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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.

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.

Pulsed air arc treatment (PAAT) was used to modify the surface morphology and phase composition of 5010 and 01 steel. The arc produced local heating and eroded the steel surface forming craters, and the subsequent modification depended on the crater density. X-ray diffraction showed that the volume fraction of ferrite decreased while that of austenite increased with crater density. Cementite was practically absent in 5010 steel, while its volume fraction in O1 steel increased with crater density. PAAT increased the microhardness of the surfaces by 10–40% over that of the virgin surfaces. PAAT increased the lifetime of a lubricant film applied to the surface by more than an order of magnitude, by forming craters on the surface which retained the lubricant.

A hot refractory anode vacuum arc (HRAVA) starts as a cathodic arc, which heats and deposits cathodic material on the anode. When the anode is hot, all the deposited cathode material is re-evaporated from the anode, forming a radially expanding plasma with reduced macroparticle (MP) contamination. This paper reports on the deposition of Sn films using this radially expanding arc plasma. The HRAVA current was I = 60–175 A and the duration was up to 180 s. Two electrode pairs were used. (1) A water-cooled Sn cathode with a diameter of D = 60 mm was used with a W anode having a thickness of d = 10 mm, diameter D = 60 mm and separated from the cathode by gaps of h = 10 and 15 mm. (2) A water-cooled Sn cathode with D = 30 mm was used with either of two graphite anodes, with d = 9 or 15 mm, D = 32 mm and h = 10 mm. The cathodes were recessed behind a boron nitride shield. A mechanical shutter controlled the deposition onset and exposure duration (15 s) on glass substrates. The distance from the arc axis to the substrate (L) was 80, 110, or 125 mm. Film thickness was measured with a profilometer. MPs on the coating surface were examined by optical microscopy. With D = 60 mm cathodes, L = 125 mm, and I = 175 A, the deposition rate Vdep monotonically increased with time from 0.5 initially to 3.0 μm/min after 120 s. With the D = 30 mm cathode, L = 110 mm, I = 175 A and d = 9 mm, Vdep increased with time to a peak of 0.84 μm/min at 20 s, and then decreased to a steady state of 0.69 μm/min at 45 s. The peak was due to re-evaporation from the hot anode of cathodic material, including MPs, which were deposited initially on the cold anode. The MP flux density decreased with I from 2.5 at I = 80 A to about 1 mm− 2 s− 1 at I = 175 A in case of D = 30 mm, L = 110 mm, d = 9 mm.

The radially expanding plasma jet generated in a Hot Refractory Anode Vacuum Arc (HRAVA) was used to deposit thin chromium and titanium films on glass substrates. The arc was sustained between a water-cooled cylindrical cathode and a non-consumed cylindrical tungsten anode separated by a 10 mm gap, for time periods up to 120 s, operating with a current (I) of 200–300 A. The chromium cathode was 30 mm diameter and 20 mm height, and was used with 32 mm diameter, 30 mm height anode. The titanium cathode was 60 mm diameter, 25 mm height and was used with a 60 mm diameter, 15 mm height anode. Thin films were deposited on glass substrates exposed to the anodic plasma. A mechanical shutter controlled the deposition onset and exposure duration time. The distance from the arc axis to the substrate (L) was varied between 80 and 145 mm. The film thickness was measured by a profilometer, and macro-particles (MP) presence on the coating surface was examined by optical microscopy. It was found that the deposition rate of Cr films increased with arc current and reached ~ 1.4 µm/min at I = 300 A, L = 80 mm. The total MP flux was less than 1 mm− 2min− 1 for I = 300 A, L = 110 mm. The deposition rate of Ti films reached 1.8 µm/min at I = 300 A, L = 100 mm.

Powders of Ni–C nanoparticles were synthesized using a pulsed arc between Ni electrodes submerged in pure ethanol. The arc was ignited by inter-electrode breakdown. The ethanol was arc treated for 5 min with 1 μs duration pulses, at a repetition rate of ∼1 kHz. The pulse energy was in a range of 0.01–0.05 J. Powder samples were obtained by extracting liquid from the treatment vessel after a pre-determined sedimentation time, or by allowing the liquid to evaporate from the vessel, and collecting the residue. The samples were examined by HRSEM, EDX and XRD. It was found that a powder of Ni–C nanoparticles was produced. The powder consisted of carbon particles and nickel–carbon alloy particles. The latter were also coated by carbon. The carbon concentration in the Ni alloy was approximately triple of the maximum equilibrium solid solubility of carbon in nickel. The production rate of carbon was greater than the erosion rate of the Ni electrodes by factors of 2–6. The ratio of carbon production rate to electrode erosion rate decreased with the discharge energy. The size distribution of the produced particles was narrow. No particles with diameter more than 0.7 μm were observed.

