Small-Scale Anode Activity in Vacuum Arcs

Coordinated high-speed movies, streak photographs, and voltage/current oscillograms have been taken for vacuum arcs on copper-based electrodes at peak currents up to 70 kA in half-cycle pulses. These results show that small-scale transient luminous anode-spot activity is associated with the strong voltage noise that precedes the establishment of the conventional large anode spots. The characteristic dimensions of the small-scale spots go below a millimeter, and may be less than 100 ?m. Unlike cathode spots of that size, these small anode spots always move in the I × B direction. This small-scale activity is especially pronounced in experimental systems initially containing surface films of volatile matter. Good correlations have been established between bursts of anode light and corresponding bursts of arc voltage noise, both of which appear to be associated with variations in the small luminous structures. The practical importance of the small transient luminous anode activity reported here is in its clear tendency to advance the formation of electrode jets, particularly under experimental conditions favoring the evolution of gas or vapor from anode surfaces. It has theoretical significance as a precursor to the formation of the usual large anode spots and jets, and as a possible source of structure within large anode spots.     

Numerical simulation of anode heat transfer of nitrogen arc utilizing two-temperature chemical non-equilibrium model

A detailed understanding of anode heat transfer is important for the optimization of arc processing technology. In this paper, a two-temperature chemical non-equilibrium model considering the collisionless space charge sheath is developed to investigate the anode heat transfer of nitrogen free-burning arc. The temperature, total heat flux and different heat flux components are analyzed in detail under different arc currents and anode materials. It is found that the arc current can affect the parameter distributions of anode region by changing plasma characteristics in arc column. As the arc current increases from 100 A to 200 A, the total anode heat flux increases, however, the maximum electron condensation heat flux decreases due to the arc expansion. The anode materials have a significant effect on the temperature and heat flux distributions in the anode region. The total heat flux on thoriated tungsten anode is lower than that on copper anode, while the maximum temperature is higher. The power transferred to thoriated tungsten anode, ranked in descending order, is heat flux from heavy-species, electron condensation heat, heat flux from electrons and ion recombination heat. However, the electron condensation heat makes the largest contribution for power transferred to copper anode.     

Anode-Spot Formation and Motion of Vacuum Arcs

This paper reports about experimental investigations on high-current vacuum-arc phenomena, especially anode-spot formation, arc states, and motion. The presented work was stimulated by lack of information about the transition process from the diffuse low-current mode to the high-current mode characterized by anode spot(s). Optoelectronic measurements, streak photographs, high-speed movies, and correlated arc voltage/current records yielded remarkable results on power-frequency vacuum arcs. Three different high-current vacuum arc modes can be observed beyond a certain threshold current. Which mode appears depends mainly on the momentary electrode distance. The modes are characterized by different anode-spot behavior and interelectrode phenomena. The transition between different arc modes is continuous. The arc modes observed on ring electrodes producing a magnetic blast field are the same as those appearing on butt-type electrodes. Anode-spot formation is preceded by congregations of cathode spots and may be initiated by thermal overload of the anode surface opposite to these cathode-spot clusters.     

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     

Distribution of Peak Temperature and Energy Flux on the Surface of the Anode in a Multi-Cathode Spot Pulsed Vacuum Arc

The distribution of the peak temperature and energy flux on the surface of a steel anode in a pulsed high-current vacuum arc was determined by studying the spatial location of the borderline separating the region of hardened steel, produced by the pulse of energy flux to the anode, and the region of the anode which did not undergo a phase transition. The arc was run between a 14-mm-diameter stainless steel cathode and a 25-mm 4340 steel anode, separated by a 4-mm gap, with peak currents up to 1000 A and 71 ms full-width half-amplitude (FWHA) duration. The phase transition of the steel occurs at 727°C and the above-mentioned borderline is thus the geometrical location of all points which reached a peak temperature of 727°C. The peak anode surface temperature was calculated from the borderline position by approximate solution of the three-dimensional heat conduction equation. The effect of an axial magnetic field on the anode surface temperature and energy flux distribution was also studied showing that with no magnetic field the distribution had a pronounced maximum on the axis of the arc, while with the presence of a magnetic field the distribution became annular with a maximum at about mid-radius. In comparison, the shape of the distribution of the cathode mass deposited by the arc on the anode was uniform without a magnetic field. The peak of the anode temperature and the energy flux amplitude also depended on the magnetic field, first decreasing and then increasing almost linearly with it.     

Anode Heat Transfer in a DC Electric ARC with Superimposed Axial Flow

A DC electric arc is operated in argon at pressures between 100 and 760 mm Hg, axial flow velocities from 0 to 100 m/sec, and currents from 50 to 200 amps to assess the influence of these parameters on anode heat transfer using a double-anode configuration consisting of two plane, parallel anodes. Since the anode arc attachment size is appreciably smaller than the anode surface area in these experiments, a method is developed using an optical and/or floating potential probes for measuring the size of the near-anode arc column. The relative variation of two perpendicular dimensions of the near-anode arc column for variation in the arc operating parameters (pressure, flow velocity, current, anode separation) are measured. The results reveal that the cross section of the nearanode column is not circular and the average local heat fluxes show a strong dependence on the gas pressure.     

