Cavity Discharge Research Articles

Continuous optical discharge in a laser cavity

Optical discharge in a laser cavity is experimentally studied. A significant increase in the absorption of laser radiation (up to total absorption) is revealed. Optical schemes for initiation and maintaining of optical discharge in the cavity are proposed for technological applications of the optical discharge.

Thermal poling of ferroelectrets: How does the gas temperature influence dielectric barrier discharges in cavities?

The influence of the temperature in the gas-filled cavities on the charging process of ferroelectret film systems has been studied in hysteresis measurements. The threshold voltage and the effective polarization of the ferroelectrets were determined as functions of the charging temperature TP. With increasing TP, the threshold voltage for triggering dielectric barrier discharges in ferroelectrets decreases. Thus, increasing the temperature facilitates the charging of ferroelectrets. However, a lower threshold voltage reduces the attainable remanent polarization because back discharges occur at lower charge levels, as soon as the charging voltage is turned off. The results are discussed in view of Paschen's law for electrical breakdown, taking into account the respective gas temperature and a simplified model for ferroelectrets. Our results indicate that the thermal poling scheme widely used for conventional ferroelectrics is also useful for electrically charging ferroelectrets.

Influences of temperature on partial discharge behavior in oil-paper bounded gas cavity under pulsating DC voltage

The oil-paper insulation of valve-side windings in a high voltage direct current (HVDC) converter transformer is subjected to both complicated voltage and severe thermal stresses. Partial discharge (PD) occurring in oil-paper bounded cavities is influenced by voltage stress type as well as temperature. This paper presents PD behaviors in an oil-paper bounded gas cavity under pulsating DC voltages. The voltage across the gas cavity and PD inception voltages at different test conditions are analyzed. Influences of temperature on statistical PD distributions and parameters are also presented. A numerical model is used to interpret PD behaviors under pulsating DC voltages and various temperatures. Experimental results and theoretical statements show that the temperature exhibits a great influence on the behaviors of cavity discharge under pulsating DC voltages. Volume resistivity, gas pressure, charge decay time constant, and electron generation rate are the primary parameters influencing PD behaviors at different temperatures.

Influences of Different Ratios of AC-DC Combined Voltage on Internal Gas Cavity Discharge in Oil-Pressboard Insulation

The degradation of insulation would be accelerated by the partial discharge (PD) incurred by an internal air-gap defect in oil-pressboard insulation. At different bridge rectifier locations in the converter transformer, the oil-pressboard insulation typically endures various ratios of combined ac and positive dc voltages. Therefore, the dielectric status of the converter transformer is closely correlated to the ratios of ac to dc voltages. It is practically significant to conduct the study of the correlation between discharge characteristics and ac-dc ratios. At the same time, it enhances the diagnosis of faults in converter transformers. According to the actual internal operation conditions of converter transformers, the study established a test platform. Furthermore, the conventional pulse current method was adopted to compare the discharge characteristics at the initiation and close-to-breakdown stages. Under these ratios, test results display distinctive PD dynamics. The PD of the air gap generates surface charges. These charges, in turn, establish an inverse electric field. As a result, the proportion of dc in the ac-dc combined voltage increases, and total initial voltage and total breakdown voltage increase continuously, but the recurrence rate of discharge pulse and discharge magnitude decrease at the initiation and close-to-breakdown stages. As the positive dc proportion increases, the discharge recurrence rate in the negative half cycle gradually exceeds that in the positive one. These research findings provide useful criteria for the fault diagnosis of converter transformers.

Improving recognition accuracy of partial discharge patterns by image-oriented feature extraction and selection technique