First results of complete filling of 100nm widex300 nm deep trenches with Cu using the expanding plasma plume from a hot refractory anode vacuum arc (HRAVA) plasma source are presented. The arc was ignited between a consumed water-cooled cylindrical Cu cathode (30mm diameter) and a non-consumed W cylindrical anode (32mm diameter, 30mm height) that was heated by the arc. An arc current of 200A was applied for periods of 180s. The films were deposited on a Si substrate with a top SiO"2 layer. The substrates were exposed to the plasma plume for 120s, while a shutter was open. The distance to the substrate from the electrode axis was varied over the range of about 74-122mm. A pulsed bias voltage of -75 or -100V, with a 60kHz pulse repetition rate and a 50-80% duty cycle was applied to the substrate. The films were examined using a scanning electron microscope. The average film resistivity was measured with a four point probe. The deposition rate was as high as 425nm/min, and the minimum average resistivity was 5.5@m@Wcm.

Copper films deposited by a radial expanding plasma plume from a tungsten hot refractory anode vacuum arc were investigated. The arc was sustained between a consumed water-cooled cylindrical copper cathode (30 mm diameter) and a cylindrical anode (32 mm diameter, 30 mm height) that was heated by the arc current. The anode temperature was measured at 2, 12 and 22 mm from its front side by high temperature W/5%Re–W/26%Re thermocouple probes ( T 1, T 2 and T 3 respectively). Coatings were deposited on glass substrates which were exposed to the plasma flux. Distances to substrate ( L ) were 80–165 mm and arc currents I = 150–300 A. The film thickness was measured by profilometry and macroparticle (MP) contamination was determined by the optical microscopy. It was found that the anode temperature increased with arc current ( T 1 ∼ 2300 K for I = 150 A and 2500 K for I = 250 A) and slightly decreases with gap distance. The number of MPs on portion of the substrate facing of the anode decreased as function of MP size, while total number of MPs was approximately 3 mm − 2 min − 1 (for I = 300 A, L = 110 mm, h = 10 mm). The MP size distribution had a similar shape as observed with a Mo anode, but the amount of MPs was about 15% less using the W anode. The deposition rate at L = 80 mm, h = 10 mm and I = 300 A was about 3.6 μm/min. The larger surface temperature was reached by using higher current with the W anode, due to its lower rate of vaporization in comparison to previously used Mo and graphite anodes.

Two vacuum arc deposition techniques were compared for metal film deposition: 1) filtered vacuum arc deposition (FVAD), which applies a magnetic field to separate the plasma from macroparticles (MPs) generated in the cathode spots, and 2) hot refractory anode vacuum arc (HRAVA) deposition, in which anode plasma plume forms by re-evaporation of cathode material from the hot anode surface and MPs are vaporized in the hot interelectrode plasma. A Cu cathode and an arc current of 200 A were used in both systems. The FVAD film thickness was axially symmetric to within 10–20%. The focused plasma jet in the FVAD system covered a circular area, where the radius for half-thickness is ∼ 30 mm. The deposition rate was constant in time and maximal (about 0.25 μm/min) in the center of the circular area. The mass deposition rate was about 9.5 mg/min assuming bulk density. The HRAVA deposition rate initially increased with time and saturated at a maximum of ∼ 2.3 μm/min after 1 min. The plasma expanded radially, and was deposited on a cylindrical area of 100 cm 2 and height ∼ 20 mm, which was co-axial with the electrode axis. The thickness distribution was axially symmetric within 10%. The steady-state mass deposition rate was 400 mg/min. The HRAVA system produced an almost MP-free radially expanded mass throughput and cathode utilization efficiency ∼ 40 times greater than with the FVAD system.

A 200 A Hot Refractory Anode Vacuum Arc (HRAVA) was studied. The arc was sustained between a cylindrical Cu cathode and an Mo anode spaced ∼ 10 mm apart. In the HRAVA, metallic plasma (from cathode material) expands radially. He, N 2 or Ar gas was introduced into the arc chamber through an electrically controlled valve. Films were deposited onto glass microscope slide substrates. The angular film distribution and the influence of different background gases on films deposited by the HRAVA were measured. The film thickness was measured by profilometry. The ion current was measured by a circular flat probe. It was shown that the film thickness was uniform with respect to the azimuthal angle around the electrode axis within approximately 10%. The film thickness was independent of gas pressure p , below a critical value p cr . For p > p cr , the film thickness decreased with p , eventually reaching 0. The value of p cr was less for gases with larger molecular weight—60, 10 and 5 mTorr (0.67, 1.33 and 8 Pa) for He, N 2 and Ar, respectively. The ion current to a 78 mm 2 probe in vacuo increased with time and reached a saturation value of approximately 4.5 mA after about 60 s from arc ignition. The ion flux fraction in the total deposition mass flux was estimated to be about 60% in the fully developed HRAVA ( t = 60–90 s).

