A review is given on arc cathode spots, mainly based on the investigation of arcs in a vacuum with cold cathodes. For the latter and after a short description of general features and theoretical concepts, experiments are presented that study the temporal and spatial behaviour of the spots with high time and space resolution of less than 10 ns and less than 5 µm, respectively. With the help of these observations the various spot types described in the literature are ordered into three levels: level A corresponding to the proper spot with typical diameters of 50-100 µm, level B associated with spot fragments having a size of 10-20 µm and level C comprising a substructure of the fragments. The structures undergo periodic fluctuations of brightness and position with characteristic times that can be arranged in a hierarchy from a few nanoseconds through about 100 µs. The analysis of these fluctuations shows that the spot operates in cycles that include both extremely non-stationary periods with time constants of less than 10 ns and more stationary periods in the microsecond range. In the presence of an external magnetic field, the latter periods lead to unstable plasma configurations that give rise to retrograde motion. Finally, for vacuum arc spots the basic parameters are summarized. After that, the peculiarities of spots in gases with cold electrodes are discussed, followed by a presentation of spots with hot cathodes at high pressures.

https://iopscience.iop.org/article/10.1088/0022-3727/34/17/202/pdf

Topical Review: Cathode Spots of Electric Arcs

A review of vacuum arc phenomena including the cathode and anode processes with cold and hot electrodes as well as the processes in the interelectrode plasma is presented In the case of cold electrodes, the current continuity mechanism, the nature of mass loss, spot motion, plasma jet generation in the spots, and the cathode potential drop are reviewed The explosive and gas-dynamical models of cathode spot operation are described In the case of hot electrodes, the diffuse current continuity mechanism is analyzed and a model for electron-current fraction calculation in discharge with cathode-anomalous electron emission is proposed The present state of electrical and plasma characteristics of high-current arcs in magnetic fields are discussed

State of the theory of vacuum arcs

In this paper, we review the state of the art in studying the physical processes that occur in the cathode spots of vacuum arcs. The now available experimental data are interpreted in the context of the ecton mechanism of the operation of vacuum arc cathode spots. Central in this mechanism is the explosive electron emission, a phenomenon discovered by the author and his co-workers in the mid-1960s while studying high-voltage pulsed vacuum breakdown. In the light of the ecton mechanism, the cathode spot of a vacuum arc consists of individual cells which are explosive emission sites each emitting a portion of electrons termed an ecton. The cathode spot processes are cyclic in nature due to the finiteness of the ecton lifetime. It is shown that an arc is self-sustained due to the explosive emission processes initiated on the interaction of the cathode plasma either with nonmetal inclusions present in the cathode surface (first-type spots) or with liquid metal jets ejected from the zone of an active cathode spot (second-type spots). Attention is focused on the physical processes occurring during the operation of a cathode spot cell. A statistical model of a vacuum arc is used to interpret the effect of the spontaneous extinction of an arc. It is shown that an increase in the arc current is accompanied by a slight increase in the number of simultaneously operating ectons; therefore, as observed in the experiments, the parameters of a vacuum do not greatly depend on the current up to the kiloampere level.

Ecton Mechanism of the Cathode Spot Phenomena in a Vacuum Arc

The review relates to low-pressure discharges with a hollow cathode and a hollow anode. The term ‘low pressure’ implies that a neutral particle density is extremely small and the electron free path for ionization is in excess of the characteristic size for the discharge gap. On the other hand, the neutral particles still play an essential role in forming the gas-discharge plasma. Thus, the discharge regimes keep an intermediate position between the classical low-pressure discharge with avalanche ionization and pure vacuum discharge that burns in the cathode metal vapor. New approaches to the interpretation of the discharge formation mechanism at the stage of delay time to breakdown, and for the discharge sustaining features at the later temporal stages, are discussed. The developed approaches offer the possibility of interpreting the discharge behavior in the systems to obtain a uniform plasma in a large volume of the cathode or anode cavity, in the sources of electron and ion beams based on the plasma cathode and in the high-current pseudospark switches.

https://iopscience.iop.org/article/10.1088/1361-6463/aacf10/pdf

Low-pressure discharges with hollow cathode and hollow anode and their applications

Cathode spot phenomena show many features of fractals, for example self-similar patterns in the emitted light and arc erosion traces. Although there have been hints on the fractal nature of cathode spots in the literature, the fractal approach to spot interpretation is underutilized. In this work, a brief review of spot properties is given, touching the differences between spot type 1 (on cathodes surfaces with dielectric layers) and spot type 2 (on metallic, clean surfaces), as well as the known spot fragment or cell structure. The basic properties of self-similarity, power laws, random colored noise, and fractals are introduced. Several points of evidence for the fractal nature of spots are provided. Specifically, power laws are identified as signature of fractal properties, such as spectral power of noisy arc parameters (ion current, arc voltage, etc.) obtained by fast Fourier transform. It is shown that fractal properties can be observed down to the cutoff by measurement resolution or occurrence of elementary steps in physical processes. Random walk models of cathode spot motion are well established: they go asymptotically to Brownian motion for infinitesimal step width. The power spectrum of the arc voltage noise falls as 1/f/sup 2/, where f is frequency, supporting a fractal spot model associated with Brownian motion.

