Cathode Spot Motion Research Articles

Thermocapillary convection and dynamics of the cathode spot at the liquid cathode

Consideration is given to the influence of the Marangoni convection, which develops in a layer of liquid metal under the cathode spot, on its properties. The structure and parameters of the thermocapillary cell formed have been determined. It is shown that the convective transfer of heat plays a dominant role in the cathode spot and the thermal model, which ignores it, will lead to highly overestimated temperature values. We describe a dynamic model of equilibrium of the liquid-metal surface in the spot which explains the possibility of appearance of high local pressures in the fluid. We discuss the absolute instability of the cathode spot rest state and the hydrodynamic mechanism of motion, which is determined by breakdown of the symmetry of a convective cell. It appears that such different (at first glance) phenomena as the random motion of the cathode spot, the repulsion of the spots during the approach and division, the detention of the spot on the wetting line and the retrograde motion of the spot in a magnetic field can be explained in unified terms with the aid of the above-mentioned hydrodynamic mechanism. The cited effects are characterized by external actions on the spot of different nature, which give rise to an asymmetry of convection with respect to the heating centre and, consequently, the ordered motion of the cathode spot. The behaviour of the cathode spot on a liquid-metal layer of variable thickness is considered and a new method of detention of the spot is proposed. We propose ideas of experiments which would make it possible to determine the size of a thermocapillary cell and to check the hydrodynamic mechanism of the cathode spot motion. We discuss the relationship of the phenomena considered with the processes of ordering and self-organization in nonequilibrium systems.

Splitting and Motion of Diffuse Arc Cathode Spots in an Axial Magnetic Field

By high-speed photography, diffuse vacuum arcs between opening contacts in axial magnetic fields were found to have, in general, a concentrated cathode spot at the beginning of arc initiation. The spot then begins to split into two or more fragments when sinusoidal arc current and axial magnetic fields rise with time. It was found that at instantaneous current the value for the first splitting of the cathode spot decreases with increasing axial magnetic fields, and the current density of the cathode spots increases with increasing arc current. Experimental results showed that the group of cathode spots under the influence of axial magnetic fields was nearly uniformly distributed in dynamic form within a definite area on the cathode surface. It was also shown that the cathode spot-covered area reached a maximum value which lagged slightly behind the peak arc current. The group of the cathode spots expanded and contracted at the speed of the so-called bi-hump characteristics, i.e., the spots expanded twice at high speed before the maximum value of the spot-covered area was reached. These experimental results are discussed.

Cathode spot motion in high-current vacuum arcs under self-generated azimuthal and applied axial magnetic fields

The motion of vacuum arc cathode spots under the influence of self-generated azimuthal and externally applied axial magnetic fields has been investigated for several metals at currents up to about 10 kA. The spot motion was recorded by means of a high-speed framing camera, and the self-generated magnetic field was measured with an inductive magnetic probe. The axial magnetic field was applied by means of two coils round the vacuum chamber in an approximate Helmholtz arrangement. The data have been used to determine the relationships between the retrograde (anti-Amperian) spot velocity and the self and axial magnetic flux densities. At low self-generated fields, the velocity is directly proportional to the flux density but at high fields velocity saturation is observed in some cases. An axial magnetic field decreased the retrograde velocities on all metals investigated. The spontaneous spot formation observed with high rates of current rise is analysed; the velocity saturation phenomenon is explained semiquantitatively and the influence of the axial magnetic field on the cathode spot velocity discussed.

Random walk of cathode arc spots in vacuum

The cathode spot movement of a vacuum arc with Al, Cd, Mo and Cu cathodes was investigated. Provided external disturbances are absent the cathode spot was found to move at random. The two-dimensional distributions of spot displacements can be described by a Rayleigh function. From the experimental data the average distances between cathode craters were deduced, leading to values of 2-3 times the crater diameter. Crater formation times, derived from the random walk data, were used to calculate the erosion rates of the metals listed. Good agreement was found between these data and earlier experimental results of cathode erosion rates and ion flows.

