Gas Switch Research Articles

Evaluation of gas switch effect on isothermal gas-solid reactions in a thermogravimetric analyzer

The effect of gas switching during isothermal gas-solid reactions were evaluated by taking CO2 gasification of graphite in a thermogravimetric analyzer as an example. The evolution behavior of the gases during gasification with or without the gas switch step were recorded and analyzed by an on-line mass spectrometer. The gasification curve from the procedure with gas switch step showed an obvious difference with that obtained from all CO2 atmosphere. For the procedure with gas switch step, the reaction indeed occurs in a mixture atmosphere of CO2 and the inert gas initially, which greatly restrict the gasification reaction and completely changes the reaction rate curve trends. Moreover, the effect of gas switching is in proportion to the gasification temperature. With the increase of the gasification temperature, the effect of gas switching becomes more significant. The activation energy calculated using the data from the gas switch method become lower for the gas diffusion effect. This research indicate that the previous study which does not consider the gas switch effect would contain some errors on the evaluation of the samples reactivity and kinetic parameters. Estimate the gas evolution behavior is needed before doing the isothermal gas-solid reaction experiments using the gas switch method.

Comparative study on breakdown characteristics of trigger gap and overvoltage gap in a gas pressurized closing switch

Gas pressurized closing switches are one of the most important elements in FLTD-based systems. Improving the trigger performance of gas switches is useful for optimizing the output parameters and the reliability of the FLTD. In this paper, the breakdown characteristics of the trigger gap and the overvoltage gap are studied experimentally. The reasons for the different breakdown performance of the two gaps are also investigated. The results show that the breakdown delay of the trigger gap is more influenced by the trigger voltage, while the breakdown delay of the overvoltage gap is more influenced by the working coefficient and always higher than that of the trigger gap. The jitter of the trigger gap is more influenced by the trigger voltage and accounts more than 60% of the total switch jitter, while the jitter of the overvoltage gap is hardly changed with the trigger voltage as well as the working coefficient and maintains less than 1.4 ns. It is proved that the discharging product from the trigger gap can effectively reduce the breakdown delay and jitter of the overvoltage gap. Based on that, the effect and improvement of pre-ionization on the two gaps are also studied. It is concluded that the jitter of the trigger gap reduces obviously when the pre-ionization is added, while the pre-ionization almost has no effect on the jitter of the overvoltage gap. The jitter of the overvoltage gap is about two times higher than the trigger gap in the pre-ionizing switch.

A compact high-current “field-distortion” gas switch with increased lifetime of sharp trigger electrode

A compact 40 kV, 80 kA low-jitter triggered gas switch of “field-distortion” type was developed. The electrode system of the switch includes two main electrodes with toroidal working surface and a disk type trigger electrode. A stable operation for a long time without degradation of dynamic parameters (time delay, jitter), which is resulted from erosion of sharp edge of the trigger electrode, was achieved by the decreasing of its diameter and removing the disk edge from zone between the tops of the main electrodes. Switch tests showed that for positive polarity of applied voltage U equal to 0.75 of self-breakdown voltage Usb and negative trigger pulse of 2/3U the total time delay including the rise time of the trigger pulse of 15 ns is about 27 ns and jitter is ≈1 ns. A stable operation of 10 simultaneously triggered switches is observed with the maximal deviation of closing time within 5 ns. No degradation of the switch parameters were registered for more than 200 pulses at switching currents of ≈60 kA in amplitude.

Corona-stabilized gas spark gap switch for a double forming line with 300 kV working voltage

A high-current self-breakdown gas switch stabilized by corona preionization was developed for closing of 300 kV repetitively operated at 10 pps water insulated double-forming line (Blumlein) of a nanosecond electron accelerator. The switch is installed at the end of the Blumlein and is used for closing of outer of two coaxial lines. High stability of the self-breakdown voltage (within ±1%) at fast charging of Blumlein (during ≈1.4 μs) is achieved by gas preionization in the spark-gap by additional pulsed corona discharge. The spark-gap electrode system includes two main toroidal electrodes and an additional corona needle that is located in the cavity of one of them (negative during the operation). It combines two parallel gas discharge gaps – the main spark gap and an additional corona discharge gap. Separating the functions of the spark and corona gaps makes it possible to optimize the geometry of the electrodes of each gap and provide both the ability to switch high currents and high dynamic characteristics for a long operating time. Dried air is used as a working gas at pressure of 3-4 atm (abs.). Gas blowing system includes 4 gas inlets located at outer flange supporting the high voltage insulator (water-gas interface) and one outlet in the cavity of the grounded main electrode. This scheme of gas flow protects the insulator from deposition of electrode erosion products and minimizes gas flow (≈20 l/min) for stable operation of the switch by removing of the hot gas throw the cavity of the grounded electrode.The design of the switch together with the main results of the working tests will be presented and discussed.

