Numerical simulation and experimental verification of plasma jet development in gas gap switch

Plasma jet triggered gas gap switch has obvious advantages in fast control switch.The development of the plasma in the ambient medium is the key factor affecting the triggering conduction of the gas switch.However,the plasma jet process and its characteristic parameters are complicated and the existing test methods cannot fully characterize its development laws.In this work,a two-dimensional transient fluid calculation model of the plasma jet process of the gas gap switch is established based on the renormalization-group k-εturbulence equation.The results show that the characteristic parameters and morphological evolution of the plasma jet are basically consistent with the experimental results,which verifies the accuracy of the simulation model calculation.The plasma jet is a long strip with an initial velocity of 1.0 km·s-1and develops in both axial and radial directions.The jet velocity fluctuates significantly with axial height.As the plasma jet enters the main gap,the pressure inside the trigger cavity drops by80%,resulting in a rapid drop in the jet velocity.When the plasma jet head interacts with the atmosphere,the two-phase fluid compresses each other,generating a forward-propelled pressure wave.The plasma jet heads flow at high velocity,a negative pressure zone is formed in the middle part of the jet,and the pressure peak decreases gradually with height.As the value of the inlet pressure increases,the characteristic parameters of the plasma jet increase.The entrainment phenomenon is evident,which leads to an increase in the pressure imbalance of the atmospheric gas medium,leading to a significant Coanda effect.Compared with air,the characteristic parameters of a plasma jet in SF6are lower,and the morphological evolution is significantly suppressed.The results of this study can provide some insight into the mechanism of action of the switch jet plasma development process.

A novel method for multichannel discharge image diagnosis in gas switches based on multi-axis tomography techniques

The discharge channel is a crucial diagnostic tool for identifying the properties of gas switches.However,single-view images cannot easily be used to inherently understand the discharge channel characteristics of trigger electrodes.A reconstruction method of the discharge channel in gas switches is proposed based on a multi-axis tomography technique,which uses very few projections of optical images to reconstruct the location of a multichannel discharge in annular electrodes.In this paper,an algorithm named TVM-OSEM(total variation minimization ordered subsets expectation maximization)is proposed,which can effectively remove artifact noise to improve the reconstruction accuracy of discharge channels.The method is validated against simulations of a radiator with high-density-difference boundary.The calculated discharge channel distribution from one experiment is presented.

The Joule–Thomson effect of (CO_(2)+H_(2)) binary system relevant to gas switching reforming with carbon capture and storage(CCS)

The Joule-Thomson effect is one of the important thermodynamic properties in the system relevant to gas switching reforming with carbon capture and storage(CCS). In this work, a set of apparatus was set up to determine the Joule-Thomson effect of binary mixtures(CO_(2)+ H_(2)). The accuracy of the apparatus was verified by comparing with the experimental data of carbon dioxide. The Joule-Thomson coefficients(μ_(JT)) for(CO_(2)+ H_(2)) binary mixtures with mole fractions of carbon dioxide(x_(CO_(2))= 0.1, 0.26, 0.5,0.86, 0.94) along six isotherms at various pressures were measured. Five equations of state EOSs(PR,SRK, PR, BWR and GERG-2008 equation) were used to calculate the μ_(JT)for both pure systems and binary systems, among which the GERG-2008 predicted best with a wide range of pressure and temperature.Moreover, the Joule-Thomson inversion curves(JTIC) were calculated with five equations of state. A comparison was made between experimental data and predicted data for the inversion curve of CO_(2). The investigated EOSs show a similar prediction of the low-temperature branch of the JTIC for both pure and binary systems, except for the BWRS equation of state. Among all the equations, SRK has the most similar result to GERG-2008 for predicting JTIC.

