Breakdown Phase Research Articles

Measurements of Corona Discharge in Sharp –to-Plane Electrodes Configuration

A Dc positive sharp- to –plane Corona phenomena have been studied in electronegative gases (air ,and SF6 ) for pressure up to (3.2bar and gap length from 0.1 to 0.5 cm), with high degree of non-uniformity electric field. The corona –breakdown phases can exhibit anomalous characteristics. The exact physical process of such phenomena is not yet fully understood complete. To better understand this process precise knowledge about corona and spark discharge is necessary. Hence the dependency of corona inception and spark breakdown voltage on the pressure, non-uniformity field factor and the type of applied voltage ,also the type of materials.
 The steady region of corona discharge is observed in the ( sharp- to- plane ), for applied voltage between the threshold value voltage and onset value , we observe an effect that attaching gases. A Townsend type of discharge develops in the region close to point. At voltages above the onset value , the burst pulses are replaced by a Dc current ,and in some cases 'pre-breakdown ' streamer are also observed.

Electrical Oscillations of the UNU-ICTP Plasma Focus Device in the Early Breakdown Phase

Recent circuit driven electromagnetic particle in cell simulation of the United Nations University-International Centre for Theoretical Physics plasma focus device revealed regular oscillations in both the voltage and current profiles. The simulated voltage waveform agreed well with the experimental profile, where similar unaccounted oscillations were also observed. In this paper, the oscillations are attributed to the plasma inductance, whose value was calculated by circuit analysis and agreed reasonably well with the computed experimental value. A circuit simulation of the prebreakdown and postbreakdown phases, with the plasma inductance incorporated, reproduced with sufficiency accuracy the electromagnetic particle in cell generated waveforms and consequently confirmed our finding.

Random telegraph noise in 2D hexagonal boron nitride dielectric films

This study reports the observation of low frequency random telegraph noise (RTN) in a 2D layered hexagonal boron nitride dielectric film in the pre- and post-soft breakdown phases using conductive atomic force microscopy as a nanoscale spectroscopy tool. The RTN traces of the virgin and electrically stressed dielectric (after percolation breakdown) were compared, and the signal features were statistically analyzed using the Factorial Hidden Markov Model technique. We observe a combination of both two-level and multi-level RTN signals in h-BN, akin to the trends commonly observed for bulk oxides such as SiO2 and HfO2. Experimental evidence suggests frequent occurrence of unstable and anomalous RTN traces in 2D dielectrics which makes extraction of defect energetics challenging.

A thermodynamic model for the plasma kernel volume and temperature resulting from spark discharge at high pressures

In today’s internal combustion engines assisted ignition is predominantly achieved by conventional spark ignition. The spark ignition starts with the breakdown phase after which complex flow dynamics govern the energy deposition into the gas. The condition of the activated plasma kernel resulting from spark breakdown, among others, is important for the development of the flame kernel to a self-sustained flame. A simple thermodynamic model is formulated based on insight gained from available detailed plasma simulations and experiments. Under the assumption that the problem is cylinder symmetrical and that the breakdown energy is supplied quasi-instantaneously as a line source, the extent of the hot plasma kernel and the time for pressure equalization with the surrounding can be given by exploiting the properties of radially expanding blast waves. The so-defined control volume is treated as an open system, and a simple energy balance leads to a relation for the mean kernel temperature. The plasma volumes and mean temperatures predicted by this simple model are shown to correspond well with published results in the open literature based on full plasma simulations of the spark discharge available for pressures ranging from 1 to 8 bar. The model allows drawing some conclusions about the requirements for experimental methods, such as calorimetry, frequently used to characterize the efficacy of the spark discharge produced by ignition systems and the influence of pressure and breakdown energy on kernel volume and temperature.