A study of the erosion rate of vacuum arcs in a transverse magnetic field

The cathode mass loss due to the action of low-current vacuum arcs under the influence of a high transverse magnetic field has been measured for titanium, stainless steel and graphite. The effects of magnetic field, arc current and cathode temperature on the erosion rate are investigated. The measured erosion rate decreases marginally with the magnetic field, the total variation being less than 20% and the rate of decrease is much lower at higher fields. Within the experimental accuracy only very small or no increase of erosion rate is observed with the arc current for currents up to 70 A. For measurements made at higher cathode temperatures (upto 500°C) there is a considerable increase in erosion rate for the stainless steel cathode and the results are discussed for titanium and graphite. The importance of clean cathode surfaces in the erosion studies is demonstrated by observing the cathode spot motion using an Image Converter Streak Camera. This method has proved to be an effective diagnostic tool in judging the surface condition before any measurements are made. Prolonged glow discharge cleaning in hydrogen is found to provide an acceptably clean surface. The earlier reported cathode spot velocity measurements are extended to higher magnetic fields.

Cathode spots and vacuum arcs

Considerable effort has been devoted to the analysis of mass flows in vacuum arcs during recent years. It is generally agreed that there are three different fluxes which orginate from the cathode; they consist of ions, molten droplets and metal vapour. A critical summary will be given of the available data on these mass flows. The question of the large discrepancies in cathode erosion rate measurements is discussed. Minimum and maximum erosion rates are analyzed and it is shown that these discrepancies can be understood only if the variation of liquid metal erosion with different parameters is taken into account. The origin of droplet-flow and ion-flow is considered and the available data pertinent to this subject is discussed. It is concluded that the origin of ionized mass flow is directly related to the generation of crater structures in the cathode surface. Droplet erosion, on the other hand, is primarily related to cathode spot movement and is a surface heating effect. Finally, a summary is given of a model in which the different observations on cathode flows are explained in a consistent way. The model is supported by a numerical analysis of ion mass production by Joule heating. Close agreement is obtained for around twenty different cathode metals.

On the Nature and the Motion of Arc Cathode Spots in UHV

AbstractCathode spot types and spot motion of arcs in ultra high vacuum have been investigated with large area cathodes that consisted of two adjacent pieces of Mo and Cu. Arc currents were 20–60 A dc and 8–20 kA pulse (duration about 1 ms). Two spot types occured with different velocities and surface erosion: Type 1 spots are typical for surfaces covered by oxides or thick adsorption layers, whereas clean surfaces show only type 2 spots.During arc‐conditioning both types exist simultaneously in a complex mutual dependence. Type 1 spots react weakly on the cathode material, while type 2 spots burn preferentially on Cu and at the boundary line between Mo and Cu. The motion of type 1 spots is determined by the expanding spot plasma, whereas type 2 spots show a step‐by step motion, determined by explosions in the arc craters. Generally a spontaneous formation of type 2 spots beneath the arc plasma takes place only with contaminated surfaces (probably by a transition from type 1 to type 2 spots). Thus a breakdown between plasma and cathode surface requires the presence of contaminations. The observed effects occur in low current dc‐arcs as well as in high current pulse arcs. They are discussed for different spot models.

Velocity Distribution of the Moving Cathode Spot in Breaking Contact Arcs

The motion of cathode spot in Au, Ag, Cu, and Ni contacts, which affects the formation of the pip and crater structures, has been precisely observed by utilizing a high-speed camera in moderate current, atmospheric pressure dc break arcs. Dependencies of contact material, current and voltage on the motion of cathode spot were examined experimentally. Behaviors of the cathode spot were evaluated by three factors: 1) the maximum length moved (l <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mx</inf> ) which was defined as a maximum shift distance of the spot in an arcing; 2) the maximum moving width (l <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mw</inf> ) which was the difference between the maximum and minimum distances on the motion of cathode spot; and 3) distributions of velocity of moving cathode spot. For these contact materials, the cathode spot moved rapidly in the sequence of Au > Ag > Cu > Ni. Moreover, the crater structures of each contact have been reasonably explained through the above behavior of the cathode spot.

Displacement of Field Emitters by Vacuum Discharges

AbstractThe location of field emitting micro‐points, produced by nanosecond discharges in UHV, has been investigated by field emission microscopy. Weak discharges (duration &lt; 5 ns, current &lt; 10 A) caused a displacement of the field emission over the cathode by a distance that corresponds to average crater diameters (4–6 μm). Thus new emitters are produced at the boundary of discharge craters. More intense discharges show sometimes a far higher displacement. This can be explained by the formation of micro‐points by splashes of molten metal that fly out of the discharge craters. The results support the model of the development of micro‐points, as it was published in [1]. They show furthermore that the motion of arc cathode spots can be related to the displacement of microscopic field emitters.

