Spot Velocity Research Articles

Dynamic characteristics of cathode spots during expansion process in a vacuum arc

Cathode spot is the source of plasma and metal vapor which affects the breaking capacity of vacuum circuit breakers. In this paper, the cup-shaped axial magnetic field contacts were used and the images of cathode spots were obtained by an ultra high-speed camera in the experiment. The dynamic characteristics of cathode spots during expansion process in vacuum arc were investigated. Under the conditions of different current amplitudes, different distributions of cathode spots were observed. During the expansion process of cathode spot, there are group spots and individual spots. The group spot only occurs in the cold electrode. The electrons emission characteristics of cathode spot as a function of temperature and electric field was calculated. The formation of group spot is related to the current and electron emission of cathode spot. The cathode spots were identified by Matlab. It was found that the variation of number of cathode spots is consistent with the current. However, at different stages, the reasons for the increase in the number of cathode spots are different, such as the increase in the area of cathode spots in the initial arcing stage and the splitting of cathode spot. It was observed that the cathode spots are not completely evenly distributed on the cathode surface. The farther away from the center of the contact, the longer the distance between the cathode spots. It is mainly effected by the transverse magnetic field (TMF) which is generated by the arc column. The closer to the contact edge, the higher the TMF at the cathode spot and the higher the expansion velocity of cathode spot.

Arc Spot on Film Cathode Using Kinetic Approach of Plasma Flow

Spot behavior on Cu film cathodes in vacuum arcs was studied theoretically. A kinetic model was developed and a system of equations was derived to understand Kesaev’s measurements of spot parameters on Cu films deposited on glass substrates. The solution showed a cathode potential drop of 12 V, lower than the value for a bulk Cu cathode, 16 V, in accordance with the measured data for film cathode, 11 V. This low value can be understood by the calculated power losses, which were lower than that for bulk cathodes, due to film heat conduction, which was comparable with the power losses by vaporization. Both increased from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 0.1$ </tex-math></inline-formula> up to about 1 W when the thickness was increased from 0.017 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.14~\mu \text{m}$ </tex-math></inline-formula> . The plasma velocity in the Knudsen layer normalized by the plasma sound speed is < 1, indicating impeded plasma flow. The spot velocity was determined using directly calculated erosion rate in the film track left by the spot motion. This velocity decreased from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 10^{4}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times 10^{3}$ </tex-math></inline-formula> cm/s, agreeing well with the measurements for film thickness larger than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.06~\mu \text{m}$ </tex-math></inline-formula> . The track width increased with film thickness and the calculated dependence was comparable with that measured.

Experimental investigation on the velocity of cathode spots in a vacuum arc with high di/dt

With the development of DC transmission and aerospace technology, the demands on the breaking capacity of vacuum circuit breakers (VCBs) under high condition are getting higher and higher. The breaking capacity of VCBs is determined by cathode spots, which provide electrons and metal vapor to maintain the arc. In this paper, the experiments of cathode spot observation were carried out based on the detachable vacuum interrupter and the cup-shaped axial magnetic field contacts. The images of cathode spot distribution were obtained by an ultra high-speed camera. In the initial expansion stage of arcing, it was discovered that the distribution of cathode spots is ring-shaped and symmetrical. The initial expansion velocity of cathode spot ring is approximately proportional to the value of and is independent of frequency. During the whole arcing process, the expansion velocity of cathode spot ring rises first and then drops with arcing time. The effects of the magnetic field and metal vapor density on the expansion process of cathode spot were analyzed. Based on the experimental conditions and results, the values of magnetic field and metal vapor density were calculated which can reasonably explain the variation of expansion velocity of cathode spot.

Tiny velocity measurement using rotating petal-like mode of orbital angular momentum.

A novel, to the best of our knowledge, tiny velocity measurement system is proposed and demonstrated. This proposed system employs an interference structure in which the reference and measurement paths are filled by two light beams carrying opposite-sign orbital angular momentum (OAM), respectively. The tiny velocity to be measured in the measurement path causes the change of the light path and results in a time-varying phase shift between the reference and measurement paths. This time-varying phase shift leads to the rotation of the petal-like light spot obtained by the interference between two paths. The rotating angular velocity of the petal-like light spot is proportional to the time-varying phase shift caused by the tiny velocity, and it is measured by a chopper and a single-point detector instead of array detectors. This proposed system has a simple structure and achieves a high-accuracy tiny velocity measurement with a measurement error rate that is less than 10 nm/s.

