Microscopic high speed investigations of vacuum arc cathode spots

The main parameters and dimensions of cathode spots have been under discussion for years. To solve these current questions, a new system was specially designed. The image converting high speed framing camera (HSFC) combines a micrometer lateral resolution with a nanosecond time resolution and a very high optical sensitivity. This camera was used to study the microscopic behavior of vacuum arc cathode spots in a pulsed high current arc discharge on copper. The direct observation of these spots with high resolution revealed that one single cathode spot, as normally observed by optical means, consists of a number of simultaneously existing microscopic subspots, each with a diameter of about 10 μm and a mean distance of 30-50 μm between them. The mean existence time of these subspots on copper was found to be about 3 μs, where the position of a subspot remains unchanged (with an upper limit of about 5 μm) during its existence time. The lower limit of the current density in the cathode spots was estimated to be on the order of 10¹⁰-10¹¹ A/m². An upper limit of the crater surface temperature was estimated by a comparison between the brightnesses of a cathode spot and of a black body radiation lamp to about 3000 K     

Pulsed dye laser diagnostics of vacuum arc cathode spots

The ignition and arc phases of vacuum arcs were investigated using differential dye laser absorption photography with simultaneous high spatial (micrometer) and temporal (nanosecond) resolution. The discharge duration was 800 ns, the current 50-150 A, the electrode material copper, and the cathode-anode distance less than 50 μm. A 0.4 ns laser pulse (tunable, γ=480-530 nm) was used to obtain momentary absorption photographs of the cathode region. During ignition, an optically thick anode plasma expanded toward the cathode, decaying within 25 ns after bridging the electrode gap. In the arc phase, a fragmentary structure of the cathode spots was observed in situ for the first time. The microspots have a characteristic size of 5-10 μm. They appear and disappear on a nanosecond time scale. The plasma density of the microspots was estimated to be greater than (3-6)×10²⁶ m^-3     

Nanosecond displacement times of arc cathode spots in vacuum

With a high speed camera consisting of a combination of framing and streak channels, arc spots on a copper cathode are imaged in the spectral range 200-800 nm with spatial and time resolution of <5 μm and ⩽10 ns, respectively. At currents of 30-70 A and sufficiently long time after ignition (3-300 μs), the spots consist of fragments with diameters of 10-20 μm. These fragments appear and disappear in a cyclic way. Formation times <50 ns and residence times <100 ns have been observed. Apparent fragment merging into one spot is due to the extinction of all of them except one, while apparent spot splitting is due to the formation of a new fragment outside the spot center. Consecutive fragment formation appears as displacement with momentary velocities up to 1000 m/s. The fragment dynamics leads to random displacement of the spot center with a ratio of mean square displacement 〈R²〉 to the observation time t of 〈R²〉/t=(2.3±0.6)×10^-3m ²/s. This holds down to t=100 ns. Thus, fragments and spots operate on nanosecond time scales. Prior to apparent spot splitting and after apparent fragment merging the spot brightness increases considerably. When analyzing time-integrated pictures, the stages of increased brightness lead to overestimating the average residence time. Because of the short formation time, the fragments do not reach a balance between surface heating and heat conduction into the bulk, i.e., there is no stationary evaporation. A further substructure of the fragments exists with size <5 μm and timescale ⩽10 ns     

On the macroparticle flux from vacuum arc cathode spots

The macroparticle contamination of vacuum-arc-deposited thin films generated by a plasma source with an optional axial magnetic field is studied. Emphasis is placed on the macroparticle flux near the discharge axis. The arc current, metal species, deposition system geometry and axial magnetic field strength are varied. Distribution functions for macroparticles of Pb, Ag, Cu, Pt, W, and Ni are determined, normalized to the film thickness deposited or the charge transferred. The application of the axial magnetic field leads to a considerable reduction of the normalized macroparticle flux since the plasma is effectively focused by the field, whereas the macroparticle production is not influenced. The macroparticle content normalized to the deposited film thickness is reduced to about 20-35% of that without an additional magnetic field     

