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

▪ Abstract Cathodic arc deposition is a plasma-based technology for the fabrication of films. The process can be carried out either at high vacuum or in a low pressure gaseous environment, and films can be formed for example of metals, ceramics, diamond-like carbon, some semiconductors and superconductors, and more. The plasma stream can be filtered to remove microdroplet contamination, and the ion energy can be controlled by substrate bias, thereby transforming the straightforward deposition method into hybrids with other surface modification processes such as ion beam–assisted deposition, ion beam mixing, and ion implantation. The method provides a versatile and powerful plasma tool for the synthesis of novel and technically important surfaces.

Cathodic arc deposition of films

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

Cathodic vacuum arc is a deposition technique which exhibits unique properties, such as high ion energy, high ionization rate and multiple ion charge states, depending on the cathodic materials. The drawback lies in the presence of macroparticles in the plasma thus preventing this technique from being widely applied. However, in the past decade, much effort had been focused on the elimination of macroparticles and the most successful approach is the use of curved magnetic filters to effectively separate the plasma from the macroparticles. The filtered cathodic vacuum arc (FCVA) refers to the combined cathodic arc with the magnetic filters. This article reports on the fundamentals related to vacuum arc, and the basic concept behind the magnetic filters and its effectiveness in the removal of macroparticles. This promising technique has been further applied to the deposition of metal oxide thin films, which is an important group of materials. The properties of various metal oxide thin films, such as titanium oxide, aluminum oxide, zinc oxide, zirconium oxide, transparent conducting oxides and other oxides, deposited by the FCVA are reviewed in this article. In addition, some applications of the metal oxide thin films will be discussed.

Review of metal oxide films deposited by filtered cathodic vacuum arc technique

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

Though the vacuum arc cathode spot has been under investigation for more than a century, its structure and operation is still discussed controversially. Experimentally measured parameters and descriptions of the visible structures given by different authors far from provide a uniform picture. In the present paper, the wide range of findings, parameter values and phenomenological descriptions are explained as different semblances of a complex structured object. The random behavior of all arc spot related parameters, mostly stated as a factor complicating their accessibility, gives the key to a better understanding of its nature. It is shown that the arc spot is mainly characterized by the statistical self-similarity of its structure. Recognizing fractal structures as one of the main features of the arc spot, its comparable appearance on different spatial scales is understandable. Scale dependent values of structure dimensions and time constants are inevitable consequences of this "fractal picture".

N. Rackwitz

Papers

Investigation of cathode spots and plasma formation of vacuum arcs by high speed microscopy and spectroscopy