Yangyang Fu

Validity of the similarity law for the glow discharges in non-plane-parallel gaps

The glow discharges in the gaps geometrically similar to that used in glow discharge cleaning of the International Thermonuclear Experimental Reactor (ITER) were numerically simulated based on a two-dimensional fluid model in which the linear dimensions of gap A are two times that of gap B and the pressure of gap A is half that of gap B. Under an applied voltage of 1000 V, the physical parameters at the corresponding point pz in these two gaps were compared. It was found that the electric potential U(pz), the reduced field E(pz)/p and the electron temperature Te(pz) are equal in values for these two gaps, but the electron density ne(pz) and ion density ni(pz) for gap B are four times that of gap A. All these parameter ratios are the same as that defined by similarity law, which confirmed that the similarity law is valid for the glow discharges in non-plane-parallel gaps.

Cathode fall thickness of abnormal glow discharges between parallel-plane electrodes in different radii at low pressure

In order to investigate the influence of electrode radius on the characteristics of cathode fall thickness, experiments of low-pressure (20 Pa ≤ p ≤ 30 Pa) abnormal glow discharge were carried out between parallel-plane electrodes in different radii keeping gap distance unchanged. Axial distributions of light intensity were obtained from the discharge images captured using a Charge Coupled Device camera. The assumption that the position of the negative glow peak coincides with the edge of cathode fall layer was verified based on a two-dimensional model, and the cathode fall thicknesses, dc, were calculated from the axial distributions of light intensity. It was observed that the position of peak emission shifts closer to the cathode as current or pressure grows. The dependence of cathode fall thickness on the gas pressure and normalized current J/p2 was presented, and it was found that for discharges between electrodes in large radius the curves of pdc against J/p2 were superimposed on each other, however...

Intersection of Paschen's curves for argon

The intersection of Paschens curves for argon with a same gap length but a different electrode radius was observed. While the breakdown voltage increases with the increase of the nonuniformity in the electric field of the gap at lower pressures, it decreases at higher pressures. The reason for the intersection of Paschens curves was given based on the mean free path length of the electrons inversely proportional to the gas pressure and the electron impact ionization coefficient exponentially increasing with the electric field. The intersection of the Paschens curves was qualitatively reproduced by a numerical simulation.

Research on Similarity Law of Glow Discharge in Argon at Low Pressure by Numerical Simulation

In order to investigate the validity of the similarity law of glow discharge between two similar discharge gaps, a 2-D fluid model of argon discharge was established and solved with finite-element method. In the two similar discharge gaps, the gap length d, electrode radius r, and gas pressure p were set to meet the relations of p1d1 = p2d2 and p1r1 = p2r2, respectively, and the reduced field E/p was kept constant as well. The parameters of the larger gap were 20 mm in gap length, 10 mm in diameter, and 1 torr in pressure, while those of the scale-down gap were 10 mm, 5 mm, and 2 torr. The simulation results indicate that the discharge was a typical normal glow discharge, and the discharge physical parameters, such as electric potential, electric field, current density, electron density, and ion density, were obtained. The voltage-current characteristic curves of the two similar gaps were approximately the same, and the physical parameters of the two similar gaps show the proportional relationships derived by similarity law. The results could offer theoretical instructions in extrapolating the discharge properties of two similar gaps.

Pressure effect on a tandem hollow cathode discharge in argon

The tandem hollow cathode discharge, formulated by arranging two discharges in series, is an important method used to increase the irradiance of a hollow cathode discharge. In this paper, based on a two-dimensional fluid model we studied a five-layer tandem hollow cathode discharge, with three hollow electrodes stacked together and separated by the insulators to obtain the configuration of anode/insulator/cathode/insulator/anode from the top to the bottom. In the model, the thickness of both electrodes and insulators is set at 1 cm and the diameter of the hollow cavity is 2 cm. The pressure effect on the discharge properties is investigated with gas pressure ranges from 100 Pa to 5 kPa. The gap voltage first decreases, reaching a minimum sustaining voltage at 1 kPa, and then increases. Based on the two-dimensional electron density distributions, the discharges parameters (including the electron density, ion density, electric potential, and electric field) of one integrated hollow cathode discharge at 1 kP...

Effect of distribution of electric field on low-pressure gas breakdown

A low-pressure gas breakdown in a gap of the non-uniform electric field between two plane-parallel electrodes was studied. The experiments were specially designed to neatly separate the effect of a centered dielectric tube in between the electrodes on the breakdown from the effect of the electric field distribution determined by the electrode geometry on the breakdown. For a given electrode radius and an interelectrode distance, when the diameter of the centered dielectric tube in between the electrodes is smaller, the breakdown voltage is lower, which is most possibly as a result of the flashover more easily happening along the surface of the smaller tube on which the more charged particles are accumulated. When the dielectric tube in between the electrodes is removed, the breakdown voltage depends not only on the product of gas pressure and gap length but also on the aspect ratio of the gas gap, i.e., Ub = f(pd, d/r). Furthermore, Ub = f(pd, d/r) was proved to automatically fulfill two necessary conditi...

