High-voltage Sphere Research Articles

수직배치 구대평판 공기 갭에서 연소화염 존재시의 절연파괴 특성

In this study, breakdown characteristics of a vertical sphere-plane air gap in the presence of combustion flame were examined under the application of a.c. and d.c. high-voltages in order to investigate the effect of flame on the dielectric strength of air. Propane gas is used as the fuel of the flame, and the temperature distribution of air around the flame is measured. The polarity effects for the d.c. voltages and breakdown voltages of air when applied both of a.c. and d.c. voltages were examined in the case of the variation of the gap length and the horizontal distance between the flame and the high-voltage sphere electrode. The influence of relative air density, electrification and shape changes of the flame as factors that affect the breakdown voltages were discussed.

Conductivity Measurements of Femtosecond Laser–Plasma Filaments

Experiments are performed to characterize the electrical properties of plasma filaments that are generated by self- guided femtosecond laser pulses propagating in air. A single plasma filament passes through a high-voltage sphere pulsed at -100 kV to a grounded electrode, which serves as a current monitor. The experiments utilize moderate electric fields to probe the filament conductivity, thereby avoiding the strong perturbations caused by electric discharges. The measured filament current decreases as ~1/R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> as the separation R between the electrodes is increased up to 1.5 m. The pulselength of the filament current signal is 2 ns (full-width at half-maximum), but the time resolution is limited by the bandwidth of the oscilloscope. The typical plasma density in the conducting filament is 9 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , which is inferred from the conductivity measurements and the size of the optical filaments. Comparisons are made with mobility values derived from electron swarm data, where the mobility depends upon the applied electric field. The conductivity of the filament is measured as the laser pulselength is varied from 50 fs to 1.5 ps. We find that relatively long laser pulses (1 ps) produce filaments with the largest conductivity. A model is used to predict the longitudinal position where the plasma filament forms and is in reasonably good agreement with measurements.

Reproduction of mimosa and clock anomalies before earthquakes. Are they "Alice in the Wonderland Syndrome"?

Some earthquake precursor phenomena told as legends or reported retrospectively for the 1995 Kobe earthquake have been reproduced by laboratory experiments. Mimosa placed on the high voltage sphere of a Van de Graaff electrostatic generator closed its leaves and bowed on charging and air-gap discharging, presumably due to the current induced in its stem. Air-gap discharging caused sudden movements of eels in a nearby plastic aquarium. Eels moved also on applying a pulsed electric field of less than 0.5V/m, while catfish responded only at around 5V/m. The higher sensitivity of eels to electric field than that of catfish is consistent with the story in the Ansei Chronicle that a man could not find eels in a river but only catfish in violent movement before the earthquake. Eels might have already hidden themselves in panic. Rapid rotation and stopping of the second hand of a clock, which were observed before the Kobe Earthquake like in the“Alice in the Wonderland”, have also been reproduced by exposing clocks to electromagnetic waves generated by air-gap discharging. Reported malfunctioning of home electronic appliances before the earthquake would have been due to some natural electromagnetic disturbance at the epicenter.

The interaction of high-voltage spheres with the ionosphere

The interaction of a high-voltage sphere with both magnetized and unmagnetized plasmas was studied with an electrostatic fluid code. Simulations were conducted for a wide range of sphere and plasma parameters. Depending on the conditions, one can observe a propagating spherical double layer, an electron ringing, a stable rapidly-rotating electron density torus, as well as spherical and toroidal discharges.

High voltage spheres in an unmagnetized plasma: Fluid and pic simulations

The basic physics involving the interaction of a high voltage sphere with a LEO-Plasma was investigated via computer experiments. Two approaches were used: the fluid description and the more general particle pushing method. Both techniques displayed qualitatively similar features. Thus, we verified the initial time-dependent response of the plasma predicted by the fluid model, including the initial current surge and sheath formation, the formation of ion and electron density shells about the sphere with a double layer potential structure, and the subsequent propagation of the density shells away from the sphere. The PIC results also displayed an oscillatory character and slightly different time constants for the growth and decay mechanisms involved.

High negative voltage spheres in an unmagnetized plasma: fluid simulation

Because of the anticipated use of high-voltage power sources on future space platforms, there is an increased interest in elucidating the phenomena associated with the interactions of a high-voltage conductor and the ionospheric plasma at low-Earth-orbit altitudes. Currently there are ongoing rocket, laboratory and modelling programs, and an electrodynamic tether will be flown on the Space Shuttle. All of these programs involve the current collection characteristics of high-voltage spheres embedded in both magnetized and nonmagnetized thermal plasmas. In support of these efforts, the authors have solved the time-dependent nonlinear fluid equations for ions and electrons and the Poisson equation in order to study the response of a nonmagnetized plasma to 'negative' high-voltage spheres. Simulations were conducted for a range of voltages (-10 to -10000 V) and for plasma densities from 104 to 106 cm-3. The temporal evolution of the plasma was followed all the way to the steady state. In all simulations, there were certain qualitative similarities in the plasma response.

