Sheath Voltage Research Articles

Non-equilibrium modelling of free-burning argon arc in different anode sheath regimes

In this paper, an anode sheath model coupled with two-dimension arc column model is performed to study the physical process in the near-anode region. In the anode sheath model, two different physical mechanisms of ion and electron dynamics, for positive and negative anode sheath voltages, are considered. The effect of the arc column on the anode is also considered. The arc column model accounts for the thermal non-equilibrium and ionization non-equilibrium. The diffusion equations and generalized Ohm’s law are used to analyze the effect of diffusion of plasma species near the anode region. The simulation results from this model are presented for an atmospheric free-burning argon arc with arc currents of 50 A and 150 A. The simulation shows that: (1) both positive and negative anode sheath voltages are obtained for I = 50 and 150 A. The distributions of electric potential in front of the anode surface predicted by the model are in good agreement with the experimental data. (2) In the negative anode sheath regime, the diffusion current density exceeds the total current density, which makes the electric field reverse in front of anode surface. In the positive anode sheath regime, the diffusion current becomes negative in the near-anode region, which makes the ohmic current increase rapidly in the near-anode region. (3) There is a significant interaction, which is positive feedback, between the diffusion current and anode sheath voltage in both positive and negative anode sheath modes. (4) The thermal conduction from the arc column plays a major role in the heat transfer to the anode for the negative anode sheath mode while it becomes much smaller for the positive anode sheath mode.

Simulations of hybrid direct current radiofrequency (dc/rf) capacitively coupled plasmas

Effects of applying a high negative direct current (dc) voltage in argon capacitive coupled plasmas on the flux of secondary electrons (SEs) to a radiofrequency (rf) biased electrode and plasma uniformity were investigated using particle-in-cell Monte Carlo collision simulation and results were compared with experimental results. High energy SEs originating at electrodes with high negative dc voltage due to ion-impact as well as electron-impact were accelerated out of the sheath through the sheath voltage drop. These SEs gain energy equal to the applied dc voltage and either travel to the wafer electrode where they are lost (dumped) or are trapped between electrodes depending on the voltage on bottom electrode. Trapping and dumping of ballistic electrons depends on the magnitudes of the voltages (dc and rf) applied to each electrode. High energy SEs alter the electron energy distribution function at the wafer, high energy electron flux to the wafer and the plasma density profile. With the application of negative dc voltage, wafer receives high energy electron flux with energies up to applied dc voltage. For the range of conditions examined in this paper, the plasma density increases with the application of the negative dc voltage due to ionization by trapping of high energy SEs. The electrons that originate at the negative dc electrode minimize the impact of enhanced radial electric fields at the rf electrode edge allowing dc/rf plasmas to be center peaked even at low (13.56 MHz) rf frequencies.

Radio-frequency wave interactions with a plasma sheath in oblique-angle magnetic fields using a sheath impedance model

The physics of interactions between waves in plasmas and sheaths for background magnetic fields which make oblique angles with sheath surfaces is studied with the use of the self-consistent finite element code rfSOL incorporating the recently developed sheath impedance model [J. R. Myra, Phys. Plasmas 24, 072507 (2017)]. The calculation based on this model employs the generalized sheath boundary condition (sheath BC), which surpasses the previously used capacitive sheath BC in reliability by taking into account the contributions of the ion and electron currents in the sheath and the displacement current. A series of numerical simulations is carried out in two-dimensional slab geometry with a flat or curved sheath surface as part of the boundary. It is shown that the sheath–plasma wave appears when the equilibrium magnetic field line angle with respect to the sheath surface is small, the absolute value of the radio-frequency (RF) sheath voltage is large, and the plasma density is slightly higher than the lower hybrid resonance density (LHR density), all of which bring the sheath property closer to being capacitive. It is also shown that the sharp variation of the magnetic field line angle along the sheath surface can sensitively affect the maximum absolute value of the RF sheath voltage at a plasma density slightly lower than the LHR density.

