Kinetic model of short vacuum arc with active anode

Measurements of spatial distribution of the intensity of radiation of different ionization sates of copper (CuI, CuII and CuIII) in a short (8 mm length) low-current vacuum arc with copper electrodes were carried out. The arc current I = 60 A = Is (Is - average current per cathode spot). That is, an arc with a single spot was investigated. The spatial and temporal resolutions are, respectively, 0.5 mm and 50 μs. Induction of the external axial magnetic field 0 ≤ B ≤ 0.1 T. The investigations showed that the state of the plasma and the shape of the channel of the short arc are determined not only by cathode process. In the near-anode region of the arc, changes take place both inside the channel and in the "coat" of low ionized plasma surrounding the arc. Interaction of high-speed ions of the cathode spot plasma jet with the anode forms a flow of atoms from that part of the surface to which the arc is attached. The atoms are ionized in the immediate vicinity of its surface. This increases the plasma concentration and decreases the average charge of ions in the near-anode part of the arc, in comparison with the average charge of the cathode spot plasma jet.

Measurements of radiation intensity spatial distribution for different copper ionization states (CuI, CuII, and CuIII) were carried out in short (8 and 4 mm long) low-current vacuum arcs with copper electrodes. The arc current was I = 60 A = I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> [where I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> is the mean current per cathode spot (CS)]. That is, the single-spot arcs were investigated. The spatial and temporal resolutions were 0.5 mm and 50 μs, respectively. The external axial magnetic field induction was 0 ≤ B ≤ 0.1 T . The studies have shown that the plasma state and the column shape and structure in short arcs are determined not only by cathode processes. In the near-anode region of the arc, changes take place both inside the arc channel and in the “coat” of weakly ionized plasma that surrounds it. The interaction of the high-speed ions of the CS plasma jet (PJ) with the anode forms a flow of atoms from the anode surface part to which the arc is attached. The atoms are ionized in the immediate vicinity of its surface. This increases the plasma concentration and decreases the average charge of ions in the near-anode part of the arc, as compared with the average charge of the CS PJ.

Generation of multiply charged metal ions in a high current short pulse vacuum arc

As the main source of the vacuum arc plasma, cathode spots (CSs) play an important role on the behaviors of the vacuum arc. Their characteristics are affected by many factors, especially by the magnetic field. In this paper, the characteristics of the plasma jet from a single CS in vacuum arc under external axial magnetic field (AMF) are studied. A multi-species magneto-hydro-dynamic (MHD) model is established to describe the vacuum arc. The anode temperature is calculated by the anode activity model based on the energy flux obtained from the MHD model. The simulation results indicate that the external AMF has a significant effect on the characteristic of the plasma jet. When the external AMF is high enough, a bright spot appears on the anode surface. This is because with a higher AMF, the contraction of the diffused arc becomes more obvious, leading to a higher energy flux to the anode and thus a higher anode temperature. Then more secondary plasma can be generated near the anode, and the brightness of the ‘anode spot’ increases. During this process, the arc appearance gradually changes from a cone to a dumbbell shape. In this condition, the arc is in the diffuse mode. The appearance of the plasma jet calculated in the model is consistent with the experimental results.

Copper film deposition and anode temperature measurements in a vacuum arc with tungsten anode

Anode activity is critical in high-current vacuum arcs in vacuum interrupters. If the anode temperature gets too high, an evaporation of metal vapor from an anode melting pool may lead to a formation of anode spots, which would lead to a failure of current interruption of vacuum interrupters. The input energy from an arc column to an anode surface is adjusted by an anode sheath. The objective of this paper is to develop a numerical simulation model of the anode sheath of vacuum arc by using a Particle-in-Cell method. A low-current case and a high-current case of arcing are simulated. The physical parameters of electrons and ions are obtained from magnetohydrodynamic simulations, and are set at the simulation boundary of the current model. The sheath thickness is 2∼4 Debye length, and the anode potential fall is about 1.5∼3 V for both case. It is found that the acceleration of ions by electric field in the anode sheath is more significant for the high-current case. This also adds to the energy flux into the anode surface.

