Voltage of the vacuum arc with a ring anode in an axial magneticfield

The plasma jet focusing and voltage distribution in the interelectrode gap of a vacuum arc with a ring anode and subjected to an axial magnetic field were studied theoretically. A two-dimensional model was developed based on the free plasma jet expansion into vacuum, and the steady-state solution of the fully ionized plasma in the hydrodynamic approximation was analyzed. It was found that the imposition of an axial magnetic field reduces the radial expansion of the plasma jet. The characteristic jet angle decreases from about 40° in the zero magnetic field case and approaches a value of about 20° with a 0.02 T magnetic field. The arc voltage consisting of the cathode drop, the plasma voltage drop, and anode sheath drop increased, with the imposition of a magnetic field, and decreased with the anode length. The model was compared to experimental measurements of the vacuum arc voltage behavior in an axial magnetic field, and good agreement was found     

Cathode spot dynamics on pure metals and composite materials inhigh-current vacuum arcs

An investigation has been carried out of cathode spot dynamics in a triggered vacuum arc in a demountable chamber. A rectangular current pulse of 1-5 kA, 1-5 ms has been used. Sufficient statistics were collected. The expansion of a cathode spot ring on a clean, pure metal surface was corroborated to be a retrograde movement in the self-magnetic field which obeys the same law as the movement of a single spot in an external magnetic field. The influence of a contact gap of 0.5-8 mm and current on the dynamics of cathode spots was investigated. The gap dependence of the proportional coefficient between the spot velocity and magnetic field in the case of a pure copper cathode was obtained. A phenomenon was discovered, where a group of cathode spots form in the short arcs on the CuCr cathodes after a transition diffuse arc stage. The follow-up investigation revealed that a close interrelation exists between the cathode and anode processes in short arcs. This interrelation is responsible for the appearance of the discovered phenomenon. Short-circuit performance tests conducted for a commercial vacuum interrupter proved cathode spot group formation to be responsible for the interruption failure at short contact gaps     

The influence of unipolar axial magnetic field on the behavior of vacuum arcs

The behavior of vacuum arcs under the influence of unipolar axial magnetic field (AMF) has been investigated. In experimental investigations, the vacuum arc mode is studied at different arc currents by using high-speed charge-coupled device (CCD) video images. In spite of the AMF, first sign of arc constriction appears at relatively small currents of about 15 kA (RMS). Three different arc modes were found. Based on generalized Ohm's law, the current density distribution in the vacuum arc with unipolar axial magnetic field is computed using three-dimensional finite-element method (FEM) software. The calculated current distribution is confirmed by the vacuum arc appearance taken from CCD video film. The distribution of AMF can be optimized by such experiments and theoretical analysis.     

Observation of tungsten particles in vacuum arcs by laser inducedfluorescence

We have clarified the relation between the decay of tungsten ion density in the vicinity of current zero and vacuum arc mode in high current period by using a laser induced fluorescence method in tungsten vacuum arcs of 60 Hz sinusoidal current with the peak value of 3.3, 6.7, and 9.8 kA. In the case of 6.7 kA, the arc mode was the anode spot mode. Because of the generation of the anode spot, the tungsten ion density near the anode was higher than near the cathode and the density near the anode was about ten times as high as the case of 3.3 kA which was the diffuse mode. In the case of 9.8 kA, which was the intense arc mode, the density near the anode was not significantly different from the case of 6.7 kA. The density near the cathode was higher than near the anode and tungsten ions were observed till about 30 μs after current zero while they disappeared at current zero in the other cases     

Plasma-assisted deposition of a three-layer structure by vacuum andgas arcs

The durability and adhesion of thin coatings often depends on the structure and properties of the layer intermediate between the coating and the substrate, especially in the case where the layer and the substrate are highly different in microhardness. With a vacuum arc and a hot-cathode arc, a process has been arranged which involves cleaning of the surface, nitration of the article, and deposition of a coating. As a result, a three-layer composition has been produced which consists of a TiN layer of thickness up to 5 μm and microhardness 20 GPa, an intermediate Fe₄N layer of thickness up to 8 μm and microhardness 7.5 GPa, and a nitrated layer of thickness up to 100 μm with a gradually varying microhardness. With the TiN layer showing high adhesion, the coating has a durability three of four times greater than that of a coating produced with the use of a conventional technology     

Single-frequency ytterbium-doped fiber laser stabilized by spatial hole burning

We have exploited spatial hole burning to achieve remarkably stable single-frequency operation and mode-hop-free tuning over 300 free spectral ranges in an ytterbium-doped fiber laser with a sample standing-wave geometry. This approach makes possible stable and narrow-linewidth single-frequency fiber lasers that do not require components such as Faraday isolators, fiber couplers, and Fabry-Perot filters.     

The origin of current constriction in vacuum arcs

The current constriction in a diffuse vacuum arc is shown theoretically and experimentally to be caused by a pressure source, to which the charged particles and neutral particles contribute.     

