Plasma stability control using dielectric barriers in radio-frequency atmospheric pressure glow discharges

It is widely accepted that electrode insulation is unnecessary for generating radio-frequency (rf) atmospheric pressure glow discharges (APGDs). It is also known that rf APGDs with large discharge current are susceptible to the glow-to-arc transition. In this letter, a computational study is presented to demonstrate that dielectric barriers provide an effective control over unlimited current growth and allow rf APGDs to be operated at very high current densities with little danger of the glow-to-arc transition. Characteristics of electrode sheaths are used to show that the stability control is achieved by forcing the plasma-containing electrode unit to acquire positive differential conductivity

Electron avalanches and diffused γ-mode in radio-frequency capacitively coupled atmospheric-pressure microplasmas

Space-, time- and wavelength-resolved optical emission profiles suggest that the helium emission at 706 nm can be used to indicate the presence of high energy electrons and estimate the sheath in helium rf discharges containing small concentration of air impurities. Furthermore, the experimental data supports the theoretical predictions of energetic electron avalanches transiting across the discharge gap in rf microdischarges and the absence of an α-mode. Nonetheless, microdischarges sustained between bare metal electrodes and operating in the γ-mode can produce diffuse glowlike discharges rather than the typical radially constricted plasmas observed in millimeter-size rf atmospheric-pressure γ discharges

Evolution of the light emission profile in radio-frequency atmospheric pressure glow discharges

Time-resolved images of the light emission profiles of RF atmospheric pressure plasmas are presented. As the current increases, the emission profile evolves from bell shaped to double humped. At moderate currents, bright layers above each of the electrodes are observed, and it is found that both layers light up and fade simultaneously during the RF cycle

Electron trapping in radio-frequency atmospheric-pressure glow discharges

In this letter, the authors present experimental evidence of electron trapping in radio-frequency rf atmospheric-pressure glow discharges. By linking electron density to nanosecond plasma images and optical emission spectroscopy, they show that electron trapping occurs under most discharge conditions. The level of electron trapping increases with increasing discharge current or/and increasing excitation frequency, and manifests itself in the change of the differential conductivity at the point of the gas breakdown. Finally, they demonstrate that electron trapping is largely related to whether the half rf period is shorter than the electron transition time across the electrode gap

Electron heating in radio-frequency capacitively coupled atmospheric-pressure plasmas

In atmospheric-pressure plasmas the main electron heating mechanism is Ohmic heating, which has distinct spatial and temporal evolutions in the α and γ modes. In γ discharges, ionizing avalanches in the sheaths are initiated not only by secondary electrons but also by metastable pooling reactions. In α discharges, heating takes place at the sheath edges and in contrast with low-pressure plasmas, close to 50% of the power absorbed by the electrons is absorbed at the edge of the retreating sheaths. This heating is due to a field enhancement caused by the large collisionality in atmospheric-pressure discharges

Mitigating plasma constriction using dielectric barriers in radio-frequency atmospheric pressure glow discharges

It is known that radio-frequency (rf) atmospheric glow discharges with bare electrodes are susceptible to plasma constriction at large discharge currents. This is undesirable for large-scale applications, even though large currents usually lead to abundant plasma reactive species and high application efficiency. In this letter, an experimental investigation is presented to demonstrate that plasma constriction can be mitigated by introducing dielectric barriers to the electrodes. The resulting atmospheric rf dielectric-barrier discharge is shown to operate in the γ mode of large discharge current while maintaining its discharge volume. This improves significantly plasma stability and the application potential

Effects of dielectric barriers in radio-frequency atmospheric glow discharges

This paper reports the effects of introducing dielectric barriers to radio-frequency (RF) atmospheric pressure glow discharges (APGD) that have hitherto employed bare electrodes. The resulting atmospheric RF dielectric barrier discharges (DBD) are experimentally shown to retain their large volume without constriction at very large currents, well above the maximum current at which conventional RF APGD with bare electrodes can maintain their plasma stability. Optical emission spectroscopy is used to demonstrate that larger discharge currents lead to more active plasma chemistry. A complementary computational study is then presented on the dynamics and structures of the RF DBD under different operation conditions. While the RF DBD and conventional RF APGD may present very different electrical signatures in the external circuit, it is shown that their discharge properties, particularly the sheath characteristics, are very similar. Finally, it is demonstrated that thinner dielectric barriers or/and larger excitation frequencies are desirable to maximize the largest permissible discharge current without compromising the plasma stability

The effect of applied electric field on pulsed radio frequency and pulsed direct current plasma jet array

Here we compare the plasma plume propagation characteristics of a 3-channel pulsed RF plasma jet array and those of the same device operated by a pulsed dc source. For the pulsed-RF jet array, numerous long life time ions and metastables accumulated in the plasma channel make the plasma plume respond quickly to applied electric field. Its structure similar as “plasma bullet” is an anode glow indeed. For the pulsed dcplasma jet array, the strong electric field in the vicinity of the tube is the reason for the growing plasma bullet in the launching period. The repulsive forces between the growing plasma bullets result in the divergence of the pulsed dcplasma jet array. Finally, the comparison of 309 nm and 777 nm emissions between these two jet arrays suggests the high chemical activity of pulsed RF plasma jet array

Classical confinement of test particles in higher-dimensional models: stability criteria and a new energy condition

We review the circumstances under which test particles can be localized around a spacetime section \Sigma_0 smoothly contained within a codimension-1 embedding space M. If such a confinement is possible, \Sigma_0 is said to be totally geodesic. Using three different methods, we derive a stability condition for trapped test particles in terms of intrinsic geometrical quantities on \Sigma_0 and M; namely, confined paths are stable against perturbations if the gravitational stress-energy density on M is larger than that on \Sigma_0, as measured by an observed travelling along the unperturbed trajectory. We confirm our general result explicitly in two different cases: the warped-product metric ansatz for (n+1)-dimensional Einstein spaces, and a known solution of the 5-dimensional vacuum field equation embedding certain 4-dimensional cosmologies. We conclude by defining a confinement energy condition that can be used to classify geometries incorporating totally geodesic submanifolds, such as those found in thick braneworld and other 5-dimensional scenarios.Comment: 9 pages, REVTeX4, in press in Phys. Rev.

A Combining Forecasting Modeling and Its Application

Part 5: Modelling and SimulationInternational audienceThe supply chain coordination has abstracted more and more attention from industries and academics. This paper studies a Bayesian combination forecasting model to integrate multiple forecasting resources and coordinate forecasting process among partners in retail supply chain. The simulation results based on the retail sales data show the effectiveness of this Bayesian combination forecasting model to coordinate the collaborative forecasting process. This Bayesian combination forecasting model can improve demand forecasting accuracy of supply chain