Effect of surface protrusion on plasma sheath properties in atmospheric microdischarges

The electric field enhancement due to the presence of cathode surface protrusion is investigated in atmospheric microdischarges with the goal of identifying the plasma sheath properties (such as cathode sheath thickness and electric field distortion). The electric field enhancement caused by surface protrusion is examined by adjusting the aspect ratio and the protrusion size. It is found that the cathode electric field enhancement depends strongly (weakly) on the aspect ratio (size) of the protrusion when it is much smaller than the discharge gap distance. In particular, the axial electric field in both vacuum and discharges becomes nonlinear with the protrusion on the cathode. The cathode sheath thicknesses obtained by two different methods are compared. With the same axial (or radial) protrusion dimension, increasing the aspect ratio will result in a significant decrease in the sheath thickness, whereas increasing the axial protrusion size with an unchanged aspect ratio will only lead to a slight decrea...

Transition characteristics of low-pressure discharges in a hollow cathode

Based on a two-dimensional (2-D) fluid model, the transition processes of discharges in a hollow cathode at low pressure are observed by changing three parameters, i.e., applied voltage U0, gas pressure p, and external circuit ballast resistance Rb. The voltage-current characteristic curves, electron density distributions, and electric potential distributions of different discharge operating points in a hollow cathode are obtained. The transition processes are characterized by the voltage-current characteristic curves, the electron density distributions, and the electrical potential distributions. The transition modes observed from the voltage-current characteristics include the low-current abnormal mode, normal mode, and high-current abnormal mode. Increasing the applied voltage U0 can have a similar effect on the discharge transition processes to decreasing the ballast resistance. By increasing U0 from 200 V to 500 V and decreasing Rb from 5000 kΩ to 100 kΩ independently, it is observed that the discharge transfers from the outside to the inside of the hollow cavity, thus forming the virtual anode potential. By increasing the gas pressure p from 50 Pa to 5 kPa, the discharge also moves into the hollow cavity from the outside; however, a further increase in the gas pressure leads to the discharge escaping from the hollow cavity. Simulation results and characterizations for different parameters are presented for the transition properties of low-pressure discharges in a hollow cathode. It is verified that the hollow cathode discharge only exists under certain ranges of the above parameters.Based on a two-dimensional (2-D) fluid model, the transition processes of discharges in a hollow cathode at low pressure are observed by changing three parameters, i.e., applied voltage U0, gas pressure p, and external circuit ballast resistance Rb. The voltage-current characteristic curves, electron density distributions, and electric potential distributions of different discharge operating points in a hollow cathode are obtained. The transition processes are characterized by the voltage-current characteristic curves, the electron density distributions, and the electrical potential distributions. The transition modes observed from the voltage-current characteristics include the low-current abnormal mode, normal mode, and high-current abnormal mode. Increasing the applied voltage U0 can have a similar effect on the discharge transition processes to decreasing the ballast resistance. By increasing U0 from 200 V to 500 V and decreasing Rb from 5000 kΩ to 100 kΩ independently, it is observed that the dischar...

Characterizing the dominant ions in low-temperature argon plasmas in the range of 1–800 Torr

The dominant ions in low-temperature rare gas plasmas can be either molecular ions or atomic ions depending on the discharge regime. In this paper, the dominant ions in low-temperature argon plasmas are characterized in a wide range of gas pressure (1–800 Torr). The channels for creation of molecular ions include atom assisted association, dissociative recombination, dissociation by atom impact (DAI), and dissociation by electron impact (DEI). The latter two were previously less often considered. It is found that the DEI reaction has a significant impact on the ion fractions, while the effect of the DAI reaction is much less important in the whole investigated gas pressure regime. As the gas pressure increases from 1 to 800 Torr, the atomic ion fraction drops rapidly in conjunction with an increase of the molecular ion fraction. This phenomenon confirms that in low-temperature argon plasmas the dominant ion will be the atomic ion in the low pressure regime but the molecular ion in the high pressure regime...

Gas breakdown in atmospheric pressure microgaps with a surface protrusion on the cathode

Gas breakdown in atmospheric pressure microgaps with a cathode surface protrusion is highly sensitive to the protrusion geometry. The breakdown voltage is identified when the discharge enters the subnormal region, according to voltage-current curves calculated by a two-dimensional fluid model. The effects of the protrusion size and the aspect ratio on the gap breakdown voltage are examined. It is found that the protrusion size can have a more profound effect on the breakdown voltage than the protrusions aspect ratio. The breakdown voltage versus the protrusion aspect ratio will show a minimum value if the aspect ratio varies in a wider range. Shrinking the size of cathode protrusion can increase the breakdown voltage faster than enlarging the gap distance in the absence of a protrusion in the same scale. The effect of the aspect ratio of the microgap on the breakdown voltage is also presented.