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Plasma Science, IEEE Transactions on

Issue 5 • Date Oct 1989

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Displaying Results 1 - 25 of 46
  • Breakdown of a coaxial diode transverse to the magnetic field

    Page(s): 789 - 792
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    It is noted that the modern development of pulsed power generators with fast energy storage systems in the range of several megajoules makes it necessary to find conditions for charge particle beam formation with the same energy characteristics. Presently, the highest parameters of electron beams (E0=140 kJ) are obtained in coaxial diodes, with magnetic insulation. However, even in these experiments the energy transport efficiency from a storage system into the beam was only ~20%. One of the main reasons for low efficiency is the breakdown in the diode transverse to the magnetic field. A breakdown centrifugal instability model is described and compared with experiments. It is shown that, in order to raise the electron-beam energy in coaxial diodes it is necessary to suppress the centrifugal instability in the plasma created on the inner diode electrode. For inner cathode diodes it is advisable to use a nonuniform magnetic field configuration with the field strength increasing rapidly enough in the direction from cathode to anode. The large radius diode with uniform magnetic field also seems to be promising for this purpose View full abstract»

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  • On the ion-acoustic soliton in a relativistic beam ion plasma system

    Page(s): 804 - 810
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    The authors analyzed the formation of ion-acoustic solitons in a beam-plasma system, where both the beam and the ion were considered to be relativistic. The system is assumed to be nonisothermal. Both lower- and higher-order computations are performed, and the change of the wave profile is discussed, both in the lower and higher order. It is observed that both the amplitude and the width of the soliton change significantly in the presence of the relativistic effect and ion beam. The variations of these quantities with respect to v0/c, the ion-beam population, and the nonisothermality of the plasma are depicted graphically and compared with other results View full abstract»

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  • Angular distribution of ion current emerging from an aperture anode in a vacuum arc

    Page(s): 713 - 716
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    The ion current distribution emerging from a vacuum arc between a Cu cathode and a conical ring anode was measured by a set of five probes. It was found that: (1) the total ion current emerging through the anode was 8.5% of the arc current; (2) the measured ion distribution without a magnetic field was a slightly flattened cosinusoidal function; (3) with an axial magnetic field, the ion current distribution became peaked along the z axis; (4) the total ion current extracted through the anode aperture slightly increased with the magnetic field; and (5) an anode with a larger aperture exhibited less magnetic collimation View full abstract»

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  • Acceleration gap behavior of flashover-type pulsed ion diodes

    Page(s): 793 - 796
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    The physical processes of the acceleration gap of flashover-type magnetically insulated pulsed ion diodes were examined. Spectroscopically measured values of the density and temperature of the anode plasma were 1016 cm-3 and 1 eV, respectively. The electron temperature increased with the diode time. The anode plasma expansion velocity was estimated to be about 3 cm/μs. Resonant laser interferometry was applied to the plasma using a tunable dye laser pumped by a nitrogen laser. The hydrogen atom number density was estimated to be 1016-17 cm-3. It filled the entire A-K gap within 200 ns from the time the voltage pulse rising started. Results are presented on the formation and evolution process of the anode plasma and the behavior of neutral particles in the gap View full abstract»

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  • Metallurgical aspects of contact materials for vacuum switching devices

    Page(s): 734 - 740
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    Both the metallurgical aspects of contact materials for vacuum switching devices and their practical applications for important electrical load cases (circuit breaker, contactor, and load switch) are discussed. Because a vacuum contact material has to fulfil conflicting requirements, an optimization of the contact material with regard to the type of application is always necessary. The selection procedure for the different types of contact materials is discussed on the basis of experimental results. Cu/Cr-type contacts yield very good values for the breaking capacity of the circuit breakers used in the medium-voltage range. Optimization must attempt to improve the welding behavior and ability to withstand a high voltage after making short-circuit conditions. W/Cu-type contacts show a long service life in high-voltage contactors. Current chopping behavior, however, has to be optimized. WC/Ag-type contacts are a good choice for low-voltage contactors. The service life and breaking capacity, however, are not always satisfactory and need optimization View full abstract»

