Field electron emission from cathode as a possible factor in the transition from a streamer to spark discharge channel

A possible scenario of the transition from a streamer to spark discharge channel during the breakdown of narrow discharge gaps between metal electrodes is considered. It is suggested that the discharge gap bridging by a streamer may create an electric field at the cathode, which may be strong enough to induce field electron emissions that, in turn, will cause the development of processes that lead to the transition from a streamer to spark discharge channel.     

Numerical analysis of flow pattern transition in a conical silo with ellipsoid particles

Gravity-driven discharge flow in conical silos is ubiquitous in manufacturing processes for numerous industries such as food, pharmaceutical and chemical industries, wherein flow pattern is the key topic to be studied. In this work, discrete element method was used to study discharge flow behaviors in a special conical silo. This work aims to understand the dynamic evolution of discharge flow pattern and establish methods for regional determination of flow pattern transition. The results indicated that both mass flow and funnel flow patterns coexist in the silo at the initial stage of discharge, and there is a definite transition process from mass flow to funnel flow. Furthermore, the flow pattern transition is revealed by the change in discharge flow characteristics. Specifically, the change in force acting on particles leads to the change of particle orientation and particle velocity field, macroscopically resulting in the flow pattern transition. Finally, based on the kinetic stress field and shear rate, the height and radial region of flow pattern transition were determined, respectively. Understanding of the flow pattern transition is useful to the design, scale-up and optimization of silos and similar structural devices.     

Discrete element modelling of a bucket elevator head pulley transition zone

This paper presents the results of discrete element simulations applied to a bucket elevator model with particular reference to the head pulley transition zone. This is the first stage in a larger study to better understand the mechanics of bucket elevator operation with reference to the discharge of particles at the head end. At the head end two issues arise; mechanically, the buckets are bolted to the conveying media (typically a fabric reinforced belt) and at the point of belt to headpulley tangency, the tip of the bucket undergoes a theoretical step change in velocity. This theoretical step change results in a classical under-damped response in the buckets tip velocity. In undergoing this motion, there are stresses that are passed to the carcass of the conveying media; understanding the magnitude of these stresses is one longer term goal of this research allowing a quantitative basis for the existing qualitative design guidelines such as (Handbook for conveyor and elevator belting, Apex Belting Pty Ltd). The discharge of the bulk material from the bucket has been addressed Beverly et al. (Bulk Solids Handling, 1983) but this analysis is dependent on simple, but common, bucket geometry and ignores the initial transition to the headpulley. Ignoring the transition with a low speed discharge elevator is not likely to impact on the predicted discharge pattern, however with high speed discharge elevators, the destabilising effect of the transition is expected to promote premature discharge of bulk material from the bucket. Depending on the design of the elevator casing this early discharge may or may not impact on the overall conveying efficiency.     

Spectroscopic study and modelling of a reactive magnetron discharge in Ar/O2 and in air

The Optical Emission Spectroscopy technique was used to monitor a magnetron discharge working in an Ar/O₂ mixture and in air. Hysteresis effects and instability, which are frequently observed when working with reactive gases, were clearly seen in the optical spectra. When the discharge power was increased, the atomic oxygen line intensity increased, saturated and then disappeared whilst, at the transition, sputtered material line intensities suddenly increased. In air, nitrogen lines were found to be less sensitive to this transition. The transition between the two regimes of the discharge is due to a change in the composition of surfaces from oxidised to metallic, as confirmed by the analysis of deposited films. In parallel with the experiment, a plasma model was developed for the magnetised region of the discharge which predicts the variations of the various plasma species in the Ar/O₂ mixture. This model explains the behaviour of line intensities on the two sides of the transition.     

