Gas Gap Research Articles

How pulse energy affects ignition efficiency of DBD plasma-assisted combustion

This work aims to find how coupled energy per pulse influences the ability of a pulsed dielectric barrier discharge (DBD) plasma to ignite fuel-lean methane–air flow. For that, experiments are performed on a custom-built DBD flow reactor with a variable dielectric thickness and the discharge is operated by bursts of 10 ns duration pulses at 3 kHz repetition rate. With an increase in dielectric thickness, we observe that the coupled energy per pulse decreases even though applied voltage conditions are similar and so more pulses are required to ignite the lean mixture. Interestingly, we observe a significant increase in the minimum ignition energy (MIE) with an increase in the thickness beyond 3 mm. Moreover, the ignition kernel growth rate is much slower in the thicker dielectric cases even though total energy coupling per burst is similar. This phenomenon is investigated further by evaluating plasma parameters using electrical and optical diagnostics. Effective dielectric capacitance, discharge current, and voltage drop across the gas gap are derived from an equivalent circuit analysis, whereas plasma gas temperature and effective reduced electric field ( E/N ) are estimated from optical emission spectroscopy. From these analyses, we conclude that a thicker dielectric limits the discharge current and so the plasma filament temperature. For more than 3 mm thick dielectric cases, the filament heating per pulse is too low to achieve strong enough plasma pulse-to-pulse coupling which eventually leads to higher MIE and slower ignition kernel growth rate or the inability to ignite at all.

A standard capsule design for structural material testing in the Advanced Test Reactor

Nuclear materials testing is commonly completed in various material test and research reactors throughout the world, including the Advanced Test Reactor (ATR), but the current capsule design, analysis, and fabrication process can take years to complete. To decrease the costs and time associated with materials testing, a standard capsule has been designed which houses various specimen geometries and allows irradiation in virtually any ATR test position. The standard capsule features locking end caps which connect and lock together to form the capsule stack, eliminating the need for a basket and maximizing the quantity of specimens which are contained within the capsule. A customizable internal gas gap provides thermal resistance between the specimens and reactor coolant, making specimen temperatures from approximately 370 to 1000 K achievable. The flexibility of the capsule design allows experimenters to choose irradiation positions based off desired neutron flux, with typical fluences per cycle ranging from 8.8x1019 to 2.3x1021 n/cm2, depending on experiment position. This paper presents and discusses the standard capsule design and analysis.

Research on dynamic characteristics and structural optimization of porous gas bearings in linear compressors

In order to study the influence of structural parameters of porous gas bearing and operating parameters of linear compressor on the static and dynamic performance of porous gas bearing, based on gas lubrication theory, Darcy's law and Reynolds equation, the mathematical model and simulation model of porous gas bearing of linear compressor are derived and established. The static and dynamic characteristics of the porous gas bearing of the linear compressor are studied by using Fluent software simulation. According to the simulation results, the effects of inlet pressure, porous material thickness and gas gap on the gas consumption and bearing capacity of the porous gas bearing under different eccentricities are analyzed. The results show that the higher the inlet pressure is, the larger the gas consumption and bearing capacity; the thicker the porous material is, the smaller the gas consumption and the larger the bearing capacity, the thicker the gas gap is, the larger the gas consumption and the smaller the bearing capacity. On the basis of simulation research, considering the difficulties of processing and assembly, multi-objective optimization of porous gas bearings is carried out based on response surface methodology. Taking the bearing capacity and gas consumption as the objective functions, the intake pressure is set between 0.3 and 0.5 MPa, the thickness of porous materials is set between 3 and 5 mm, and the thickness of gas gaps is set between 10 and 20 μm. While ensuring the stable operation of the linear compressor, the optimal combination of design parameters is provided for the optimal design of gas bearings used in linear compressors.

Discharge Randomness and Attraction Process of Epoxy Surface on Discharge Channel in SF6 Gas Under Uniform Electric Field

