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Thin films were deposited using a vacuum arc with a refractory anode. The arc was sustained between a water cooled Zr cathode and a graphite anode. Both electrodes with diameter 32 mm and 30 mm length, with a gap of 10 mm between them. The distance from the arc axis to the substrate was 110 mm. The arc was operated either 40 or 60 s before a glass substrate was exposed to the plasma for 15 s. The film thickness was measured by profilometry. The deposition rate was obtained from the film thickness and exposition time. The visual radiation emitted by the plasma plume was photographed with a digital camera. The highest measured deposition rate was 0.71 μm/min, for an arc current of 225 A and time before exposure of 60 s.

A physical model and a mathematical approach were developed to describe ponderomotive acceleration a body pushed by plasma column of high-current arc in a railgun. In contrast to the previously published studies, the arc plasma generation was considered by taking into account kinetics of heavy particle fluxes in a non-equilibrium layer near the vaporizating (ablating) rails and the phenomena of heat and mass transfer and electric sheath at rail surfaces. The self-consistent numerical analysis has been conducted for a wide range of arc currents (up to 2MA). It has been shown that the electron temperature was determined mainly by the Joule energy dissipation supporting high degree of ionization. It has been determined that the arc voltage is about 102 V. The rails temperatures are relatively low (~ 3000 K) and therefore, the electron flux emitted from the cathode manly due to thermionic emission enhanced by Schottky effect is lower than the flux of ions from the plasma, which supported the current continuity at the cathode region. The calculated velocity vs of the plasma-solid body increased with the arc current (I) showing tendency to saturation. In range of I = 0.5–2 MA the vs slightly increased from 5 × 105 to 10 × 105 cm/s when the rail length increased from 1 to 3 m respectively. The dependence of the velocity on the arc current and the calculated plasma parameters are in range of those values observed experimentally. The nature of the velocity with arc current saturation is explained by presence of the dynamic pressure resulting in predicted velocity dependence on the arc current as 4-th root.

A physical model of spot operation on film cathodes was developed and a system of equations was derived to understand Kesaev’s measurements of spot parameters on Cu films of 0.017 to 0.14 μm thickness deposited on glass substrates. Kinetics model of cathode vaporization was used to describe the spot phenomena. The plasma velocity normalized by the plasma sound speed is determined by the rate of film evaporation, dependent on the spot action. The power losses due to film heat conduction was calculated comparable with that power losses by vaporization. The calculations and measurements were comparable, in particular, for low cathode potential drop (~ 12 V).

The hot refractory anode vacuum arc (HRAVA) plasma source was previously developed with a consumed cathode and a refractory anode to reduce the macroparticle (MP) contamination in vacuum arc deposited films. The HRAVA had open cylindrical electrodes and demonstrated that it not only reduced MP sizes and numbers but also converted MP material to plasma, and thus increased the deposition rate. This paper presents a new HRAVA configuration with a double arc between a common water-cooled Cu cathode with two active surfaces and two refractory graphite or tungsten anodes, with the aim of depositing film over a wider area. The arc current in each of the two arcs was 125 A for graphite and 150 A for tungsten. The radially expanding plasma plumes from each of the cathode–anode gaps merged, forming a wider common plasma. Cu films were deposited and over a 150 mm length parallel to the arc axis, while the corresponding length from a single-HRAVA source was only 75 mm.

Plasma is an ionized gas that is typically formed under high-temperature laboratory conditions. Recent progress in atmospheric plasmas has led to cold atmospheric plasma (CAP) devices with ion temperatures close to room temperature. The unique chemical and physical properties of CAP have led to its use in various biomedical applications including cancer therapy. CAP exhibits a spontaneous transition from a spatially homogeneous state to a modifiable pattern that is subject to self-organization. In this Opinion article, we discuss some new applications for plasma in cancer therapy based on plasma self-organization, which enables adaptive features in plasma-based therapeutic systems.