Microstructural investigation after vacuum electrical breakdown of the W-30wt.%Cu contact material

In the present work, characteristics regarding the sub-micron scale microstructure of the W-30 wt%Cu contact material after vacuum electrical breakdown have been investigated by SEM and EDS. EBSD was also utilized to reveal the change of grain size, misorientation and micro-texture for both Cu and W phases after arc erosion. The results revealed that the arc erosion led to distinct grain refinement and intensified local misorientation strain. Besides, with the help of orientation distribution function analysis, strong peak type {112}<4–61> component was identified to be an important arc erosion texture for the first breakdown phase of Cu, which would complicate the following breakdown process during service due to the work function anisotropies.

Numerical characterization of plasma breakdown in reversed field pinches

In the reversed field pinch, there is considerable interest in investigating the plasma breakdown. Indeed, the plasma formed during the breakdown may have an influence on the confinement and maintenance in the latter process. However, up to now there has been no related work, experimentally or in simulation, regarding plasma breakdown in reversed field pinch (RFP). In order to figure out the physical mechanism behind plasma breakdown, the effects of the toroidal and error magnetic field, as well as the loop voltage have been studied. We find that the error magnetic field cannot be neglected even though it is quite small in the short plasma breakdown phase. As the toroidal magnetic field increases, the averaged electron energy is reduced after plasma breakdown is complete, which is disadvantageous for the latter process. In addition, unlike the voltage limits in the tokamak, loop voltages can be quite high because there are no requirements for superconductivity. Volt-second consumption has a small difference under different loop voltages. The breakdown delay still exists in various loop voltage cases, but it is much shorter compared to that in the tokamak case. In all, successful breakdowns are possible in the RFP under a fairly broad range of parameters.

Time-resolved imaging of the plasma development in a triggered vacuum switch

Triggered vacuum switches (TVS) are particularly used in pulsed power technology as closing switches for high voltages and high charge transfer. A non-sealed-off prototype was designed with a side-on quartz window to investigate the evolution of the trigger discharge into the main discharge. The image acquisition was done with a fast CCD camera PI-MAX2 from Princeton Instruments. The CCD camera has a maximum exposure time of 2 ns. The electrode configuration of the prototype is a conventional six-rod gap type, a capacitor bank with C = 16.63 μF, which corresponds at 20 kV charging voltage to a total stored charge of 0.3 C or a total energy of 3.3 kJ. The peak current is 88 kA. According to the tremendously highly different light intensities during the trigger and main discharge, the complete discharge is split into three phases: a trigger breakdown phase, an intermediate phase and a main discharge phase. The CCD camera images of the first phase show instabilities of the trigger breakdown, in phase 2 three different discharge modes are observed. After the first current maximum the discharge behavior is reproducible.

A PIC-MCC code RFdinity1d for simulation of discharge initiation by ICRF antenna

Discharges produced and sustained by ion cyclotron range of frequency (ICRF) waves in absence of plasma current will be used on ITER for (ion cyclotron-) wall conditioning (ICWC, eV, ). In this paper, we present the 1D particle-in-cell Monte Carlo collision (PIC-MCC) RFdinity1d for the study the breakdown phase of ICRF discharges, and its dependency on the RF discharge parameters (i) antenna input power Pi, (ii) RF frequency f, (iii) shape of the electric field and (iv) the neutral gas pressure . The code traces the motion of both electrons and ions in a narrow bundle of magnetic field lines close to the antenna straps. The charged particles are accelerated in the parallel direction with respect to the magnetic field BT by two electric fields: (i) the vacuum RF field of the ICRF antenna and (ii) the electrostatic field determined by the solution of Poisson’s equation.The electron density evolution in simulations follows exponential increase, . The ionization rate varies with increasing electron density as different mechanisms become important. The charged particles are affected solely by the antenna RF field at low electron density ( , ). At higher densities, when the electrostatic field is comparable to the antenna RF field , the ionization frequency reaches the maximum. Plasma oscillations propagating toroidally away from the antenna are observed. The simulated energy distributions of ions and electrons at correspond a power-law Kappa energy distribution. This energy distribution was also observed in NPA measurements at ASDEX Upgrade in ICWC experiments.