Development of a High-Current Vacuum Arc in a Rod Electrode System

The development of a high-current pulse electric discharge in a rod electrode system with the current amplitude changing from 10 to 100 kA is investigated using a sectional model of a vacuum chamber with the simultaneous recording of the electrical characteristics of the discharge, of the glow of discharge plasma, and of the spots on the electrodes. A phenomenon is revealed of a rapid transition of the discharge from the ignition unit to interrod gaps on reaching an instantaneous value of current from 5 to 9 kA. A correlation is established between the moment of appearance of an abrupt drop of voltage across the vacuum arc and the moment of formation of a new discharge channel in the interrod gap. It is demonstrated that various vacuum arc modes in the interrod gap and the multiplication of discharge channels in all interrod gaps are observed with increasing current. The effect of the rate of current increase on the development of the vacuum arc is investigated.     

Effective generation of electron beams in an abnormal discharge with increased photoelectron emission from a cathode

An abnormal discharge is realized in which electron beams with energies in a kiloelectronvolt range are generated at an efficiency of η∼1. The results are explained within the framework of a photoelectron discharge model. A transition to this mechanism is possible due to a significant increase in the power of self-illumination from a large electron beam drift volume. The passage to a discharge with predominant photoemission leads to a decrease in the cathode potential fall region, a drop in the ion current to the cathode, and an increase in the efficiency.     

Testing steady and transient velocity scalings in a silo

Gravity-driven discharge experiments were performed in a perspex 3D flat bottomed silo which was filled with a granular material, and had a variable discharge orifice size. The granular material used was amaranth seed with an average diameter of 1?mm. Particle Image Velocimetry (PIV) analysis was performed on a high-speed video recording of the discharge, and used to quantify the velocity field within the silo both at steady state and during the development of flow. We verified not only that the steady-state velocity of the granules in the silo scales with the flow rate, but, additionally, the transition to a steady-state regime is also rate-controlled by the volumetric discharge. We present evidence that, away from the discharge orifice, the flow behaves identically, regardless of the orifice diameter, in a scaled time. We discuss these results with reference to the physics and mathematical modelling of granular flows.     

Observations on the operation of a planar magnetron sputtering system by target erosion patterns

The discharge characteristics in a planar magnetron sputtering system was studied by means of the target erosion pattern. The erosion pattern was affected not only by the magnetic flux distribution but also by the discharge power and the sputtering gas pressure. A model analysis revealed that this behaviour was due to interaction between the vertical component of the field and electrons moving in a sideways direction. It was found from its definite correlation with the pattern that the transition in the discharge characteristic from a magnetron mode to a high impedance mode was brought about by the escape of trapped electrons from the target edge.     

On a criterion of the appearance of an upward leader from ground-based objects

Results of an experimental study of processes forming an upward leader discharge from a grounded rod electrode in the electric field of an artificial cloud of charged water aerosol are presented. The transition of the streamer corona flash to an upward leader discharge was found to be of a probabilistic character and to be determined, first of all, not by the current amplitude, and not only by the flash charge passing through the stem of the pulse corona, but by a set of parameters: duration of the current passage at a rather high level and optimum value of the corona charge passed through the stem base. The range of radii of curvature of the rod-shaped lightning arrester tip (from 20 to 25 mm) at which the highest probability of the appearance of an upward leader is observed has been revealed.     

Transverse bulk solid behaviour during discharge from troughed belt conveyors

Guidelines for the calculation of bulk solid material cross sectional dimensions and the influence of the belt conveyor transition length on the inclination of the trajectory at discharge are well established. However, not a great deal of research has been conducted on the influence of bulk solid material properties and conveyor belt transition geometry on the bulk solid material cross section at discharge. As such, assessment of cross section break-up associated with cohesive materials and transverse spreading of free flowing materials is missing. Conversely, the majority of discharge trajectory analysis techniques focus on analysis in a single vertical plane along the length of the belt.This paper presents an analysis of high speed conveyor discharge trajectories in three dimensions, taking into account transverse spreading of free flowing materials and shearing, or cross section discontinuity, exhibited by cohesive materials. Transverse bulk solid material behaviour and trajectory discharge is evaluated using a combination of experimental laboratory tests, a continuum mechanics approach incorporating CAD and Discrete Element Modelling (DEM). The work presented shows that bulk solid material behaviour at discharge is directly influenced by material characteristics and interactions resulting from the geometry of the belt conveyor transition zone.     