High-voltage spheres in an unmagnetized plasma: long-term evolution and rise-time effects

A fluid model was used to study the response of a nonmagnetized plasma to a high-voltage sphere for a time interval up to 20000 plasma periods. Different applied voltages, background plasma densities and voltage rise times were considered. It was found that after the transient response to the applied voltage the system evolves to a steady state. In the steady state, the ion density satisfies the Boltzmann relation. The thicknesses of both the electron sheath and the presheath increase with the applied voltage. The sheath thickness, estimated from the electron and ion density profiles, agrees with the simple Langmuir-Blodgett (LB) theory, but the simulation current is 2-5 times higher. If the voltage is applied according to an exponential law with a rise time tau , only the electron motion at very early times is affected if tau <or= omega pe-1. A finite tau reduces the initial acceleration and density build-up of the electrons around the sphere. The overshoot oscillation seen at early times for small tau does not occur for tau =10 omega pe-1. The formation of the spherical double layer is delayed, and at a given time, it is located closer to the sphere compared to the case of tau <or= omega pe-1. For different tau s, the system reaches the steady state in slightly different ways.

High-voltage spheres in an unmagnetized plasma: long-term evolution and rise-time effects

High-voltage spheres in an unmagnetized plasma: long-term evolution and rise-time effects

High negative voltage spheres in an unmagnetized plasma: Fluid simulation

High negative voltage spheres in an unmagnetized plasma: Fluid simulation

Ionization in the magnetized ionosphere surrounding a high voltage sphere

The interaction of a high-voltage element with the ionospheric plasma at LEO altitudes is an area of current interest because of the anticipated use of high-voltage power sources on future space platforms. Depending on the current collection characteristics, an exposed high-voltage element can lead to power losses or arcing. In an effort to elucidate high-voltage current collection characteristics, there is an ongoing series of rocket and laboratory experiments involving the interaction of high-voltage spheres with an ambient plasma. In support of this effort, we solved the time-dependent, three-dimensional, nonlinear fluid equations for ions and electrons and the Poisson equation in order to study the initial response of a partially-ionized, magnetized plasma to positive high-voltage spheres. Our emphasis was on the effect that collisions and impact ionization have on the sphere-plasma interaction. Different neutral densities (10 11–10 14 cm −3), species (Ar, Ba, O, N 2), and magnetic field strengths were considered. Our simulations indicate that a rapidly rotating electron density torus tends to form around a high-voltage sphere embedded in magnetized partially-ionized plasma. Since the electrons orbit the sphere before striking it, the high toroidal density and long travel time lead to enhanced collisional and ionization rates (relative to the no B -field case), which act to modify the torus, sheath, and current collection characteristics. The high toroidal density and long travel time also reduce the neutral density (pressure) requirement for a discharge, by an order of magnitude. Depending on the applied voltage, neutral gas pressure, and magnetic field, we obtained steady toroidal configurations both with and without ionization, a toroidal discharge, and a spherical discharge. These results are in qualitative agreement with measurements.

Electrical breakdown of vacuum insulation at cryogenic temperature

An experiment on the breakdown of vacuum insulation was performed for the development of cryocables and other cryogenic electrical apparatus. The influence of conditioning, pressure, gap spacing, electrode material and voltage waveshape on the breakdown voltage of vacuum insulation was measured at room temperature and at cryogenic temperature respectively. The results indicate that cooling the high-voltage sphere electrode enhances the breakdown strength of vacuum insulation, and that in general, almost all effects of electrical breakdown of vacuum insulation at room temperature are reproduced at cryogenic temperature at somewhat higher voltage. The close correlation between the low-temperature and room-temperature breakdown voltages of large vacuum gaps is explained from the point of view of microparticle-initiated breakdown theory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A fluid model of high voltage spheres in an unmagnetized plasma

In the coming decade, large space structures with high-power sources will be orbiting the Earth at ionospheric altitudes. Exposed voltages on any part of the structure will cause a current flow between the exposed element and the ambient plasma, which in turn may cause a power loss or arcing. In an effort to better understand high-voltage current collection, a series of rocket and laboratory experiments will be conducted whereby, initially, positive high-voltage spheres will be exposed to an ambient plasma. In support of this program, the authors have solved the time-dependent nonlinear fluid equations for ions and electrons and the Poisson equation in order to study the response of a plasma to positive high-voltage spheres. Simulations were conducted for a range of voltages and for both low and high density surrounding plasmas. In all of the simulations, the sheath and current collection were found to be highly dynamical and the plasma response exhibited four distinct phases. The phases include a rapid initial current surge, the formation and subsequent propagation of both ion and electron density shells away from the sphere, a burst of localized large-amplitude wave activity, and eventually a quasi-steady state. The steady state results are consistent with the classical Langmuir-Blodgett theory.