Study of the arc voltage in gas metal arc welding

In this paper, the voltage drop in pulsed gas metal arc welding (GMAW) is studied, focussing on the contributions of the different sections of wire and arc along the current path and their dependence on arc length and current. A typical pulsed GMAW process of mild steel in the one drop per pulse modus is considered as a reference. Voltage and current have been measured in experiments with fixed wire feed speed for varying arc length and free wire length. The temporally changing geometry of the arc and the molten wire parts has been studied carefully by high-speed imaging and image processing. In particular, the shielding gas and the metal vapour dominated parts of the arc have been captured simultaneously, and conclusions on the length of the current path in the arc column have been drawn. Comparing voltage measurements for the same current and different arc lengths, finally, the sum of cathode and anode sheath voltages as well as the mean electric field strength of the arc column have been determined. Sheath voltages increasing from 18 to 23 V for pulse currents from 350 to 650 A and a mean electric field of about 1.1 V mm−1 in the arc column, which is almost independent of the current, have been obtained. As a consequence, the Ohmic heating in the arc column contributes less than 20% to the overall power budget whereas the contribution of the electrode sheathes is larger than 60%.

Experimental and computational investigations of electron dynamics in micro atmospheric pressure radio-frequency plasma jets operated in He/N2 mixtures

The electron power absorption dynamics in radio frequency driven micro atmospheric pressure capacitive plasma jets are studied based on experimental phase resolved optical emission spectroscopy and the computational particle in cell simulations with Monte Carlo treatment of collisions. The jet is operated at 13.56 MHz in He with different admixture concentrations of N2 and at several driving voltage amplitudes. We find the spatio-temporal dynamics of the light emission of the plasma at various wavelengths to be markedly different. This is understood by revealing the population dynamics of the upper levels of selected emission lines/bands based on comparisons between experimental and simulation results. The populations of these excited states are sensitive to different parts of the electron energy distribution function and to contributions from other excited states. Mode transitions of the electron power absorption dynamics from the Ω- to the Penning-mode are found to be induced by changing the N2 admixture concentration and the driving voltage amplitude. Our numerical simulations reveal details of this mode transition and provide novel insights into the operation details of the Penning-mode. The characteristic excitation/emission maximum at the time of maximum sheath voltage at each electrode is found to be based on two mechanisms: (i) a direct channel, i.e. excitation/emission caused by electrons generated by Penning ionization inside the sheaths and (ii) an indirect channel, i.e. secondary electrons emitted from the electrode due to the impact of positive ions generated by Penning ionization at the electrodes.

Dynamic simulation of induced voltages in high voltage cable sheaths: Steady state approach

This paper presents a novel approach to the modeling of high voltage underground cables. Its main contribution is that it considers induced effects. Indeed, it incorporates the estimate of induced voltages and currents in cable sheaths in steady state due to the nearby cables and sheaths, for different types of sheaths connections and for various single-phase short-circuit configurations and three-phase short-circuits. Furthermore, it allows multiple circuits to be coupled automatically in a simple way. An intuitive and friendly simulation tool has been implemented that allows the automatic generation of multiple coupling circuits and to calculate all these induced effects caused by the connection of the sheaths and the distance between cables. It has been validated by comparing it with the expected theoretical data and to other simulators with satisfactory results.

Effect of self-bias on cylindrical capacitive discharge for processing of inner walls of tubular structures—Case of SRF cavities

Cylindrical capacitive discharge is a convenient medium for generating reactive ions to process inner walls superconductive radio-frequency (SRF) cavities. These cavities, used in particle accelerators, presents a three-dimensional structure made of bulk Niobium, with axial cylindrical symmetry. Manufactured cavity walls are covered with Niobium oxides and scattered particulates, which must be removed for desired SRF performance. Cylindrical capacitive discharge in a mixture of Ar and Cl2 is a sole and natural non-wet acid choice to purify the inner surfaces of SRF cavities by reactive ion etching. Coaxial cylindrical discharge is generated between a powered inner electrode and the grounded outer electrode, which is the cavity wall to be etched. Plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external compensating dc power supply and corrugated design of the inner electrode. The dc bias potential difference is established between two electrodes to make the set-up favorable for SRF wall processing. To establish guidelines for reversing the asymmetry and establishing the optimal sheath voltage at the cavity wall, the dc self-bias potential and dc current dependence on process parameters, such as gas pressure, rf power and chlorine content in the Ar/Cl2 gas mixture was measured. The process is potentially applicable to all concave metallic surfaces.