Erosion and Surface Morphology of the Graphite Electrodes in High-Current, High-Coulomb Transfer Gas Switch

A two-electrode graphite gas switch has been widely used in power conditioning systems for high-power laser pumping due to its simple structure, robustness, and low cost. Previous studies have shown that the lifetime of the graphite gas switches mainly depended on the graphite erosion performance, while different types of graphite materials displayed a great difference in erosion performance. In order to understand the erosion mechanism of the graphite electrodes under high-current, high-Coulomb transfer working conditions, the electrode’s surface morphology was obtained during the lifetime experiments, and the thermophysical process on the electrode surface was studied. The results showed that the energy derived from arc plasma was mainly used for graphite sublimation, and the surface morphology of the graphite electrodes was obviously different. The reason could be that the heat flux on anode and cathode surfaces was different due to convection process generated by electron flow during discharge. The erosion rate of the anode was heavier than that of the cathode, and the proportion was around 2.15:1. The measured erosion rates of the electrodes were linearly increased with the number of discharges; this was significant for predicting the lifetime of the two-electrode graphite gas switch.

THE MEASURES FOR SUPPRESSION APERIODIC CURRENT COMPONENT IN EXTRA-HIGH VOLTAGE LINES 750 kV

At present, the networks of extra-voltage received the SF6 switches with the auto-compression principle of extinguishing the arc. The experience of operation showed the inability of these switches to perform commutation-type quick "on/off" for lines with a high degree of compensation of charging power due to the existence of a long durable aperiodic component in the current flowing in the switch. The results of investigation of the causes of accidents of SF6 circuit breakers during commutation of compensated transmission lines of 750 kV are presented. In the article have been reported, that the presence of a large aperiodical component of current prevent the current from crossing zero for several milliseconds, resulting in serious damage to one pole of circuit breaker.In the work the analysis of abnormal modes of extra high voltage transmission lines with different measures is performed. The article deals with the cycle of three-phase automatic re-inclusion on the 750 kV line with two groups of shunt reactors installed. Electromagnetic processes in compensated transmission lines depending on parameters of the line are investigated. The moments of a sharp change in parameters of transient processes during switching in extra-high voltage lines are revealed. Measures to limit the duration of the existence of the aperiodic component of the current are considered.In the main power Ukrainian grids have replaced air switches for SF6 gas switches. This upgrade was caused by the fact that modern electrical industry does not produce air switches and in the event of damage to repair and restore power would be impossible. Compared to SF6 gas air circuit breakers have certain advantages, the main ones are speed and high breaking capacity, and the possibility of an extra device controlled switching. This upgrade was carried out from 2010 year to 2014.Nevertheless new SF6 breakers have a lot advantages, but some nuances were discovered in the process of using 750 kV on long overhead power lines with shunt reactors installed on them, due to which there were several accidents at the substations. Damage 750 kV circuit breakers, which in recent years have occurred on several foreign substations and also in Ukraine, served as an excuse for starting the necessary development of normative documents, establishing regulations and norms of operation.

Note: Multi-gap gas switch with low trigger-threshold voltage by mounting resistors and capacitors in parallel with switch gaps.

To reduce the trigger threshold voltage of the multi-gap gas switch used for linear transformer drivers, a method is proposed by mounting resistors and capacitors in parallel with the switch gaps. Based on the circuit model of the six-gap gas switch, the gap voltage distribution during the triggering process is analyzed. When the multi-gap gas switch is triggered, the voltage distribution between gaps is mainly determined by the stray capacitance between electrodes. In such condition, the trigger voltage is not fully applied on the trigger gap, and as a consequence, a higher trigger voltage is required for obtaining a low jitter. The effects of capacitor parameters on the triggering characteristics of the switch are experimentally investigated. Compared with the original switch design, the results indicate that at a charging voltage of ±80 kV and operating at 60% of the self-breakdown voltage, the trigger voltage is reduced from 110 kV to 75 kV while the 3.2 ns jitter of the switch is preserved.