Why vacuum arc cathode spots can appear larger than they are

The visual appearance of arc cathode spots in vacuum is studied experimentally and theoretically. Emission photographs of the spots taken with line radiation have a broad light profile with a rather flat slope (proportional to r^-β with β≈2, r being the distance from the spot center), while photographs taken in absorption are small, having a sharp edge with a steep slope of the profile (β⩾4). Emission photographs from the continuum are similar to absorption photographs. Theoretical analysis shows that the emitted line radiation cannot stem from the locus of excitation. As a consequence, the particles are excited at the edge of the dense spot core, but they radiate at a greater distance due to the finite lifetime of the excited levels and the plasma expansion. Thus, emission photographs from line radiation indicate a greater spot size (about 100 μm) than corresponding to the active spot radius which amounts to ⩽10 μm. This statement holds for discharge durations from 10 ns up to at least 100 μs. The spots exist not only at ignition but during the whole time of the discharge, the location varying due to the spot movement. Absorption photographs show a small size of 10-20 μm still 200 μs after ignition     

Influence of microstructure of CuCr25 cathode on the motion of vacuum arc spots

The motion of vacuum arc spots on nanocrystalline and coarse-grained CuCr25 alloys were observed by a digital high speed video camera and SEM. Experimental results show that without an external magnetic field a spot can move a long distance in a direction and leave a long straight arc trace (50–100 μm) on the surface of nanocrystalline CuCr25 cathode. This kind of spot motion can be defined as sub-directional motion. The spot motion is totally random and restricted on coarse Cr particles for coarse-grained CuCr25 cathode. Arc spots move sub-directionally and more easily on nanocrystalline cathode maybe results from an active surface formed by the special electronic structure due to Cu–Cr internal electric field.     

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     

Ecton mechanism of the vacuum arc cathode spot

A new mechanism for the operation of a cathode spot in a vacuum arc, based on ecton processes, is proposed. An ecton is formed by the explosion of the tip of a jet of molten metal as it interacts with plasma. The time of ecton operation is assumed to be limited by the thermal conductivity of the liquid metal. For copper electrodes, the theoretical expressions are derived for the specific mass removal, ion erosion characteristics, current density, and the diameters of craters. The results agree well with the experimental data available     

Time dependence of vacuum arc parameters

Time-resolved investigations of the expanded plasma of vacuum arc cathode spots are described, including the study of the ion charge state distribution, the random cathode spot motion, and the crater formation. It was found that the ion charge state distribution changes over a timescale on the order of hundreds of microseconds. For the random spot motion two timescales were observed: a very short spot residence time of tens of nanoseconds which gives, combined with the step width, the diffusion parameter of the random motion, and a longer timescale on the order of 100 μs during which the diffusion parameter changes. Crater formation studies by scanning electron microscopy indicate the occurrence of larger craters at the end of crater chains. The existence of a timescale much longer than the elementary times for crater formation and spot residence can be explained by local heat accumulation     

Effect of the cathode spots of a vacuum arc on the properties of the surface layer of structural materials

The application of vacuum-arc discharge provides great opportunities for metallic-surface cleaning and the development of products with the required functional surface properties. The effect of vacuumarc treatment on the roughness of the surface, its mechanical properties, and the structure and composition of the surface layer is studied. The processing and cleaning of the surface of steel samples by cathode spots of a vacuum-arc discharge form a structurally changed layer with a thickness of up to 10 μm and a microhardness exceeding the initial one by 1.4–1.5 times. The deposition of 20% solution of KOH and NaOH alkalis (substances which reduce the electronic work function of the scale surface) on the processed steel surface reduces the specific power inputs of vacuum-arc cleaning by a factor of 1.5–2.4. The depletion of the surface layer of low-grade steels by impurity elements enhances its anticorrosion properties.     