It is generally believed that due to the electric field distortion effect and the photoemission process of the solid surface, the discharge channel near the solid surface trends to develop along the surface than that in bulk gas. One way to improve insulation performance is to avoid the solid surface from participating in the gas discharge process. However, the mechanism of the solid surface’s effect on the gas discharge process is still unclear. In this paper, we recorded high-speed framing pictures of the discharge process in SF6 gas near the epoxy surface under lightning impulse voltage in the uniform electric field. The arc pictures show that the attraction effect of the solid surface to the discharge channel is related to the surface shape, gas gap height, and distance from the discharge channel to the solid surface. Besides, discharge randomness also plays a role, which is more obvious in the cylinder insulator than in the conical insulator, for the surface with conical shape could easily attract and arrest the discharge channel to its surface. According to the pictures of discharge process near the cylinder surface, we found that the photoemission process occurs in the streamer stage but has little effect on the attraction effect. The leader initiates after the streamer bridges the gas gap, and its random stepped walk could induce the attraction effect from the solid surface: only when the random stepped leader deflects close to the solid surface, the discharge channel could be further attracted to the solid surface by the electrostatic force, especially its normal component. The result indicates that there are three points to prevent the solid surface from participating in the gas discharge process and further improve insulation performance: choose a cylinder shape (avoid a conical shape), keep the surface away from the high electric field area where discharge may start, and reduce the normal component of the surface electric field.

Chemical footprints of giant planet formation

Context. Protoplanetary disks, the birthplaces of planets, commonly feature bright rings and dark gaps in both continuum and line emission maps. Accreting planets interact with the disk, not only through gravity, but also by changing the local irradiation and elemental abundances, which are essential ingredients for disk chemistry. Aims. We propose that giant planet accretion can leave chemical footprints in the gas local to the planet, which potentially leads to the spatial coincidence of molecular emissions with the planet in the ALMA observations. Methods. Through 2D multi-fluid hydrodynamical simulations in Athena++ with built-in sublimation, we simulated the process of an accreting planet locally heating up its vicinity, opening a gas gap in the disk, and creating the conditions for C-photochemistry. Results. An accreting planet located outside the methane snowline can render the surrounding gas hot enough to sublimate the C-rich organics off pebbles before they are accreted by the planet. This locally elevates the disk gas-phase C/O ratio, providing a potential explanation for the C2H line-emission rings observed with ALMA. In particular, our findings provide an explanation for the MWC 480 disk, where previous work identified a statistically significant spatial coincidence of line-emission rings inside a continuum gap. Conclusions. Our findings present a novel view of linking the gas accretion of giant planets and their natal disks through the chemistry signals. This model demonstrates that giant planets can actively shape their forming chemical environment, moving beyond the traditional understanding of the direct mapping of primordial disk chemistry onto planets.

On the Frequency Dependence of the PDIV in Twisted Pair Magnet Wire Analogy in Dry Air

This paper presents an experimental verification of a PDIV model for twisted-pair magnet wires (TPMW). The model is physically rooted in an inception criterion based on secondary electron feedback and parametrizes the PDIV of wedge-shaped gas gaps (such as TPMW) solely in terms of its reduced coating thickness—the ratio of coating thickness to its relative permittivity. According to the literature, the coating’s permittivity and thickness should play a significant role in determining the PDIV. Still, no explicit quantification of their concurrent effects is often given, and they are frequently treated as decoupled parameters. Therefore, extensive measurements of PDIV of samples analogous to TPMW coated with dielectric materials of different permittivity (sealed and unsealed alumina, silicone polyester, and perfluoroalkoxy alkane) and thickness (≈45 to 80 μm) are performed. The PDIV of the test samples is determined under sinusoidal test voltage in the frequency between 10 to 60 000 Hz. It is shown that the PDIV can be predicted based on the relative permittivity and thickness of the coating, and the frequency dependence is due only to the change of permittivity with frequency. Temperature and pressure are kept approximately constant at their ambient values (ca. 23 °C & 970 mbar), and only dry air is used to exclude the effect of humidity on the insulation’s permittivity. Ultimately, the role of surface charge is discussed qualitatively by comparing the PDIV with the partial discharge extinction voltage (PDEV) and their repetitive counterparts (RPDIV, RPDEV).

FEATURES OF ELECTRON RUNAWAY IN A GAS GAP WITH A CONICAL CATHODE

The conditions and dynamics of electron runaway in a gas gap with an inhomogeneous distribution of the electric field due to the use of a conical cathode are analyzed. Significant differences in the behavior of electrons at cone opening angles larger and smaller than the threshold value of 98.6 degrees are revealed. For a weakly inhomogeneous field distribution (cones with angles exceeding the threshold), electrons are continuously accelerated throughout the gap. In a strongly inhomogeneous field (cones with relatively small opening angles), electrons can gain maximum energy inside the gap, and begin to decelerate in the near-anode region. Despite this, they reach the anode with high energies comparable to the work of the electric field. This difference leads to the need to use different criteria for the runaway of free electrons in the gas gap.

