Gas Breakdown Research Articles

Investigation and diagnostics of plasma flows in a pulsed plasma accelerator for experimental modelling of processes in tokamaks

This paper presents the experimental results on electron, ion temperatures and densities in a pulsed plasma accelerator. The values of electron densities and temperatures were computed using the methods of relative intensities of Hα and Hβ lines, Hβ Stark broadening, and the technique is based on Faraday cup beam current measurements. In this work, a linear optical spectrometer S-100 was used to acquire the emission spectra of hydrogen and air plasmas. In this spectrum, there are some lines due to Fe, Cu, N2, O2, and H2. The series of visible lines in the hydrogen atom spectrum are named the Balmer series. The spectral emissions of iron and copper occur throughout the gas breakdown and ignition of an arc discharge, during the erosion and sputtering of materials. The vacuum chamber and coaxial electrodes were made. The electron temperatures and densities in a pulsed plasma accelerator, measured via relative intensities of spectral lines and Stark broadening, at a charging voltage of a capacitor bank of 3 kV and a working gas pressure in a vacuum chamber of 40 mTorr, were 2.6 eV and 1.66 · 1016 cm−3 for hydrogen plasma. These results were compared with the Faraday cup beam current measurements. However, no match was found. Considering and analyzing this distinction, we concluded that the spectral method of plasma diagnostics provides more accurate results than electrical measurement. The theory of probe measurements can give approximate results in a moving plasma.

Effects of gas adsorbed on solid surface during gas breakdown in electron cyclotron resonance discharges

The microwave breakdown power (P wb) in an ECR plasma source was not merely determined by pressure (gas flow rate), but found to vary with the time interval between two successive breakdowns. The measured P wb dropped rapidly from a high value at a short time interval to a low level at a long time interval. The obtained dependence of P wb on pressure (gas flow rate) exhibited distinct features: the normal monotonicity and abnormal non-monotonicity at the short and long time intervals, respectively. The effective zone in the antenna’s surface, bombarded by hot electrons heated in the ECR layer, was validated by (1) masking the antenna with a film having a variable radius; (2) calculating the distribution of the vertical component of the microwave electric field with respect to the static magnetic field; and (3) imaging glows of transient breakdown discharges with a fast camera. The reduction in P wb was mainly attributed to the enhanced emission of δ-electrons from the gas-adsorbed antenna under the bombardment of energetic electrons coming from the ECR layer. The correlation between the dynamic gas coverage and the emission coefficient of δ-electrons was established to understand the abnormal ECR breakdown features.

Design, fabrication, and testing of microelectromechanical system spark gap switch based on streamer theory

In order to realize that the fuze micro system has both high security and miniaturization characteristics, the spark gap research of Micro-Electro-Mechanical System safety system is carried out. So that to solve the safe and reliable function of the spark gap switch under the low power supply voltage (35 V) of the fuze micro system, the gas gap size and electrode radius are shown to significantly affect the gas breakdown voltage using streamer theory. Based on these results, a spark gap switch with triggering electrodes is designed. The triggering electrode gap is 2 μm and the main electrode gap is 10 μm. A spark gap switch test circuit is designed based on the RLC circuit. Through finite element simulation, it is verified that the gas breakdown voltage increases nonlinearly with increasing gap size. Pre-breakdown spark gap switches were fabricated based on the surface silicon process and tested. The test results show that the conduction voltage values of the triggering electrode and the main electrode are basically consistent with the simulation and calculation results. The breakdown voltage of the main electrode can be greatly reduced by applying a certain voltage to the triggering electrode, realize the reliable function in the micro fuze system.