Numerous traces of arcs have been found on W-covered graphite tiles of ASDEX Upgrade after exposure. The distributions of number density, lengths and orientation are calculated and compared to pure graphite tiles at comparable locations. It was established that arcs perforate a 1 μm tungsten layer down to the carbon substrate. The amount of removal should rise with arc current, but a surface fraction of about 8% is eroded at 10 A already. At tiles of the divertor baffle the layer is continuously removed along the entire track pointing to higher currents. The carbon of the stripped parts is subject to subsequent erosion processes. The distribution of materials in and around arc tracks was investigated by sputter depth profiling (SIMS and AES) and the characteristic geometry was studied using an electron microscope. Observations are interpreted using results from laboratory vacuum arcs on the same material.

The temperature distribution in an asymmetric graphite anode and deposited copper film characteristics in a Hot Refractory Anode Vacuum Arc (HRAVA) were measured. The material to be deposited originates from the cathode and is re-evaporated from the hot anode. The discharge was between a water-cooled copper cathode (diameter 30 mm) and a thermally isolated anode (diameter 32 mm) with arc currents I=120–225 A, gap distances of h=5–18 mm and arc durations up to 150 s. The front surface of the asymmetric anodes was inclined so that the maximal and minimal anode lengths (L1, L2) were: (1) (30, 25) and (2) (30, 20) mm, whereas the length of the symmetrical anode was 30 mm. The anode temperature was measured using thermocouple probes located near the front and rear surfaces. The steady-state temperature at the front and rear anode surfaces for all anode geometries increased approximately linearly with current. The surface temperature for asymmetric anodes was not symmetric and the difference in steady-state temperature measured by thermocouples on the surface near the anode length L1 and L2 was ∼130 C for anode (2), and ∼100 C for anode (1), when I=175 A and h=18 mm. The macroparticle (MP) contamination in thin films deposited on substrates facing the anode decreased with arc current. The deposition rate was about 1.5 times greater using asymmetric anode (2) than with a symmetric anode.

Copper films, deposited by radially expanding plasma plume from a hot refractory anode vacuum arc, were studied. The arc was sustained between a consumed, water-cooled Cu cathode, and a non-consumed Mo anode, which was heated by the arc. The plasma plume was created by re-evaporation of the cathode material from the hot anode. Arc currents of 175–300 A were sustained for periods of up to 120 s. The coatings were deposited on glass substrates. A shutter controlled when and how long the substrate was exposed to the plasma. Macroparticle (MP) presence on the coating surface was examined by optical microscopy and the coating thickness was measured by profilometry. The MPs size distribution function was determined. It was found that the morphology of the film in the region facing the anode (anode region) differed from that on the region facing the cathode (cathode region). The cathode region had a matt appearance, and the surface contained many MPs, while in the anode region, a mirror-like film was obtained. The film thickness in the anode region increased linearly with current. Film deposition rate in a 200 A arc in the anode region on a substrate positioned 11 cm from the electrodes initially increased linearly with time and in 1 min reached 0.5 μm/min.

A new mode of the vacuum arc, the Hot Refractory Anode Vacuum Arc (HRAVA), was investigated as a plasma source for depositing coatings. Arc currents of 155–340 A were sustained for periods of up to 120 s between a water-cooled Cu source cathode, and a non-consumable refractory anode, which was heated by the arc. Cu coatings were deposited on ground stainless steel and glass substrates. A shutter controlled when and how long the substrate was exposed to the plasma. The coating rates were measured by weighing the substrates, and the macro particles (MPs) presence on the coating surface was examined by optical microscopy. Films that formed in a 30-s exposure at the beginning of a 175-A arc, when it operated in the cathode spot mode, were heavily contaminated with MPs. The density of MPs with diameters of between 3 μm and 50 μm was ∼10 3 mm −2 . However, with a 30-s exposure, which began 30 s after arc initiation, by which time the arc was in the HRAVA mode, the MP density was reduced to ∼1 mm −2 . The HRAVA deposition rate was ∼1–2μm/min onto substrates placed at distances of ∼110–120 mm from the arc axis. The HRAVA deposition rate is comparable to filtered cathode spot vacuum arc deposition, but over a much larger deposition area.