Ultimate turbulent Taylor-Couette flow

The flow structure of strongly turbulent Taylor-Couette flow with Reynolds numbers up to Re(i)=2×10(6) of the inner cylinder is experimentally examined with high-speed particle image velocimetry (PIV). The wind Reynolds numbers Re(w) of the turbulent Taylor-vortex flow is found to scale as Re(w)∝Ta(1/2), exactly as predicted by Grossmann and Lohse [Phys. Fluids 23, 045108 (2011).] for the ultimate turbulence regime, in which the boundary layers are turbulent. The dimensionless angular velocity flux has an effective scaling of Nu(ω)∝Ta(0.38), also in correspondence with turbulence in the ultimate regime. The scaling of Nu(ω) is confirmed by local angular velocity flux measurements extracted from high-speed PIV measurements: though the flux shows huge fluctuations, its spatial and temporal average nicely agrees with the result from the global torque measurements.     

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     

Contact temperature and erosion in high-current diffuse vacuum arcson axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I_P, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I_P. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion     

激光聚变黑腔中等离子体的热流研究

辐射流体采用限流的局域Spitzer-Harm(S-H)电子热流近似,在预估等离子体状态时可能与实验观察存在偏差.利用一维(1D3V)含碰撞的粒子模拟程序,研究了激光聚变黑腔中金等离子体的电子分布函数和电子热流.分析表明,在等离子体的冕区,α=Z(νos/νte)^2>1,电子分布函数表现为超高斯分布(m=3.34),克努森数λe/Le=0.011大于局域S-H理论的临界值2×10^-3.这导致了局域S-H电子热流远大于实际热流.这种实际热流受限现象将导致辐射流体模拟给出的冕区电子温度高于神光实验测量值.而在等离子体的高密度区域,电子分布函数仍表现为超高斯分布(m=2.93),克努森数λe/Le=7.58×10^-4小于局域S-H理论的临界值,限流的局域S-H电子热流具有一定的适用性.但电子热流严重依赖于限流因子,辐射流体模拟需要根据不同位置的光强和电子温度调整的大小.     

Flux-limited nonequilibrium electron energy transport in warm dense gold

An abrupt change in energy transport has been observed in femtosecond laser heated gold when the absorbed laser flux exceeds ~7×10(12) W/cm(2). Below this value, the absorbed flux is carried by ballistic motion of nonthermal electrons produced in interband excitation. Above this value energy transport appears to include ballistic transport by nonthermal electrons and heat diffusion by thermalized hot electrons. The ballistic component is limited to a flux of ~7×10(12) W/cm(2). This offers a unique benchmark for comparison with theory on nonequilibrium electron transport.     

Thermal effects on laser-ablated particle velocities from thin film targets

The thermal history of the target surface on the plume dynamics was studied by high-speed streak photography. A thin aluminum layer was deposited on a thick substrate material. By choosing substrate materials with different thermal conductivity, it is possible to control the heat flux inside the target material, which in turn controls the particle ejection process. In all the experimental conditions tested, the plume was found to have two different velocity clouds. By a series of experiments with different substrate materials, their velocities were shown to depend on the substrate material. The heat conduction equation was solved to simulate the temperature history of the target materials. The result was discussed with those from streak photography and burn patterns, showing an appreciable dependence on the heat conductivity of the material.     

Numerical modelling of unsteady heating and melting of the anode by electric arc. Part 1. Mathematical model and computed characteristics of the arc column

The two-dimensional mathematical model in the approximation of a partial local thermodynamic equilibrium of plasma and the technique for numerical computation of the unsteady electric arc characteristics, including the conjugate heat exchange of the electric-arc plasma flow with the treated product (anode) are considered. The results of overall testing of the numerical algorithm point to the correctness of the model and computational technique. The results of the computation of the arc characteristics from the conventional ignition moment until the passage to stationary regime are presented. It is found that a single thermal torus, which scatters fairly quickly in the ambient medium, forms around the arc column.     

离子注入层的X射线光电子能谱分析

采用金属蒸汽真空弧（MEVVA）离子源发出的强束流脉冲钨离子,对H13钢进行了离子注入表面改性研究,借助X射线光电子能谱（XPS）考察了注入表面层中钨,氧,铁的化学状态,研究发现,钨离子注入层中钨元素以替位钨和三氧化钨形式存在,铁元素以金属铁和三氧化二铁形式出现,而且各价态元素的原子比随深度而变化。     

Enhancement and stabilization of cathodic arc using mesh anode

The performance and characteristics of a cathodic arc deposition apparatus consisting of a titanium cathode, an anode with and without a tungsten mesh, and a coil producing a focusing magnetic field between the anode and cathode arc investigated. The arc voltage V_a is measured with a fixed arc current for an anode diameter of 40 mm. The relationship between V_a and the magnetic field B with and without a mesh is obtained. In addition, the relationship between the arc current I_a and V_c, the voltage to which the artificial transmission line was charged, is measured with and without the mesh to determine the minimum ignition voltage for the arc when the anode hole diameter is 40 mm. The arc resistance increases with the focusing magnetic strength B and decreases when using the mesh. Our results indicate that the high transparency and large area of the mesh allows a high plasma flux to penetrate the anode from the cathodic arc. The mesh also stabilizes the cathodic arc and gives better performance when used in concert with a focusing magnetic field     