Multi-physics Coupling Simulation of Heat Transfer and Structural Optimization for a Three-phase Gas Insulated Switchgear Busbar

Gas insulated switchgear (GIS) is one of the most important equipment in the power system. Excessive temperature of the GIS busbar conductor will lead to the loss of insulating gas performance and equipment life. In the present paper, based on the finite element method, the temperature distribution and power loss density of a 252 kV three-phase GIS busbar is numerically studied using electromagnetic-heat-flow coupling model, and the structure parameters of the rotation angle, center distance and conductor thickness of the busbar are optimized with Taguchi method. The results show that the conductor thickness has the greatest influence on the maximum temperature and power loss, and the influence proportion is more than 70 %. When the combination scheme of (A1, B5, C5) is adopted, the temperature rises and power loss performance of GIS has been optimized. The maximum temperature is reduced by 9.56 K and the power loss is reduced by 21.7 %. In addition, according to the analysis of the gas breakdown margin, it is found that the SF6 gas still has a good insulation performance after structure optimization. This study is of great significance to the heat transfer and optimization design of three-phase GIS busbar.

Microparticle‐initiated electron emission and subsequent gas breakdown

AbstractMicroparticle‐initiated breakdown phenomena in gas gap have obtained extensive research interest and micro‐gap discharge is convinced to play a key role. To illustrate this, this paper focused on microparticle‐initiated electron emission and subsequent gas breakdown process in a nitrogen gap. Analytic results of the electric field distribution between the charged microparticle and electrode surface are obtained by using the image method first. Results indicate that for the typical microparticles (R < 20 μm) from electrode erosion, when the micro‐gap distance h is lower than 2 μm, the maximum enhanced field strength could exceed 3 × 107 V/m. Based on this, a two‐dimensional axis‐symmetry numerical model is calculated by using the particle‐in‐cell coupling with Monte Carlo method and the charged microparticle is positioned near anode and cathode separately. In both cases, the local enhanced electron emission between microparticle and electrode surface is the key to initiate the final gas breakdown. The breakdown near cathode is electron‐dominated, which could accomplish in several nanoseconds. The breakdown near anode is ion‐dominated, which needs stronger field enhancement factor or longer breakdown time and would be harder to occur in reality.

Runaway of electrons and initiation of explosive electron emission during pulse breakdown of high-pressure gases

We propose a scenario of the initiation of explosive electron emission on the boundary of the electrode and a high-pressure gas. According to this scenario, positive ions are formed due to the gas ionization by field-emission electrons and accumulated in the vicinity of protrusions of micron size at the cathode. The distance between the ion cloud and the emitting surface decreases with increasing pressure which results in a growth of the local field. As a consequence, an explosive growth of the emission current density occurs for a dense gas (the gas with the pressure of tens of atm). As a result, explosive-emission centers can be formed in dozens of ps. These centers give a start to plasma channels expanding towards the anode. Runaway electron flow generated near the channel heads ionizes the gas gap, causing its subnanosecond breakdown.

94 GHz microplasma sustained by a photonic crystal

A photonic crystal (PhC) with a single vacancy sustains an argon microplasma at 94 GHz. The PhC consists of a 7 × 13 array of small alumina rods, each 250 μm in radius. The PhC has a forbidden bandgap from 70–105 GHz. The vacancy creates an internal electromagnetic resonance at 94 GHz that intensifies the electric field of an incident wave to 5 × 105 V m−1. Breakdown of low-pressure argon gas (80 mbar) occurs and forms a stable, ellipsoidal plasma inside the crystal. The central core of the microplasma is probed using a 200 μm optical fiber inserted into the PhC. The rotational temperature of the CH molecule is estimated from a numerical fit of the plasma emission and is found to increase from 450–650 K with both pressure (200–1000 mbar) and incident wave power (100 mW–1100 mW). Under these conditions, the electron density increases with both power and pressure from 0.5–1.5 × 1020 m−3. This plasma density range brackets the electron plasma frequency at 94 GHz. The results are compared with a similar microplasma operating at 44 GHz.