Study on the Characteristics of the Gas Switch Electrode Erosion

Gas spark switch is one of the key parts in pulsed power technology.Electrode erosion has great influence on the switch performance and lifetime.In this paper,a field distortion gas switch is selected for the experiment and a great deal of discharging experiments have been conducted in different test conditions.The forming process of etch pit as well as its influencing factors is discussed briefly and surface roughness coefficient(SRC) of the electrode is put forward to evaluate the state of electrode erosion.Experimental results show that current peak plays an important role in electrode erosion when waveforms of discharge current are the same,and electric charge and oscillation frequency of discharge current also have great effect on the electrode erosion when waveforms of discharge current are different.With the increase of discharge times,SRC decreases slowly at first and then decreases quickly after three thousand of discharge times.

Experimental Study of the Recovery Times of Spark Gap Switch with Different Gases

A two-pulse method is used to determine the insulation recovery time of the gas spark gap switch with different types of gas applied in a high power accelerator with a water dielectric pulse forming line. At the breakdown voltage of 450 kV, with the vacuum diode voltage of about 200 kV, and a current of 30 kA, recovery characteristics of H2, N2, SF6 were studied. The recovery percentages of the gas breakdown voltage and vacuum diode voltage were determined. The results show that hydrogen has the best recovery characteristics. At a pulse interval of 8.8 ms, the recovery percentages of both the gas breakdown voltage and vacuum diode voltage for hydrogen exceed 95%. For SF6 and N2 with an interval of 25 ms and 50 ms respectively, a 90% voltage recovery was obtained. The experiments also proved that the repetitive rate of the high power accelerator with a pulse forming line is mainly restricted by the gas switch repetitive rate; the recovery percentages of the vacuum diode voltage are limited by the recovery percentages of the gas switch breakdown voltage. The hydrogen switch can be employed for a high repetitive rate-high power accelerator with a pulse forming line.

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 Multi-Gap Multi-Channel Gas Switch for the Linear Transformer Driver

A multi-gap and multi-channel gas switch with convexo-convex discal planet electrodes was designed and investigated. Eight gaps are formed in series by a trigger electrode, six intermediate electrodes and two high voltage electrodes with a uniform gap length of 5 ram. The self breakdown and triggered breakdown performance of the switch are reported. Both the delay time and jitter decrease with the increase in the trigger voltage, switching coefficient and the decrease in the trigger isolating resistor. The delay time of the switch is about 40 ns, and the jitter is less than 2 ns when charged with 4-85 kV and triggered by a voltage pule of -75 kV. The inductance of the switch is about 30 nH.

Investigation on the electrode surface roughness effects on a repetitive self-breakdown gas switch

In this work,the influence of the electrode surface roughness on the self-breakdown gas switch is investigated by physical analysis,computer simulation and experiment.Cu-W electrodes of different surface roughness were tested under the conditions of a pulse repetitive frequency of50 Hz,self-breakdown voltage of~30 kV,and peak current of~2 k A for~93000 shots(the total charge transferred was~15 C).The coefficients of variation of the self-breakdown voltage of Cu-W 0.8,Cu-W 3.2 and Cu-W 12.5 electrodes were~2.95%,~1.62%and~1.16%,respectively.With the increase of electrode roughness,the erosion area decreased continuously,indicating that the breakdown positions were more stable and the coefficient of variation of breakdown voltage decreased.The method showed that decreasing the coefficient of variation of the self-breakdown voltage by increasing the surface roughness of electrode greatly improves the stability of the self-breakdown switch,which is significant for their application in compact highpower pulse power devices over a long time with stable operation.

A calculation model for breakdown time delay and jitter of gas switches under hundred-nanosecond pulses and its application in a self-triggered pre-ionized switch

In this paper, a calculation model for the breakdown time delay and jitter of gas switches under hundred-nanosecond pulses is proposed and applied in a self-triggered pre-ionized switch. The effects of injection time of pre-ionization, pulse rise time, and the pre-ionization jitter are discussed and verified through experiments. It indicates that the pre-ionization should be injected when the electric field is high enough in the gap, injection after 80% peak-time can ensure its effectiveness.Then the statistical time delay jitter will be determined by the pre-ionization jitter, which is an intrinsic restriction of the self-triggered switch. However, when the changing rate of the pulsed electric field exceeds a certain value, the breakdown time delay jitter can be partly offset in the formative stage because the formative time delay has an exponential relationship with the electric field. Therefore, lower time jitter can be obtained under pulses with a shorter pulse rise time. In general, the results of the calculation model agree with the experimental results, and the experimental parameters which lead to a low jitter can also be used as a reference.

