Cathode Spot Region Research Articles

Plasma Oscillations at Cathode Spot Region of Vacuum Arc

The model of ionization processes in high-temperature plasma of the near-cathode region of vacuum arc is given. The model is mathematically represented as a system of second-order nonlinear differential equations of drift diffusion for the electron density and mean energy of electrons. The solution of the presented system of equations was derived using the finite element method according to two coordinates in numerical form using the COMSOL Multiphysics program. It is shown that Langmuir waves, which result in a multiple-step ionization of metallic vapor, are generated in plasma owing to nonstationary processes.

Three-fluid simulation of a cathode spot jet in vacuum arc discharge with a metal deuteride cathode

To study the physical process of a cathode spot jet in vacuum arc discharge with a metal deuteride cathode (TiD), a two-dimensional hydrodynamic model based on three-fluid theory is established in this paper. The ionization and recombination of the plasma are considered in this model. The total current of the cathode spot is set to 2–4 A. The simulation shows that: (1) for Ti ions, there are huge pressure gradients and D–Ti ion collisions (1013–1014 Pa m−1) in both the axial and radial direction near the cathode spot region, therefore the plasma jet will expand rapidly in all directions. For D ions, the ion collisions (D–Ti) play an important role in the whole region of the cathode spot jet, resulting in the velocity of a deuterium ion () being limited to a level close to the velocity of a titanium ion (). (2) Due to the expansion of the cathode spot jet, the ion density decreased rapidly from to . Meanwhile, ion speed quickly reached a level of at a distance of 2 from the cathode spot. (3) When the current of the cathode spot is small (I = 2 A), deuterium ions obtained by the reaction path are significantly less than that obtained by the reaction path. However, with an increase in the cathode spot current, deuterium ions obtained by the two reaction paths are almost the same. (4) Finally, the plasma parameters of the cathode spot jet predicted by our numerical model are in good agreement with the experimental results.

Ionization features in cathode spot region of vacuum arc

A model of ionization processes in a low-temperature plasma within the cathode region of a vacuum arc is presented. This mathematical model is represented by a system of nonlinear differential equations of drift-diffusion for the electron density and the average energy of second-order electrons. The system of equations was solved by the finite element method in two coordinates in numerical form using the COMSOL Multiphysics software program. It was shown that due to nonequilibrium processes, Langmuir waves appeared at the plasma boundary, which leads to multistage ionization of metal vapor. The arc discharge was presented in the form of a cylindrical plasma column with a length of 200 μm and a diameter of 50 μm. At the interface between the cathode and the plasma column, a double electric layer has appeared which holds hot electrons in the central zone of the discharge. High specific power was focused close to the cathode surface, which provided the continuous supply of metal vapor. The numerical results of the model have appeared to be in good agreement with the experimental data.

2D simulation of a copper cathode spot jet considering the charge composition in vacuum arc

Plasma in the vacuum arc is usually generated by numerous cathode spots on the cathode surface, each of which produces a supersonic plasma jet. This work studies the physical mechanisms of supersonic ion flow in a cathode spot jet of vacuum arc by using a two-dimension magneto-hydro-dynamic numerical model that considers ionization and recombination processes. Simulation results are presented for different currents of the copper cathode spot (I = 3, 4, and 5 A). The results indicate that in the cathode spot region, there are huge electron and ion pressure gradients (1013–1014 Pa/m) in both axial and radial directions, resulting in the rapid expansion of the plasma jet in all directions. The expansion of the plasma jet decreases the ion density rapidly from 1026 m−3 to 1022 m−3, and the ions reach supersonic speed within 0.4 μm from the cathode spot. Moreover, within 0.2 μm from the cathode spot, vaporized copper atoms quickly become completely ionized because of high electron temperature and electron density. Beyond a certain distance from the cathode spot, the recombination and ionization rates of the ions will be insufficient to alter the ion charge state. In most of the region, the average charge state varies from 1.84 to 2.05. Finally, the plasma parameters of the cathode spot jet predicted by the proposed numerical model are consistent with the experimental and theoretical results.