Study of Shock Wave Development in Cold Flow of Gas Circuit Breaker for Improving Capacitive Current Switching

Abstract Capacitive current switching performance of SF6 circuit breaker is determined by combined action of gas flow and electric field in interruption chamber. Computational fluid dynamics method is used to study the development of shock waves inside a SF6 circuit breaker under the condition of capacitive current switching. It was found that a shock wave exists and travels with the pin contact during the contact opening. Existence of shock waves reduces the local gas density around contact head, while the contact head is also stressed by high electric field at the same time. If the region of low gas density coincides with the region with high electric strength, local dielectric performance will be significantly reduced. Using E/ρ as the dielectric performance index, when the local value of E/ρ exceeds the gas breakdown criterion, it will result in restrike to the capacitive current switching. The causes of high E/ρ are studied by analyzing the shock waves during the sequential periods of contact travel. Since the gap between the pin contact and nozzle throat strongly affects the strength of shock wave, the behavior of gap is studied with the calculation of E/ρ to find its influence on capacitive current switching.

Characterization of Atmospheric-Pressure DC-Glow Discharge in Contact with Liquid with a Miniature Argon Flow

Atmospheric pressure dc glow discharges in contact with liquid with a miniature argon flow were created between a needle cathode and an electrolyte anode (NaCl solution) in the open air. The effect of argon flow in stabilizing the obtained discharge was explored. Electric characterization of the discharge was studied for electrode separation range from 1 to 12 mm at atmospheric pressure. The electrical characterization of the discharge revealed that the obtained Paschen curve showed a bi-straight line for the relation between the gas breakdown voltage and the product of the gas pressure 𝑝 with the electrode separation distance 𝑑. The current-voltage characteristics of the discharge revealed that the discharge was operated in the normal glow mode. Optical characterization of the discharge showed that the discharge was strongly non-equilibrium, which is suitable for electrochemistry applications such as synthesizing magnetic nanoparticles from the iron electrode. Proposed chemical reaction routes for different electrochemical applications of DC glow discharge in contact with the liquid electrode were discussed.

Spatiotemporal Characteristics of Radio Frequency Dielectric Barrier Glow Discharge at Atmospheric Pressure

In this paper, argon was used in radio frequency (13.56 MHz) dielectric barrier discharge (rf-DBD) at atmospheric pressure. The IV curve was recorded after gas breakdown, and discharge photos were captured by ICCD camera. Discharges of α mode and γ mode were observed based on IV curve and ICCD photos. As the existence of negative glow in γ mode, the luminescence intensity of different position of the discharge gap was analyzed. It was found that in the α mode, the electron avalanche occurs from negative to positive and negative glow appeared after the discharge changed into γ mode. In every half cycle, the peak position of negative glow is 13 ± 1 ns later than that of electron avalanche on cathode surface.

Computational analysis of direct current breakdown process in SF6 at low pressure

The breakdown of SF6 gas at low pressure is of vital importance to both aerospace and microelectronics industries. However, the breakdown characteristics of SF6 in direct current at low pressure are still seldom studied. In this work, one-dimensional implicit particle-in-cell/Monte-Carlo collision algorithm is used to study the entire direct current breakdown process of low-pressure SF6. The ion-molecule collision, recombination, and external circuit are considered in the model. According to the results, the breakdown process can be divided into three stages: pre-breakdown stage, breakdown stage, and post-breakdown stage. In the pre-breakdown stage, the cathode sheath is not yet formed so the constant electric field exists in the entire area. In the breakdown stage, the formation mechanism of the cathode sheath is analyzed and the electrodes as a whole changes from capacitive to resistive, sharing the voltage with the external resistance. In the post-breakdown stage, the continued growth of positive ions leads to the formation of a thin anode sheath, which further causes the negative plasma potential, different from electropositive gas. The energy production terms including heating power and secondary electron emission (SEE) power are equal to the energy loss terms including collision loss power and boundary loss power, where collision loss power and boundary loss power are almost equal, while SEE power is negligible. In the final, plasma parameters gradually evolve to the last steady-state.