开式单喷嘴喷雾冷却均匀性实验研究

针对高热流密度激光介质高效散热与均匀冷却技术需求,设计并搭建了以去离子水为冷却工质的开式单喷嘴喷雾冷却实验平台,实验研究获得了不同热流密度（16～110 W/cm2）、不同冷却工质流量（200～300 mL/min）以及不同喷雾高度（15～25 mm）下单相喷雾冷却换热系数及其冷却均匀性效果。结果表明:该实验工况下,不同热流密度条件下喷雾高度及工质流量对于单相喷雾冷却换热效率及温度均匀性影响显著;喷雾高度15 mm、工质流量200 mL/min时获得最大对流换热系数为5.93 W/（cm2·K）;喷雾高度15 mm、工质流量250 mL/min时面积20 mm×20 mm的热源表面温度均匀性最佳可优于0.6 ℃。     

Vacuum Arc Anode Phenomena

This paper briefly reviews anode phenomena in vacuum arcs, specially experimental work. It discusses, in succession, arc modes at the anode, anode temperature measurements, anode ions, transitions of the arc into various modes (principally the anode spot mode), and theoretical explanations of anode phenomena. The two most common anode modes in a vacuum arc are a low current mode where the anode is basically passive, acting only as a collector of particles emitted from the cathode, and a high current mode with a fully developed anode spot. Characteristically this anode spot has a temperature near the atmospheric boiling point of the anode material and is a copious source of vapor and energetic ions. However, other anode modes can exist. A low current vacuum arc with electrodes of readily sputterable material may emit a flux of sputtered atoms from the anode. Usually this sputtered flux will have little effect upon the vacuum arc, but in certain circumstances it could be significant. A vacuum arc doesn't always transfer directly from a low current mode to the anode spot mode. In appropriate experimental conditions, formation of an anode spot may be preceded by the formation of an anode footpoint. This footpoint is luminous, but much cooler than a true anode spot. Finally, (again in appropriate circumstances) several small anode spots may form instead of one large anode spot. With sufficient increase in arc current or arcing time these will usually combine to form a single large active spot.     

Bouncing Expansion of the Arc-Cathode Plasma in Vacuum along the Transverse Applied B Field

Measurements of the angular flux distribution of the cathodic arc plasma (I ? 100 A) subjected to a transverse magnetic field (B ? 8.5 × 10-2 T) ae presented. The angular distribution without magnetic field approximately follows the cosine law. Expansion with transverse field is found to deviate strongly from the cosine law, the plasma being confined close to the cathode plane and expanding along the magnetic-field lines. Furthermore, time-resolved photographs of the expansion reveal the predicted pulsating behavior of the expanding plasma.     

Behavior of a High Current Vacuum Arc between Hollow Cylindrical Electrodes in a Radial Magnetic Field

Experimental observations were conducted on the behavior of a high current vacuum arc on cylindrical electrodes in a radial magnetic field. The arc was sustained between the ends of two cylindrical Cu electrodes, 54-mm diam and 1.5-mm wall thickness separated by 5 mm. Arc current pulses with peak values in the range 4-15 kA with a half amplitude full width (HAFW) duration of 8 ms were investigated with radial magnetic fields proportional to the instantaneous current with proportionality constants of 4.0 and 6.5 × 10-6 T/A. The arcs were photographed simultaneously with a streak camera and by a high speed framing camera and the arc voltage was recorded on a digitizing transient recorder. The results indicated that the arc in this geometry, both with and without an imposed radial magnetic field, can be characterized by three development stages: a) arc formation, b) diffuse arc along the electrode perimeter, and c) simultaneous existence of several concentrated arc columns. When a radial magnetic field was imposed two changes were noted: 1) the arc appeared somewhat more distributed in that a greater number of constricted columns were observed, and they were distributed more evenly; and 2) the constricted columns moved in the J? × B? direction with velocities in the range 5-35 m/s.     

全固态碟片激光器的多孔泡沫热沉传热特性研究

高功率全固态碟片激光器在运行中产生的热透镜效应会引起激光器输出功率降低、光束质量退变,针对该问题本文将多孔碳化硅泡沫和毫米通道引入到全固态碟片激光器的换热热沉中,并将其应用于多冲程泵浦的全固态碟片激光器.利用有限元分析软件对其结构模型参数进行了优化,当碳化硅厚度为2 mm,孔隙率为40%,入水口压力为4 kg(0.4 MPa)时,系统理论换热系数为1.51×10^5 W/m^2·K,实验测量结果为1.45×10^5 W/m^2·K,理论和实验结果较为接近,验证了理论模型的正确性.最后利用该新型热沉搭建了基于Yb∶YAG的24冲程全固态碟片激光器实验装置,获得输出功率为393 W,波长为1030 nm的连续激光输出,光-光转换效率达到52%,光束参数乘积为5.918 mm·mrad.