Recent Progress in Development New Generation Erosion Plasma Sources

This is a brief review of the current status of study and progress in devices for a new generation of plasma-dynamic tools based on combining a vacuum-arc plasma source with an axially symmetric cylindrical electrostatic plasma-optical lens into a single device. The combined system operates using the basic plasma-optical principles of magnetic electron isolation and equipotentialization along magnetic field lines and could be attractive for many practical applications of high current, moderate energy plasma sources of heavy metal ions and electrons. The hardware is relevant to processing equipment using dense cleaned plasma streams for the synthesis of coatings and thin films with given functional properties. A combination of the plasma lens with vacuum-arc ion plasma sources like the MEVVA allows control of low-energy dense plasma flow from the cathode spot region toward the substrate (for film deposition) or toward an extraction system (for ion beam generation). The application of the plasma lens for the transport of low-energy plasma is shown to improve plasma delivery to a substrate and provide microdroplet evaporation and elimination by fast electrons within the lens region. These results open up a new and attractive way for further development and application of erosion plasma sources for the synthesis of functional coatings, as well as for the creation of upgraded vacuum-arc ion sources with improved characteristics.

The influence of external magnetic field on the plume of vacuum arc thruster

The vacuum arc thruster (VAT) is considered to be a potential micro-thruster because of its simple structure and high specific impulse. The plume downstream from the cathode spot region of a co-axial type thruster with external magnetic field is analyzed by numerical simulation. Plasma velocity, temperature and density are used to represent the influence of the cathode spot region. When the magnetic field is applied, the plume is significantly affected even at the intensity of 0.03 T. Most ions move around the magnetic induction line, forming a concentrated beam. The beam expansion angle and the ion backflow are controlled therefore. The ion beam total velocity increment is about 20% in the plume while the beam direction is determined significantly by the cathode spot position and the external magnetic field. The deflection angle of the magnetic induction line in the near-field region will bring axial velocity loss. The magnetic field intensity affects little on the ion beam velocity increment while the influence of the initial plasma density change caused by the magnetic field cannot be ignored. Ground experiment data show a large increment of ion beam velocity between the conditions with and without the magnetic field (0.03 T and 0 T), but nearly no obvious variation from 0 to 15 cm downstream of the thruster in the former condition.

Characteristic of a triple-cathode vacuum arc plasma source

In order to generate a better ion beam, a triple-cathode vacuum arc plasma source has been developed. Three plasma generators in the vacuum arc plasma source are equally located on a circle. Each generator initiated by means of a high-voltage breakdown between the cathode and the anode could be operated separately or simultaneously. The arc plasma expands from the cathode spot region in vacuum. In order to study the behaviors of expanding plasma plume generated in the vacuum arc plasma source, a Langmuir probe array is employed to measure the saturated ion current of the vacuum arc plasma source. The time-dependence profiles of the saturated current density of the triple vacuum arc plasma source operated separately and simultaneously are given. Furthermore, the plasma characteristic of this vacuum arc plasma source is also presented in the paper.

The heating sources of spots on the vacuum arc cathodes

We have considered two stages in the development of a spot on the surface of a vacuum arc cathode: the stage of ignition and the stage of the remaining arc lifetime which consists of arc spot operation until ultimately it is extinguished. A model for the near cathode region of electric arc has been applied to obtain different parameters involved in the cathode spot region during the spot ignition stage. This model is based on the construction of a thin sheath near the cathode wall and electron emission upon the thermo-field theory of electron emission. The model is time-dependent and hence the different parameters of the cathode, plasma and sheath can be derived as a function of time. In particular, we determine the required values of the plasma and sheath parameters for spot ignition on the smooth cathode surface. Based on these calculations and taking into consideration the fluctuation in plasma and sheath parameters, the contribution of different heating mechanisms at the spot initiation stage is evaluated. After spot ignition, some spot parameters such as temperature and mass density of spot material, electron current density and vapour pressure considerably increase. High values of the electron current density and low values of the electrical conductivity of the dense plasma in the spot region significantly increase Joule heating during the evolution stage of spot operation.

Continuous phase transition in the region of the vacuum arc cathode spot

A model for the near-cathode region of electric arcs is presented to investigate the liquid-plasma phase transition in the cathode spot region. Due to the high values of pressure and temperature after spot ignition, a “continuous phase transition” occurs in the liquid-vapor interface. A set of fluid equations with suitable boundary conditions have been solved to obtain diagrams of the spot plasma in the temperature-density plane during the spot evolution for a typical spot. To evaluate the model, the magnitude of some essential quantities such as the mean ion charge state of plasma and current density have been calculated, which are in accordance with experimental results.