Specific Features of Electron Runaway in a Gas Gap with a Conical Cathode

Specific Features of Electron Runaway in a Gas Gap with a Conical Cathode

Development and testing of secondary ion accelerator for ITER neutral particle diagnostic system.

This paper presents the results of the development and testing of a secondary ion accelerator for the diagnostic complex of neutral particle analyzers at ITER. The accelerator is part of an analyzer designed to measure fluxes of neutral deuterium and tritium particles escaping from reactor plasma in a total range of 10-200keV. The aim of the accelerator is to improve the signal/background ratio of the analyzer. It is especially important for the region with a relatively low energy of 10-50keV. The accelerator has the function of converting atoms into secondary ions and accelerating them in an electrostatic field with a voltage of up to +100kV. The accelerator is based on two high-voltage accelerating tubes. A special unit is built into the central electrode of the accelerator, which provides the replacement of stripping foils used for the conversion of the flux of neutral particles into a flux of secondary ions. The high-voltage tube assembly is encompassed by a sealed steel housing, in which electrical insulation is provided by a gas gap (nitrogen at 6bar). The results of tests of the accelerator under ITER relevant conditions, namely, under high-intensity gamma irradiation and seismic loads, have shown its high reliability for use in the diagnostic complex of the fusion reactor.

Analysis of different modeling approaches for simulation of glow discharge in helium at atmospheric pressure

One-dimensional numerical models of a direct current atmospheric pressure glow discharge in helium are developed and examined. The models use a fluid description of charged and neutral particles and a drift-diffusion approximation for particle fluxes. The effects of plasma-chemical models, the form of the electron energy distribution function (Maxwellian vs non-Maxwellian), the energy loss due to gas heating and the width of the gas gap on the discharge characteristics are analyzed. The performance of different modeling approaches is examined by superimposing computed current–voltage characteristic (CVC) curves with each other and with measured and computed CVCs available in the literature.

Constraining the gas distribution in the PDS 70 disc as a method to assess the effect of planet-disc interactions

Context. Embedded planets are potentially the cause of substructures, such as gaps and cavities, observed in the continuum images of several protoplanetary discs. Likewise, gas distribution is expected to change in the presence of one or several planets, and the effect can be detected with current observational facilities. Thus, the properties of the substructures observed in the continuum as well as in line emission encode information about the presence of planets in a system and how they interact with the natal disc. The pre-transitional disc around the star PDS 70 is the first case of two young planets being imaged within a dust-depleted gap that was likely carved by the planets themselves. Aims. We aim to determine the spatial distribution of the gas and dust components in the PDS 70 disc. The axisymmetric substructures observed in the resulting profiles are interpreted in the context of planet-disc interactions. Methods. We developed a thermo-chemical forward model for an axisymmetric disc to explain a subset of the Atacama Large Millimeter/Submillimeter Array (ALMA) band 6 observations of three CO isotopologues plus the continuum towards PDS 70. The model accounts for the continuum radiative transfer, steady-state chemistry, and gas thermal balance in a self-consistent way and produces synthetic observables via ray tracing. Results. We demonstrate that the combination of a homogeneous dust size distribution across the disc and relatively low values of viscosity (α ≲ 5 × 10−3) can explain the band 6 continuum observations. For the gas phase, analysis of the synthetic observables points to a gas density peak value of ~0.1 g cm−2 located at 75 au and a minimum of ~10−3 g cm−2 at 20 au. The location of the minimum matches the semi-major axis of the innermost planet PDS 70 b. Combining the gas and dust distributions, the model results in a variable gas-to-dust ratio profile throughout the disc that spans two orders of magnitude within the first 130 au and shows a step gradient towards the outer disc, which is consistent with the presence of a pressure maxima driven by planet-disc interactions. Particularly, the mean gas-to-dust ratio within the dust gap between 16 and 41 au is found to be ~630. We find a gas density drop factor of ~19 at the location of the planet PDS 70 c with respect to the peak gas density at 75 au. Combining this value with results from the literature on the hydrodynamics of planet-disc interactions, we find this gas gap depth to be consistent with independent planet mass estimates from infrared observations. Our findings point towards gas stirring processes taking place in the common gap due to the gravitational perturbation of the two planets. Conclusions. The distribution of gas and dust in the PDS 70 disc can be constrained by forward modelling the spatially resolved observations from high-resolution and high-sensitivity instruments like ALMA. This information is a key piece in the qualitative and quantitative interpretation of the observable signatures of planet-disc interactions.