Cathode Spot Velocity of Atmospheric Arc Plasma after Arc Ignition Affected by Self and External Electromagnetic Force between Parallel Electrodes Using 3D Numerical Simulation

For visually and quantitatively elucidating the contribution of self and external electromagnetic forces of the cathode spot area to the cathode spot movement, a three‐dimensional electromagnetic thermal fluid simulation program was used to simulate the arc movement between parallel electrodes. According to the analysis results, the cathode spot first step occurred when the electromagnetic force balance of cathode spot area was broken, and the direction of total electromagnetic force at the right side control volume pointed forward. When the current path changes from straight to reentry model, the cathode spot movement became faster because of the self electromagnetic force derived from electrodes. The rapid of cathode spot first step significantly increased along with the arc current increasing, and the increment of self electromagnetic force was more significant than the external electromagnetic force. In conclusion, the application of external magnetic field or the self magnetic field derived from the parallel electrodes benefit to break the electromagnetic force balance of cathode spot area and promote the rapid of cathode spot first step. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Effects of surface wettability on pool boiling process: Dynamic and thermal behaviors of dry spots and relevant critical heat flux triggering mechanism

In this paper, we investigate experimentally and numerically the influences of surface wettability on the boiling process from nucleate boiling to critical heat flux (CHF). The boiling surfaces with four different wettabilities (apparent contact angles) are examined. Using a simple optic setup, sequential phase distribution images on the boiling surfaces were acquired, and we analyzed the dynamics of microlayer, bulk liquid region, and dry spots with the processed images. The variation of surface wettability significantly affected the liquid and vapor phase behaviors on the boiling surface, such as the growth and shrinkage of the microlayer and dry spot, wetting velocity and area portion of the dry spot. As a result, the CHF increased with the surface wettability, and we found the CHF is triggered from a local dry spot where the liquid wets the spot no longer and is violently scattered by strong evaporation at the triple contact line. Our numerical simulation results, in which the three-dimensional transient heat diffusion equation was solved for the boiling substrate using the phase distribution images as boundary conditions, indicated that certain dry spots (critical dry spot) can be highly overheated under high heat flux condition, and they lead significant heat transfer degradation and, eventually, the CHF. With the numerical results and observation of the phase distribution images at the CHF conditions, we postulated the allowable wetting temperature and formulated it as a function of surface wettability through considering the balance between vapor recoil and surface tension forces. The wetting temperature increased with surface wettability, and it was confirmed by the numerical results. Finally, we proposed a qualitative relation to elucidate the influence of surface wettability on the CHF based on the heat conduction via the vapor on a critical dry spot and heat flux partitioning concept.

Evidence of Three-Dimensional Asymmetries Seeded by High-Density Carbon-Ablator Nonuniformity in Experiments at the National Ignition Facility.

Inertial confinement fusion implosions must achieve high in-flight shell velocity, sufficient energy coupling between the hot spot and imploding shell, and high areal density (ρR=∫ρdr) at stagnation. Asymmetries in ρR degrade the coupling of shell kinetic energy to the hot spot and reduce the confinement of that energy. We present the first evidence that nonuniformity in the ablator shell thickness (∼0.5% of the total thickness) in high-density carbon experiments is a significant cause for observed 3D ρR asymmetries at the National Ignition Facility. These shell-thickness nonuniformities have significantly impacted some recent experiments leading to ρR asymmetries on the order of ∼25% of the average ρR and hot spot velocities of ∼100 km/s. This work reveals the origin of a significant implosion performance degradation in ignition experiments and places stringent new requirements on capsule thickness metrology and symmetry.

Control of traveling localized spots

Traveling localized spots represent an important class of self-organized two-dimensional patterns in reaction–diffusion systems. We study open-loop control intended to guide a stable spot along a desired trajectory with desired velocity. Simultaneously, the spot’s concentration profile does not change under control. For a given protocol of motion, we first express the control signal analytically in terms of the Goldstone modes and the propagation velocity of the uncontrolled spot. Thus, detailed information about the underlying nonlinear reaction kinetics is unnecessary. Then, we confirm the optimality of this solution by demonstrating numerically its equivalence to the solution of a regularized, optimal control problem. To solve the latter, the analytical expressions for the control are excellent initial guesses speeding-up substantially the otherwise time-consuming calculations.