Optical Observation of Arc Discharges between Electrical Contacts Breaking at Low Speed in DC42 V Resistive Circuit

Breaking arcs occurring between silver electrical contacts are observed in DC42 V resistive circuit using a high-speed camera. The motion and current densities of the cathode and anode spot regions are investigated for different interrupt currents (1=7 A, 10 A and 14 A). Results indicate that the arc length at which the motion of arc spots becomes stable depends on the interrupt current, and the current densities of the cathode spot region are almost constant immediately before arc extinction for each interrupt current.

The effect of axisymmetric two-dimensional magnetic field on the configuration of vacuum-arc plasma

An experimental investigation is made of the effect of axisymmetric two-dimensional magnetic field on the forming of plasma and on the configuration of cathode spots in a vacuum-arc discharge. It is demonstrated that a magnetic field with a transverse (relative to the discharge axis) component has a significant effect on the shape of plasma column and on the rate of expansion of the cathode spot region. In a magnetic field, arc plasma has the form of truncated cone expanding toward the anode. The cathode spots take up a part of the cathode area which decreases with increasing magnetic field. Arguments are given in support of the assumption that the arrangement of cathode spots and the form of arc plasma are defined by the minimum principle similar to the Steinbeck principle. In so doing, the displacement of spots is caused by their emergence in a new region corresponding to a lower arc voltage. Also discussed is the mechanism associated with retrograde motion of cathode spot in view of the effect of azimuthal magnetic field on the axial component of current and of the effect of axial magnetic field on the azimuthal component of current.

Plasma Flow Crisis and the Limiting Electron Temperature in a Vacuum Arc in Axial Magnetic Field

The possibility of supersonic motion of cathode plasma in a low-current vacuum arc in an axial magnetic field has been studied. It is shown that an increase in the electron temperature unavoidably leads to a plasma flow crisis, whereby the plasma velocity decreases to the sound velocity. The dependence of the limiting length of a stationary flow on the magnetic field has been studied. The maximum possible electron temperature Tcr in the plasma is determined by the initial ion energy and can be estimated as Tcr ≈ 3Tm, where Tm is the maximum electron temperature in the cathode spot region.

Ion energy distribution functions of vacuum arc plasmas

The velocity distribution function of vacuum arc ions can be measured by a time-of-flight technique. The measuring principle makes use of the well-justified assumption that the ion drift velocity from the cathode spot region to a collector is approximately constant. It is shown that the negative time derivative of the collector current is directly proportional to the ion distribution function provided that the time-averaged emission of ions from cathode spots is constant until the arc is rapidly switched off. In the experiment, arc termination took about 700 ns, which is much faster than the decay of the ion current measured at the collector placed more than 2 m from the cathode. The experimental distribution functions for most cathode materials show one large peak with a tail and one or more small peaks at higher ion velocities. The typical peak position is at about 104 m/s, with the precise values being material specific. The distribution functions for some materials exhibit not one but several peaks. No conclusive answer can be given about the nature of these peaks. Arguments are presented that the peaks are not caused by different charge states or plasma contamination but rather are due to insufficiently averaged source fluctuations and/or acceleration by plasma instabilities.

An Analysis of the Instability Phenomena of a Low-Current Vacuum Arc for Silver Cathode-A Comparison of Electron Temperature between Probe Measurement and Analysis-

In order to clarify the instability phenomena of a low current vacuum arc, characterized by noise on the current trace prior to the actual current chopping, an analysis of silver cathode spot has been made. For the analysis, cathode spot region is recognized as the collisionless space charge sheath connected with singly ionized collisional plasmas.It is proposed that a current level below that no real solution exists is unstable current and this current corresponds to the point at which a vacuum arc turns into unstable. It is found that there is no real solution when the current is below 19.4A. The physical explanation is considered that the electrons returning to the sheath region from the plasma one dominate over positive ions and, consequently, the electric field at the cathode surface is F02<=0 below 19.4A. Similar results have already been obtained for copper cathode. In order to examine the analytical result for its validity, the probe measurements of electron temperature in the diffused arc plasma has been performed. The electron temperature-arc current characteristics were found to be similar between experimental and analytical results.