The Helical Resonator: A Scheme for Radio Frequency Plasma Generation

The helical resonator is a scheme for the production of high voltage at radio frequency, useful for gas breakdown and plasma sustainment, which, through a proper design, enables avoiding the use of a matching network. In this work, we consider the treatment of the helical resonator, including a grounded shield, as a transmission line with a shorted end and an open one, the latter possibly connected to a capacitive load. The input voltage is applied to a tap point located near the shorted end. After deriving an expression for the velocity factor of the perturbations propagating along the line, and in the special case of the shield at infinity also of the characteristic impedance, we calculate the input impedance and the voltage amplification factor of the resonator as a function of the wave number. Focusing on the resonance condition, which maximizes the voltage amplification, we then discuss the effect of the tap point position, dissipation and the optional capacitive load, in terms of resonator performance and matching to the power supply.

Kinetic simulation of electron cyclotron resonance assisted gas breakdown in split-biased waveguides for ITER collective Thomson scattering diagnostic

For the measurement of the dynamics of fusion-born alpha particles Eα≤3.5 MeV in ITER using collective Thomson scattering (CTS), safe transmission of a gyrotron beam at mm-wavelength (1 MW, 60 GHz) passing the electron cyclotron resonance (ECR) in the in-vessel tokamak “port plug” vacuum is a prerequisite. Depending on neutral gas pressure and composition, ECR-assisted gas breakdown may occur at the location of the resonance, which must be mitigated for diagnostic performance and safety reasons. The concept of a split electrically biased waveguide (SBWG) has been previously demonstrated in C.P. Moeller, U.S. patent 4,687,616 (1987). The waveguide is longitudinally split and a kV bias voltage is applied between the two halves. Electrons are rapidly removed from the central region of high radio frequency electric field strength, mitigating breakdown. As a full scale experimental investigation of gas and electromagnetic field conditions inside the ITER equatorial port plugs is currently unattainable, a corresponding Monte Carlo simulation study is presented. Validity of the Monte Carlo electron model is demonstrated with a prediction of ECR breakdown and the mitigation pressure limits for the above-quoted reference case with 1H2 (and pollutant high Z elements). For the proposed ITER CTS design with a 88.9 mm inner diameter SBWG, ECR breakdown is predicted to occur down to a pure 1H2 pressure of 0.3 Pa, while mitigation is shown to be effective at least up to 10 Pa using a bias voltage of 1 kV. The analysis is complemented by results for relevant electric/magnetic field arrangements and limitations of the SBWG mitigation concept are addressed.

Simulation of high-power microwaves gas breakdown with a modified multi-physical model

In this paper, a modified multi-physics method for transient analysis of high-power microwaves (HPM) gas breakdown is proposed. Distinguished from previous works, the proposed method couples the plasma fluid equations with Maxwell's equations to fully consider the interaction between plasma and electromagnetic waves. To perform the numerical simulation, the spectral-element time-domain method is employed, which has the advantages of spectral accuracy and block diagonal mass matrix. Numerical simulations are conducted to demonstrate the accuracy of the proposed method. Moreover, with an external DC magnetic field, HPM breakdown can be effectively delayed by increasing its breakdown threshold. Simultaneously, the phase shift of electromagnetic waves during the HPM breakdown can also be controlled by the external DC magnetic field, which can improve the quality of the phase-modulated signal in HPM illumination. This proposed framework is expected to provide an effective numerical tool for analyzing the microwave propagation characteristics and suppressing the HPM breakdown in gas-filled microwave devices.