Detection of Cracks in a Cantilever Beam Using Signal Processing and Strain Energy Based Model

Structure health monitoring is one of the most important aspects in an industry, as structures should work safely during their service life. Cracks are the most common damage that initiates a breakdown phase and hence timely and accurate detection of these cracks is imperative. In this article, a vibration based non-destructive technique is presented to detect one or multiple edge cracks in beam like structures. This model is based on variation in mode shapes and natural frequencies that provide accuracy in results as well as ease in practical applications. The crack location is identified using mode shapes of damaged beam wherein an appropriate signal processing technique is implemented by using which the noise in the signal can be reduced. Along with this, the crack severity is also determined using a strain energy based mathematical model. The model presented in this study is capable of detecting an arbitrary number of cracks in cantilever or simply supported configuration. The results obtained using the proposed method is also validated by considering few case studies.

The effects of pre-ionization using a shunt resistor on reproducibility of the x-ray emission in a dense plasma focus device

In this research, the effects of pre-ionization using a shunt resistor on reproducibility of x-ray emission in a Mather type plasma focus device have been studied. This technique increased the intensities of the emitted x-rays from argon as the filling gas of the device and made the x-ray yields with similar intensities reproducible. A Mirnov coil was also used to record the variations of the plasma's magnetic field, and the wavelet spectrums of these recorded signals showed the reduced instabilities due to the application of the pre-ionization technique. Moreover, it was demonstrated that this technique was capable of reducing the number of initial runaway electrons that could increase the impurities and instabilities inside the plasma. In addition to the above-mentioned features, this technique could improve the uniform formation of the current sheath during the breakdown phase that might later lead to a high quality pinch and high intensity emitted x-rays.

First results from protective ECRH diagnostics for Wendelstein 7-X

Wendelstein 7-X (W7-X) is a steady state capable optimised stellarator. The main heating system is electron cyclotron resonance heating (ECRH) operating at 140 GHz providing up to 9 MW microwave power. The power is launched into the machine by front steerable quasi-optical launchers in X- or O-mode. While in X-mode the first pass absorption is 99%, it is only 40... 70% in O-mode. O2-mode heating is forseen for high density operation above the X2 cutoff density of m−3. A set of diagnostics has been developed to protect the machine from non absorbed ECRH power which can easily damage in vessel components. The non absorbed power hitting the inner wall is measured by waveguides embedded in the first wall (ECA diagnostic). In order to prevent the inner wall from overheating or arcing, a near-infra red sensitive video diagnostic with a dynamic range of 450...1200 °C was integrated in the ECRH launchers. Thermal calculations for the carbon tiles predict a temperature increase above the detection threshold for scenarios of plasma start-up failure or poor absorption on a time scale of 50 ms. However, the temperature increase measured by an IR camera in experiments with failed break down, i.e. no ECRH absorption for up to 50 ms, was only C. In discharges with 5% transmission the measured temperature increase was comparable. The stray radiation level inside the machine is measured by so called sniffer probes resembling microwave diode detectors which were designed to collect all radiation approaching the probing surface independent of incident angle and polarization. Five sniffer probes are installed at different toroidal positions. They were integrated in the ECRH interlock system. During the first operational phase of W7-X this was the only available plasma interlock system. The signal quality proofed to be high enough for a reliable termination in case of poor absorption. After a breakdown phase of 10 ms, the sniffer probe signals dropped by more than an order of magnitude. Especially in the very first days of operation, most discharges died by a radiative collapse due to impurity influx. In this case the heating power was reliably switched off due to the increased level of stray radiation. Moreover, ECRH bolometers with a slower response time in the launcher ports and an empty diagnostic port were used to estimate the stray radiation level in the ports. In the launcher ports it could be shown that the stray radiation could lead to an overheating of the bellows in long discharges. Possible counter measures are discussed.