The objective of the present work was to calculate the influence of positive bias on macroparticle (MP) flow in a curved magnetized plasma duct. A non-stationary model for MP charging and motion in the duct wall sheath was developed. Two cases of MP charging were considered: for large MPs (radii ≥1 μm) and small MPs (

Anode erosion during Pulsed Air Arc Deposition was investigated. The depositions were conducted with a series of pulses with a current amplitude of 300 A, a pulse duration of 150 μs, and a pulse repetition rate of 100 Hz. Slab anodes of pure Al, Ti, Cu, Fe, W, and WC based hard alloys were used to coat low carbon steel and Ti cathode-substrates. The influence of the anode material and deposition time on the anode mass loss were studied. The anode mass loss increased in the following order: W, Cu, WC based hard alloys, Ti, Al. A criterion was proposed to explain the material influence on the anode mass loss, based on heat flux required for the anode to reach either the melting or boiling temperature. The anode material order obtained by this criteria agrees with the experimental data.

WC-based hard alloys were deposited from a slab source anode on to low carbon steel and pure Ti substrates using a pulsed air arc. The depositions were conducted with a series of pulses with current amplitudes of 75 A, 300 A, and 500 A, a pulse duration of 150 μs, and a pulse repetition rate of 100 Hz. The influence of the electrode material, discharge energy and deposition time on the electrode erosion rate, mass transfer direction, and the wear resistance and friction coefficient of the coatings was investigated. Anodic and cathodic mass change characteristics depended on both the anode and cathode materials. Using a steel cathode and a WC anode, depositions of the anode material always formed on the cathode. For the same WC anode but a Ti cathode, coatings only formed locally in the craters of the eroded cathode. The anodic mass loss when using the steel cathodes was about a factor of 3 larger than when using the Ti cathodes. The microhardness of the coated substrates was larger by factors of 5–12 than the uncoated Ti and low carbon steel substrates. The coatings increased the wear resistance of the steel and Ti substrates by factors of 4.3 and 1.4, respectively. The poorer performance of the coated Ti substrates is attributed to the intensive erosion of the Ti cathodes (compared to the steel cathodes) and the discontinuous nature of the coatings (i.e. only in the erosion caters).

The presence of macroparticles (MP) in the vacuum arc cathodic jet is the critical problem of cathodic vacuum arc deposition technology. The MP distribution on the substrate depends on MP generation, MP transport to the substrate, and the MP—substrate interaction. In the present work different aspects of the interaction of the MP with a substrate are studied theoretically. The MP temperature evolution during transport to the substrate was studied and it was found that the MP is liquid in the case of low melting point cathode materials. It was found that the MP temperature does not depend on the initial MP temperature in the near cathode region. Possible MP acceleration in the substrate direction resulting from ion friction was calculated by solving the equation of motion for the MP taking into account the plasma density and the velocity distribution obtained in previous analyses of the 2-D plasma expansion. The voltage drop in the substrate sheath as a function of the background pressure was calculated assuming charge exchange between ions and gas atoms. It was found that the voltage drop in this sheath increases with increasing background pressure pg by factor of 2 for pg ∼ 1 Pa. The MP motion in this sheath was calculated taking into account MP charging and reflection from the substrate. The probability of reflection decreases with MP size, velocity, angle of the MP velocity vector with respect to the substrate, and increases with the background pressure.

During this decade significant advances have been made both in the understanding and implementation of filtered vacuum are deposition. Rigid rotor models have been analyzed statistically, and new models which treat the mutual influence of the electrons and ions on each other self-consistently, take into account the centrifugal force on the ions, and take into consideration collisions, have been formulated. It was shown that the plasma transport efficiency is limited by drifts caused by the centrifugal force and by the electric field generated by charge separation in the plasma. For a range of magnetic fields strengths for which the ions are not magnetized, i.e., confined to a Larmor radius less than the duct radius, the transport efficiency for Cu plasma is about 10%, and depends only weakly on the magnetic field strength. Increased transmission is found when the ions are magnetized, reaching about 50% for a 36–60 mT field in typical configurations. The plasma transport efficiency and spatial distribution has been measured over a large parameter range, and correlated with the various theories. The plasma beam may be approximated as a Gaussian distribution which is displaced in the B × G direction, where G is in the direction of the centrifugal force, while a displacement in the plane of symmetry is surprisingly found in the − G direction. The total convected ion current decreases exponentially with distance from the toroidal filter entrance. Macroparticle transport within the magnetic filter has been analyzed, and it has been shown that electrostatic reflection from the walls can occur if the magnetic field is weak. Filtered arc sources with improved throughput performance and novel geometries have been built, and are now available commercially. The range of coatings deposited with FVAD has been expanded to include metals, oxides, and nitrides, as well as diamond-like carbon. In several cases, coatings having the highest quality reported in the literature have been fabricated with the FVAD technique, and one commercial application has been reported.