Mechanism of ion flow generation in vacuum arcs

The ecton model of the cathode spot is used to analyze the main parameters of ion flow in vacuum arcs (ion erosion, mean charge, and velocity). It is shown that the arc plasma is formed as a result of microexplosions at the cathode surface, induced by the Joule heating by the high-density current of explosive electron emission. Ionization processes are localized in a narrow region of the order of a micrometer near the cathode and the ionization composition of the plasma subsequently remains unchanged. Under the action of the electron pressure gradient, ions acquire directional velocities of the order of 10⁶ cm/s even over small distances of the order of several micrometers.     

Experimental investigations of the currents collected by metalshields in high-current vacuum arcs

This paper presents experimental results of currents collected on the three-element condensation shield connected to the cathode potential in high-current vacuum arcs. The arc current had 900 Hz, 150 Hz, or 50 Hz half-cycle sinusoidal shapes and was conducted between the CuCr40 contacts with a diameter of 50 mm (cathode) and 30 mm (anode) spaced 10 mm apart. Most of the measurements were made for the current of 900 Hz with peak values up to 9 kA. Arc voltage, floating shield potential, and distribution of shield currents were measured. It was found that the current collected by the shield and also the arc voltage and floating shield potential are greater for higher frequency currents, and that they are affected by the arc mode. Considerable shield current is observed during a high-amplitude (HA) oscillation sequence of arc voltage while its mean value is increased. For 900-Hz arcs at the 9-kA peak value (I_am), the ratio of shield current (i_s) to an instantaneous value of arc current (i_a) reaches even 40% near I_am value. In the initial half-cycle period (before the initiation of high-voltage oscillation), the ratio of i_s/i_a increases with i_a and current frequency. A close relationship was found between arc voltage and current distribution on a three-element shield     

Cathode jets and the retrograde motion of arcs in magnetic fields

The influence of a transverse magnetic field on the jets of an impulse discharge was investigated. It was found that the jet trajectories show a curvature. The amount of the curvature depends on the pressure, the magnetic field intensity, the discharge current, the material and the polarity of the electrodes. The curvature of the jets was explained as a result of the action of forces arising in a plasma flow through a magnetic and electric field; their analysis was carried out on the bases of a MHD model. On hand of a derived solution, a possible influence of the jets on the origin of a retrograde motion is discussed. It is shown that the existence of certain discharge parameters and of a certain magnitude of the magnetic field brings about, in a plasma flying through a magnetic field, an induction of fields and currents under the action of which, on the one hand, the plasma itself flows in a retrograde direction and, on the other hand, there are influenced the trajectories of the carriers (particularly those of ions) of the current in the channel of discharge in front of the cathode spot. The proposed model of the retrograde motion is discussed and compared to the known results.     

洛伦兹力对自由燃烧弧和壁稳非转移弧流场的影响

通过耦合迭代求解流体力学方程和电磁场方程,数值模拟了转移式自由燃烧电弧和具有细长中间段及突扩阳极结构的壁稳式非转移直流电弧的流场,分析了洛伦兹力对这两种典型直流电弧流场的影响。结果显示:在自由燃烧电弧情况下,电流自感磁场的洛伦兹力对流场特性有显著影响,自磁压缩是约束电弧的主要机制;而在壁稳式非转移直流电弧情况下,相对于强壁面约束和气动力作用而言,洛伦兹力对流场的影响有限。特别在中间段出口以后,洛伦兹力与气动力的比值小于0.010,因此,当主要考虑壁稳式非转移直流电弧发生器出口参数时,为了提高数值模拟效率,可忽略洛伦兹力的作用。     

洛伦兹力对自由燃烧弧和壁稳非转移弧流场的影响

通过耦合迭代求解流体力学方程和电磁场方程,数值模拟了转移式自由燃烧电弧和具有细长中间段及突扩阳极结构的壁稳式非转移直流电弧的流场,分析了洛伦兹力对这两种典型直流电弧流场的影响。结果显示：在自由燃烧电弧情况下,电流自感磁场的洛伦兹力对流场特性有显著影响,自磁压缩是约束电弧的主要机制;而在壁稳式非转移直流电弧情况下,相对于强壁面约束和气动力作用而言,洛伦兹力对流场的影响有限。特别在中间段出口以后,洛伦兹力与气动力的比值小于0.010,因此,当主要考虑壁稳式非转移直流电弧发生器出口参数时,为了提高数值模拟效率,可忽略洛伦兹力的作用。     

Thin free arcs in alkali-seeded noble gases

Thin arcs (e.g. those observed in noble gas magnetohydrodynamic generators) are analyzed. The spatial distributions of the important plasma quantities are calculated by solving the appropriate balance equations. Solutions are obtained for cesium-seeded argon as well as for pure argon. The solutions yield characteristic radial dimensions which are in agreement with known experimental values. The effects of variations of important arc parameters are discussed in the case of cesium-seeded argon. It appears that the effects of the parameter variations can be explained to a large extent by a simplified expression for the radial dimension of the arc. Deviation from local thermodynamic equilibrium is only significant in the outer region of the arc and has practically no influence on the arc properties     

Secondary effects on solid surfaces stimulated by the cathode plasma flux in vacuum arcs