This paper aims to improve recognition accuracies of single sources of partial discharge (PD) by employing image-oriented feature extraction and selection algorithms. 419 diversified samples of PD data acquired from typically artificial defect models, where the defect size, applied voltage and insulation aging are taken into account, are employed for algorithms validation. In the proposed method, the PD gray images are decomposed into various vectors by two-dimensional principal component analysis (2DPCA) on horizontal and vertical directions, respectively, in which 9 representative parameters are extracted from each image vector. Based on the 419 PD samples, the proposed image features are confirmed to achieve better PD recognition performances than typical image compression methods relevant to 2DPCA. To further improve the recognition performance and reduce the feature dimension, the feature selection technique based on non-dominated sorting genetic algorithm II (NSGA-II) is adopted to search the optimal 2DPCA features. By using the proposed image-oriented feature extraction and selection algorithm, fuzzy k-nearest neighbor classifier (FkNNC), back-propagation neural network (BPNN) and support vector machine (SVM) with 16-dimensional 2DPCA features can now achieve 96.4%, 94.4% and 97.5% accuracies, respectively. The increment of average recognition accuracies of 5%–7% are obtained by different classifiers compared with phase-resolved partial discharge (PRPD) statistical features in previous studies. The investigations on the recognition results relevant to image distortion, cavity discharge change with time and PD contaminated by random noises with different level confirm the robustness of the 16-dimensional 2DPCA features, which signify a great potential in applying the proposed method to field PD pattern recognition in the future.

Surface discharges in silicone gel on AlN substrate

AlN substrate is widely used along with silicone gel to encapsulate power electronic circuits. It is known that the weakness of the insulation system is surface discharges propagating at the gel-substrate interface. In this research, the nature of surface discharges in gel, on various substrates was investigated. The results are as follows: the maximum stopping length of cavities on AlN substrate was more than twice than that on other substrates, and light emission due to discharges in cavities on AlN substrate was different from that on other substrates; for AlN substrate, tip of the cavity emitted light while other part did not emit light, and for other substrates, all the path along the cavity emitted light. These results indicated that the surface of the AlN substrate is degraded by discharges and becomes conductive. We have confirmed this assumption by measuring the conductivity of the cavity path: 5 kΩ /100 μm for that on AlN and above 1 MΩ /100 μm for Al2O3 and glass substrates. The high electric field at the tip of conductive path elongates the cavity stopping length on AlN substrate compared with others. In order to investigate the degradation process of AlN substrate surface, energy dispersive X-ray spectroscopy analysis was carried out. Reduction of nitrogen component on the cavity path was seen, indicating that generation of Al increases the conductivity of cavity path. We have experimentally shown that oxidation treatment of the AlN substrate significantly reduces the cavity stopping length.

Self-pulsing in a low-current hollow cathode discharge: From Townsend to glow discharge

We investigate the self-pulsing phenomenon of a low current cavity discharge in a cylindrical hollow cathode in pure argon. The waveforms of pulsed current and voltage are measured, and the time-averaged and time-resolved images of hollow cathode discharge are recorded by using high-speed intensified charge coupled device camera. The results show that the self-pulsing is a mode transition between low-current stage of Townsend discharge and high-current stage of glow discharge. During the self-pulsing, the current rising time relates to the dissipation of space charges, and the decay time relates to the reconstruction of the virtual anode by the accumulation of positive ions. Whether or not space charges can form and keep the virtual anode is responsible for the discharge mode and hence plays an important role in the self-pulsing phenomenon in low current hollow cathode discharge.

Partial discharge characteristics and trap parameters of aged oil-impregnated paper

The degradation of oil-impregnated paper induced by partial discharge (PD) seriously affects the life of power equipment. In order to investigate the relationship between partial discharge and the degradation of oil-impregnated paper, PD characteristics and trap parameters of the impregnated paper are measured. In this work, a cavity discharge model is adopted and samples with different degrees of polymerization (DP) are fabricated using thermally accelerated aging to simulate transformers with different lifetimes. For each sample, phase resolved partial discharge (PRPD) spectrograms are recorded for various partial discharge times. The characteristics of partial discharge are analyzed, and the trap parameters of specimens are obtained by the thermally stimulated depolarization current (TSDC) method. Following this, the microstructures of the samples are observed using a scanning electron microscope (SEM). It is found that the damaging breakdown process of oil-immersed paper can be divided into three stages: initial stage, developing stage and pre-breakdown stage. There also exist two levels of trap on the surface of oil-impregnated paper: shallow (0.10-0.59 eV) and relatively deep (0.60-0.90 eV) energy. Importantly, the thermal aging makes the trap energy deeper while electrical aging makes the trap energy shallower.

Microwave induced plasma discharge in multi-cell superconducting radio-frequency cavity.