Optical Diagnostics of Multi-Gap Gas Switches for Linear Transformer Drivers

The trigger characteristics of a multi-gap gas switch with double insulating layers,a square-groove electrode supporter and a UV pre-ionizing structure are investigated aided by a high sensitivity fiber-bundle array detector, a UV fiber detector, and a framing camera, in addition to standard electrical diagnostics. The fiber-bundle-array detector is used to track the turn-on sequence of each electrode gap at a timing precision of 0.6 ns. Each fiber bundle, including five fibers with different azimuth angles, aims at the whole emitting area of each electrode gap and is fed to a photomultiplier tube. The UV fiber detector with a spectrum response of 260-320 nm,including a fused-quartz fiber of 200 μm in diameter and a solar-blinded photomultiplier tube, is adopted to study the effect of UV pre-ionizing on trigger characteristics. The framing camera,with a capacity of 4 frames per shot and an exposure time of 5 ns, is employed to capture the evolution of channel arcs. Based on the turn-on light signal of each electrode gap, the breakdown delay is divided into statistical delay and formative delay. A decrease in both of them, a smaller switch jitter and more channel arcs are observed with lower gas pressure. An increase in trigger voltage can reduce the statistical delay and its jitter, while higher trigger voltage has a relatively small influence on the formative delay and the number of channel arcs. With the UV pre-ionizing structure at 0.24 MPa gas pressure and 60 kV trigger voltage, the statistical delay and its jitter can be reduced by 1.8 ns and 0.67 ns, while the formative delay and its jitter can only be reduced by 0.5 ns and 0.25 ns.

Experimental Study of the Multi-gap Multi-channel Gas Spark Closing Switch

A coaxial multi-gap multi-channel spark switch with stainless-steel-spring ring gap electrodes is designed and investigated. The switch is triggered by a pulse applied to the cylindrical electrode outside the discharging channel through a parasitic capacitance coupling. The jitter of the switch is reduced by several short-distance gas gaps in series, and its inductance is reduced by a multi-channel discharge on account of the inductance isolation between the coils of the spring ring electrode. The experimental results indicate that the switch is of low inductance (15-30 nH), low jitter (-3 ns), and stable breakdown performance

Experimental investigation on the development characteristics of initial electrons in a gas pressurized closing switch under DC voltage

As one of the most important elements in linear transformer driver(LTD) based systems, the gas pressurized closing switches are required to operate with a very low prefire probability during the DC-charging process to ensure reliable operation and stable output of the whole pulsed power system. The most direct and effective way to control the prefire probability is to select a suitable working coefficient. The study of the development characteristics of the initially generated electrons is useful for optimizing the working coefficient and improving the prefire characteristic of the switches. In this paper an ultraviolet pulsed laser is used to generate initial electrons inside the gap volume. A current measuring system is used to measure the time-dependent current generated by the growth of the initial electrons so as to study the development characteristics of the electrons under different working coefficients. Experimental results show that the development characteristics of the initial electrons are influenced obviously by the working coefficient. With the increase of the working coefficient, the development degree of the electrons increases consequently. At the same times, there is a threshold of working coefficient which produces the effect of ionization on electrons. The range of the threshold has a slow growth but remains close to 65% with the gas pressure increase. When the working coefficient increases further, γ processes are starting to be generated inside the gap volume. In addition, an optimal working coefficient beneficial for improving the prefire characteristic is indicated and further tested.