Effect of airflow on the discharge uniformity at different cycles in the repetitive unipolar nanosecond‐pulsed dielectric barrier discharge

AbstractThis study explores the discharge uniformity at different cycles under different airflow velocities in nanosecond‐pulsed dielectric barrier discharge. Based on a proper choice of the airflow velocity and discharge parameters, diffuse discharges can be maintained at an air gap of 5–7 mm. At higher airflow velocities, the discharge may transition to a complex mode, in which the diffuse and filamentary regions are maintained simultaneously. When the discharge is operated at a higher frequency, higher voltage, or smaller gas gap, the area of diffuse discharge can be expanded to a certain extent. The discharge characteristics indicate that airflow lowers the gas temperature and modulates the preionization degree, which induces the generation of a diffuse discharge for a properly adjusted airflow velocity.

Dimensional analysis on microscale gas breakdown with electric field nonuniformity and positive space charge effects

A dimensional method was employed to evaluate the microscale gas breakdown characteristics at atmospheric pressure, resulting in a universal breakdown curve applicable to different types of gases (e.g., Ar, Xe, Ne, and N2). As the gap distance decreases, the breakdown mode transitions from ion-induced secondary electron emission to the field emission regime. In the field emission regime, the positive space charge effect becomes more significant. We discovered that incorporating the positive space charge effect in the field emission regime can be achieved by modifying the local electric field enhancement factor β. Consequently, we propose an effective electric field enhancement factor, βeff, which scales linearly with β, to accurately reproduce the breakdown curve while considering the positive space charge effect. This proposed approach significantly simplifies the numerical model. Additionally, we examined the effects of gas pressure, gap distance, cathode properties (e.g., work function and secondary electron emission coefficient), and electric field nonuniformity.

Development of an 1-m long prototype TOF-RPC for the J-PARC [formula omitted]20 beam line

We have developed a prototype multi-gap Resistive Plate Chamber (RPC) for a time-of-flight (TOF) detector at the J-PARC π20 beam line. It is a double stack RPC with five 0.26-mm gas gaps in each stack. Its readout strips are 25 mm wide and 940 mm long. We have also developed an amplifier board which can be used for TOF-RPCs in general and carried out a beam test to evaluate the performance of the RPC and this new board. Timing measurements and a time-walk correction using a time-over-threshold (TOT) method were found to work well without stretching signals. We obtained time resolutions of 60-70 ps when the test particle’s position was determined using 5 mm × 10 mm trigger detectors and 65-80 ps without this position determination.

Static breakdown threshold modeling of quasi-uniform gas gaps with a focus on the PDIV of contacting enameled wire pairs

This paper provides a theoretical and conceptual framework for the determination of static breakdown inception thresholds in quasi-uniform gas gaps bounded by dielectric layers of thickness s and relative permittivity . The special case of uncoated metallic electrodes is included in the limit . Moreover, a review of breakdown mechanisms and the underlying physical processes in quasi-uniform gas gaps is provided, and the applicability of the associated breakdown criteria is discussed. The results include a parametrization of the partial discharge inception voltage of wedge-shaped air gaps at atmospheric pressure as a function of the reduced coating thickness . The predicted results for this prototypical insulation geometry agree well with a broad array of literature data and own measurements. Finally, a physically motivated parametrization of the ionization threshold K(d) is suggested for atmospheric pressure air in terms of secondary electron feedback by energetic photons and ion-enhanced field emission. This contrasts with the ad hoc introduction of modified ‘streamer constants’ often found in literature to explain breakdown voltages that deviate from the predictions of the classical Raether–Meek–Loeb criterion.

Experimental and numerical investigation on the uniformity of nanosecond pulsed dielectric barrier discharge influenced by pulse parameters

Nanosecond (ns) pulsed dielectric barrier discharge (DBD) is considered as a promising method to produce controllable large-volume and high activity low-temperature plasma at atmospheric pressure, which makes it suitable for wide applications. In this work, the ns pulse power supply is used to excite Ar DBD and the influences of the pulse parameters (voltage amplitude, pulse width, pulse rise and fall times) on the DBD uniformity are investigated. The gas gap voltage (U g) and conduct current (I g) are separated from the measured voltage and current waveforms to analyze the influence of electrical parameters. The spectral line intensity ratio of two Ar excited species is used as an indicator of the electron temperature (T e). The time resolved discharge processes are recorded by an intensified charge-coupled device camera and a one-dimensional fluid model is employed to simulate the spatial and temporal distributions of electrons, ions, metastable argon atoms and T e. Combining the experimental and numerical results, the mechanism of the pulse parameters influencing on the discharge uniformity is discussed. It is shown that the space electric field intensity and the space particles’ densities are mainly responsible for the variation of discharge uniformity. With the increase of voltage and pulse width, the electric field intensity and the density of space particles increased, which results in the discharge mode transition from non-uniform to uniform, and then non-uniform. Furthermore, the extension of pulse rise and fall times leads to the discharge transition from uniform to non-uniform. The results are helpful to reveal the mechanism of ns pulsed DBD mode transition and to realize controllable and uniform plasma sources at atmospheric pressure.