Plasma diagnostic by emission spectroscopy during vacuum arc remelting

The plasma produced during vacuum arc remelting of a Zircaloy4 electrode has been investigated by optical emission spectroscopy. Spatial variations of plasma emission along the arc axis has been measured with a specific apparatus consisting of nine aligned optic fibres. The plasma consists of zirconium atoms, of singly and doubly charged zirconium ions and of chromium atoms. The non-observation of emissions of tin and iron particles, which are, with chromium, the three main alloy components of Zircaloy4, suggests that the concentrations of these two species in the plasma are negligibly small. Distribution temperatures of atomic and ionic species of the order of 1 eV and high ionization degree of the plasma (greater than 70%) have been determined. The similar decay of the line intensities of the various species with increasing axial distance from the cathode surface indicates that the plasma composition remains approximately unchanged within the interelectrode region. Synthesis of the spectroscopic results has shown that the emission of vapour into the plasma cannot be accounted for by a mechanism of metal volatilization from the cathodic and anodic liquid surfaces only. It also involves emission mechanisms occurring in the cathode spot region, like the expulsion of metal droplets which volatilize or the ejection of particles.

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.

Integrated ion flux emitted from the cathode spot region of a diffuse vacuum arc

This paper reviews the properties of the cathode ion flux generated in the vacuum arc, concentrating on the characteristics of the ion energy distributions of the cathode ions. The cathode 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 authors' calculations are based mainly upon the energy distribution data of the different ion fluxes, the data available for various cathode materials are summarized. They assume that the integrated energy distribution (IED) of the ion flux for a particular material may be treated as the sum of all the fractional ion energy distributions and present plots of such IEDS. They further assume that a shifted Maxwellian type velocity distribution gives the best fit to the IED, and calculate the characteristic parameters (shifted velocity v0, thermal velocity beta and normalization constant C1) for each material. IEDS calculated using these parameters agree reasonable well with the experimental data. The ratios vp/v0 (vp is the most probable velocity) and v0/ beta are evaluated and used in discussing theoretical ion acceleration mechanisms. Their analysis suggests that the mechanism making the greatest contribution to the energy of the ion flux is the electro-ion collision mechanism, but that the contributions of the electric field force and flow interaction (pressure gradient) mechanisms are also significant. Their results should be applicable to any vacuum arc where the cathode spots may be treated as individual emitting sites, i.e. where collective interactions between cathode spots may be neglected.

Fluctuations of charged particle and light emission in vacuum arcs

The high-frequency fluctuations, superimposed on arc current, ion current, light intensity and arc voltage of a single cathode spot Cu-vapour vacuum discharge are investigated. For a current above approximately 40 A, fluctuations in arc current are smaller than 1%. In arc voltage, variations on a MHz frequency scale are observed in the order of 10% of the time averaged voltage. A temporary increase of arc voltage is correlated to an increase of up to 100% in the ion current, drawn by a grounded probe 3.6 cm away from the cathode spot. A delay between the two signals is interpreted as the time of flight of a quasi-neutral high-density front (at a velocity of 1.1*104 m s-1), resulting from a pulsed production of ionised mass in the cathode spot region of the discharge. A synchronous correlation is established between the HF component of arc voltage and light intensity, originating from the cathode spot. The time-averaged light intensity, however, is proportional to arc current. Analysis shows that the fluctuations in light intensity-reaching amplitudes up to 100% of the average level-are probably due to a varying cathode spot density, rather than to a varying electron temperature. In the frequency spectra of all the signals involved, no peaks are found.

On sputtering in vacuum arcs

The effective condensation coefficient for the ion flux coming from the cathode spot region of a vacuum arc will be influenced by sputtering. Calculations for a copper vacuum arc indicate that sputtering alone will limit the effective ion condensation coefficient to, at most, 0.94 or 0.80 for copper surfaces at anode or cathode potential, respectively.

3876. Erosion products from the cathode spot region of a copper vacuum arc. (USA)

3876. Erosion products from the cathode spot region of a copper vacuum arc. (USA)