1254. Cathode spot motion in high-current vacuum arcs on copper electrodes: J C Sherman et al,J Phys D: Appl Phys, 8 (6), 1975, 696–702

1254. Cathode spot motion in high-current vacuum arcs on copper electrodes: J C Sherman et al,J Phys D: Appl Phys, 8 (6), 1975, 696–702

Cathode spot motion in high current vacuum arcs on copper electrodes

The motion of vacuum arc cathode spots under the influence of the self-generated magnetic field has been investigated for copper electrodes at currents up to 6.9 kA. The spots were photographed with a high-speed framing camera. The magnetic flux density was measured. Surface contamination caused large changes in spot velocity. For velocities up to 15 ms-1 and corresponding flux densities up to 2*10-2T the velocity is proportional to the flux density. Above 20 ms-1 and 4*10-2T the velocity increases relatively slowly with increasing flux density. Velocities of more than 30 ms-1 were not observed.

プラズマ・アーク切断現象に関する基礎的研究(第1報)

In a plasma arc cutting, the behaviour of anode or cathode spots in a cutting kerf affects not only the distribution of heat input in the work piece but also the quality of the kerf or cut surface. Using a stack plate of water-cooled copper, the current in each plate was measured with arc voltage in order to clarify the fundamental behaviour of such a spot. Moreover, the spot motion in actual cutting was observed by an optical method, and analyzed from the correlation of the fluctuations between radiation intensity from the spot and arc voltage.Essentially, the sopt is carried away from the upper to lower side on the front surface of the cutting zone or the edge surface of the work piece due to the momentum of plasma and cold surrounding gas streams and due to the magnetic force acting on the spot and arc column. When the voltage between the arc column and the front surface of the kerf reaches a breakdown voltage, a new spot is suddenly made on the upper side. Consequently, the spot periodically repeats such a motion, so that the arc voltage fluctuates as sawteeth waves in several kHz to several 10 kHz corresponding to the spot motion.The above consideration shows that the wandering region of the spot is governed by the breakdown voltage of the sheath and the potential gradient of the arc column. Especially, effect of the former is considerably large. At reverse polarity, the breakdown voltage is higher than at straitgh polarity, because the voltage to make a new cathode spot can not be neglected as well known from the restrike phenomena in A.C. arc. Thus, the spot wandering regon at reverse polarity is extended in comparison with that at straight polarity.It is a noticeable fact that the cathode spot motion is remarkably affected by the condition of the plate surface and surrounding gas. If air mixing into the plasma stream increases, the cathode sopt seeks out an oxide film, and behaves the same as in cleaning phenomena at reverse polarity of TIG arc welding. Arc voltage, therefore, shows the characteristic triangle waves in several 100 Hz.

Dynamics of Arc Ignition and Cathode Spot Movement of Thermionically Emitting Cathode Surfaces

If a thermionically emitting cathode is used in an arc discharge the heat feedback from the gaseous plasma to the solid surface provides the continuity of the arc process. The response time of the temperature fluctuation of the solid-surface material is found to be at least an order-of-magnitude longer than that of the plasma, therefore, the rate process is necessarily controlled by the slowest branch in the process, namely, the solid material. Based on this logic, the initial stages of electric arc ignition and cathode spot growth are analyzed for a thermionically emitting cathode. A feedback heat-transfer process which couples the plasma and the solid surface is hypothesized to govern these phenomena. While the model chosen has been simplified to the extent that quantitative significance may be lost, however, several previously observed qualitative features of arcs are explained, one of which is the need for a minimum current density for the persistence of a thermionic arc. The cathode spot growth can also be accounted for by this hypothetical model.

