Motion of high-current vacuum arcs on spiral-type contacts

Motion of vacuum arcs on spiral-type contacts is not only controlled by self-induced magnetic fields, but also by heating phenomena. An expression is derived which permits the calculation of the speed of the arc from a computation of the time needed to heat the surface up to boiling temperature. The heat flux density of the constricted arc at the anode is required as input for the calculation. Good coincidence is achieved with experimental data. The speed of the arc varies from 5 to 400 m/s, depending on experimental conditions     

Numerical model of the anode region of high-current electric arcs

A two-dimensional, two-temperature axisymmetric numerical model has been formulated for the flow-affected region and the boundary layer in front of high-intensity electric arc anodes. The plasma flow is laminar, steady, incompressible, and the plasma composition is found from the diffusion equation because chemical nonequilibrium is expected. Computational results are obtained for an atmospheric pressure argon arc considering two different situations: a free-burning electric arc and an arc with a constrictor tube. The solutions indicate two different anode attachments modes-a constricted and a diffuse attachment. It is found that under the conditions considered in the calculations, the gradient-induced current densities become significant at distances in the order of 1 mm from the anode surface. The thermal anode boundary layer is compressed with increasing current. The thickness of the thermal boundary layer for the constricted mode is approximately three times smaller than for the diffuse mode. A reversal of the electric field strength occurs over the entire thickness of the boundary layer in all calculated cases. A satisfactory agreement is reached between the calculated heat flux values and experimental results obtained for a 200-A free-burning electric arc     

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.     

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 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     

Electrical and pyrometric measurements of the decay of the anodetemperature after interruption of high-current vacuum arcs andcomparison with computations

Vacuum arcs of up to 20-kA peak current were investigated. The surface temperature of the anode area melted during the anode spot mode was determined by pyrometry and the evaluation of thermionic currents. The measurements confirm the computations of heating and cooling of the anode, taking into account heat conduction melting/solidification, and evaporation. Pyrometrically obtained temperatures agree well with theory. This gives confidence in the heat conduction model and also shows that the boiling temperature was reached during arcing. Another method evaluates currents of a milliampere value after arcs of several kiloamperes and postarc currents of amperes. Experimental observation (e.g., loading of the shield surrounding the contacts) and theoretical analysis of the interfering effects support the idea that the currents measured are due to thermionic emission     

Macroparticle filtering of high-current vacuum arc plasmas

The transport of vacuum arc plasmas through a 90° curved magnetic macroparticle filter was investigated using a high-current pulsed arc source with a carbon cathode. The peak arc current was in the kiloampere range, exceeding considerably the level of what has been reported in the literature. The main question investigated was whether magnetic macroparticle filters could be scaled up while maintaining the transport efficiency of small filters. In front of the cathode, we found that arc current dependent total ion saturation currents were in the range from 10% to 23% of the arc current. The best relative transmission was 25% (time integrated output/time integrated input) at a duct wall bias of 12.5 V and at an axial magnetic field of about 100 mT. The measured relative transmission of the used high-current arrangement is comparable to what has been observed with other low-current filters. The absolute measurable ion saturation currents at the filter exit reached 70 A at an arc current of about 1000 A     

毛细管放电X光激光装置中的预脉冲电源

针对目前毛细管放电X 光激光装置产生的预脉冲电流幅值过大、持续时间较短的问题，提出了增加预脉冲开关抑制原有预脉冲，再外加由脉冲成形网络组成的预脉冲发生器，产生所需预脉冲的改造方案。可在主脉冲来临之前产生幅度10~50A，持续时间约17μs的方波预脉冲电流，来满足毛细管放电泵浦类氖氩X光激光实验的需要。     

Experimental observation of high-voltage, low-current vacuum arcs

A poorly explored type of discharge has been investigated in high vacuum (10^-7 to 10^-6 torr), with a DC high voltage across 0.2- to 0.8-mm gaps. The discharge has been found to be quite different from other widely known types of vacuum and gas discharges by the combination of its voltage-current characteristics (hyperbola-type), source and carriers of current (mostly electrons), and spatial potential distribution (a considerable electric field across the gap and a steep potential fall near the cathode)     