High intensity, macroparticle-free, aluminum ion beam formation

We describe a vacuum arc based system for the generation of repetitively pulsed metal ion beams with very high current density, and the results of experimental investigations of the performance of this new system, as well as the results of numerical simulations. Our approach uses a DC vacuum arc as a metal plasma source, and the beam-forming technique is a hybrid method using features of conventional gridded ion extraction together with some plasma immersion attributes. A hemispherical grid is located some distance from the plasma source so as to intercept part of the metal plasma flow and repetitively pulse-biased to the requisite ion beam energy. Ions are accelerated in the high voltage sheath that forms in front of the biased grid and focused into a converging beam that can be of a very high current density. Space charge neutralization is provided by cold plasma that streams through the grid during the bias-off part of the cycle. At the same time, macroparticles in the vacuum arc plasma are blocked from viewing the target by a metal disk positioned centrally on the hemispherical grid. In the work outlined here, we formed 5 keV aluminum ion beams with a current density up to 470 mA/cm2 at a negative bias amplitude of 3 kV and a pulse repetition rate of 105 pulse per second. The experiments and numerical simulations demonstrate the formation of high-intensity macroparticle-free aluminum ion beams for material surface modification and other possible applications.

Control of residual stress of tetrahedral amorphous carbon thin film deposited on dielectric material by filtered cathodic vacuum arc source by using mid-frequency pulse bias voltage

It was shown that the residual stress of the tetrahedral amorphous carbon (ta-C) thin film deposited on insulating material by filtered cathodic vacuum arc (FCVA) source can be controlled with mid-frequency pulse bias voltage. The residual stress of ta-C thin film decreased monotonically with the substrate pulse bias when the base material to be coated was insulating. There was not the apparent peak of residual stress about the substrate pulse bias which appeared for the electrically conducting base material. It was observed that the residual stress increased with the arc current and dielectric thickness at some pulse voltage range (100 V–1000 V). The sp3 contents calculated by using Raman spectra and the surface hardness values as a function of substrate pulse voltage showed the similar trend with the residual stress values. For ta-C thin film deposited on the polished alumina sample, it was shown that the critical load in the scratch test decreased with the residual stress. The residual stress of ta-C on the insulating base material was explained with the change of the sheath voltage in the equivalent circuit model.

275 kV cable discharge field measurement and analysis of SVLs chain failure using ATP

Results of field measurement of trapped charges on a 275kV cable will be presented. The instrumentation and calibration method used in the measurement will be discussed. It will be shown that if the cable is not force-discharged by means of earth switches, the trapped charges will decay very slowly. The atmospheric parameters such as humidity and temperature affect the discharge time, which also depends on how much of the circuit is exposed to open air.The measurement results are used in EMTP-ATP simulation to confirm that trapped charges may be sufficient to cause sheath voltage limiters (SVL) of the cable to fail. The number of SVL breakdown depends on the mode of SVL failure, i.e. whether it attains low impedance (short circuit) or open circuit when the energy absorbed exceeds the rating.