Estimation of Surface Roughness due to Electrode Erosion in Field-Distortion Gas Switch

Field distortion gas switch is one of the crucial elements in a Marx generator, fast linear transformer driver and other pulsed power installations. The performance of the gas switch, which is dramatically affected by the surface roughness due to electrode erosion during the discharge process, directly influences the output parameters, stability and reliability of the pulsed power system. In this paper, an electrode surface roughness （ESR） calculation model has been established based on a great deal of experimental data under operating current. The discharge current waveform, the peak height of the burr, the radius and the depth of etch pits in the electrode erosion region were used to predict the ESR. Also, experimental results indicate that this calculation model can effectively estimate the ESR of the test gas switch.

A low-jitter self-triggered spark-discharge pre-ionization switch: primary research on its breakdown characteristics and working mechanisms

MV pulsed switch plays a key role as the transfer switch in large electromagnetic pulse simulators. To broaden the range of self-triggering time, a novel spark-discharge pre-ionization switch, in which the main gap electric field is superposed at the trigger gap to let the electrons in its spark channel also become initial electrons, is proposed and tested. The design idea is: as electrons in the spark channel of the trigger gap always exist after its breakdown, the injection time of pre-ionization should have a more negligible effect on reducing the switch jitter. The experiment results under pulses with a rise time of ~100 ns support the above assumptions.When the operating voltage is from ~300 to ~800 kV and the self-triggering time is ~45% to~75% of the peak time, the breakdown time delay jitter is less than 2 ns, and the breakdown voltage jitter is smaller than 1.25%. Under specific self-triggering time, the breakdown time delay jitter is less than 1.5 ns, and the breakdown voltage jitter is smaller than 0.8%.

Comparative study of pulsed breakdown processes and mechanisms in self-triggered four-electrode pre-ionized switches

This work investigates the pulsed breakdown processes and mechanisms of self-triggered preionized switches with a four-electrode structure in nitrogen through intensified charge coupled device photographs.The diameter of the trigger plane hole mainly determines the switch’s electric field distribution.Two configurations with minimum and maximum trigger plane holes are adopted for comparison.In the switch with a minimum trigger plane hole,the maximum electric field distributes at the surfaces of the main electrodes.Although charged particles in the triggering spark channel cannot drift out,homogeneous discharges can be stimulated from both the cathode and anode surfaces through ultraviolet illumination.Two sub-gaps are likely to break down simultaneously.In the switch with a maximum trigger plane hole,the maximum electric field locates near the trigger electrodes.Discharges in both sub-gaps initiate from the trigger electrodes in the form of a positive or negative streamer.Due to the lower breakdown voltage and electric field threshold for discharge initiation,the cathode side sub-gap breaks down first.The analysis of two extreme examples can be referenced in the future design and improvement of self-triggered four-electrode switches with different trigger electrode structures.

Characteristics of SF_6 Switch with a Small Gap under High Pressure and Nanosecond Pulse

Structural design and tests on the characteristics of the SF6 gas switch with a small gap are presented. This kind of switch often works under high pressure and nanosecond pulse for getting pulse with faster risetime. The breakdown voltage and breakdown delay of a number of switches with different geometries, gas pressures and pulse waveforms were investigated. Experimental results suggested that the breakdown voltage increases linearly with the gas pressure, and the breakdown delay decreases with an increase in the gas pressure and a reduction in the gap distance of the switch under the same applied pulse. By using this kind of switch with a gap of 3 mm as a peaking switch, a pulse generator can provide an output voltage with a peak voltage of 300 kV and a risetime of 3 ns on a resistance load of 150Ω.

Fifth-Order A-WENO Schemes Based on the Adaptive Diffusion Central-Upwind Rankine-Hugoniot Fluxes

We construct new fifth-order alternative WENO(A-WENO)schemes for the Euler equations of gas dynamics.The new scheme is based on a new adaptive diffusion centralupwind Rankine-Hugoniot(CURH)numerical flux.The CURH numerical fluxes have been recently proposed in[Garg et al.J Comput Phys 428,2021]in the context of secondorder semi-discrete finite-volume methods.The proposed adaptive diffusion CURH flux contains a smaller amount of numerical dissipation compared with the adaptive diffusion central numerical flux,which was also developed with the help of the discrete RankineHugoniot conditions and used in the fifth-order A-WENO scheme recently introduced in[Wang et al.SIAM J Sci Comput 42,2020].As in that work,we here use the fifth-order characteristic-wise WENO-Z interpolations to evaluate the fifth-order point values required by the numerical fluxes.The resulting one-and two-dimensional schemes are tested on a number of numerical examples,which clearly demonstrate that the new schemes outperform the existing fifth-order A-WENO schemes without compromising the robustness.