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  • Theory of the cathode sheath in a vacuum arc

    Page(s): 641 - 643
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    A previous theory of the plasma sheath transition starting from the charge exchange model for ion collisions is extended to account for ionization and recombination. It is applied to the quasi-neutral boundary layer (presheath) in front of the cathode sheath of a vacuum arc. An essential potential and density difference between the sheath edge and cathodic plasma ball is found. This difference is accounted for in a unified theory of the arc cathode based on G. Ecker's (1971) existence diagram method, which indicates possible areas of arc operation in the Tcj plane, where Tc is the spot temperature and j is the current density. A numerical evaluation for Cu gives the results which are qualitatively similar to Ecker's theory. The existence areas are quantitatively enlarged and shifted to lower current densities View full abstract»

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  • A model for a uniform steady-state vacuum arc with a hot anode

    Page(s): 661 - 665
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    A model is formulated and evaluated for a Uniform electrical discharge sustained in vapor evaporated from an arc-heated anode. The plasma potential is positive with respect to both the cathode and anode. For a Cu anode, the anodic vapor dominates the plasma for current densities exceeding 8 kA/m2. The anode heating potential is approximately 6.5 V, and the dominant cooling mechanism is evaporation for current densities exceeding 20 kA/m2. Over the range 10 to 10000 kA/m2, the electron density increases from 8×1017 to 5×1023 m-3, while the ionization fraction rises from 0.3% to 4%. At the lower end of this current range the electrical resistivity of 4 mΩ-m is determined primarily by electron-neutral collisions, while with increasing current the resistivity decreases to 0.7 mΩ-m, with electron-ion collisions contributing an equal share. This hot-anode vacuum arc may have potential for industrial application as a macroparticle-free high-deposition-rate coating source View full abstract»

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  • Recent technical developments in high-voltage and high-power vacuum circuit breakers

    Page(s): 717 - 723
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    Explanations are given regarding recent technical topics in the development of vacuum circuit breakers. The area-effect concept is introduced to aid in insulation designs of vacuum interrupters. By using Cu-Cr prepared through a special method, the voltage performance of vacuum interrupters has been remarkably improved. Through the development of axial magnetic field electrodes, large-current interruption has been made possible. The design concept for determining contact radii has been established. A computer simulation method for generating switching overvoltage is described, thereby indicating the possibility of realizing large-current interrupting vacuum interrupters with low switching overvoltages View full abstract»

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  • Study of recovery phenomena in a high-current pulse-triggered vacuum switch

    Page(s): 781 - 785
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    The recovery capacities of a triggered vacuum switch after the crossing of a 25-μs high-current pulse were investigated. The recovery time is less than 100 μs. An experiment was conducted with several electrodes with different gaps, and the influence of an axial magnetic field on the switch was studied. A diffuse arc mode was achieved, making possible a commutation power as high as 109 VA View full abstract»

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  • Vacuum arcs for plasma centrifuge isotope enrichment

    Page(s): 695 - 700
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    Requirements for economic plasma centrifuge isotope enrichment are reviewed. A distributed-discharge vacuum arc in chromium, as reported by A.I. Vasin et al. (1979), is described and a qualitative model for its operation is given. Experiments were performed on a modified distributed-discharge vacuum arc in which the parameters appear to be well suited for plasma centrifuge operation. A 2-m-long, 10-cm-diameter fully ionized DC chromium arc was produced with ion density and temperature in the range of 1013 cm-3 and 0.16 eV. Chromium mass throughput rates in the range of 1 g/min are reported, with continuous running times of up to 40 min., limited only by chromium supply View full abstract»

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  • Wave propagation in a switched-on time-varying plasma medium