2p-A-13 クロストフィールド放電における電子群の回転運動とシース電圧

2p-A-13 クロストフィールド放電における電子群の回転運動とシース電圧

Evaluation of high‐voltage AC cable grounding systems based on the real‐time monitoring and theoretical calculation of grounding currents

For high-voltage (HV) AC cable transmission systems, the metal sheaths of three separate single-core cables are cross-bonded at each end of the cable section, to suppress the induced voltages in sheaths. The status of HV cable grounding systems is, therefore, an important consideration for power utilities. This study presents the status evaluations of 93 cable grounding systems in Beijing, obtained using the standards established by State Grid Corporation of China. As a result, 13 of these systems were classified as abnormal or defective. Additionally, a theoretical calculation of grounding currents was proposed and these 13 systems are re-evaluated by comparing the calculated and measured values of certain parameters. Based on this work, the authors propose a modified evaluation standards. It is indicated that the absolute value of grounding current (|I|) during a given 24-hr period has no relationship with the status of grounding system, and the ratio of grounding current to load current (K) at the time of maximum load current could be used as a status indicator instead of K at some other time, in order to avoid possible incorrect evaluations due to light load conditions.

The role of electron induced secondary electron emission from SiO2 surfaces in capacitively coupled radio frequency plasmas operated at low pressures

The effects of electron induced secondary electron (SE) emission from SiO2 electrodes in single-frequency capacitively coupled plasmas (CCPs) are studied by particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations in argon gas at 0.5 Pa for different voltage amplitudes. Unlike conventional simulations, we use a realistic model for the description of electron-surface interactions, which takes into account the elastic reflection and the inelastic backscattering of electrons, as well as the emission of electron induced SEs (δ-electrons). The emission coefficients corresponding to these elementary processes are determined as a function of the electron energy and angle of incidence, taking the properties of the surface into account. Compared to the results obtained by using a simplified model for the electron-surface interaction, widely used in PIC/MCC simulations of CCPs, which includes only elastic electron reflection at a constant probability of 0.2, strongly different electron power absorption and ionization dynamics are observed. We find that ion induced SEs (γ-electrons) emitted at one electrode and accelerated to high energies by the local sheath electric field propagate through the plasma almost collisionlessly and impinge on the opposing sheath within a few nanoseconds. Depending on the instantaneous local sheath voltage these energetic electrons are either reflected by the sheath electric field or they hit the electrode surface, where each γ-electron can generate multiple δ-electrons upon impact. These electron induced SEs are accelerated back into the plasma by the momentary sheath electric field and can again generate δ-electrons at the opposite electrode after propagating through the plasma bulk. Overall, a complex dynamics of γ- and δ-electrons is observed including multiple reflections between the boundary sheaths. At high voltages, the electron induced SE emission is found to strongly affect the plasma density and the ionization dynamics and, thus, it represents an important plasma-surface interaction that should be included in PIC/MCC simulations of CCPs under such conditions.

Fault detection of HVDC cable in multi‐terminal offshore wind farms using transient sheath voltage

With respect to rapid growth of offshore wind power, multi-terminal HVDC system is going to be an attractive way of the power transmission for remote and large offshore wind farms. Since the HVDC cables are the vital component of the multi-terminal HVDC transmission system, an accurate and fast fault detection method is necessary for protection and maintaining uninterrupted power delivery to the main grid. This study presents a novel method to diagnose HVDC cable faults and unbalancing of DC capacitor bank in multi-terminal voltage source converter HVDC systems for offshore wind farms. Transient voltage of cable sheath is investigated as a main parameter for detecting types of cable faults. Extensive simulations are performed on EMTDC-PSCAD platform and the results show the effectiveness of the proposed technique.

Velocity boundary conditions for positive ions entering radio-frequency sheaths in electronegative plasmas

Under certain conditions in radio-frequency (rf) plasmas, the amplitude of the low-energy peak in ion energy distributions (IEDs) measured at an electrode depends sensitively on the velocity at which ions approach the sheath. By measuring IEDs, incident ion velocities can be determined. Here, IEDs were measured in inductively coupled plasmas in 1.3 Pa of CF4, at rf sheath voltages up to 100 V at 1 MHz, obtained by biasing a counterelectrode. From measured IEDs and sheath voltages, we determined the incident velocities of all significant positive ions: CF3+, CF2+, CF+, and F+. At higher bias voltages, we detected essentially the same velocity for all four ions, suggesting that some collisional process keeps different ions at the same velocity as they emerge from the presheath. For all four ions, measured velocities were significantly lower than the Bohm velocity uB and the electropositive ion sound speed cs, because of negative ion effects. From the measured velocities, an upper bound for negative ion temperature is obtained. The velocities determined here do not agree with boundary conditions that have been previously proposed, because the latter neglect either the reduction in ion sound speed due to negative ions or the acceleration that occurs as ions pass from the point where quasineutrality is violated to the point where electron density becomes negligible. Both of these effects are treated to fair approximation, for collisionless sheaths, by setting the initial velocity to twice the ion sound speed modified by negative ions.