The anode in a vacuum arc with a blackbody assembly was measured as a function of time. A 30-mm-diameter Cu cathode and a hollow Ta anode, 50 mm in diameter, were used for a plasma cavity. Cu plasma was ejected through an array of 250 holes of 0.6 mm diameter in the anode. The arc currents were $I = 175$ , 200, and 225 A and the arc duration was 160 s. The anode temperature was measured using high-temperature thermocouples in the top and side of the anode. The anode temperature increased sharply during a transient time of ~70–90 s and then, it slightly increased with time up to 160 s reaching at 1650 K ( $I = 175$ A) and 1850 K ( $I = 225$ A) at the top, and 150–200 K less in the side.

Analysis of the experimental works published previously shows the presence of relatively large forces and as result generation of the high-velocity jet. These works mainly investigated the electrode material loss as vapor flow and some separate works considered also the presence of the ions. On other hand, the electrode material flow is a highly ionized complicated structure, which consists of a multi-charged and represented a supersonic plasma flux.

This chapter outlines some recent advances in plasma application for cancer therapy including cancer cell sensitization by plasma, recently discovered effect of activation and development of new plasma sources.

The vacuum arc has generated a great interest as very promising mechanism for micropropulsion is capable of providing orbit maneuvers of a spacecraft weighing on the order of few ten kg or smaller.

Different aspects of experimental investigation of cathode spot motion in a vacuum arc under transverse- and oblique-oriented magnetic fields will be reviewed and analyzed.

This chapter considers transport phenomena of the plasma particle, and their fluxes produced due to contact of highly ionized dense plasma with the electrodes and study the hydrodynamic flow of the mass and energy of different plasma components.

During the vacuum-arc burning, a high-pressure plasma is produced in the spot near the electrode surface causing an ejection of electrode material in form of the macroparticle (MP) as a liquid droplets or solid particles. The erosion mass in form of MPs is substantially determined by the arc current, thermophysical properties of the electrode material, and arc duration.

According to the considered analysis in above chapters, the cathode spot is a complicated subject that is difficult to investigate not only experimentally, but also hardly studied theoretically. Therefore, the previously involved approaches used arbitrary parameters that are preferable to calculate. The main problem consists in perception for long time by the researches that the cathode spot is completely non-equilibrium formation and of extremely material state, which cannot be considered by traditional approaches [1]. The goal of any complete spot theory is to indicate mechanism that can determine the difference between non-equilibrium and equilibrium regions and to understand the charge particles’ motion near the surface.

For long time, traditionally, the anode mechanism of an electrical arc commonly was accepted to be less complicated than the processes in cathode region. Nevertheless, in the second half of the twentieth century and, especially, in the last few decades, the experimental investigations showed different important forms of existence of vacuum arcs at the anode. A discussion of the importance for anode phenomena study is presented in this chapter.

Limited number of publications was presented concerning to the theoretical study of the anode region of vacuum arcs up to 70–80 years of the twentieth century. The most of arcs were operated with relatively small arc current, and therefore, without or at small contraction of the electrical current at the anode surface. This provokes for long time an opinion that the anode plays only a passive role as electron collector, while the cathode mechanism was traditionally studied as a complicated object supporting a source of electrons. The further arc applications were connected with development plasma accelerators, high-current electro-commutation apparatus, plasma jet generators, and welding plasma devices.

Numerous applications indicate that laser–matter interaction is technologically important (Hoffmann et al. in Laser Part Beams 23:47–53, 2005, [1]). In particular, the interaction of intense laser beams with plasmas is highly relevant to laser fusion (Miller et al. in Nucl. Fusion 44:S228–S238 (2004), [2]), material science and medicine (Gamaly et al. in Phys Plasmas 9:949–957, 2002, [3]), and nanoparticles generation (Eliezer et al. in Laser Part Beams 23:15–19, 2005, [4]).

Before an electrical discharge occurs, two electrodes are insulated by the vacuum from each other. In a vacuum, an arc can ignited when a conducting material will be appeared in the electrode gap, to which a voltage is applied.

Igor Tamm and Andrei Sakharov in 1950 are the first physicists who proposed an idea to use toroidal camera for high-temperature plasma confinement named then as Tokamak. The first tokamak, the T-1, began operation in 1958. Also in 1958, the model of the Harwell controlled Zero Energy Thermonuclear Assembly (ZETA) was demonstrated in London. High-temperature plasma exposes a high heat load the facing a divertor material and first wall in fusion reactors. Different materials were used for tokamak construction. One of these materials, tungsten, was used as a metal with high durability against the high heat load and low sputtering yield. When the sprayed tungsten was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure.