Dynamics and energy coupling of gas puff Z-pinches on a fast linear transformer driver

Gas puff Z-pinch experiments with annular Ar and Ne gas shells have been conducted on the Compact Experimental System for Z-pinch and Ablation Research (CESZAR) linear transformer driver (LTD) with 500 kA current and 160 ns rise time. Here, we present results from the first systematic gas puff Z-pinch experiments using a fast (≤200 ns) LTD as a driver, in which we show that 7% of the stored energy in the capacitors is coupled to plasma kinetic energy as estimated via self-emission and laser schlieren images. 0D and 1D simulations—which do not allow instability growth and thus reach greater maximum average velocities—using initial conditions inferred from experimental implosion trajectories predict coupling in excess of 10% of the stored energy. The Ar and Ne implosions were comparably massed and thus achieved similar maximum kinetic energies, though the Ne pinches were more stable and the x-ray pulses were longer and produced higher yield: 2–5 ns and 0.21–0.52 J (0.15–0.37 J/cm) of Ar K-shell and 12–25 ns and 2.2–3.9 J (1.6–2.6 J/cm) of Ne K-shell, respectively. The difference in stability is most likely attributed to variations in initial conditions such as density distribution and gas breakdown initiation.

Comprehensive understanding of the ignition process of a pulsed capacitively coupled radio frequency discharge: the effect of power-off duration

The effect of the pulse-off duration on the time evolution of the plasma and electrical parameters during the ignition phase in a pulsed capacitively coupled radio frequency argon discharge operated at 450 mTorr and 12.5 MHz is investigated synergistically by multifold experimental diagnostics, particle-in-cell/Monte Carlo collision simulations and an analytical model. In the experiment, the electron density is measured time-resolved by a hairpin probe, the spatio-temporal distribution of the electron impact excitation dynamics is studied by phase resolved optical emission spectroscopy, and the amplitudes and the relative phase, φ vi, of the discharge voltage and current are determined based on the waveforms measured by a voltage and a current probe. The experimental results show that the plasma and electrical parameters during the ignition process depend strongly on the duration of the afterglow period, T off, primarily because of the dependence of the remaining charge density on this parameter. Computed values of φ vi show a similar time-dependence compared to the experiment, if the simulations are initialized with specific initial charged particle densities, n ini. This allows us to further understand the time evolution of φ vi for different values of T off based on the simulation results together with an analytical model. In particular, the optical emission intensity is found to change with time in the same fashion as the power deposition into the system at T off ⩾ 100 μs, suggesting that the power is primarily absorbed by the electrons, which dissipate their energy via inelastic collisions. The system goes through different mode transitions of electron power absorption during the ignition phase depending on T off. Specifically, for short T off (high n ini), the α mode dominates during the entire ignition process, as the electric field is largely shielded by the abundant charge located in the interelectrode space. For intermediate values of T off (moderate n ini), another excitation pattern caused by an enhanced drift electric field at the center of the gap is observed, since a large fraction of the externally applied potential can penetrate into the central region in the absence of high charged particle densities. For longer T off (very low n ini), the ignition of the pulsed plasma behaves like a gas breakdown.

Main modes of runaway electron generation during a breakdown of high-pressure gases in an inhomogeneous electric field

Using our recently proposed method, we have identified three modes of runaway electron generation. This method is based on the recording of a dynamic displacement current using a current collector. It makes it possible to unambiguously interpret the dynamics of ionization processes occurring at the stage of formation of a high-voltage nanosecond discharge in a strongly inhomogeneous electric field and dense gaseous media. Simultaneous and separate recording by the current collector of a runaway electron beam current and the dynamic displacement current as well as the recording of discharge characteristics and high-speed framing of plasma glow at the breakdown stage made it possible to interpret the runaway electron generation modes in terms of the rate of ionization processes. It has been shown how the reduced electric field strength, the gas kind, the design and material of a high-voltage cathode with a small radius of curvature, the amplitude and rise time of a voltage pulse as well as the delay in the onset of the explosive emission process at the cathode affect the implementation of one or another generation mode.