A finite element procedure for radio-frequency sheath–plasma interactions based on a sheath impedance model

A finite element code that solves self-consistent radio-frequency (RF) sheath–plasma interaction problems is improved by incorporating a generalized sheath boundary condition in the macroscopic solution scheme. This sheath boundary condition makes use of a complex sheath impedance including both the sheath capacitance and resistance, which enables evaluation of not only the RF voltage across the sheath but also the power dissipation in the sheath. The newly developed finite element procedure is applied to cases where the background magnetic field is perpendicular to the sheath surface in one- and two-dimensional domains filled by uniform low- and high-density plasmas. The numerical results are compared with those obtained by employing the previous capacitive sheath model at a typical frequency for ion cyclotron heating used in fusion experiments. It is shown that for sheaths on the order of 100 V in a high-density plasma, localized RF power deposition can reach a level which causes material damage. It is also shown that the sheath–plasma wave resonances predicted by the capacitive sheath model do not occur when parameters are such that the generalized sheath impedance model substantially modifies the capacitive character of the sheath. Possible explanations for the difference in the maximum RF sheath voltage depending on the plasma density are also discussed.

Overview of ASDEX Upgrade results

The ASDEX Upgrade (AUG) programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of 3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF operation. As predicted, a factor two reduction on the ICRF-induced W plasma content could be achieved by the reduction of the sheath voltage at the antenna limiters via the compensation of the image currents of the central and side straps in the antenna frame. There are two main operational scenario lines in AUG. Experiments with low collisionality, which comprise current drive, ELM mitigation/suppression and fast ion physics, are mainly done with freshly boronized walls to reduce the tungsten influx at these high edge temperature conditions. Full ELM suppression and non-inductive operation up to a plasma current of MA could be obtained at low plasma density. Plasma exhaust is studied under conditions of high neutral divertor pressure and separatrix electron density, where a fresh boronization is not required. Substantial progress could be achieved for the understanding of the confinement degradation by strong D puffing and the improvement with nitrogen or carbon seeding. Inward/outward shifts of the electron density profile relative to the temperature profile effect the edge stability via the pressure profile changes and lead to improved/decreased pedestal performance. Seeding and D gas puffing are found to effect the core fueling via changes in a region of high density on the high field side (HFSHD).The integration of all above mentioned operational scenarios will be feasible and naturally obtained in a large device where the edge is more opaque for neutrals and higher plasma temperatures provide a lower collisionality. The combination of exhaust control with pellet fueling has been successfully demonstrated. High divertor enrichment values of nitrogen have been obtained during pellet injection, which is a prerequisite for the simultaneous achievement of good core plasma purity and high divertor radiation levels. Impurity accumulation observed in the all-metal AUG device caused by the strong neoclassical inward transport of tungsten in the pedestal is expected to be relieved by the higher neoclassical temperature screening in larger devices.