Electrode spots support the current continuity in vacuum arcs. Due to extremely high-density energy dissipation in the spots, a large energy flux to the electrode is formed. As result, the electrode is intensely vaporized and conductive metallic plasma being generated. The electrodes are locally damaged and an erosion mass flux extracted in form of directed plasma jet was produced on the one hand. On the other hand, the reaction of the jet, in turn, invokes a force acting on the electrode. The nature of electrode force is important for understanding the vacuum-arc phenomena. In this chapter, the results of measurements illustrating the principle of the force phenomena and early experimental data about vapor pressure and it expansion near the electrode will be presented (mainly in chronological order).

Two important phenomena at electrode surfaces occurred during the plasma formation in the arcs. These processes will be described in this chapter. When the electrode is heated, the neutral atoms evaporate from the hot surface, a vapor plume is generated, and due to its expansion, it has a pressure lower than saturated pressure. The rarefied collisions determine the kinetics of the plume flow immediately near the surface. Vaporization of the heated electrode material will be analyzed using the kinetic approach. Simultaneously, the electrons are also emitted from the hot electrode surface. The current of electron emission is enhanced by the electric field produced at the surface in case of cathode plasma generation. In this Chapter, we will describe the electron emission caused by different mechanisms due to high body temperature and electric field at the surface as well as the influence of an individual field and explosive phenomena.

Vacuum arc is a discharge of relatively high current (>1 A for bulks) where the conductive material in the interelectrode gap arises in course of mass loss from the electrodes during the arcing. Different phenomena of the electrode degradation occurred due to the local and high intensive electrical energy dissipation in the arc gap and at the electrode surfaces.

In this chapter, the arc definition and the experimental data characterizing the cathode spot including arc voltage, cathode voltage, threshold (minimal) arc current, spot definition, and problem of spot dynamics and the current density are described.

Metallic plasma formed in the cathode spots of a vacuum arc expands into vacuum as highly ionized and considerably accelerated plasma jets.

The near-cathode phenomena in electrical arcs such as spot motion and spot dynamics, especially in a magnetic field, determine the arc performance and playing an important role in different applications. The current per spot and spot motion depend on the arc current, gap length, interelectrode pressure

Since the cathode is negatively charged with respect to the adjacent dense plasma, the electrons were repelled from the cathode, while a high-density ion flux toward the cathode is produced. As a result, a relatively large positive space charge in the vicinity of the cathode arises leading to cathode potential drop uc formation and to occurrence of a strong electric field E at the cathode surface.

The plasma phenomena are widely described in the literature focused on general plasma physics and on specific applications [1, 2, 3]. More recent book [4] described the plasma and particle collision effects that is relevant to some emerging applications in aerospace, nanotechnology, and medical applications. This Chapter presents some details of plasma characteristics and determines the elementary particle processes in plasma volume and at the plasma–surface interface, which mainly related to understand the specifics of metallic plasmas formed in low pressure or vacuum arcs.

In the last decades metal plasmas deposition techniques have progressed and have been utilized in different applications. Ion bombardment is an important tool in producing new materials and structures. Thin metallic films are extensively used in microelectronics.

Most of published works (Chap. 15) used a system of equation, in which the number of unknown not equal to the number of equations. In this case, a number of arbitrary parameters, characterizing the cathode plasma, the cathode surface, and the spot current structure, added to the mathematical description. The cathode potential drop uc was used as input parameter traditionally referred to its measurement with high accuracy. Another parameter is the cathode erosion rate G was calculated by evaporation rate using Langmuir–Dushman (or Frenkel) formula that is incorrect not only by describing the mass loss in vacuum (with rarefied collisions), but also, it does not account the returned mass flux to the surface in the cathode dense plasma. Nevertheless, the study of transient spot parameters by gasdynamic model (GDM, Chap. 16, [1]) shows that the values uc and G can influence the spot parameters during its transient development. The presence of two values in solution when these parameters are fixed requested a physical reason to choose the correct data. In addition, the observed voltage oscillations indicate that there is more complicated behavior of cathode spot at low arc current. Thus, an investigation of the cathode spot behavior requires use of mathematical formulation in a physical closed approach.