/pdf/the-fractal-nature-of-vacuum-arc-cathode-spots-79c4c3p5f4.pdf

The fractal nature of vacuum arc cathode spots

The erosive structures formed on a tungsten cathode as a result of the motion of the cathode spot of a vacuum arc over the cathode surface have been examined. It has been found that the average mass of a cathode microprotrusion having the shape of a solidified jet is approximately equal to the mass of ions removed from the cathode within the lifetime of a cathode spot cell carrying a current of several amperes. The time of formation of a new liquid-metal jet under the action of the reactive force of the plasma ejected by the cathode spot is about 10 ns, which is comparable to the lifetime of a cell. The growth rate of a liquid-metal jet is ∼104 cm/s. The geometric shape and size of a solidified jet are such that a new explosive emission center (spot cell) can be initiated within several nanoseconds during the interaction of the jet with the dense cathode plasma. This is the underlying mechanism of the self-sustained operation of a vacuum arc.

http://ecton.ru/system/resources/W1siZiIsIjIwMTQvMDUvMDkvMDBfNDhfMDVfNzMwX2FwcGwucGh5cy5sZXR0XzE0LnBkZiJdXQ/appl.phys.lett-14.pdf

On the mechanism of operation of a cathode spot cell in a vacuum arc

The operation of the cathode spot of a high-current vacuum arc is treated in terms of the ecton model. It has been demonstrated that, in this case, the cathode spot cells are grouped to adjoin one another. The existence of such,a collective spot is due to the cumulative effect of a number of microexplosions, resulting in that the current density in the spot increases by-almost an order of magnitude. Moreover, in such a spot, conditions are realized which are energetically profitable for the repetition of ecton processes, namely, an elevated temperature of the surface and an increased density of the near-cathode plasma. The parameters of a collective cathode spot, such as the crater diameter, the current density, and the lifetime, have been related to the arc current. The predicted relationships are in good agreement with experimental results. It has been shown that as the arc current increases, the current density in the cathode spot drops, resulting in division of the spot. An estimate of the limiting arc current per unit spot has been obtained.

The cathode spot of a high-current vacuum arc as a multiecton phenomenon

The interaction of plasma generated by a vacuum arc with the anode surface was numerically simulated by using a particle-in-cell (PIC) method. It has been found that the anode potential fall remains negative even at high electron drift velocities. The plasma–anode-surface interaction has been analyzed for a 'steady-state' regime and for a 'turbulent' regime with microinstabilities developing in the plasma. For the steady-state regime, approximate formulas for the anode potential fall and energy flux have been derived from the simulation results. For the turbulent regime, naturally associated with high electron drift velocities, the negative anode fall voltage and the effective plasma electron temperature have been found to be larger than that for the steady-state regime.

http://iopscience.iop.org/article/10.1088/0963-0252/23/6/062004/pdf

On the anode potential fall in a vacuum arc: PIC simulation

The paper reports on a study of the mass-charge state of the plasma produced in a vacuum arc discharge with composite cathodes which were copper-disk coated with a hydrogenated Zr film of thicknesses 9, 22, and 35 μm. The cathodes allow the generation of multicomponent gas and metal ion beams with a hydrogen ion content from several to several tens of percent. Also investigated is the dependence of the H ion fraction in a beam on the Zr film thickness during erosion to the point of disappearance of Zr peaks in mass-charge spectra. The ability of the vacuum arc system to produce H ions is analyzed by analyzing the cathode lifetime as a function of the film thickness and pulse repetition frequency.

Lifetime of hydrogenated composite cathodes in a vacuum arc ion source

The generation and disruption of the picosecond runaway electron beam in atmospheric pressure strongly overvolted gas gap is considered with emphasis on the runaway kinetics, the increase in emission current and plasma density, and beam instabilities. It has been shown that a few-nanosecond ten-kV prepulse gives rise to a streamer. Application of the main pulse (∼2 MV/ns) results in the runaway electron beam generation with the streamer electrons involved in the acceleration, and in increase of the electron emission from the cathode and the plasma density. At the high enough plasma density, fast beam instability disrupts the runaway electron beam.

On the generation and disruption of a picosecond runaway electron beam during the breakdown of an atmospheric-pressure gas gap

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