/s through a gap not longer than 3 mm, a stable Townsend discharge was always obtained. For a 2-mm nitrogen gap, the Townsend discharge was generated in a limited range of the applied voltage, from V min to V max . While V min keeps almost unchanged at a value of about 9.75 kV, V max decreases from 12.4 kV to 9.75 kV as the frequency of the applied voltage increases from 1.5 kHz to 7 kHz. When the flow rate increases from 0 to 140 cm/s, the discharge current decreases from 3 mA to 2.5 mA and the breakdown voltage of the nitrogen gap increases from 5.3 kV to 5.9 kV, which is attributed to the nitrogen pressure in the gap rising up with the flow rate. The release of the trapped electrons from the dielectric surface plays an important role not only in the initiation of the Townsend discharge but also in the extinction of the discharge. The metastables N 2 (A3Σ u +) impacting on the dielectric surface release sufficient primary seed electrons for the nitrogen gap to be broken down at the Townsend breakdown voltage, a much lower voltage than the streamer breakdown voltage, which is necessary for getting a Townsend discharge rather than a filamentary streamer discharge. With the nitrogen flow, the density of the impurity oxygen is much reduced in the discharge gap, which allows much more N 2 (A3Σ u +) to survive to the time of the succeeding discharge for initiating a Townsend breakdown. It was found that the discharge is extinguished while F gas continues rising up. The extraordinary distinction of the Townsend discharge could be explained with the limited number of the trapped electrons that could not provide the long-time lasting Townsend discharge with sufficient secondary electrons.

Determination of ionization coefficient of atmospheric helium in DBD

Dielectric barrier discharge of helium at atmospheric pressure was investigated. Two plane-parallel electrodes, each covered by a 1-mm thick quartz plate, are 50 mm in diameter and the gas gap is 5 mm in length. Powered by an alternative voltage with a frequency of 33 kHz, a homogenous discharge was produced and characterized by one current pulse per half cycle of the applied voltage. The development of the discharge during one current pulse was recorded by taking a series of side-view photographs of 20 ns exposure time using an ICCD camera. It was important to find that a weakly luminous layer close to the anode was observed even at the time far ahead of the current pulse, which was considered as the result of a weak Townsend discharge. The distribution of light intensity in the gap was obtained by processing the photograph taken at the time of this weak Townsend discharge. The curve of this light distribution shows a shape quite similar to that of the total electron number in an electron avalanche as a function of the distance through which the avalanche passes. This suggested us that the curve could be used to determine ionization coefficient alpha in the Townsend discharge. The method is based on the proportionality of light intensity to the electron density in a discharge gap of a uniformly distributed electric field. By fitting a theoretically derived formula with the measured curve of light distribution, alpha was determined. It was found that the value of alpha is quite high even at relatively low reduced field. For instance, alpha =31 cm-1 for E/p = 3.6 V. cm-1 . Torr-1. The reason for this higher value of alpha may lie in the contribution of Penning ionization of helium metastables with impurities, especially with nitrogen molecules.

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, im, and pulse width (FWHM), tw, 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, tw 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 tw, 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 and 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.

Evolution of Helium DBD at Atmospheric Pressure

Summary form only given. Dielectric barrier discharge (DBD) of helium at atmospheric pressure was experimentally investigated. The discharge device was powered by a high voltage AC source with a frequency varying from 20 kHz to 50 kHz. Two parallel-plane electrodes are 50 mm in diameter. One electrode was made from aluminium and the other from ITO (indium tin oxide) film that is transparent for taking discharge picture end-on. Both electrodes were covered by one quartz plate which is 120 mm times 120 mm in size and 1 mm in thickness. The gas gap was kept at 5 mm. The applied voltage was measured with a capacitive divider VD305A from Pearson Electronics and the discharge current was obtained by using a current-viewing resistor of 50 ohm. For plotting Lissajous figures, the transported charges during discharge were measured with a capacitor connected in between the ITO electrode and the ground. The evolution of discharge pattern was taken end-on and side-on with an intensified CCD camera (PIMAX2: 1003RB-FG-43 from Princeton Instruments). The time-resolved spectrum from discharge was recorded with a spectrometer (SP-2558 from Acton) coupled to the ICCD camera. The transition from a homogenous glow discharge to a filamentary discharge was observed with the help of fast gated ICCD (exposure time of 10 ns) as the applied voltage or air additive was increased. The roles of He metastable atoms play in getting a homogeneous glow discharge were discussed.