One of the important issues is the electrical arc initiation and development in devices with flowing plasmas or in high-pressure systems. Such arcing occurred in generators of low-temperature plasmas, in plasma accelerators, in plasma of products of combustion, and in a rail gun [1, 2, 3, 4, 5, 6, 7, 8]. The plasma devices consist of complicated designs and different types of electrode assemblies. In this case, different problems arise by the projection of the assemblies and their optimization

In this chapter, the electrode heat regime under energy flux dissipation in the solid from a vacuum arc was analyzed. Specifics of electrode heat conduction and electrode heating by concentrate heat sources are studied. The local power density from the minute area arc spot is modeled. The mathematical formulation and methodology of solutions of heat conduction equation for different target configurations (plate, film, and bulk) are presented; as well, analytical expressions are derived.

The near-cathode plasma is relatively dense and hot. The plasma generation supported by thermal phenomena in the cathode body and in region of the plasma cathode transition. The conductive plasma expanding up to the anode executes the electrical contact needed for vacuum arc burning. As result, a large heat fluxes outputted by the both cathode and anode bodies. The ratio of the heat power (in Watt) to the arc current is named as equivalent or effective voltage for cathode ucef and for anode uaef. The measurements of electrode heat losses and the effective voltages were conducted by different methods. One of them is a calorimetric approach measuring the maximal temperature of water and its mass flux in systems of water cooling electrode or measuring the temperature of electrode having a high thermal resistance lead. The energy input in the electrode was obtained using the mass and heat capacity of the water flux or electrode. Another approach involved calculating the heat conduction flux using an analytical expression for the temperature distribution in one-dimensional approximation and corresponding electrode temperature distribution measured by thermocouples. Let us consider the measuring effective electrode voltages by different approaches and conditions.

Thin film deposition using hot anode vacuum arcs developed in the last decade is described. Two configurations were used: (i) with an open gap—the hot refractory anode vacuum arc (HRAVA) and (ii) with a closed gap—the vacuum arc with black body assembly (VABBA). In both configurations, the anode was heated by the arc with current I = 145–340 A, and a relatively dense plasma plume of cathode material (Cu, Ti, Cr, Al, Sn, Mo, Nb), was formed by re-evaporation of cathode material from the hot (2000–2500 K) anode, which was fabricated from graphite, Mo, Ta, or W. A steady state mode was reached when the anode was sufficiently hot and a plasma plume expanded, either radially (HRAVA) or directly from the front hot anode surface. As an example, the deposition rate measured in 300 A HRAVAs at distances of 80 mm from the arc axis, to be 3.6; 1.4 and 1.8 μm/min for Cu, Cr and Ti cathodes respectively. Interconnector trenches (100 nm wide ×300 nm deep) on microelectronic wafers were filled using a Cu HRAVA at a rate of 0.5 µm/min.

Vacuum arc cathode spot theories are reviewed, beginning from the primary studies in second half of the 19th century up to the present. The evolution of the main ideas is presented, starting from those, which developed, for separate phenomena, up to systematic inclusive models. New spot type classifications are proposed based on observed characteristics, instead of the previous simple numbering. Different approaches were considered for generating plasma comprised of cathode material, based on cathode vaporization and local explosions. Models with closed systems of equations were developed. A kinetic model advantageously allows calculating the cathode potential drop. These closed models explain various phenomena and two base rules were derived: 1) impeded plasma flow enables cathode spot operation and 2) the heat loss in the cathode must be smaller than the energy input to the cathode spot. Based on these rules, mechanisms for the spot types were derived and observed spot behavior in transverse and oblique magnetic fields was described.

Vacuum arcs are used as metallic plasma sources to deposit thin films, but conventional arcs also produce macroparticles (MPs) which degrade film quality. To reduce the MPs, the Hot Refractory Anode Vacuum Arc (HRAVA) plasma source was developed with a consumed cathode, which supplies the coating material, and a refractory anode. To obtain a wider deposition area, this work presents a new configuration with a double-arc between a common cathode and two refractory anodes. Experiments were conducted using two anodes from graphite or tungsten and a water-cooled consumable Cu cathode with two active surfaces. Cu film depositions were obtained over a length of 150 mm, in contrast to the deposition from a single BRAVA that produced a deposition length of 75 mm.