Kinetic simulations of gas breakdown in the dense plasma focus

The first fully kinetic, collisional, and electromagnetic simulations of the breakdown phase of a MA-scale dense plasma focus are described and shown to agree with measured electrical characteristics, including breakdown time. In the model, avalanche ionization is driven by cathode electron emission, and this results in incomplete gas breakdown along the insulator. This reinforces the importance of the conditioning process that creates a metallic layer on the insulator surface. The simulations, nonetheless, help explain the relationship between the gas pressure, the insulator length, and the coaxial gap width. Previously, researchers noted three breakdown patterns related to pressure. Simulation and analytical results show that at low pressures, long ionization path lengths lead to volumetric breakdown, while high pressures lead to breakdown across the relatively small coaxial electrode gap. In an intermediate pressure regime, ionization path lengths are comparable to the insulator length which promotes ideal breakdown along the insulator surface.

Design of an ICRF system for plasma–wall interactions and RF plasma production studies on TOMAS

Ion cyclotron wall conditioning (ICWC) is being developed for ITER and W7-X as a baseline conditioning technique in which the ion cyclotron heating and current drive system will be employed to produce and sustain the currentless conditioning plasma. The TOMAS project (TOroidal MAgnetized System, operated at the FZ-Juelich, Germany) proposes to explore several key aspects of ICWC.For this purpose we have designed an ICRF system made of a single strap antenna within a metallic box, connected to a feeding port and a pre-matching system. We discuss the design work of the antenna system with the help of the commercial electromagnetic software CST Microwave Studio®. The simulation results for a given geometry provide input impedance matrices for the two-port system. These matrices are afterwards inserted into various circuit models to assess the accessibility of the required frequency range.The sensitivity of the matching system to uncertainties on plasma loading and capacitance values is notably addressed. With a choice of three variable capacitors we show that the system can cope with such uncertainties. We also demonstrate that the system can cope as well with the high reflected power levels during the short breakdown phase of the RF discharge, but at the cost of a significantly reduced coupled power.

Amplitude-time characteristics of runaway electron beams during the breakdown phase in high-pressure gases

Results of experimental studies of the amplitude-temporal characteristics of a supershort avalanche electron beam (SAEB) with a picosecond time resolution are presented. It is shown that the maximum SAEB current and the voltage drop in the gap are timed to tens of picoseconds and that the use of sharp-ended cathode improves the stability of SAEB generation.

The influence of the cathode array and the pressure variations on the current sheath dynamics of a small plasma focus device in the presence of an axial magnetic probe

In this research, the influence of the cathode array and the pressure variations on the current sheath dynamics of a small plasma focus device (450 J) was investigated. For this purpose, the signals of an axial magnetic probe for two different gases (argon and nitrogen) were studied. The magnetic probe signals showed the slower movement of the current sheath layer when the number of cathode rods decreased. This was related to the increase in the circuit inductance, which caused the longer discharge time of the capacitor bank followed by the creation of runaway electrons. These electrons in turn produced the impurities that led to the appearance of the instabilities inside the plasma. On the other hand, in order to investigate the effect of the cathode array variation on the instabilities produced inside the plasma, the wavelet technique was used. With the aid of frequency analysis, this technique showed the increase in these instabilities, which was due to the non-uniform formation of the current sheath layer during the breakdown phase, and finally, the higher values of the pressure caused the slower movement of the current sheath due to the inverse relation of the current sheath velocity to the square root of the pressure.

Streak instability in near-wall turbulence revisited

ABSTRACTThe regeneration cycle of streaks and streamwise vortices plays a central role in the sustainment of near-wall turbulence. In particular, the streak breakdown phase in the regeneration cycle is the core process in the formation of the streamwise vortices, but its current understanding is limited particularly in a real turbulent environment. This study is aimed at gaining fundamental insight into the underlying physical mechanism of the streak breakdown in the presence of background turbulent fluctuation. We perform a numerical experiment based on direct numerical simulation, in which streaks are artificially generated by a body forcing computed from previous linear theory. Upon increasing the forcing amplitude, the artificially driven streaks are found to generate an intense fluctuation of the wall-normal and spanwise velocities in a fairly large range of amplitudes. This cross-streamwise velocity fluctuation shows its maximum at λ+x ≈ 200 − 300 (λ+x is the inner-scaled streamwise wavelength), but it only appears for λ+x ≲ 3000 − 4000. Further examination with dynamic mode decomposition reveals that the related flow field is composed of sinuous meandering motion of the driven streaks and alternating cross-streamwise velocity structures, clearly reminiscent of sinuous-mode streak instability found in previous studies. Finally, it is shown that these structures are reasonably well aligned along the critical layer of the secondary instability, indicating that the surrounding turbulence does not significantly modify the inviscid inflectional mechanism of the streak breakdown via streak instability and/or streak transient growth.