A fluid model of high voltage spheres in the ionosphere

In the coming decade, large space structures with high-power sources will be orbiting the Earth at ionospheric altitudes. Exposed voltages on any part of the structure will cause a current flow between the exposed element and the ambient plasma, which in turn may cause a power loss or arcing. In an effort to better understand high-voltage current collection at LEO altitudes, a series of rocket experiments will be conducted whereby, initially, positive high-voltage spheres will be exposed to the ionospheric plasma. In support of this program, we have solved the time-dependent, three-dimensional, nonlinear fluid equations for O + and electrons and the Poisson equation in order to study the initial response of the ionosphere to positive high-voltage spheres. Simulations were conducted for both low and high voltage spheres in both low and high density background ionospheres. The magnetic field strength was also varied in order to determine its effect on the current collection. In all simulations, an electron density torus formed around the sphere in the equatorial plane at early times. The torus rotated about the sphere in the E × B direction. The characteristic time for both the torus formation and the rotation was determined by the potential on the sphere and the electron cyclotron frequency. The torus size and density were larger for higher sphere voltages and lower ionospheric densities. At later times, the outer edge of the torus became elongated along the magnetic field, and the bulk of the region perturbed by the sphere was contained within a cylindrical volume.

Self-Triggering Spark Gap ``Crowbar''

A self-triggering spark gap has been developed for use as a simple ``crowbar'' at the output of a high-voltage dc power supply to protect the load in case of load failure. The system consists of three spheres arranged in a triggered spark gap system, with the center sphere maintained at a fixed potential with respect to the other two spheres (one at ground, one at high voltage) under steady-state conditions. When the load fails, capacitive coupling drives the triggering sphere to a voltage that causes break-down between the high-voltage sphere and the triggering sphere, with subsequent firing of the entire protective gap system. Advantages over thyratrons for this ``crowbar'' application include the following: 1) the voltage rating is unlimited, 2) the current rating is of the order of tens of thousands of amperes, 3) reliability is greater than that of a thyratron, due to lack of filament supply requirements, auxiliary triggering circuits, etc., 4) less costly than the thyratron circuit.

The influence of irradiation on the measurement of impulse voltages with sphere-gaps

The impulse breakdown of short gaps between spheres is greatly affected by the amount of irradiation present, and recommendations are made in the British Standard “Rules for the Measurement of Voltage with Sphere-Gaps”1 that appropriate precautions should be taken when voltages of less than 50 kV (peak) are being measured. These precautions are generally considered to be unnecessary in the measurement of higher voltages, and, in most laboratories, no deliberate steps are taken to provide irradiation.In a previous publication the author has shown that appreciable errors may be incurred if irradiation is not used with spheres of 6.25 cm diameter in the measurement of voltages up to 100 kV (peak). Further measurements have now been made up to 400 kV (peak) with 1/5 and 1/50 impulse voltage waves, of positive and negative polarities, for spheres of 6.25 cm, 12.5 cm and 25 cm diameter. The present results confirm and extend the previous observations.The irradiation provided by the ultra-violet illumination from the impulse generator spark-gaps has a marked influence on the behaviour of the sphere-gap, in the absence of other forms of irradiation, and consequently the position of the sphere-gap relative to the impulse generator is observed to affect the results. This factor does not appear to have been considered in other laboratories, and it may explain some of the differences between the results obtained by various investigators in the calibration of the sphere-gap with impulse voltages.As it is not always convenient to illuminate the gap with the light from the impulse generator spark-gaps, the irradiation provided by the insertion of 0.5 mg of radium in the high-voltage sphere has been examined and is found to give satisfactory results. Measurements of the impulse breakdown of the unirradiated sphere-gap show that there is a gradual transition from the voltage which causes breakdown for 10% of the applied impulses to that which causes 90% breakdown, whereas a sharply defined breakdown voltage is obtained when the gap is irradiated. The mean breakdown voltage of the unirradiated gap exceeds that of the irradiated gap by an amount which varies with the gap length. For a 3 cm gap between 12.5 cm diameter spheres, breakdown of the unirradiated gap takes place at a mean voltage about 13% in excess of that for the irradiated gap. With larger gaps, for which a difference between the breakdown voltages for positive and negative impulses is recorded when one sphere is earthed, the difference is found to be largely influenced by irradiation. The mean breakdown voltage for an unirradiated gap of 9 cm between 12.5 cm diameter spheres is 20% higher than the irradiated value for a positive impulse, and 1.5% higher for a negative impulse. The negative impulse breakdown voltages agree closely with the corresponding values given in the B.S. 358 Tables, whereas the voltages given in the Tables for the positive impulse breakdown are intermediate between those obtained in the present experiments for the unirradiated and the irradiated gaps.The results show that some revision of the Standard Rules for the use of sphere-gaps is necessary, and, in particular, that more definite recommendations should be made concerning irradiation. Further, it is evident that the influence of irradiation is an important factor to be considered in the breakdown of other forms of discharge gaps in which corona is not observed to precede breakdown. Also, the breakdown of discharge gaps on the wavefront may be expected to be governed largely by the amount of irradiation present, and, though his has been realized by many workers for short gaps, no consideration appears to have been given to irradiation in the measurement of wavefront breakdown for longer gaps.