Scientific Reports 5: Article number: 9946; published online: 16 April 2015; updated: 19 March 2018. The original PDF version of this Article contained a typographical error in the volume number ‘5’ was incorrectly given as ‘4’. This error has now been corrected in the PDF version; the HTML version was correct from the time of publication.

Refractory metal plasma generated by a vacuum arc was used to deposit thin films with different arc currents $I$ . The deposition rate $V_{\rm dep}$ was measured for electrode configurations including a planar Zr cathode and a planar W anode; cylindrical W or Mo electrode pairs and a cylindrical Nb cathode closed by a BN plate and a W or Nb shower-head cup anode or with one-hole Ta cup anode. $V_{\rm dep}$ for a Mo electrode pair with $I=275$ A at a distance $L=110$ mm from the electrode axis reached 2.2 $\mu \text{m}$ /min, 60 s after arc ignition. For the W electrode pair $V_{\rm dep}$ was $\sim 1\mu \text{m}$ /min at 80 s ( $I=200$ A and $L=110$ mm), while for W film deposition with shower-head anode $V_{\rm dep}$ was $\sim 0.6\mu \text{m}$ /min ( $I=200$ A and $L=60$ mm). For Nb films deposited with the closed electrode configuration, $V_{\rm dep}$ was 0.3 $\mu \text{m}$ /min at 30 s after arc ignition ( $I=275$ A and $L=80$ mm from the anode front).

Chapter 1 covers basic plasma concepts and builds the foundation for plasma engineering. This chapter gives brief general classification of main plasma phenomena and describes different types of plasma particle collisions, waves and instabilities, plasma-wall transition, electron emission mechanisms and wall ablation in the presence of relatively hot plasmas.

This Chapter describes main mechanisms of different types of electrical discharges. After a brief introduction into electrical breakdown based on the Townsend mechanism, the Chapter deals with the broad range of discharges including sparks, streamers, glow and atmospheric and vacuum arcs. The physics of gas breakdown and the Paschen law are detailed. Towards the end, simulation of the cathode phenomena of the vacuum arc is presented.

The local cathode heating, vaporization, electron emission, cathode sheath, and so on were considered taking into account the kinetics of mass flow at the vaporized cathode surface. A self-consistent model was developed and simulation was conducted for given spot current $I$ to obtain the cathode erosion rate $G$ and jet velocity $V$ determined the force on the cathode. The time-dependent cathode and plasma characteristics were calculated for Cu, Ag, Al, and W cathodes. The primary plasma needed to initiate the spot was taken into account. Various primary plasma parameters were defined by different spot ignition time $\tau$ at which then the spot can be developed. The model shows that the time-dependent range of $G$ agrees well with the measured data and the jet velocity $V$ calculated from the experimental data of cathode force also close to that velocity obtained experimentally in a literature. The trust performance was obtained larger for W cathode than that for Cu and Ag cathodes and this and other results were discussed.

Experiments on plasma-liquid interaction and formation of thinly stratified self-organized patterns at plasma-liquid interface have revealed a nontrivial cancer-inhibiting capability of liquid media treated at self-organized interfacial patterns. A pronounced cancer suppressing activity towards at least two cancer cells, breast cancer MDA-MB-231 and human glioblastoma U87 cancer lines, was demonstrated in vitro. After a short treatment at the thinly stratified self-organized plasma-liquid interface pattern, the cancer inhibiting media demonstrate pronounced suppressing and apoptotic activities towards tumor cells. Importantly, this would have been impossible without interfacial stratification of plasma jet to thin (of several µm) current filaments, which plays a pivotal role in building up the cancer inhibition properties. Furthermore, thinly stratified, self-organized interfacial discharge is capable to efficiently control the ROS and RNS concentrations in the cancer-inhibiting media. In particular, abnormal ROS/RNS ratios are not achievable in discharges since they do not form stratified thin-filament patterns. Our findings could be tremendously important for understanding the cancer proliferation problem and hence, the potential of this approach in tackling the challenges of high cancer-induced mortality should be explored.