Experimental study of atmospheric pressure surface discharge in helium

The surface discharge generated at atmospheric pressure in helium was examined by monitoring the current and voltage at the discharge electrode. The discharge generated in helium behaves differently when compared to that generated in other gases (e.g. air). The single discharge duration and the time between consecutive discharges are longer because there is a different mechanism of discharge evolution. The metastable helium atoms play the most important role for discharge generation. Streamer-like and glow types of discharge were observed. The decay of helium metastables concentration determines the discharge regime. Hence, operation conditions have strong influence on the discharge regime. The introduction of gas flow removes metastable quenchers (gaseous products from dielectric and electrode surfaces) and transition to glow discharge is observed. Also covering the discharge electrode with thin dielectric foil to suppress Auger de-excitation of metastables at metal surface leads to generation of atmospheric pressure glow surface discharge. Properties of this discharge are comparable with properties of glow discharge at low pressure (e.g. the electron concentration).     

Experimental study of atmospheric pressure surface discharge in helium

The surface discharge generated at atmospheric pressure in helium was examined by monitoring the current and voltage atthe discharge electrode. The discharge generated in helium behaves differently when compared to that generated in other gases (e.g. air). The single discharge duration and the time between consecutive discharges are longer because there is a different mechanism of discharge evolution. The metastable helium atoms play the most important role for discharge generation. Streamer-like and glow types of discharge were observed. The decay of helium metastables concentration determines the discharge regime. Hence, operation conditions have strong influence on the discharge regime. The introduction of gas flow removes metastable quenchers /gaseous products from dielectric and electrode surfaces) and transition to glow discharge is observed. Also covering the discharge electrode with thin dielectric foil to suppress Auger de-excitation of metastables at metal surface leads to generation of atmospheric pressure glow surface discharge. Properties of this discharge are comparable with properties of glow discharge at low pressure (e.g. the electron concentration).     

D.C. cathode sputtering: influence of the oxygen content in the gas flow on the discharge current

A qualitative physical model is proposed to explain the influence of the oxygen content in the gas flow on the discharge current for reactive cathode sputtering in a d.c. diode system. According to this model the increase in the oxygen content affects the discharge current density in two main ways: (i) by a sharp change in the coefficient of secondary ion-electron emission from the target surface as a result of its oxidation; and (ii) by the influence of the oxygen content on the elementary ionization processes in the discharge region near the cathode as a result of the transition from a glow discharge in a noble gas to a glow discharge in oxygen.The results of the experimental investigations with targets of different materials under different sputtering conditions were in good agreement with the conclusions from the proposed physical model. Experiments aimed at elucidating the mechanism of formation of an oxide layer by sputtering of a silicon target have also been carried out. The conclusions can be useful for thin film deposition as well as for investigations of secondary emission from surfaces in a glow discharge.     

Alpha-gamma transition in RF capacitive discharge in low-pressure oxygen

We report the recorded current-voltage characteristics of a RF capacitive discharge in oxygen. Low-frequency oscillations of the plasma potential in a kilohertz frequency range are observed to accompany the transition of the discharge from a weak- (α-) to a strong-current (γ-) regime in the low-pressure range. The weak-current regime of the RF capacitive discharge is observed within the pressure range limited not only from the medium pressure side but also from the lower-pressure one. Electron temperature and plasma density are registered with a probe technique.     

Experimental study of the transition to high-current regime of discharge with a hollow self-heated titanium cathode in nitrogen

It is established that rapid formation of a nitride layer on the surface of a self-heated hollow titanium cathode in arc discharge in nitrogen leads to an increase in the thermal stability and emissive properties of the cathode. Optimum regimes of cathode training that ensures rapid Ti → TiN transition over the entire cathode wall thickness in the active zone are determined, which allows the operation temperature to be increased above 2200 K at a discharge current of up to 50 A for the cathode with an inner diameter of 8 mm and a wall thickness of 1 mm. Results of measurements of the current-voltage characteristics of discharge, thermal regimes of operation, and chemical composition of the cathode are presented. The rates of erosion of the cathode operating in nitrogen and argon have been determined.     