View factor estimation of hot spot velocities in inertial confinement fusion implosions at the National Ignition Facility

Inertial confinement fusion (ICF) experiments at the National Ignition Facility suffer from asymmetries in the x-ray drive, which degrade capsule performance compared to expectations for a symmetric one-dimensional implosion. Mode 1, or pole-to-pole, drive asymmetry can reduce confinement and implosion efficiency, driving a bulk motion of the hot spot that is detectable by neutron diagnostics. Understanding and removing sources of mode 1 asymmetry in ICF implosions is important for improving performance, and the three-dimensional nature of the problem makes high-resolution radiation-hydrodynamic modeling extremely computationally expensive. This work describes a reduced order view factor model that calculates the drive asymmetry induced by beam-to-beam variations in laser delivery and Hohlraum diagnostic windows along the equator. The capsule response is estimated by coupling to a Green's function that relates final hot spot velocity to the applied time-varying mode 1 asymmetry. The model makes several predictions about the impact of mode 1 drivers such as laser delivery and target misalignment and achieves good agreement in both the magnitude and the vector direction for several shots in three families of high-performance platforms. However, notable discrepancies suggest that other potential sources of mode 1 asymmetry not captured by the model are also at play.

Cathodic Arc Evaporation of Polycrystalline Silicon

The paper presents the experimental results of silicon cathodic arc evaporation in a continuous mode with the superimposed external arched magnetic field. Volt-ampere characteristics are obtained for arc discharge and the cathode spot velocity is determined at different external magnetic field. It is shown that the erosion rate of the silicon cathode surface depends on the magnetic field. Silicon coatings obtained by using a continuous mode of cathodic arc deposition, demonstrate the silicon droplet diameter ranging from 30 nm to 5 μm. Experiments show a decrease in the droplet number density and the volume fraction in coatings with increasing magnetic field.

Modeling the temperature distribution during laser hardening process

A mathematical model has been developed here to calculate the temperature distribution on the surface and bulk of a steel plate under the laser hardening process. The model starts from the basic heat equation, is then developed to a volumetric form and is connected to the various solid phases existing. The model is based on two strongly influencing parameters of the laser hardening process: velocity of the laser spot and irradiation time. The results are compared to the available experimental data reported in the literature. The volumetric model provides an assessment of temperature distribution in both the vertical and horizontal axis. Laser irradiation at sufficiently high fluence can be used to create a solid state phase change on the surface. Primary calculations show that the temperature profile has a Gaussian distribution in horizontal x-and y-axis and presents an exponentially decreasing distribution in the horizontal and vertical depth directions.

Increasing efficiency of plasma hardening by local cooling of surface by air with negative temperature

The martensitic transformation interval of some hypoeutectoid, all eutectoid and all hypereutectoid steels covers to a large extent the region of negative temperatures. Due to the fact that the plasma hardening operation is carried out in workshops where the minimum temperature is +20 °С, the surface temperature of the part after plasma heating cannot reach negative values. Because of this, the temperature range of the martensitic transformation is not fully used and in the hardened structure there is a certain amount of austenite, which has not undergone martensitic transformation. This circumstance reduces the hardness of the hardened layer and often low tempering is required to convert residual austenite to tempered martensite, which lengthens and makes the heat treatment more expensive. Complete or almost complete martensitic transformation is possible if the surface heated by the plasma beam is immediately cooled to a negative temperature.It is shown that local cooling of the hardened surface to a temperature of –40 °C can be carried out by air using the Ranque-Hilsch tube, which significantly expands the possibilities of full hardening for eutectoid and hypereutectoid steels. The studies consisted in heating the surface with a plasma stream to a temperature of 750 °C and 900 °C. The temperature was changed by the plasma torch current and by changing the velocity of the plasma flow spot moving along the sample surface. The experiments were carried out on steels 45 (0.45 % C), U8 (0.8 % C) and U10 (1 % C). The study of the structures was carried out on a MIM-7 microscope with a video camera and with the image displayed on the screen. The approximate quantitative composition of austenite, martensite, and associated structures was determined by the areas on the screen.During plasma hardening of steel 45 from a temperature of 900 °C using the Ranque-Hilsch tube, there is practically no residual austenite in the structure. When hardening U8 steel, residual austenite is detected in a small amount. When hardening U10 steel, the amount of residual austenite is approximately 15 %. Local surface cooling allows high-quality hardening of steels of most grades, regardless of the carbon content.