Spatial profiles of interelectrode electron density in direct current superposed dual-frequency capacitively coupled plasmas

We present experimentally determined spatial profiles of the interelectrode electron density (ne) in dual-frequency capacitively coupled plasmas in which the negative direct current (dc) bias voltage (Vdc) is superposed; in the experiment, 13 MHz (Plow) was applied to the lower electrode and 60 MHz (Phigh) to the upper electrode. The bulk ne increased substantially with increases in the external power, Phigh, Plow, and with increases in Vdc. When Plow was insufficient, the bulk ne decreased as the Vdc bias increased. The bulk ne increased due to its dependence on Vdc, especially for |Vdc| > 500 V. This may correspond to the sheath voltages (Vs) of the lower electrode. The ne values in front of the upper electrode were coupled with the Vdc: the Vdc dependence first decreased and then increased. The dc currents (Idc) of the upper electrode were collected when a large Plow was applied. The value of Idc at the threshold value of Vdc ≈ Vs (e.g. −500 V) increased with an increase in ne. When |Vdc| exceeded the threshold, the spatial ne profile and the Idc dependence were changed relative to the electrical characteristics of the dc superposition; this led to a change in the location of the maximum ne, the width of the area of ne depletion in front of the electrodes, and a transition in the electron heating modes.

RETRACTED ARTICLE: Applications of artificial intelligence and hybrid neural network methods with new bonding method to prevent electroshock risk and insulation faults in high-voltage underground cable lines

The sheath current causes cable faults and electroshocks in the high-voltage underground cable lines. Thus, the sheath current must be determined before a new high-voltage cable line is installed to take the required precautions against the sheath current effects. In this study, PSCAD/EMTDC is used for simulation of high-voltage underground cable line to examine the sheath current. The sheath current is forecasted with artificial neural network (ANN) by using the results of simulation studies, and differential evolution algorithm (DEA), genetic algorithm (GA), clonal selection algorithm (CSA), hybrid DEA-CSA, artificial bee colony (ABC), particle swarm optimization (PSO) and inertia weight PSO (iPSO) are used to find the best weights of ANN. The sheath current formation factors which are obtained in PSCAD/EMTDC are used as inputs of ANN. It is seen at the end of simulation studies that many factors affect the formation of the sheath current, so Pearson correlation is used to determine the most effective sheath current formation factors for inputs of ANN. In the literature, three types of bonding methods are used to reduce the sheath current and voltage, but these methods are not sufficient to reduce both sheath current and the sheath voltage simultaneously. Therefore, in this study, the sectional solid bonding method is developed as new bonding method to reduce the sheath voltage and current in this study. When the sectional solid bonding is used, the sheath voltage can be reduced under touch voltage, and the sheath current can be reduced under the desired value.

Spatial proximity effects on the excitation of sheath RF voltages by evanescent slow waves in the ion cyclotron range of frequencies

We investigate theoretically how sheath radio-frequency (RF) oscillations relate to the spatial structure of the near RF parallel electric field E∥ emitted by ion cyclotron (IC) wave launchers. We use a simple model of slow wave (SW) evanescence coupled with direct current (DC) plasma biasing via sheath boundary conditions in a 3D parallelepiped filled with homogeneous cold magnetized plasma. Within a ‘wide-sheath’ asymptotic regime, valid for large-amplitude near RF fields, the RF part of this simple RF + DC model becomes linear: the sheath oscillating voltage VRF at open field line boundaries can be re-expressed as a linear combination of individual contributions by every emitting point in the input field map. SW evanescence makes individual contributions all the larger as the wave emission point is located closer to the sheath walls. The decay of |VRF| with the emission point/sheath poloidal distance involves the transverse SW evanescence length and the radial protrusion depth of lateral boundaries. The decay of |VRF| with the emitter/sheath parallel distance is quantified as a function of the parallel SW evanescence length and the parallel connection length of open magnetic field lines. For realistic geometries and target SOL plasmas, poloidal decay occurs over a few centimeters. Typical parallel decay lengths for |VRF| are found to be smaller than IC antenna parallel extension. Oscillating sheath voltages at IC antenna side limiters are therefore mainly sensitive to E∥ emission by active or passive conducting elements near these limiters, as suggested by recent experimental observations. Parallel proximity effects could also explain why sheath oscillations persist with antisymmetric strap toroidal phasing, despite the parallel antisymmetry of the radiated field map. They could finally justify current attempts at reducing the RF fields induced near antenna boxes to attenuate sheath oscillations in their vicinity.