Extension of the X2 VVER-1000 benchmark by a control rod cluster ejection exercise

This paper proposes a new benchmark for VVER-1000 control rod ejection transient. The benchmark is designed for code-to-code comparison and its purpose is testing and verifications of time-dependent solvers, core models and cross section generation methodology. Four solutions obtained using different combinations of three lattice transport, two nodal neutronics, and two thermo-hydraulic codes are analyzed, demonstrating good agreement in predicting transient reactivity and power peaks and highlighting the impact of gas gap modeling and DNBR correlation on simulation results.

Heat-transfer modeling of the gas gap under a wafer

The wafer temperature is a critical observable in semiconductor manufacturing. One of the various mechanisms determining this temperature is the heat transfer in a gas gap between the wafer and the electrostatic chuck (ESC). Various correlations for this heat transfer are available. However, to calculate more accurately the spatial distribution of this temperature, computing the flow in this gap is necessary. With this motivation in mind, this paper presents a computational fluid dynamics model (CFD) for the flow in the wafer-ESC gap that is designed to be easy to implement in industrial CFD codes. This model is tested in various channel-flow problems and then applied to a generic wafer-ESC configuration. For this configuration, CFD results show that varying the flow rate split between three zones, or the total flow rate, or the rugosity of the wafer affect the heat transfer coefficient and its spatial variation. This is important since controlling this variation would allow to maintain a uniform wafer temperature. The model could be used in more realistic wafer-ESC configurations to consider many other parameter variations, such as the size of injection holes, a radially varying gap distance, or the use of many injection zones. From a broader viewpoint, the model is applicable to vacuum problems other than the wafer-ESC configuration.

Dispersion characteristics of switching transients from large‐scale experiments in a 1100 kV gas insulated switchgear circuit

AbstractThe switching operation of gas insulated switchgear (GIS) disconnector will produce multiple random spark discharges between the gas gaps of the disconnector. Each spark discharge can be affected by various random factors, such as trapped charge on the load side, the initial operating phase, gap distance and operating speed of the disconnector, leading to dispersion characteristics. At present, some studies have mentioned this dispersion, but its impact is still unclear due to the lack of experimental data, resulting in a deviation between the simulation results and the measured results. In this study, thousands of switching operations were carried out based on the 1100 kV full‐scale GIS circuit in Wuhan ultra‐high voltage alternating current Test Base. The maximal information coefficient analysis, linear regression fitting and other statistical methods were used to discuss the dispersion characteristics of very fast transient overvoltage (VFTO) and transient enclosure voltage (TEV) from large‐scale experimental data. The results show that the dispersion of VFTO and TEV in amplitude aspects both increase gradually as the breakdown voltage increases, with the maximum difference of five times of TEV amplitude at normalised breakdown voltage. In terms of frequency characteristics, the dispersion of different frequency components in TEV is always greater than that of VFTO, especially for the high‐frequency components. Moreover, by comparing the frequency characteristics of TEV at different positions, we notice that the dispersion is almost independent of the spatial position but only determined by the randomness of spark discharges. These discoveries reveal the importance of the dispersion in switching transients and make up for the lack of theoretical understanding of the correlation among them.

Study of RPC ionic signal properties with different read-out methods

The simultaneous measurement of the ionic charge (induced by the slow motion of positive ions inside the gas gap) and of the fast/prompt charge (induced on the read-out electrodes by the drift of electrons) is fundamental for the understanding of the working principles of the RPC detector. Typical RPC electrodes are made of resistive material with the voltage applied on a thin graphite layer, which however alters the reading of the ionic charge depending on the discharge position. We have therefore realized a small RPC detector with a metallic grid deployed in one of the resistive electrodes. The prompt charge is measured by means of typical read-out copper strips with 1.4 cm pitch. The purpose of the study described in this presentation is to test different ion signal read-out systems and the comparison with the prompt signal read out with traditional techniques, to improve the detector knowledge and for possible future applications.