A R&D effort for in situ cleaning of 1.5 GHz Superconducting Radio Frequency (SRF) cavities at room temperature using the plasma processing technique has been initiated at Jefferson Lab. This is a step toward the cleaning of cryomodules installed in the Continuous Electron Beam Accelerator Facility (CEBAF). For this purpose, we have developed an understanding of plasma discharge in a 5-cell CEBAF-type SRF cavity having configurations similar to those in the main accelerator. The focus of this study involves the detailed investigations of developing a plasma discharge inside the cavity volume and avoids the breakdown condition in the vicinity of the ceramic RF window. A plasma discharge of the gas mixture Ar-O2 (90%:10%) can be established inside the cavity volume by the excitation of a resonant 4π/5 TM010-mode driven by a klystron. The absence of any external magnetic field for generating the plasma is suitable for cleaning cavities installed in a complex cryomodule assembly. The procedures developed in these experimental investigations can be applied to any complex cavity structure. Details of these experimental measurements and the observations are discussed in the paper.

Plasma instability in fast spherical discharge induced by a preionization

As it was shown earlier, fast discharge (dI/dt ∼ 1012 A/s and Imax ≈ 40 kA) in a spherical cavity (Al2O3, inner diameter 11 mm, 4 mm apertures for the current supply) filled with working gas (Ar and Xe, pressure 80 Pa), results in the formation of a plasma with the form close to spherical. The physical mechanism can be the cumulation of a convergent shock wave, which was originated near the inner surface of the discharge cavity. It was also shown for the cylindrical fast discharge that the preionization influences the dynamics of the cylindrical convergent shock wave, its evolutions becomes faster. The present work is devoted to the study of the influence of the preionization on the plasma formation in the fast discharge with spherical geometry (Ar, 80 Pa). The inductive storage with plasma erosion opening switch was used as a current driver. The spatial structure of the discharge plasma was studied by means of a pin-hole camera with the microchannel plate (MCP) detector with time gate of 5 ns. The extreme ultra violet spectra were studied by means of the grazing incidence spectrometer with the same MCP detector with time gate of 20 ns. Beside the expected effects (reduction of the spherical plasma formation time and some increase of the electron temperature), the preionization of the discharge by the current 500 A results also in the development of the plasma instabilities and destruction of the compact plasma ball in several tens of nanoseconds. Possible mechanism of the instability is discussed.

Enhanced distinction of surface and cavity discharges by trapezoid-based arbitrary voltage waveforms

Identification of different partial discharge (PD) sources may be improved by the use of non-sinusoidal waveforms of the testing voltages. For instance, surface discharge (SD) and cavity discharge (CD) are not always easy to directly distinguish from the phase resolved PD pattern at traditional 50 or 60 Hz alternating sinusoidal voltage. This study compares PD patterns stimulated by sinusoidal voltage and by several forms of trapezoid-wave voltage, including the limiting cases of triangular and approximately square-wave voltages. Trapezoid-based voltage waveforms are considered as a potential new off-line diagnostic method for PD sources. Partial discharge measurements were performed with each different waveform in two test cells representing canonical cases of SD and CD, with polycarbonate plates as the solid insulating material. The results show that the applied voltage of arbitrary waveform could more clearly distinguish between these PD sources� behavior than normal sinusoidal voltage. The constant-voltage period of peak value in trapezoidal and square voltage waveforms played an important role in the distinction of the two discharge sources. Compared with the cavity discharge�s symmetric features, surface discharge produced in the asymmetric test cell shows strong asymmetric behavior during the constant-voltage period between two polarities under trapezoidal and square voltage waveforms. A faster rise time and increased duration of the constant peak-voltage part of the waveform caused more obvious asymmetry of the surface discharge.

Resonant-frequency discharge in a multi-cell radio frequency cavity

We are reporting experimental results on a microwave discharge operating at resonant frequency in a multi-cell radio frequency (RF) accelerator cavity. Although the discharge operated at room temperature, the setup was constructed so that it could be used for plasma generation and processing in fully assembled active superconducting radio-frequency cryo-module. This discharge offers a mechanism for removal of a variety of contaminants, organic or oxide layers, and residual particulates from the interior surface of RF cavities through the interaction of plasma-generated radicals with the cavity walls. We describe resonant RF breakdown conditions and address the issues related to resonant detuning due to sustained multi-cell cavity plasma. We have determined breakdown conditions in the cavity, which was acting as a plasma vessel with distorted cylindrical geometry. We discuss the spectroscopic data taken during plasma removal of contaminants and use them to evaluate plasma parameters, characterize the process, and estimate the volatile contaminant product removal.