Dependence of the piezoactivity of zinc oxide films on the conditions of synthesis in critical regimes of glow discharge

The influence of the gas pressure in a magnetron sputtering system on the efficiency of volume hypersound generation in the synthesized ZnO films has been experimentally studied at pressures close to the transition from glow discharge to Townsend regime, whereby the ion and electron mean free path length increases and a drift component appears in the flux of deposited particles. It is established that the efficiency of hypersound generation (at a frequency above 1 GHz) in ZnO films deposited under these conditions increases. Based on a comparison to the properties of films deposited in a classical diffuse glow discharge, this result is explained by a lower nucleation texture and higher density of the films grown in a critical regime.     

Dynamics of a charged gas suspension with an initial spatially nonuniform distribution of the average dispersed phase density during the transition to the equilibrium state

Results of calculations are presented for the transition of a charged gas suspension from the initial nonequilibrium state with a spatially nonuniform distribution of the average density to a state with a uniform distribution of the average density and charge in a bounded volume. A numerical solution is obtained for the system of equations describing the motion of a polydisperse multirate and multitemperature gas suspension, the particles of which carry an electric discharge and create a self-consistent electric field.     

Transition of an atmospheric-pressure glow discharge from the diffuse to spark phase

The dynamics of glow and the electrical characteristics of a nanosecond glow discharge in the course of a transition from the diffuse to spark phase have been studied in a rod-plane electrode geometry in air at atmospheric pressure. For this purpose, a sequence of alternating-polarity voltage pulses with an amplitude decaying from 135–170 kV was applied to the interelectrode gap with a length of 5–6 cm. It is established that the spark channel formation is preceded by the stage of diffuse glow and that the spark parameters are correlated with the statistical features of the stage of gap shortage (bridging) preceding the diffuse glow. This analysis also revealed the important role of thermal processes in the dynamics of glow discharge development.     

音速喷嘴喉径对边界层过渡影响研究

滞止压力0.1 ～2.5 MPa下,喉径1.921 ～12.444 mm范围内,21支音速喷嘴的518组流出系数测量结果表明:滞止压力对同一块音速喷嘴流出系数的影响可超过2.3％;2.721 mm音速喷嘴边界层的过渡出现在雷诺数0.48×106;喉径12.444 mm音速喷嘴的边界层过渡出现在雷诺数0.94 ×106,音速喷嘴边界层过渡雷诺数与喉径直接相关.基于测试结果,提出了流出系数与雷诺数之间的经验公式,该公式对流出系数预测偏差小于0.3％.此外,得到边界层过渡雷诺数与音速喷嘴喉径之间的经验公式,该公式外延到日本NMIJ-2013实验工况,预测结果与实验结果具有很好的一致性.     

Transverse Electric Discharges in Supersonic Air Flows: Space-Time Structure and Current-Voltage Characteristics of Discharge

The space-time evolution of potential distribution in a pulsed transverse discharge in a supersonic jet of air is studied in a mode close to the current generator mode. The current dependences of the longitudinal electric field intensity and of the discharge channel diameter are measured for different values of pressure in the jet. It is demonstrated that the electric field intensity decreases with increasing discharge current by the power law, with the exponent being close to that for a highly contracted glow or arc discharge in the absence of flow. The increase in current is accompanied first of all by an increase in the discharge channel cross section. The characteristics of the oscillatory mode of discharge burning are studied for discharge power supplies, which are close to the current generator mode. The obtained dependences of the oscillation period on the external parameters of discharge, namely, current, pressure, and interelectrode spacing, are interpreted. An expression is derived which describes the maximal extent of discharge along the flow in the case of instability due to external electric circuit. This extent may increase further only in the case of transition to supplies close to current generators; in this case, the extent is restricted by the mechanism of repeated breakdown.