Flying focus: Spatial and temporal control of intensity for laser-based applications

An advanced focusing scheme, called a “flying focus,” uses a chromatic focusing system combined with a broadband laser pulse with its colors arranged in time to propagate a high intensity focus over a distance that can be much greater than its Rayleigh length while decoupling the speed at which the peak intensity propagates from its group velocity. The flying focus generates a short effective pulse duration with a small diameter focal spot that co- or counter-propagates along the optical axis at any velocity. Experiments validating the concept measured subluminal (−0.09c) to superluminal (39c) focal spot velocities with a nearly constant peak intensity over 4.5 mm. Experiments that increased the peak intensity above the ionization threshold for gas demonstrated ionization waves propagating at the velocity of the flying focus. These ionization waves of any velocity overcome several laser-plasma propagation issues, including ionization-induced refraction. The flying focus presents opportunities to overcome current fundamental limitations in laser-plasma amplifiers, laser wakefield accelerators, photon accelerators, and high-order frequency conversion.

Using multiple neutron time of flight detectors to determine the hot spot velocity.

An important diagnostic value of a shot at the National Ignition Facility is the resultant center-of-mass motion of the imploding capsule. This residual velocity reduces the efficiency of converting laser energy into plasma temperature. A new analysis method extracts the effective hot spot motion by using information from multiple neutron time-of-flight (nToF) lines-of-sight (LoSs). This technique fits a near Gaussian spectrum to the nToF scope traces and overcomes reliance on models to relate the plasma temperature to the mean energy of the emitted neutrons. This method requires having at least four nToF LoSs. The results of this analysis will be compared to an approach where each LoS is analyzed separately and a model is used to infer the mean energy of the emitted neutrons.

The Possibility of High-Speed Laser Glass Cutting

In recent years, the appearance of high-power fiber lasers, more efficient and easier to operate than other types of lasers, made it necessary to conduct systematic studies and develop a technology for cutting glass by the method of controlled thermal splitting. The paper considers the question of the possibility of high-speed cutting of glass by modern lasers with a power of up to several tens of kilowatts in the near-IR radiation range. In order to achieve the goal set in the work, the problem of determining the field of elastic stresses arising in a glass plate when it is heated by laser radiation has been solved. The method of investigation is based on the theory of thermoelasticity. We have proposed a mathematical model for calculating the three-dimensional temperature distribution and elastic stresses caused by the irradiation of the glass by a moving laser beam and have presented the results of numerical simulation of temperature fields and elastic stress profiles at different irradiation spot velocities. At high radiation powers, the maximum achievable cutting speed of glass is shown to be directly proportional to the power. The data on the speed of Bohr glass at low radiation powers (less than 400 W) available in the literature we have extrapolated to a high power (up to several tens of kilowatts). The investigation shows that the cutting speed can reach 70 cm / s at a laser radiation power of 20 kW. For cutting large-sized glass sheets in mass production, the fiber ytterbium lasers having a wavelength of 1.065 m are recommended. These lasers meet the requirements for power level, irradiation stability and service life. In addition, they have the ability to transport radiation through a fiber optic cable and the ability to automatically control the spatial location of the radiation spot. The results obtained can be used to develop new technologies for cutting glass on industrial lines for the production of float glass, in the process of cutting glass sheets and cutting 3D articles. The method of laser controlled thermal splitting, in comparison with other methods of laser cutting glass, is more efficient and stable, and also has a much wider range of applications, because allows increasing in several times the thickness of the divided glass.

Short-Term Trend Forecast of Different Traffic Pollutants in Minnesota Based on Spot Velocity Conversion.

Because traffic pollution is a global problem, the prediction of traffic emissions and the analysis of their influencing factors is the key to adopting management and control measures to reduce traffic emissions. Hence, the evaluation of the actual level of traffic emissions has gained more interest. The Computer Program to calculate Emissions from Road Transport model (COPERT) is being downloaded by 100 users per month and is being used in a large number of applications. This paper uses this model to calculate short-term vehicle emissions. The difference from the traditional research was that the input velocity parameter was not the empirical value, but through reasonable conversion of the spot velocity at one point, obtained by the roadside detector, which provided new ideas for predicting traffic emissions by the COPERT model. The hybrid Autoregressive Integrated Moving Average (ARIMA) Model was used to predict spot mean velocity, after converted it to the predicted interval velocity averaged for some period, input the conversion results and other parameters into the COPERT IV model to forecast short-term vehicle emissions. Six common emissions (CO, NOX, CO2, SO2, PM10, NMVOC) of four types of vehicles (PC, LDV, HDV, BUS) were discussed. As a result, PM10 emission estimates increased sharply during late peak hours (from 15:30 p.m.–18:00 p.m.), HDV contributed most of NOX and SO2, accounting for 39% and 45% respectively. Based on this prediction method, the average traffic emissions on the freeway reached a minimum when interval mean velocity reduced to 40 km/h–60 km/h. This paper establishes a bridge between the emissions and velocity of traffic flow and provides new ideas for forecasting traffic emissions. It is further inferred that the implementation of dynamic velocity guidance and vehicle differential management has a controlling effect that improves on road traffic pollution emissions.