Self-pulsing of hollow cathode discharge in various gases

In this paper, we investigate the self-pulsing phenomenon of cavity discharge in a cylindrical hollow cathode in various gases including argon, helium, nitrogen, oxygen, and air. The current-voltage characteristics of the cavity discharge, the waveforms of the self-pulsing current and voltage as well as the repetition frequency were measured. The results show that the pulsing frequency ranges from a few to tens kilohertz and depends on the averaged current and the pressure in all gases. The pulsing frequency will increase with the averaged current and decrease with the pressure. The rising time of the current pulse is nearly constant in a given gas or mixture. The self-pulsing does not depend on the external ballast but is affected significantly by the external capacitor in parallel with the discharge cell. The low-current self-pulsing in hollow cathode discharge is the mode transition between Townsend and glow discharges. It can be described by the charging-discharging process of an equivalent circuit consisting of capacitors and resistors.

Fast discharge in a spherical cavity

The work is devoted to the study of the plasma, created by a fast discharge in a spherical cavity. The discharge was driven by an inductive storage with plasma erosion opening switch (dI/dt ∼1012 A/s). The plasma was produced in a spherical cavity (alumina, 11 mm diameter). Xe, Ar, and He at the pressure 80 Pa were used as working gases. The time evolution of the spatial structure and of extreme ultraviolet (EUV) spectra of the discharge plasma was studied by means of micro channel plate detector. The discharges with Xe and Ar resulted in the stable appearance of the spherically shaped plasma with the diameter about 1–3 mm. The plasma emission in the EUV region lasts ∼500 ns. The EUV spectrum of Ar discharge at the moment of maximum of the electron temperature Te contains the lines of Ar X (ionization potential 478.7 eV), that indicates a value of Te in the range 50–100 eV. The mechanism of plasma appearance can be the cumulation of the convergent spherical shock wave, generated by fast heat deposition and magnetic pressure in working media near the inner surface of the discharge volume.

Experimental study on the characteristics of a two-electrode plasma synthetic jet actuator

Performance of a two-electrode plasma synthetic jet actuator has been experimentally studied by discharge measurements with high-speed shadowgraphy technology. Results show that the breakdown voltage and the peak discharge current of the actuator may be decreased by decreasing the ambient pressure and increasing the discharge frequency. The discharge developed in the actuator cavity is a spark-arc discharge. In the actuator could be created a strong “precursor shock” and a high kinetic energy jet. During the development of the plasma synthetic jet, the speed of the “precursor shock” is invariable and the jet propagates with an approximately local sonic velocity (350 m/s). But with decreasing jet exit diameter and ambient pressure, the increase of the cavity volume and the discharge frequency could lead to decreasing strength of the “precursor shock”. Heating efficiency of the gas in the cavity will decrease with the increase of the cavity volume and discharge frequency, and the jet velocity is decreased as well. The jet exit diameter and the ambient pressure can have their optimal values for affecting the velocity of the jet. Under all the working conditions we have studied, the plasma actuator may create a strong “precursor shock” and a high-speed jet, and also may have the potential to be used in high-speed active flow control.

Alternating current-generated plasma discharges for the controlled direct current charging of ferroelectrets

The standard charging process for polymer ferroelectrets, e.g., from polypropylene foams or layered film systems involves the application of high DC fields either to metal electrodes or via a corona discharge. In this often-used process, the DC field triggers the internal breakdown and limits the final charge densities inside the ferroelectret cavities and, thus, the final polarization. Here, an AC + DC charging procedure is proposed and demonstrated in which a high-voltage high-frequency (HV-HF) wave train is applied together with a DC poling voltage. Thus, the internal dielectric-barrier discharges in the ferroelectret cavities are induced by the HV-HF wave train, while the final charge and polarization level is controlled separately through the applied DC voltage. In the new process, the frequency and the amplitude of the HV-HF wave train must be kept within critical boundaries that are closely related to the characteristics of the respective ferroelectrets. The charging method has been tested and investigated on a fluoropolymer-film system with a single well-defined cylindrical cavity. It is found that the internal electrical polarization of the cavity can be easily controlled and increases linearly with the applied DC voltage up to the breakdown voltage of the cavity. In the standard charging method, however, the DC voltage would have to be chosen above the respective breakdown voltage. With the new method, control of the HV-HF wave-train duration prevents a plasma-induced deterioration of the polymer surfaces inside the cavities. It is observed that the frequency of the HV-HF wave train during ferroelectret charging and the temperature applied during poling of ferroelectrics serve an analogous purpose. The analogy and the similarities between the proposed ferroelectret charging method and the poling of ferroelectric materials or dipole electrets at elevated temperatures with subsequent cooling under field are discussed.