低圧雰囲気におけるアーク現象(第1報)

Systematic research has been made on arc phenomena at low pressure atmosphere to clarify the electrode mechanism. Distinguished features of low pressure arc were found in cathode and anode phenomena, which appear eminently in the arc characteristics and appearance. For example, pure graphite cathode, which used to behave as a typical thermionic cathode (hot cathode), exhibits so called cold cathode under the condition of low pressure and small current. This cold cathode is characterized by random movement of a constricted cathode spot and strong vapour jet from the spot area.This paper discusses the result of the systematic analysis on arc characteristics and their relation to transition of the cathode mode. The experiment was done under the condition of 10-150 Torr of pure Hydrogen atmosphere and 10-120 A of arc current. The electrode material was pure graphite rods (99.99%) of 6 mmφ.Conclusions obtained are as follows:(1) Transition of the cathode mode from the hot cathode to the cold cathode is mainly governed by arc current I, ambient gas pressure p and ionization p4tential Vi of the gas in the cathode fall region. Arc voltage suddenly falls by a few volts when the hot cathode changes to the cold one, though the average electric field of arc column Ep increases abruptly because of the shrinkage of column. This fact suggests that, in the cold cathode mode, Carbon rather than Hydrogen is ionized in the cathode fall region, and consequently the cathode drop Vσ reduces.(2) Sum of the anode and cathode drops, (VA+VC), increases gradually in the range of the hot cathode with the decrease of ambient gas pressure and arc current. This value of (VA+VC) suddenly falls by 3-4 volts under the critical condition of ambient gas pressure and arc current when the cathode mode changes to the cold cathode. This abrupt change of (VA+VC) is due to the decrease of the cathode drop VC.(3) In the range of the cold cathode, (VA +VC) decreases with the reduction of ambient gas pressure, while (VA+VC) remains almost constant against arc current.(4) Variation of the cathode drop VC in both cathode. modes is explained qualitatively by Slepian's ionization mechanism in the cathode fall region.

Motion and Spectrum of Arc Cathode Spot in a Magnetic Field

The velocity of the cathode spot of a mercury arc at the junction between liquid and metal in a transverse magnetic field has been measured for magnetic field strengths between 0 and 20 700 oersteds. The approximate doubling of retrograde velocity at about 11 000 to 15 000 oersteds was followed by an additional rapid rise of velocity at about 15 000 oersteds. Spectra of the arc showed Hg ii and Hg iii lines at the stronger magnetic fields. Radiation from the cathode spot showed mercury lines and a continuous spectrum which is especially intense at the lines. Some Hg lines are broadened symmetrically and others asymmetrically. If the broadening is due to a Stark effect, the electric field strength in the cathode spot region is greater than 6\ifmmode\times\else\texttimes\fi{}${10}^{6}$ volts/cm.The arc mechanism previously proposed is extended to explain the rapid velocity rises with increasing magnetic field strength by associating them with the effect of ${\mathrm{Hg}}^{++}$ and ${\mathrm{Hg}}^{+++}$ ions.

Motion of Arc Cathode Spot in a Magnetic Field

The motion of the cathode spot of a mercury arc was observed under various transverse magnetic fields, arc currents, and inert gas pressures. With no inert gas, the retrograde velocity increased as the magnetic field strength increased. Addition of an inert gas at low pressures reduced the retrograde velocity and at higher pressures caused a forward (direction of magnetic force) motion.The motion of the cathode spot in a magnetic field can be associated with electrostatic forces on positive ions produced by electron impact. A cathode spot assembly consisting of a group of electrons in the cathode, a positive ion sheath separated by a dark space from the cathode, and an electron cloud between the positive ion sheath and anode is postulated.Electrons after acceleration through the cathode dark space would travel a curved path due to the magnetic field and produce new positive ions on the forward side of the cathode spot. These new positive ions would be attracted by the cathode spot assembly with a resultant force that would cause them to overshoot the positive ion sheath before being pulled into the cathode to start a new spot in the retrograde direction.Addition of an inert gas reduces the mobility of positive ions to the order of that of the cathode electrons thus permitting forward motion of the cathode spot assembly to reduce the overshooting. Higher pressures would cause a forward spot motion.

Origin of Retrograde Motion of Arc Cathode Spots

Origin of Retrograde Motion of Arc Cathode Spots

The Retrograde Motion of the Arc Cathode Spot

The motion of the cathode spot in a direction opposite to that predicted by Ampère's law depends on the arc current, gas pressure, kind of gas, and magnetic field strength. Studies of the effects of these variables have been made, including measurements of velocity and the critical pressure at which reversal of motion occurs. The phenomena observed have not been clearly explained by any of the pictures presented to date. The existence of the retrograde motion indicates very strongly that the positive-ion space charge outside the cathode is all important in determining the mechanism of current transfer.

A Note Concerning the Motion of Arc Cathode Spots in Magnetic Fields

A Note Concerning the Motion of Arc Cathode Spots in Magnetic Fields