Comparison of parameters of the electric strength of air in a surface antenna during the radiation of single microwave pulses of various shapes

When substantiating the requirements for the radiation parameters of high-power microwave generators it becomes necessary to assess their maximum permissible values, conditioned to air breakdown in the radiating surface antenna. To predict the parameters of the dielectric strength of air, a theory based on a model of microwave pulses with a rectangular envelope is widely used. However, in practice, there are cases when the shape of the pulse envelope of the generator differs significantly from the rectangular. The aim of this work is to evaluate and compare the amplitude and energy parameters of the electric strength of air in the surface antenna of a high-power relativistic microwave generator when pulses are emitted with envelopes of various shapes. Pulses with rectangular, trapezoidal, parabolic and Gaussian envelopes were selected as the initial ones. The breakdown parameters were found based on the criterion of pulsed breakdown of gases and the regularity connecting the electron density with the pulse amplitude. The problem is solved numerically. In the range of pulse durations typical for high-power relativistic microwave generators, the dependences of the breakdown field and the maximum allowable energy on the pulse duration under normal atmospheric conditions are determined. The regularities connecting the maximum permissible energy and the breakdown field for the considered pulses are determined. With the same breakdown fields, a pulse with a Gaussian envelope has the highest maximum permissible energy, and a pulse with a rectangular envelope has the lowest.

Numerical simulation of microplasma field-effect transistor at low voltages

In this study, two-dimensional simulation of the microplasma field-effect transistor (MOPFET) was performed using COMSOL Multiphysics along with BOLSIG $$+$$ software and LXCat database. At first, argon plasma generation in the electric discharge channel was investigated in the case of an RF-biased drain terminal. The Townsend criterion for self-sustaining discharge was modified by including ion-enhanced secondary ionisation concerning to the micrometer scale of the discharge gap. The simulation results showed that the level of the gate electrode voltage serves as a tuner for the MOPFET current, and it changes the gas breakdown voltage. The results of the 2D simulation showed that the polarity of the gate voltage is crucial for controlling the MOPFET current and the operation of switching circuitry. To increase current, a new gas mixture was proposed and simulated. The $$I_{\mathrm {DS}}{-}V_{\mathrm {DS}}$$ characteristic of the MOPFET was obtained with Ar/Hg (98:2). Static transconductance and $$h_{\mathrm {FE}}$$ of the MOPFET were also evaluated. The results of this study can be used for designing and operating the MOPFET in an electronic circuit.

Linkage of electron emission and breakdown mechanism theories from quantum scales to Paschen's law

Numerous applications such as micro- and nanoelectromechanical systems, microplasmas, and directed energy increasingly drive device miniaturization to nanoscale and from vacuum to atmospheric pressure. This wide range of operating conditions and relevant mechanisms complicates the derivation of a single scaling law for electron emission and gas breakdown; therefore, theoretical studies often unify two or three mechanisms piecemeal. This study defines a common set of scaling parameters across the range of dominant mechanisms to derive a theory that links electron emission and breakdown mechanism theories from quantum scales to Paschen's law and yields asymptotic solutions for quantum space-charge limited emission (QSCL), classical space-charge limited emission (CSCL), space-charge limited emission with collisions (MG), Fowler–Nordheim field emission (FN), field emission driven gas breakdown, and classical gas breakdown defined by Paschen's law (PL). These non-dimensionalized equations are universal (true for any gas) across all regimes except for PL, which contains a single, material-dependent parameter. This approach reproduces various nexuses corresponding to the transitions across multiple mechanisms, such as QSCL to CSCL, CSCL to FN, CSCL to MG to FN, and field emission-driven breakdown as described by FN to PL, using a single non-dimensionalization scheme to facilitate experimental designs concerned with crossing these regimes. Furthermore, we demonstrate the conditions for more complicated nexuses, such as matching QSCL, CSCL, MG, and FN. This provides valuable information to experimentalists concerning regimes where slight perturbations in conditions may alter the electron emission mechanism and to theorists concerning the applicability of the asymptotic solutions or reduced nexus theories.