English

INTRODUCTION: The maxillary sinus is a rare anatomic site for Hodgkin’s lymphoma. Lymphoma in a patient who presents with disease in an unusual site is difficult to diagnose. We report the case of a 70-year-old male who presented with unilateral nasal obstruction and facial pain. CASE DESCRIPTION: A 70 year old male, known diabetic and hypertensive, presented to OPD with complaints of left sided nasal obstruction and facial pain since 4 months. Examination revealed mucopurulent discharge in the left nasal cavity, swelling of left cheek and enlarged left submandibular lymph node. CT PNS revealed erosion of walls of left maxillary sinus with partial erosion of nasal septum. Patient underwent total maxillectomy left side. The histopathology was reported as Hodgkin’s lymphoma of the maxillary sinusnodular sclerosis type. Post operative recovery was uneventful. Patient has been planned for further chemo and radiotherapy as per the guidelines stated by national cancer institute. CONCLUSION: The maxillary sinus is a rare anatomic site for the presence of Hodgkin’s lymphoma. Because making such a diagnosis is difficult, close attention to radiologic and pathologic findings is important.

Feature extraction and stage classification of cavity discharge development in transformer oil-paper insulation

SUMMARY Characteristic study of cavity discharge is important in detecting and diagnosing internal failure of power transformers. Based on the kernel principal component analysis method, eight new feature parameters are extracted from 29 statistical parameters, which are commonly used in the characteristic study of partial discharges. Redundancy and computational complexity for applications are greatly reduced. Using cluster analysis method, new parameters are proved to be effective in the application of stage classification in the cavity discharge development. Four stages are classified: incipient discharge, unstable discharge, consistent discharge, and disruptive discharge. Copyright © 2013 John Wiley & Sons, Ltd.

A critical review of partial-discharge models

Partial-discharge (PD) measurements in compliance with the standard IEC 60270 are based on the main quantity apparent charge. This term is deduced from a simple PD model comprising a capacitive network. However, from a physical point of view, a PD defect cannot be represented by a capacitance, as argued by Pedersen and his coworkers. As an alternative, Pedersen proposed a field theoretical approach that is based on a dipole model. This more sophisticated concept has been ignored in the past, whereas the capacitive PD model is accepted also nowadays. Apparently, the reason for that is that the charge transfer can easily be calculated for the capacitive equivalent circuit, which is not the case if the field theoretical approach based on the dipole concept is adopted. As will be reported in this article, the analysis can substantially be simplified if instead of gaseous discharges in spherical, elliptical, or cylindrical cavities, as commonly regarded in the relevant literature, the establishment of a dipole moment in a uniform field between solid dielectric layers is considered.

Numerical model of the submarine fresh-water discharge in a karst cavity

We discuss the results of numerical experiments aimed at the reconstruction the structure of hydrophysical fields in a karst cavity (in the region of Cape Aiya on the South Coast of Crimea) observed in the course of the second specialized expedition in September 2008. To describe the process of submarine fresh-water discharge, we use a three-dimensional hydrostatic model with a priori specified boundaries of the layers (σ- and/or Z- levels) in the “rigid-lid” approximation including the procedure of “convective adjustment.” It is shown that only a good vertical resolution enables one to give a quantitatively correct description of a thin upper freshened layer of the sea. It is shown that, in order to get an adequate picture of the influence of bottom topography on the upper layer of the sea, it is reasonable to use a mixed grid along the vertical combining the σ- and Z- coordinates. We also present the results of numerical experiments allowing us to estimate the effects of location, number, and power of fresh-water sources placed on the lateral boundary of the cavity.