The value of the voltage fall across the anode sheath is found as a function of the current density. Analytic solutions are obtained in a wide range of the ratio of the directed velocity of plasma electrons v 0 to their thermal velocity v T . It is shown that the voltage fall in a one-dimensional collisionless anode sheath is always negative. At the small values of v 0/v T , the obtained expression asymptotically transforms into the Langmuir formula. Generalized Bohm’s criterion for an electric discharge with allowance for the space charge density ρ(0), electric field E(0), ion velocity v i (0), and ratio v 0/v T at the plasma-sheath interface is formulated. It is shown that the minimum value of the ion velocity v * (0) corresponds to the vanishing of the electric field at one point inside the sheath. The dependence of v * (0) on ρ(0), E(0), and v 0/v T determines the boundary of the existence domain of stationary solutions in the sheath. Using this criterion, the maximum possible degree of contraction of the electron current at the anode is determined for a short high-current vacuum arc discharge.

Interelectrode phenomena related to intense-mode vacuum arc interruption were systematically compared between CuCr and AgWC electrodes by using various optical techniques mainly including two-color pyrometry for anode temperature determination after current zero, optical emission spectroscopy for the plasma composition assessment, and a Shack-Hartmann method for simultaneous 2-D electron and metal vapor density imaging. At the current peak, the widths of the radial electron density distributions for CuCr were narrower than those for AgWC. The electron density for CuCr was 3-5 times higher than that for AgWC. At current zero, the metal vapor density for CuCr was slightly lower than that for AgWC, which was specifically demonstrated for CuCr by a locally reduced metal vapor density below the minimum detectable limit of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . During and after the arcing time, mixing ratios of the copper vapor existing in the CuCr gap was about two times higher than expected from the CuCr electrode composition. After current zero, the decaying time constants of the metal vapor densities on the anode surfaces were Cr <; Cu <; Ag. This order was consistent with the decaying time constants of the anode evaporation rates estimated from the anode temperature measurement. The consistency suggests that the supplying process of the metal vapor after the intense vacuum arc interruption was the evaporation of the anode surfaces.

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.

This paper discusses the issues of a possible change of the type of cathode attachment of high-current vacuum arcs (HCVA) with an average cathode current density of more than 105 A/cm2. This type of HCVA is used as pumping plasma gun in experiments with plasma puff z-pinches. These experiments showed that the measured linear mass of the HCVA plasma jet is much higher (by a factor of 10 or more) than the expected mass, which can be obtained from the assumption that cathode attachment occurs only through a multitude of cathode spots emitting supersonic plasma jets. It is shown that in HCVA of the type under consideration, at some time instant there are two types of cathode attachments - cathode spots and thermionic erosion attachment (TEA). It can be said that HCVA of this type have a hybrid cathodic attachment. Unlike cathode spots, TEA produces a subsonic plasma flow, which contributes to an increase in the linear mass of the HCVA plasma jet.

The objective of this article is to determine the anode surface temperature during and after current zero (CZ) with single and dual vacuum arc columns. Both the alternating current (ac) and pulsed direct current (dc) waveforms were applied with a peak value of 5–16 kA. A butt-type electrode and a horseshoe-type electrode were utilized to generate single and dual vacuum arcs. In the draw arc experiment, three spectrometers with working wavelengths of: 1) 200–1000 nm; 2) 880–1700 nm; and 3) 1600–2400 nm, a fast thermography camera, and two high-speed cameras were used for optical analysis. Based on the experimental results, it was certified that spectroscopic analysis with the Planck fitting procedure is an effective method to determine the anode surface temperature after CZ. The most preferable wavelength of the spectrometer in anode temperature determination was 800–1900 nm. The temperatures of anode spot 1, anode spot type 2, and footpoint mode were 3200–3300, 3800–4000, and 3000–3100 K, respectively. Two kinks appeared in the anode temperature cooling curve. The corresponding temperature was the liquidus temperature of the electrode material and the melting point of copper. Besides, two hot areas on the anode surface were observed for the horseshoe-type electrode. The anode temperature tended to be homogeneous at about 0.8 ms after CZ.