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 observations of steady anodic vacuum arcs withthermionic cathodes

Stationary plasma discharges have been investigated in a high vacuum ambient (background gas pressure <10^-2 Pa), with an externally heated cathode and a consumable hot evaporating anode. With various anode materials like chromium or copper, and electrode separations between 0.5 and 3 mm, the nonself-sustained discharge operates with DC arc currents in the range of 220 A. The waveform of the arc voltage is strongly influenced by the magnetic field of the cathode heating current, and arc voltages between a minimum of 3 V and a maximum exceeding 100 V have been observed. The voltage-current characteristics (V_CC) and the influence of the electrode separation have been measured separately for the minimum and the maximum of the arc voltages and show a different behavior. The metal plasma expands into the ambient vacuum toward the walls of the vacuum vessel and offers a macroparticle free deposition source of thin films. The arc voltage can be varied by external manipulations of the arc discharge, and the mean ion energy of the expanding metal plasma shows a linear dependence of the mean arc voltage     

强流二极管阳极靶温度和热形变模拟

以电子束在靶中的能量沉积剖面为桥梁,建立了二极管阳极靶温度和热形变模拟方法。该方法可获知二极管不同工作状态下靶的温度分布和热形变情况,为靶热-力学损伤研究提供基础数据,为二极管构型设计和寿命提升提供技术支撑。将该方法应用于“强光一号”短γ二极管,计算结果显示:当阳极离子密度大于1014 cm?3时（强箍缩）,靶表面温度最高可达5500～6000 ℃,热形变量达约4.5 mm;无离子流时（弱箍缩）,温度处在4500 ℃左右,形变为2.8～3.5 mm。     

Investigation of arc roots of constricted high current vacuum arcs

The anodic and cathodic arc roots of constricted high current vacuum arcs were investigated with a fast framing charge-coupled device camera of 1 μs exposure time. The experiments were performed with cup-shaped contacts, with sinusoidal currents of amplitudes between 20 and 100 kA, and a sine halfwave duration of 10-12 ms. The arcs were drawn by contact separation and accelerated by the Lorentz force between the arc current and the transverse magnetic field generated by the contrate contact. The anode and cathode arc roots behave reproducibility and arc scaleable within the range of currents investigated. Both types of arc roots are elliptical, with a major to minor axis ratio of 1.4. The major axis points are in the direction of arc propagation. Anodic and cathodic arc root cross-sectional areas as a function of current can both be described by a potential law with a common exponent of 0.76. For currents of 20-100 kA, mean current densities of 81-121 and 41-60 kA/cm ² were found in anode and cathode arc roots, respectively. Estimations of their temperature and vapor densities were performed. For the investigated current range T_A≈3300-3600 K, n_A ≈1.6*10¹⁹-2.2*10¹⁹cm^-3 and T _C≈3200-3400 K, n_C≈0.8*10¹⁹-1.2*10 ¹⁹ cm^-3 were found for anode and cathode, respectively     

Application of vacuum arc cathode spot model to graphite cathode

A model of near-electrode processes is applied here to describe the behavior of cathode spots on graphite cathode in vacuum arc. The physical model is based on a kinetic treatment of cathode evaporation, electron emission from the cathode, and plasma production. The model consists of physical assumptions and a system of equations that are formulated in the paper. Spot parameters, such as cathode erosion rate, cathode potential drop, cathode surface temperature, current density, electric field, and plasma density, temperature, and velocity in the near-electrode region are calculated numerically. The calculation includes the dependence of spot parameters on spot current and spot lifetime. The variation of spot parameters as a function of spot lifetime are very strong at lifetimes shorter than 10 μs. The calculations indicate that Joule heating in the cathode body is significant, and may exceed cathode heating by the ion heat flux. Calculated spot parameters are compared with the corresponding experimental data for relatively low arc currents (<100 A) and their agreement is discussed     

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     

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