Theory of the cathode sheath in a vacuum arc

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 T_cj plane, where T_c 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     

Ion flux from the cathode region of a vacuum arc

The properties of the ion flux generated in a vacuum arc are reviewed. The structure and distribution of mass erosion from individual cathode spots and the characteristics of current carriers from the cathode region at moderate arc currents are described. An appreciable ion flux (~10% of the total arc current) is emitted from the cathode of a vacuum arc. This ion flux is strongly peaked in the direction of the anode, although some ion flux may be seen even at angles below the plane of the cathode surface. The observed spatial distribution of the ion flux is expressed quite well as an exponential function of the solid angle. The ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply charged ions. The average ion potential and ion multiplicity increase significantly for cathode materials with higher arc voltages but decrease with increasing arc current for a particular material. The main theories concerning ion acceleration in cathode spots are the potential hump theory and the gas dynamic theory. Experimental data indicate that these theories serve reasonably well when used to predict the mean values of the charge state, ion potential, and ion energies for the ion flux, but are quite insufficient when compared with the results for the potentials and energies of individual ions     

Parameters of plasma layer within the vacuum arc cathode spot

The paper gives a formulation of the closed loop model developed on the basis of the integral balances method to determine parameters of the plasma layer and boundary conditions at the cathode surface for the processes of energy, mass, and current transportation. Presented are the results of the self-consistent calculations for the copper cathode     

Gyrocenter shift of low-temperature plasmas and the retrograde motion of cathode spots in arc discharges

The gyrocenter shift phenomenon explained the mechanism of radial electric field formation at the high confinement mode transition in fusion devices. This Letter reports that the theory of gyrocenter shift is also applicable to low temperature high collisional plasmas such as arc discharges by the generalization of the theory resulting from a short mean free path compared with the gyroradius. The retrograde motion of cathode spots in the arc discharge is investigated through a model with the expanded formula of gyrocenter shift. It is found that a reversed electric field is formed in front of the cathode spots when they are under a magnetic field, and this reversed electric field generates a rotation of cathode spots opposite to the Amperian direction. The ion drift velocity profiles calculated from the model are in agreement with the experimental results as functions of magnetic flux density and gas pressure.     

Influence of a current jump on vacuum arc parameters

Based on time of flight method, influence of short time vacuum arc current jump on arc plasma parameters were investigated. Superposition of the current pulse of a vacuum arc with a high operating voltage results in the appearance of ions of higher charge state in the discharge plasma and in an increase in the mean ion charge state for most of the cathode materials used in the experiment. The method of a “short-time current jump” can be also used to investigate the parameters of a vacuum arc, in particular to estimate the ion direct velocities in vacuum arc plasmas. Our estimates show that in the presence of a current step the ion velocities are almost identical for all differently charged ions and depend only on the peak current and the ion mass     

Thermionic cathode parameters versus pressure in a sustained argon arc

Experimental data on the influence of the argon pressure on the performance of a rodlike thoriated-tungsten cathode in a sustained arc discharge are reported for the first time. The integral parameters of the cathode, namely, the electron work function, temperature, current density, and specific surface area of erosion at currents of 20–150 A in the pressure range 2.66 × 10³?1.00 × 10⁵ Pa, are measured. Taken together, these parameters provide comprehensive information about the operating conditions of the activated cathode. A significant influence of the pressure on the specific erosion of the cathode is noted.     

Interaction of plasma jets and droplets in the cathode region of a vacuum arc

The interaction of liquid-metal droplets with plasma jets in the cathode region of a vacuum arc is considered in the context of an ecton model. It is shown that heating of a droplet in the cathode spot region can initiate the droplet transition to the plasma state.     

Structure of group cathode spots on the surfaces of hot-rolled steels

The results of investigations of cathode spots on steel surfaces are presented. Their behavior is found to have some distinctive features in comparison with the results obtained on clean metals. Zh. Tekh. Fiz. 68, 57–62 (November 1998)