A high-current vacuum arc subjected to an axial magnetic field is maintained in a diffuse status. With an increase in arc current, the energy carried by the arc column to the anode becomes larger and finally leads to the anode temperature exceeding the melting point of the anode material. When the anode melting pool is formed, and the rotational plasma of the arc column delivers its momentum to the melting pool, the anode melting pool starts to rotate and also flow outwards along the radial direction, which has been photographed by some researchers using high-speed cameras. In this paper, the anode temperature and melting status is calculated using the melting and solidification model. The swirl flow of the anode melting pool and deformation of the anode is calculated using the magneto-hydrodynamic (MHD) model with the volume of fraction (VOF) method. All the models are transient 2D axial-rotational symmetric models. The influence of the impaction force of the arc plasma, electromagnetic force, viscosity force, and surface tension of the liquid metal are all considered in the model. The heat flux density injected into the anode and the arc pressure are obtained from the 3D numerical simulation of the high-current vacuum arc using the MHD model, which gives more realistic parameters for the anode simulation. Simulation results show that the depth of the anode melting pool increases with an increase in the arc current. Some droplets sputter out from the anode surface, which is caused by the inertial centrifugal force of the rotational melting pool and strong plasma pressure. Compared with the previous anode melting model without consideration of anode deformation, when the deformation and swirl flow of the anode melting pool are considered, the anode temperature is relatively lower, and just a little more than the melting point of Cu. This is because of liquid droplets sputtering out of the anode surface taking much of the energy away from the anode surface. The azimuthal velocity of the anode melting pool for arc current 12.5 kA root-mean-square (rms) is larger than that for 17.5 kA (rms), which is likely to be caused by the thinner liquid layer, and also a smaller melting pool mass of 12.5 kA.

A mechanism of plasma plume generation and plasma jet formation in a microscale vacuum arc (MVA) was proposed. The uniqueness of the subject is the force generated by a common plasma plume extracted from cathode and anode spots in a MVA. A calculating model for microplasma origin was developed taking into account the atom ionization and the phenomena in the nonequilibrium layers near evaporated surface in the cathode and anode regions. The plasma parameters as well as the electrode erosion rate were studied. It was found relatively large anode erosion rate and force at the anode surface in MVA in comparison to the force that usually occurs at the cathode in arcs with large gap. This important result of MVA was proposed to use in an exceptional microthruster concept for spacecraft propulsion and for anode microplasma source.

Summary form only given. In our investigation of ion charge state distributions (CSD) in vacuum arc plasmas, good correlation between increase in arc operating voltage and mean ion charge state has been established (Oks et al., 1996). Therefore, to increase the mean charge state of a vacuum arc plasma, it is necessary to find ways to increase the arc operating voltage. The voltage can be increased via transients associated with the arc current by means of which a rather high operation voltage can be established across the discharge gap. Experiments were performed both in Tomsk and Berkeley with the discharge system of a vacuum arc ion source. To effect a step current rise an additional power supply was connected to the usual vacuum arc supply. This power supply made it possible to increase the vacuum arc current up to 1 kA for several /spl mu/s. As a rule the current jump was produced after 100-200 /spl mu/s into the main discharge pulse when all principal parameters of the vacuum arc were already established. To measure the CSD a time-of-flight method was used.

The importance of understanding the energy loss specifics by the cathode for vacuum arc metallic plasma generation and its applications were emphasized. To this end, the heat conduction losses per unit current were characterized by the cathode effective voltage uef, which is weakly dependent on the current. In this paper, a physical model and a mathematical approach were developed to describe the energy dissipation due to heat conduction in the cathode body, which is heated by energy outflowed from the adjacent plasma. The arc plasma generation was considered by taking into account the kinetics of the heavy particle fluxes in the non-equilibrium layer near the vaporizing surface. The phenomena of electric sheath, heat and mass transfer at the cathode were taken into account. The self-consistent numerical analysis was performed with a system of equations for a copper cathode spot. The transient analysis starts from the spot initiation, modeled by the plasma arising at the initial time determined by the kind of arc triggering, up to spot development. The results of the calculations show that the cathode effective voltage uef is determined by the cathode temperature as a function of spot time. The calculated evolution of the voltage uef shows that the steady state of uef is approximately 7 V, and it is reached when the cathode temperature reaches a steady state at approximately one microsecond. This essential result provides an explanation for the good agreement with the experimental cathode effective voltage (6–8 V) measured for the arc duration from one millisecond up to a few seconds.