Energy deposition in discharge chamber of lightning protection multichamber system

The experimental data of energy deposition distribution along discharge chamber of lightning protection multichamber system in initial stage of discharge process aimed to model lightning current impulse up to 10 kA is presented. A multichamber system is a series connection of discharge chambers. According to our experiments the shock wave formation occurs during the breakdown phase between electrodes located at the bottom of discharge chamber. The consequent energy deposition during discharge development goes in the whole volume bounded by shock wave front.

Numerical modeling of tokamak breakdown phase driven by pure Ohmic heating under ideal conditions

We have simulated tokamak breakdown phase driven by pure Ohmic heating with implicit particle in cell/Monte Carlo collision (PIC/MCC) method. We have found two modes can be differentiated. When performing breakdown at low initial gas pressure, we find that it works at lower density and current, but higher temperature, and requires lower heating power, compared to when having a high initial pressure. Further, two stages can be distinguished during the avalanche process. One is the fast avalanche stage, in which the plasma is heated by induced toroidal electric field. The other is the slow avalanche stage, which begins when the plasma density reaches 1015 m−3. It has been shown that ions are mainly heated by ambipolar field and become stochastic in the velocity distribution. However, when the induced electric field is low, there exists a transition phase between the two stages. Our model simulates the breakdown and early hydrogen burn-through under ideal conditions during tokamak start-up. It adopted fewer assumptions, and can give an idealized range of operative parameters for Ohmic start-up. Qualitatively, the results agree well with certain experimental observations.

Origin and Evolution of Spontaneous Rotation in Plasma Under Different Magnetic Field Geometries in Tokamak QUEST

Spontaneous toroidal rotation of the plasma is observed in the spherical tokamak QUEST with the help of electron cyclotron resonance (ECR) heating and without the use of any externally injected momentum. Several vertical magnetic field ( $B_{z}$ ) configurations with varying mirror ratio ( $M$ ) (a measure of field curvature) are applied and evolution of rotation is studied with the help of Doppler spectroscopy of bulk and impurity ions. Significant toroidal rotation ( $V_{\phi } \sim 6$ km/s) is initiated in the open magnetic field configuration during the initial plasma breakdown phase, which is later sustained ( $V_{\phi } \sim 20$ km/s) in a closed magnetic field configuration in steady state. Rotational velocity is primarily along the cocurrent direction and is found to be proportional to the $B_{z}$ strength and the resulting plasma current. High $M$ and $B_{z}$ are demonstrated to be the two specific external controls by which rotation can be initiated in the plasma. The rotation in open field lines is found to be initiated at the ECR layer in the slablike plasma, which is evolved to produce a sustained rotation in the natural divertor inboard poloidal field null equilibrium in QUEST.

Using High-Power Gyrotrons in the T-10 Tokamak

In this work, we briefly review the T-10 tokamak experiments on the interaction of microwaves with the plasma in the electron-cyclotron resonance frequency range. The basic results on the electron-cyclotron heating and the noninductive current generation in the plasma at both the first and second cyclotron harmonics are presented along with the typical values of the heating and current-generation efficiencies. The possibility of using the local contribution of the high-frequency power to control the sawtooth-oscillation instability and the amplitudes of the neoclassical tearing modes is demonstrated. Using the injection of the high-frequency waves for the working-gas preionization, allows one to optimize both the breakdown phase in the absence of a rotational electric field and the discharge-current increase stage.