Arcing Model of a Disconnector and its Effect on VFTO

In the computational process of very fast transient over-voltage （VFTO）, it is essen-tial to find an accurate model for a gas insulated substation. The arcing model of the disconnector is particularly important. The general arcing model is not able to give a good description of the arc development process. In this paper, based on the physical process of arcing and existing arc models （the exponential time-varying resistance model and the segmental arcing models）, a dy- namic arcing model is proposed, which is divided into two stages before and after the zero crossing. The dynamic arcing model combines hyperbola time-varying resistance and the Mayr model to describe the dynamic process of arcing. The present paper creates an arc model blockset upon the Matlab/Simulink software platform. Moreover for a specific 1100 kV station, VFTO is simulated in detail based on different arcing models. It is demonstrated that the dynamic arcing model can describe the physical arc process precisely and is useful for improving the accuracy of VFTO simulations.

Experimental investigation of a novel micro gas turbine with flexible switching function for distributed power system

Micro gas turbine(MGT)is widely used in small-scale distributed power systems because of its low emissions and fuel flexibility.However,the under-utilization of its exhaust heat and the low electric efficiency are the main bottlenecks that restrict its application.Additionally,the flexible switching between the power generated by the MGT and the power grid is also a key factor for keeping the secure operation of a distributed power station.Therefore,this paper conducted some experimental investigations of a 30 kW MGT to provide reference solutions for the above issues.This MGT is located at Shanghai Jiao Tong University(SJTU),which is designed by the Gas Turbine Research Institute of SJTU,and is manufactured by a turbo machinery factory in Chongqing,China.The demonstration prototype is mainly composed of a single stage centrifugal compressor,a radial turbine,a combustor,a high-speed pennanent magnet generator,and a control system.The results show that the MGT can achieve steady operation at a low rotational speed from 10000 r/min to 34000 r/min in the case of using oil lubricated bearings,which can greatly reduce the economic cost compared with the use of air bearings.At the same time,the ignition success rate of combustion chamber(CC)reaches 98%at a low rotational speed,and a wide range of stable combustion area can be obtained,because of the novel design method of combustor by referencing the way applied in an axial flow aero-engine.The MGT generating set can achieve functions,such as starting up,ignition,stable operation,loaded operation,grid-connection and stopping.This system also can realize flexibly switching from the start motor mode to the generator mode,and from grid-connected mode to off^grid mode,because the innovative multi-state switching control system is adopted.The above research work can make our state master independent intellectual property rights of micro gas turbine,rather than continue to be subject to the technological monopoly of the developed states,which can provide theoretical and experimental support for the industrialization of MGT in China.

Refined analysis and prediction of natural gas consumption in China

In view of the abrupt and phased features of natural gas consumption,this paper attempts to predict natural gas consumption in China with a refined forecasting approach.First,we establish a Markov switching(MS)model to identify the phase characteristics after eliminating change points in the natural gas consumption sequence,using the product partition model(PPM).The results show that there are"rapid growth"and"slow growth"regimes in the development process of natural gas consumption in China.Second,the Bayesian model average(BMA)method is employed to determine the core determinants of natural gas consumption under sub-regimes,and it is determined that there are significant differences in the influencing factors under different regimes and periods.Third,this paper establishes the BMA model of the"rapid growth"regime after predicting the state of future natural gas consumption in China.We find that,compared to some other models,the BMA model that fully recognizes the regime without considering change points has the best predictive performance.Finally,the results of static and dynamic scenario analyses show that natural gas consumption continues to rise in 2019 and has obvious seasonal charac-teristics,while possible ultra-rapid growth of consumption in the future provides a new requirement for the supply of natural gas.