    Page(s): 828 - 833
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    The interaction of an electromagnetic wave with a switched-on time-varying plasma medium is considered. The characteristic features of wave propagation in such a medium are the conservation of wave number and the generation of new waves having frequencies different from that of the incident wave. Initially, a plane wave is assumed to be propagating in free space, and at time t=0 the entire space is converted to a plasma having a time-varying number density. The solution to this problem is obtained through a WKB (Wentzel-Kramer-Brillouin) approximation. The condition of validity of the solution is discussed. Identification of the reflected and transmitted waves constitutes the chief advantage of the method. The ratio of the amplitudes of the electric fields of the reflected and the transmitted waves is shown to depend upon the initial rate of rise of the particle density. This result is useful in diagnosing the initial rate of rise of the number density. To illustrate the method, an exponentially varying number density is considered. Numerical results of the solution of the time-varying differential equations are found to be in good agreement with the proposed solution View full abstract»

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  • Experimental investigation of a probe-induced localized electron temperature elevation near electron-cyclotron resonance

    Page(s): 818 - 827
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    A spatially localized electron temperature elevation is observed near the electron-cyclotron resonance region. The properties of this phenomenon are examined in detail. The dependence of the temperature elevation upon launched wave amplitude and frequency relative to the local electron-cyclotron frequency, plasma density, neutral pressure, microwave cavity plasma source tuning, and probe type is discussed. These results are combined with detailed wave electric and magnetic field, emissive probe and spectral measurements and the use of different Langmuir probe types and orientations to characterize the temperature elevation, background plasma, and wave properties. It is found that the magnetic field and wave absorption near the electron-cyclotron resonance, probe type, and probe tip input impedance as seen from the plasma have important influences on the temperature elevation process. Two-probe correlation measurements are used to show that the elevation process exists in the radial neighborhood (<1 cm) of the probe tip and along a flux tube parallel to the magnetic field. This process can be present in all Langmuir probe-diagnosed ECRH experiments and can be suppressed by appropriate probe design. Measurements suggest that the electron temperature elevation is due to the interaction of sufficiently strong wave electric fields near electron-cyclotron resonance with an appropriate probe tip input impedance as seen by the fields View full abstract»

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  • The fundamentals of the pseudospark and its applications

    Page(s): 748 - 753
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    The pseudospark, an axially symmetric gas discharge at low pressures of typically 10 to 100 Pa and high voltages (of some hundred volts up to several hundreds of kilovolts), has become of interest in fast high-power switching, in producing well-pinched high intensity electron or ion beams, and as an X-ray source. The essential features of this type of discharge are described. The results of fast time-resolved spectroscopic investigations and breakdown delay statistics of the pseudospark are discussed View full abstract»

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  • Measurements of the behaviour of neutral atom density in a diffuse vacuum arc by laser-induced fluorescence (LIF)

    Page(s): 666 - 671
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    The method of laser-induced fluorescence was used to study the behavior of the absolute neutral vapor density of a diffuse vacuum arc on FeCu contacts. The local and temporal resolutions were 1 mm3 and 10 μs, respectively. The arc current had a sinusoidal shape of 5.8-ms duration with peak values of 90 and 510 A. It was found that the maximum densities of the iron and copper atoms are 1.2×10 17 m-3 and 7.5×1017 m-3, respectively. During the arc the density was correlated with the current. At current zero the measured densities decreased to 10 16 m-3. After current zero, an exponential density decay with a time constant of about 100 μs was observed, indicating the recovery of dielectric strength after current zero. Measurements of the neutron iron vapor density at different spatial positions in the electrode gap reveal a nonisotropic distribution. From the measurements of the population distribution of the iron ground-state multiplet a 5D, the excitation temperature was derived. This temperature was low compared with the cathode spot temperature 2500-4000 K and decreased from 1600 K at the current maximum to 1000 K at current zero. The results indicate that the generation of neutrals is caused by flying evaporating metal droplets rather than by molten surface areas View full abstract»

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  • Theoretical analysis of the current and energy flow to the anode in the diffuse vacuum arc

    Page(s): 649 - 652
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    The current energy flow to the anode in a diffuse vacuum arc is investigated using a simple multicathode-spot fluid description of the interelectrodic plasma. Decisive for the current constriction and the energy flow to the anode center is the space-charge sheath in front of the anode. This sheath not only determines the current-density distribution at the anode, but also may lead to a strong energy gain of the electrons before they reach the anode surface (in case the anode drop appears to be positive in the anode center). Numerical results are given and two possible scenarios for the transition from the diffuse to the anode-spot mode are discussed View full abstract»