The conducting plasma region between the electrodes of a vacuum arc contains single-and multiple-charged ions of the cathode material. Investigation into the behaviour of this flux when it impinges upon a surface should take into account secondary processes. From among various processes on the surface in vacuum arc, particular attention is paid to the sputtering of the surface and to the secondary electron emission by positive ions emitted from the cathode spots. On the basis of the sputtering yield characteristicsS _r as a function of ion kinetic energyE _i and their energy distribution in the cathode flux, the total sputtering yieldsS _re for atomicaly clean surface have been determined. Experimental verification ofS _re was performed by measuring the ratio of mass deposited on the collectors to mass carried by the particle incident flux. It was found that the calculated value ofS _re is comparable to the evaluated one from the experiments. The emission of secondary electrons from clean metal surfaces due to a bombardment by multiple-charged ions is considered. The secondary electron emission _c depends on excess energyE _ex of incident ion and its average value _av for multiple-charged ions depends both on the fractional distribution of ion flux _zi and on the excess energyE _ex of each ion. It is proposed that the relationship derived is applicable to most combinations of atomic ions and pure metal surfaces.Presented at 17^th Symposium Plasma Physics and Technology, Prague, June 13–16, 1995.This work was supported by the State Committee for Scientific Research within the research project No. 3P40101507.     

Two-dimensional observation of copper vapor in vacuum arcs bylaser-induced fluorescence

The density distribution and the velocity of copper neutral atoms emitted from a single cathode spot in 40 A vacuum arc were measured by the two-dimensional (2-D) laser-induced fluorescence method. The density was calibrated from the two-dimensional fluorescence image observed by a CCD camera. The gap space was almost filled with the copper atoms, and the density reached 5×10¹⁹/m³. We varied the wavelength of the laser light and measured the velocity of the copper atoms emitted from the cathode spot using the effect of Doppler shift. The velocity of the copper atoms was about 10 km/s     

X-ray radiography of aluminum cathodes eroded in high-current vacuum arcs

The aim of the study presented in this paper was to characterize quantitatively the erosion of aluminum cathodes in high-current vacuum arcs. The experimental setup comprised two current generators. The first one, capable of generating a current of amplitude up to 350 kA, was used to produce a plasma jet, that is, the object to be investigated. The second generator was used to produce a source of probe radiation for imaging the object under investigation in soft x rays of energy ?ν ≈ 0.5–3 keV. The findings of the study are based on experimental data obtained by electrophysical and radiographic methods. It has been shown that the cathode erosion rate in a high-current vacuum arc is a function of the charge passed through the cathode.     

Cathode erosion rate in high-pressure arcs: influence of swirling gas flow

Cathode erosion in a 400-A arc burning in oxygen at 4-atm pressure was measured at different gas dynamic conditions near the cathode. The flow pattern near the cathode was changed by varying the swirl of the gas entering the discharge chamber. Experiments showed that the erosion rate depends on the swirl in a very strong way. Model calculations of velocity distribution showed that the gas velocity near the cathode increases as the swirl intensifies. According to our model, it leads to an increase of the net evaporation rate (evaporation rate minus rate of previously evaporated particles returning back to the cathode).     

Investigation of ion behavior associated with anode-spot modetransition in vacuum arcs

The ion behavior phenomenon associated with transitions of the anode discharge mode to the anode-spot mode is studied by measuring the wall ion current and by spectroscopic observation in vacuum arcs. The anode mode transfers when the wall ion current attains a certain magnitude that is independent of the cathode, but dependent on the anode. The ion-current function to the arc current increases when the arc current increases in the diffuse arc. Spectral-line intensity of Cu III emitted from the plasma in the anode region increases with an instantaneous arc current of a 5-kA peak (kAp) sinusoidal half-wave. These findings suggest an idea for the mode transition, that an ion generation region appears, and that an increase in the ion density produces a positive potential hump near the anode, which results in the negative anode voltage drop triggering the mode transition. After the mode transition, an arc current is found to reduce the ion current near the crest of a sinusoidal current in a copper arc. This appears to be significant for the arc on a small anode. The decrease in the ion current is attributed to the recombination of ions decelerated by anode vapor with electrons emitted from the hot spot on the anode     

On the ion energy distribution of high current arcs in vacuum

With the experiments presented in this paper, applications of a retarding field analyzer (RFA) for the measurement of the ion energy E _i in a vacuum arc plasma are discussed. The examined plasma was produced by a sinusoidal half-wave vacuum arc current. The experiments were concentrated on evaluating the plasma parameters at the last three milliseconds before current zero. In a current range from 300 A_rms to 10 kA_rms, the ion energy distributions and their peak values were evaluated. With the increase of the arc current, a decrease of the ion energy was found. By additional investigations of the angular distribution of the ion energies, a transition from a collision dominated interelectrode plasma to a freely expanding plasma was observed, depending on the arc current     

Investigation of the role of self-absorption of radiation in freely burning arcs in copper vapors by laser diagnostics

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 51, No. 4, pp. 571–575, October, 1989.