Erratum: “Note: Multi-gap gas switch with low trigger-threshold voltage by mounting resistors and capacitors in parallel with switch gaps” [Rev. Sci. Instrum. 89, 096105 (2018)

First Page

New low-inductance capacitor-switch assembly

We have developed a new type of capacitor - switch assembly that are designed to be used with linear transformer drivers. This assembly can be DC charged to 160 kV and has a capacity of 110 nF. Due to the low inductance of 62 nH, this assembly is able to develop an output power of 10.2 GW. Also, we have designed and tested a three-electrode gas switch which is able to operate at the rated voltage and triggered with a jitter of 2 ns or less. This design of the capacitor-switch assembly allows creating new more compact and more powerful linear transformer driver.

A plasma-triggered gas switch.

This paper presents a low-jitter, low-trigger-threshold, high-dielectric-constant ceramic, plasma-triggered gas switch developed for pulsed power generators. We also discuss the principles behind the switch and the experiments and numerical calculations used. The electron emission and surface flashover in the trigger structure are used to create the initial plasma for switch breakdown. The experimental results show that the switch is triggered stably with a negative pulse of -50 kV peak and has a breakdown delay time jitter of 2-3 ns. In this jitter performance, the switch has a switch coefficient below 20%, and a peak current-carrying ability of at least ±5 kA in dry air at atmospheric pressure. The switch can also be reliably triggered with a switch coefficient of 49.7% and an over-voltage ratio of only 0.16 in dry air.

Experimental and Theoretical Studies on the Effect of Electrode Area on Static Performance of Gas Switches

Based on the self-breakdown experiments of gas switches, the relationship between the electrode area and the switch static performance has been studied. In the experiments, the structure of the switch is fixed, but the disc electrodes of the switch are replaceable to change the electrode area. Diameters of the electrodes are, respectively, 30, 35, 40, and 45 mm. The gas pressure, gas type, and discharge circuit are constant. As the electrode diameter has grown, distribution curve of the self-breakdown voltage has been regularly changed and both average and deviation of the voltage decreased. In order to describe the experimental results, a semiempirical model based on mathematics has been proposed. The calculated self-breakdown voltage distribution curves agree well with those in experiments.

Marx Generators Based on MOS-Gated Switches With Magnetic Assist for Accelerator Applications

In the field of pulsed power, gas switches have been widely used due to their high operating voltages and currents. One example of these gas switches is the thyratron, which is in use for some particle accelerators even today. At the Extreme Energy-Density Research Institute, the Nagaoka University of Technology, Nagaoka, Japan, such devices are in use. In recent times, semiconductor switches have been considered as possible replacements due to their ease of maintenance and control. In general, it is difficult to achieve the same ratings for maximum switching current and voltage of a gas switch with a single semiconductor device, so connecting many in series, parallel, or a different scheme is often applied. We propose a study to evaluate a Marx topology using MOS-gated thyristors capable of high current output. The attractive factors of these semiconductors are the low gate current required for turning ON, a low resistance during the ON-state, a four-stage prototype was built, and it achieved a peak output of 1 kV and 1 kA with a $2~\mu \text{s}$ pulsewidth. To improve the output voltage and current, a ferrite core was implemented as a magnetic switch on every stage; this method has been used before to improve the efficiency of thyristors, however, not combined with circuit topologies like a Marx generator. Adding these magnetic switches increased the efficiency to 89% (from 70%) and allowed an output of 2.2 kV and 2.2 kA without changing the shape of the pulse or the rise time.