A vacuum arc with black body electrode configuration is used to deposit thin Cu films on glass substrates. In this configuration, a cup shaped anode, a refractory insulator, and a water cooled cathode form an enclosed volume from which dense plasma of cathode material was extracted through different types of apertures (one hole, frontal or radial shower-head) in the anode. All materials emitted from the cathode (including macroparticles) were re-evaporated from the hot anode producing in the enclosed volume a dense plasma which extracted through the anode aperture. The Cu deposition rate with the frontal shower anode was about 3.6 μm/min at a distance L s = 80 mm from the anode for current 200 A. Deposition with the radial shower anode produced a film which was azimuthally equal. The cathode erosion rate with this electrode configuration was a factor of 2–3 lower than that in a conventional cathodic arc indicating higher cathode material utilization.

A simple approach to modeling the plasma layer similar to that appearing in the vicinity of a hypersonic vehicle is demonstrated in a laboratory experiment. This approach is based on the use of a hypersonic jet from a cathodic arc plasma. Another critical element of this laboratory experiment is a blunt body made from a fairly thin foil of refractory material. In experiments, this blunt body is heated by the plasma jet to a temperature sufficiently high to ensure evaporation of surface deposits produced by the metallic plasma jet. This process mimics reflection of gas flow from the hypersonic vehicle in a real flight. Two-dimensional distributions of the hypersonic plasma flow around the blunt body were measured using electrostatic Langmuir probes. Measured plasma density was typically 1012 cm−3, which is close to the values measured for real hypersonic flight. The demonstrated laboratory experiment can be used to validate numerical codes for simulating hypersonic flight and to conduct ground-based tests...

Micropropulsion systems are rapidly gaining attention from the small satellite community as they can increase the mission lifetime and allow the satellite to perform complex maneuvers and precise attitude control. These systems need to be fully operational with the low power available on satellites. Various thruster concepts based on vacuum arcs are currently under development, predominantly in the pulsed regime due to the power constraints on small spacecraft. Pulsed vacuum arc thrusters are capable of efficiently producing highly-ionized supersonic plasma at very low average power. This Perspective article provides a critical analysis and a review of various aspects of electric propulsion technology based on vacuum arcs. Furthermore, we give a personal assessment of the present status and provide an outlook on the field, including the growing role in small satellites such as CubeSats. Vacuum arc micropropulsion systems could play an important role in mitigating the problem of space debris. Such a system could be integrated with a satellite so that, at the end of its mission and using metal components as solid fuel, it will lower the satellite’s orbit and accelerate reentrance into the atmosphere faster than by its natural decay rate.

The energy dissipated in the cathode in a vacuum arc black body assembly (VABBA) is investigated. A closed volume is formed by a water-cooled cylindrical Cu cathode and a cup-shaped W or Ta anode. Cu cathode material is emitted into the closed volume and the anode is heated by the arc. The VABBA confined and evaporated macroparticles while the plasma is extracted through small anode apertures. Arc currents are $I=175\hbox{--}250~{\rm A}$ and the arc time is 150 s. The effective cathode voltage $U_{\rm cef}$ is determined calorimetrically, using a thermocouple probe. It is observed that $U_{\rm cef}$ increases with time from ${\sim}{\rm 6.5}~{\rm V}$ for cold anodes (i.e., during conventional cathodic arcing) to a steady-state value of ${\sim}{\rm 11.2\hbox{--}12.0}~{\rm V}$ when the anode is sufficiently hot and the arc operated in the VABBA mode.

Anode temperature in a Vacuum Arc with a Black Body Assembly (VABBA) was measured as a function of time. The arc was ignited in an electrode assembly operated as a black body for the macroparticles (MPs). The cathode was a 30 mm diameter Cu rod and the refractory anode was 50 mm in diameter, and constructed from Ta. Plasma was ejected through an array of 250 holes of 0.6 mm diameter in the anode. The arc currents were I=175 & 225 A and the arc duration was 180 s. The anode temperature was measured using high-temperature thermocouples at two points (TC-top, TC-side) inside the anode body. The observed anode temperature increased sharply during a transient time of ∼90 s and then it slightly increased with time up to 180 s reaching at TC-top 1650K (I=175 A) and 1850K (I=225 A). The anode temperature at the top surface exceeded that at side surface by ∼150–200C.