Experimental Investigation on Triggered Vacuum Arc and Erosion Behavior Under Different Contact Materials

In order to improve the breaking capacity of contacts, it is very important to find a better contact material. In this paper, the triggering experiments of plate contact with different materials are studied to investigate the characteristics of vacuum arc. The current range is from 1 to 9 kA (rms). The gap distance is selected as 3 and 5 mm. The arc appearance and the behavior of cathode spots are captured by high-speed camera with an exposure time of $2~\mu \text{s}$ . The surface topography of contacts after experiments is evaluated by scanning electron microscope. The arc current and arc voltage are measured at the same time by Rogowski coil and high-voltage probe. The diameter of the contact is 25 mm. There are seven kinds of materials (CuCr10, CuCr25, CuCr30, CuCr40, WCu10, WCu30, and CuW–WC) conducted in this paper. Compared with the experimental results, the arc of WCu alloy distributes more evenly on the contact plane with many bright cathode spots. The small gap distance makes the burning point more fixed. The arc of CuW–WC is unstable. During the arc trigger moment to the arc stable burning stage, the radial diffusion velocity of WCu10 cathode spot is the lowest with about 25% of CuCr10. CuCr10 alloy has the worst melting resistance. WCu alloy has good melting resistance and bad toughness. The toughness of CuW–WC is even worse. Under the low current, CuCr30 might be preferred for vacuum interrupters from the cathode spots’ radial diffusion speed, arc appearance, and erosion result. When the current increasing, the arc can easily spread to the entire surface of WCu contact, and WCu10 would be the prefer material with the diffusion arc and good erosion resistance.

Nanostructure patterns on stainless-steel upon ultrafast laser ablation with circular polarization

Ripple-like nanostructure patterns are known to occur in ultrafast laser ablation with linearly polarized radiation. The observation of similar features at the bottom of grooves produced during laser ablation of stainless-steel with circular polarization is reported here. A comprehensive morphological analysis of the machined surfaces, carried out by electron and scanning probe microscopes, reveals a marked dependence of feature shape on process parameters, in particular on the scanning velocity of the laser spot. Such a dependence is interpreted based on the occurrence of an inclined surface during the ablation and on the consequent differential absorption of s- and p-polarized components of the laser radiation by stainless-steel. The resulting anisotropy of the absorbed energy mimics irradiation with elliptical polarization that can drive formation of ripple-like, elongated nanostructures.

Scattering of Ginzburg–Frank and Cherenkov Types Under Self-Focusing of Nanosecond Laser Pulses in Liquids

We study the dynamics of nonlinear optical processes such as self-focusing, self-phase modulation, and stimulated Raman scattering in Kerr-liquids under the nanosecond laser pulse excitation. The results prove the existence of the transition Ginzburg–Frank-type effect, which promotes the appearance of new spectral components of the laser radiation at the medium boundary. The generation of extended anti-Stokes frequency-angular bands of stimulated Raman scattering is explained. When the velocity of a self-focusing focal spot matches the phase velocity of the non-linear polarization at the anti-Stokes Raman frequency and the phase velocity of the scattered axial radiation, the most intense frequency-angular bands appear. They are described by the equations typical of the Cherenkov radiation.

Laser Beam Steering Along Three-Dimensional Paths

This paper deals with the development of a vision-based control scheme for three-dimensional (3-D) laser steering. It proposes to incorporate a simplified trifocal constraint inside a path following scheme in order to ensure intuitively a 3-D control of laser spot displacements in unknown environment (target). The described controller is obtained without complex mathematical formulation nor matrix inversion as it requires only weak camera and hand-eye calibration. The developed control law was validated in both simulation and experimental conditions using various scenarios (e.g., static and deformable 3-D scenes, different control gains, initial velocities, etc.). The obtained results exhibit good accuracy and robustness with respect to the calibration and measurement errors and scene variations. In addition, with this kind of laser beam steering controller, it becomes possible to perfectly decouple the laser spot velocity from both the path shape and time. These features can fit several industrial applications (welding, micromachining, etc.) as well as surgical purposes (e.g., laser surgery) requirements.