Nanosecond bipolar pulse generators for bioelectrics

Biological effects caused by a nanosecond pulse, such as cell membrane permeabilization, peripheral nerve excitation and cell blebbing, can be reduced or cancelled by applying another pulse of reversed polarity. Depending on the degree of cancellation, the pulse interval of these two pulses can be as long as dozens of microseconds. The cancellation effect diminishes as the pulse duration increases. To study the cancellation effect and potentially utilize it in electrotherapy, nanosecond bipolar pulse generators must be made available. An overview of the generators is given in this paper. A pulse forming line (PFL) that is matched at one end and shorted at the other end allows a bipolar pulse to be produced, but no delay can be inserted between the phases. Another generator employs a combination of a resistor, an inductor and a capacitor to form an RLC resonant circuit so that a bipolar pulse with a decaying magnitude can be generated. A third generator is a converter, which converts an existing unipolar pulse to a bipolar pulse. This is done by inserting an inductor in a transmission line. The first phase of the bipolar pulse is provided by the unipolar pulse's rising phase. The second phase is formed during the fall time of the unipolar pulse, when the inductor, which was previously charged during the flat part of the unipolar pulse, discharges its current to the load. The fourth type of generator uses multiple MOSFET switches stacked to turn on a pre-charged, bipolar RC network. This approach is the most flexible in that it can generate multiphasic pulses that have different amplitudes, delays, and durations. However, it may not be suitable for producing short nanosecond pulses (<100 ns), whereas the PFL approach and the RLC approach with gas switches are used for this range. Thus, each generator has its own advantages and applicable range.

A novel compact low impedance Marx generator with quasi-rectangular pulse output

In this paper, a novel low impedance compact Marx generator with near-square pulse output based on the Fourier theory is developed. Compared with the traditional Marx generator, capacitors with different capacity have been used. It can generate a high-voltage quasi-rectangular pulse with a width of 100 ns at low impedance load, and it also has high energy density and power density. The generator consists of 16 modules. Each module comprises an integrative single-ended plastic case capacitor with a nominal value of 54 nF, four ceramic capacitors with a nominal value of 1.5 nF, a gas switch, a charging inductor, a grounding inductor, and insulators which provide mechanical support for all elements. In the module, different discharge periods from different capacitors add to the main circuit to form a quasi-rectangular pulse. The design process of the generator is analyzed, and the test results are provided here. The generator achieved pulse output with a rise time of 32 ns, pulse width of 120 ns, flat-topped width (95%-95%) of 50 ns, voltage of 550 kV, and power of 20 GW.

Resonant Gas Sensor and Switch Operating in Air With Metal-Organic Frameworks Coating

We demonstrate a resonant gas sensor uniformly coated with a metal-organic framework (MOF) and excited near the second vibration mode for enhanced sensitivity. The possibility of realizing a smart switch triggered upon exceeding a threshold mass is demonstrated when operating the resonator near the dynamic pull-in instability. The resonator is based on an electrostatically excited clamped–clamped microbeam. The microbeam is fabricated from a polyimide layer coated from top with Cr/Au and from bottom with Cr/Au/Cr layer. The geometry of the resonator is optimized to reduce the effect of squeeze film damping, thereby allowing operation under atmospheric pressure. The electrostatic electrode is designed to enhance the excitation of the second mode of vibration with the minimum power required. Significant frequency shift in kilohertz is demonstrated for the first time upon water, acetone, and ethanol vapor exposure due to the MOF functionalization and the higher order mode excitation. The adsorption dynamics and MOF selectivity are investigated by studying the decaying time constants of the response upon gas exposure. [2017–0218]

Free microparticles-An inducing mechanism of pre-firing in high pressure gas switches for fast linear transformer drivers.

Microparticle initiated pre-firing of high pressure gas switches for fast linear transformer drivers (FLTDs) is experimentally and theoretically verified. First, a dual-electrode gas switch equipped with poly-methyl methacrylate baffles is used to capture and collect the microparticles. By analyzing the electrode surfaces and the collecting baffles by a laser scanning confocal microscope, microparticles ranging in size from tens of micrometers to over 100 μm are observed under the typical working conditions of FLTDs. The charging and movement of free microparticles in switch cavity are studied, and the strong DC electric field drives the microparticles to bounce off the electrode. Three different modes of free microparticle motion appear to be responsible for switch pre-firing. (i) Microparticles adhere to the electrode surface and act as a fixed protrusion which distorts the local electric field and initiates the breakdown in the gap. (ii) One particle escapes toward the opposite electrode and causes a near-electrode microdischarge, inducing the breakdown of the residual gap. (iii) Multiple moving microparticles are occasionally in cascade, leading to pre-firing. Finally, as experimental verification, repetitive discharges at ±90 kV are conducted in a three-electrode field-distortion gas switch, with two 8 mm gaps and pressurized with nitrogen. An ultrasonic probe is employed to monitor the bounce signals. In pre-firing incidents, the bounce is detected shortly before the collapse of the voltage waveform, which demonstrates that free microparticles contribute significantly to the mechanism that induces pre-firing in FLTD gas switches.