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  • Consequences of balance equations applied to the diffuse plasma of vacuum arcs

    Page(s): 657 - 660
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    By means of balance equations, the partial currents in vacuum arcs are represented by some simple coefficients, and it is shown that the surprising properties of the expanding diffuse arc plasma can easily be explained if the plasma temperature near the cathode is 4.5±0.5 eV (in the case of Cu), i.e., by a factor three times higher than that measured and presumed in most studies. Evidence for this high temperature comes from both the plasma energy balance and the Saha equation View full abstract»

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  • The study of laser-induced flare discharge on dielectric and semiconductor macroparticles in vacuum: plasma production and ablative acceleration

    Page(s): 801 - 803
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    An injector was used that allowed the multiple triggering of a beam of particles (of a size of several to 100 μm) in a region where laser irradiation was focused. By varying the experimental conditions, the particles could be either completely or partially evaporated and strongly accelerated by ablation. The maximum measured velocities were 4×106 cm/s, plasma velocities were about (1-2)×10 7 cm/s, and the temperature was about 100-150 eV. Only 30% of the substance (particles of B4C with dimensions greater than 40 μm) was evaporated before destruction. The particles of B4 C that were smaller than 30 μm were completely evaporated under a laser power density of about 1012 W/cm2 and a half-duration of 30 ns. For the particles smaller than 100 μm and made of materials with a low evaporation temperature, such as paraffin, LiH, and polystyrene, no accelerated fragments were detected, i.e. these particles were completely evaporated during less than 10 ns View full abstract»

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  • Cathode spot energy transfer simulated by a focused laser beam

    Page(s): 638 - 640
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    Minimum conditions for the formation of surface craters by laser irradiation were studied experimentally and theoretically for various metals. The critical power density for crater formation within 20 ns was about 1011 W/m2. It is therefore concluded that crater formation by ion bombardment requires an ion current density on the order of 1010 A/m2 View full abstract»

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  • Measurements of vacuum arc ion charge-state distributions

    Page(s): 679 - 682
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    The charge-state distribution of ions generated in the metal vapor vacuum arc under a wide range of experimental conditions was measured. The experiments were carried out using an ion source in which the metal vapor vacuum arc is used as the method of plasma production and by which a high-quality, high-current beam of metal ions is produced. Charge-state spectra were measured using a time-of-flight diagnostic; arc voltages were also measured. Parameters that were varied include cathode material, arc current, axial magnetic field strength, neutral gas pressure, and arc geometry View full abstract»

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  • Principles and applications of vacuum arc coatings

    Page(s): 705 - 712
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    The development of vacuum arc coatings, commencing a century ago with Thomas Edison and continuing through the recent development of industrial-scale batch coating machines, is reviewed. Most of the work exploited the high ionization, plasma production rate, and ion energy intrinsic in the cathode spot arc to deposit metals, diamondlike carbon Si, and, with the presence of a background gas, various ceramics. Deposition rates of up to 400 μm/s were achieved in pulsed operation. Various techniques were developed to control the motion and location of the cathode spots and to reduce the macroparticle contamination of the coatings. Hot electrode vacuum arc modes were investigated recently as well. Simple models for the plasma transport to the substrate based on known properties of the cathode spot plasma jets are presented, as well as a description of current industrial practice View full abstract»

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  • The influence of contact surface microstructure on vacuum arc stability and voltage

    Page(s): 727 - 729
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    The authors present the results of DC vacuum arc lifetime and voltage measurements for contacts with different surface microstructures. This was realized by treating the contact surface with emery paper of varying roughness. The contact surface microstructure was found to have a large effect on the DC arc lifetime (several tens of times difference at the most) and arc voltage (30% difference at the most). The mechanism of the surface microstructure effect on DC arc stability was analyzed and is explained preliminarily. The rougher the contact surface is, the longer the arc lifetime and the more stable. The arc itself has a tendency to condition the surface in a way that is unfavorable for a sustained arc lifetime. The crater size for a rough surface is smaller than for a smooth one. The rougher the contact is, the lower the arc voltage (both DC and HF components). The arc erosion has a tendency to increase the arc voltage. It is probable that type I cathode spots (fast motion, small crater size) correspond not only to the surface contamination, but also to the surface roughness View full abstract»