The development of shock wave overpressure driven by channel expansion of high current impulse discharge arc

During the formation of a high current impulse discharge arc, objects near the discharge arc will be strongly impacted. In this paper, a high power, high current gas switch is used as the site of the impulse discharge arc. The explosion wave theory and the arc channel energy balance equation are introduced to analyze the development of the shock wave overpressure driven by the high current impulse discharge arc, and the demarcation point of the arc channel is given, from which the energy of the arc channel is no longer converted into shock waves. Through the analysis and calculation, it is found that the magnitude of the shock wave overpressure caused by impulse discharge arc expansion is closely related to the arc current rising rate. The arc shock wave overpressure will undergo a slow decay process and then decay rapidly. The study of this paper will perform the function of deepening the understanding of the physical nature of the impulse arc discharge, which can be used to explain the damage effect of the high current impulse discharge arc.

Effects of the injected plasma on the breakdown process of the trigatron gas switch under low working coefficient

Based on the surface flashover discharge, the injected plasma was generated, and the effects on the breakdown process of the trigatron gas switch were studied in this paper. The breakdown model caused by the injected plasma under the low working coefficient (&amp;lt;0.7) was established. The captured framing images showed that the injected plasma distorted the electrical field of the gap between the frontier of the injected plasma and the opposite electrode, making it easier to achieve the breakdown critical criterion. The calculation results indicated that the breakdown delay time was mainly decided by the development of the injected plasma, as without considering the effects of the photo-ionization and the invisible expansion process, the breakdown delay time of the calculation results was 20% higher than the experimental results. The morphology of the injected plasma generated by polyethylene surface flashover was more stable and regular than ceramic, leading to a 30% lower breakdown delay time when the working coefficient is larger than 0.2, and the difference increased sharply when the working coefficient is lower than 0.2. This was significant for improving the trigger performance of the trigatron gas switch under low working coefficient.

Electrode erosion properties of gas spark switches for fast linear transformer drivers

Fast linear transformer drivers (FLTDs) are a popular and potential route for high-power devices employing multiple “bricks” in series and parallel, but they put extremely stringent demands on gas switches. Electrode erosion of FLTD gas switches is a restrictive and unavoidable factor that degrades performance and limits stability. In this paper, we systematically investigated the electrode erosion characteristics of a three-electrode field distortion gas switch under the typical working conditions of FLTD switches, and the discharge current was 7–46 kA with 46–300 ns rise time. A high speed frame camera and a spectrograph were used to capture the expansion process and the spectral emission of the spark channel was used to estimate the current density and the spark temperature, and then the energy fluxes and the external forces on the electrode surface were calculated. A tens of kilo-ampere nanosecond pulse could generate a 1011 W/m2 energy flux injection and 1.3–3.5 MPa external pressure on the electrode surface, resulting in a millimeter-sized erosion crater with the maximum peak height Rz reaching 100 μm magnitude. According to the morphological images by a laser scanning confocal microscope, the erosion crater of a FLTD switch contained three kinds of local morphologies, namely a center boiling region, an overflow region and a sputtering region. In addition, the crater size, the surface roughness, and the mass loss were highly dependent on the current amplitude and the transferred charge. We also observed Morphology Type I and Type II, respectively, with different pulse parameters, which had an obvious influence on surface roughness and mass loss. Finally, the quantitative relationship between the electrode mass loss and the pulse parameter was clarified. The transferred charge and the current amplitude were proved to be the main factors determining the electrode mass loss of a FLTD switch, and a least squares fitting expression for mass loss was also obtained.

Stability of Triggering of the Switch with Sharply Non-Uniform Electric Field at the Electrode with Negative Potential

Results of investigations into a two-electrode high-pressure gas switch with sharply non-uniform field at the electrode with negative potential operating in the self-breakdown regime with pulsed charging of a highvoltage capacitive energy storage for ~100 μs to voltage exceeding 200 kV are presented. It is demonstrated that depending on the air pressure and the gap length, the corona-streamer discharge, whose current increases with voltage, arises in the switch at a voltage of 0.2–0.3 of the self-breakdown voltage. At the moment of switch self-breakdown, the corona-streamer discharge goes over to one or several spark channels. The standard deviation of the triggering moment can be within 1.5 μs, which corresponds to the standard deviation of the self-breakdown voltage less than 2 kV. The voltage stability can be better than 1.5%.