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  • Some interruption criteria for short high-frequency vacuum arcs

    Page(s): 741 - 743
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    If the contacts of a vacuum interrupter open shortly before a current zero, the transient recovery voltage (TRV) can cause a reignition and reestablish the arc. When the current in a diffuse vacuum arc passes through zero, there is a distinct pause before the TRV builds up (approximately 40 ns for copper). During this pause the gap carries conduction current only with an ion component which depends on dI /dt, varying between 3 A for dI/dt=60 A/μs and 60 A for dI/dt=1235 A/μs. The ion current subsequently decays in tens or hundreds of nanoseconds. It can be distinguished from the displacement current at this time by varying dV/dt, keeping the other parameters constant. Among the interruption criteria for short high-frequency vacuum arcs, dI /dt prior to current zero and initial dV/dt are the most important. High values of dI/dt are more likely to precipitate reignitions, but breakdowns can occur after lower dI/dt's if the gap has been subjected to a high current for a relatively long time (>100 μs) View full abstract»

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  • Investigations of pulsed surface flashovers for the triggering of pseudospark high-power switches

    Page(s): 762 - 765
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    Surface discharges over insulators in vacuum under rectangular high-voltage pulses are investigated. These discharges are among the methods used to trigger pseudospark switches. Low breakdown voltage and long lifetime are the requirements these insulators have to meet to be used as trigger materials. For this purpose the breakdown behavior of certain ceramics stressed by high-voltage pulses was investigated in an oil-free vacuum of 10-5 Pa. The erosion of the insulator by the discharge plasma was investigated with a scanning electron microscope. An evaluation of the measurement and a comparison of the properties of the investigated insulators show that SiN, Al2O 3, and CaTiO3 have low breakdown voltages and exhibit the lowest erosion rates. First results obtained with these materials as trigger insulators in pseudospark switches are promising with regard to the achieved lifetimes. A lifetime of four million triggered discharges under 90% switch current reversal was reached at a peak switch current of 25 kA without a serious reduction in trigger performance View full abstract»

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  • Vapor shield potential in vacuum interrupters after clearing of short circuits

    Page(s): 834 - 838
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    Measurements have revealed that, after a short circuit is cleared, the potential of an isolated vapor shield in a vacuum interrupter is initially coupled with the contact, which is the anode after current zero. This due to the fact that the residual plasma between the anode and shield first has to be depleted during that initial period. The duration of that coupling depends upon the arc current, the arcing time, and the interrupter geometry. Experiments using a shield with additional external circuitry make it possible to determine the number of charge carriers that have to be depleted at current zero. Shield potential measurement can be used, therefore, in the development of vacuum interrupters to facilitate the evaluation of a contact material or a contact configuration View full abstract»

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  • Creation and properties of a high-current, high-velocity plasma discharge

    Page(s): 786 - 788
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    A device constructed to study the behavior of high-current, high-velocity plasma arcs similar to those present in plasma-driven electromagnetic launchers is described. An electromagnetic launcher plasma armature is a magnetically driven arc with a current of several hundred kiloamps and a velocity of a few km/s. The device described in the paper is designed to produce such a plasma, with a peak current of 300 kA and a variable linear velocity. The current is produced by pulse-forming networks (PFNs) placed at each end of the device, and the arc velocity is varied by adjusting the currents delivered to the arc from each PFN. The device is used to study the behavior or the moving arc and to develop diagnostic techniques that can be applied to such systems View full abstract»

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IEEE Transactions on Plasma Sciences focuses on plasma science and engineering, including: magnetofluid dynamics and thermionics; plasma dynamics; gaseous electronics and arc technology.

 

 

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