Ecton Model Research Articles

Arc erosion characteristics of W-fuzz samples with different thicknesses of the nanostructured layer

An analysis of the results of measuring the erosion rates of W-fuzz samples with different fuzz thicknesses subject to arcing has been performed. Based on the examination of the structure of arc tracks, it has been concluded that a sharp increase in the erosion rate was accompanied by melting of the nanostructured layer. At the same time, numerous frozen droplets were observed at the boundary of the cathode spot track on the samples with a large fuzz thickness. Individual droplets were also found at some distance from the boundary. The experimental results have been interpreted theoretically using the explosive emission (ecton) model and the results of the analysis of thermal processes occurring in the cathode spot.

Threshold parameters of vacuum arcs with W-fuzz cathodes

An experimental technique was developed and performed to investigate the threshold currents and operation times of vacuum arcs with tungsten cathodes having a nanostructured (so-called “fuzz”) surface. The fuzz layer, pre-formed due to exposure of the tungsten samples with helium plasma, reduced the threshold current for arc initiation by an order of magnitude. For currents of tenths of an ampere, the average arc operation time was in the microsecond range. The experimental data were interpreted using the explosive emission (ecton) model of a vacuum arc cathode spot.

Theoretical Model of the Retrograde Motion of Cathode Spots of Vacuum Arc

The phenomenon of cathode spot (CS) retrograde motion has been attracting extensive research in the field of vacuum arc. Although a number of theoretical models have been proposed to explain this phenomenon, up to now, the first-principle understanding is still missing. In this work, based on the generalized Ohm’s law, it is proposed that there is a transport current in the explosive emission center when a transverse magnetic field (TMF) is applied. The investigation of the influence of the transport current on the strength of the electric field in the cathode sheath suggests that the transport current affects the ignition of new CS in the retrograde direction. In the framework of the ecton model, combined with the dynamics of explosive emission plasma, the relationship between the strength of applied TMF and the ignition probability of new CS in different directions is established, and the retrograde motion characteristics of CS are simulated using a statistical model. The simulation results show that the directionality of retrograde motion of CS is enhanced with an increase in TMF, and the retrograde motion speed of CS increases with the increase of the applied TMF. When the strength of TMF is small, the motion speed of a single CS has a linear relationship with the strength of TMF, but with the increase of the strength of TMF, the motion speed of a single CS is gradually saturated, which is consistent with experimental facts. The feasibility of the model is verified by comparing the simulation results with relevant experimental results.

Самоорганизация газоразрядной плазмы в SF-=SUB=-6-=/SUB=- и смесях на его основе

It was shown that the dynamics of a self-initiated volume discharge in SF6-based mixtures can be explained on the base of the ecton model of the cathode spots birth and the current density limiting effect due to the gas mixture molecular components dissociation.

The sequence of processes in the ecton cycle of a vacuum arc

The paper deals with the temporal structure of the explosive electron emission cell of the vacuum arc cathode spot. Essential features of the cathode spot cells operation - the ignition, burning and extinction have been considered in frames of the ecton model.

Ignition and erosion of materials by arcing in fusion‐relevant conditions

This study investigated the characteristics of arcing initiated on nanostructured fuzzy tungsten (W) surfaces in helium (He) plasmas. Under the same He plasma conditions, the relationship between arc voltage and current was characterized as linear due to plasma resistance across the He plasma from the target surface to the plasma source. Properties of the erosion caused by arcing were investigated by focusing on two parameters: arc current and fuzzy layer thickness. The erosion per charge was significantly dependent on the thickness of the fuzzy layer but did not depend on the arc current. Sample surface analysis revealed that drastic changes in arc spot grouping features were accompanied by the fuzzy layer thickness variation. The ecton model indicated that the erosion as ions does not play a main role, but mechanical destruction by momentum transfer from the explosive electron emission centre to neighbouring nanowires seems to enhance the erosion greatly.

Transition in velocity and grouping of arc spot on different nanostructured tungsten electrodes

Behavior of arc spots was investigated in detail using a nanostructured tungsten specimen with different thicknesses of the nanostructured layer. From the observation using a fast framing camera, it was found that the velocity of the arc spots significantly altered as passing the boundary of the two layers. The changes in spot velocity and spot width were discussed theoretically using the ecton model. The fractal dimension of the arc trail evaluated by using a box-counting method was significantly changed. Also, the width of arc trail was increased with the nanostructured layer thickness. From the SEM analysis of the specimen, the amount of tungsten eroded by arcing for two different thickness cases was estimated, and the erosion rates were discussed.

Charge distribution of ions in low-current vacuum-arc plasma

The charge distribution of vacuum-arc ions has been investigated for the arc between copper electrodes at currents varied from 8 to 60 A (with the arc voltage increased from 21 to 24 V). Plasma contains copper ions with charge states from +1 to +4. The mean ion charge state increases from 2.15 to 2.24 (i.e., by only 4%) due to a small increase in the fractions of multiply charged ions (Cu2+ and Cu3+) in the total ion flux. This conclusion does not contradict that of the ecton model of vacuum-arc cathode spot, according to which the number of cells in the spot that are due to microexplosions on the cathode increases proportionally to the arc current.

A new approach for explanation of specimen rupture under high electric field

Failure of needle-shaped specimens under the action of high electric fields is one of the most serious problems in many field-ionization devices and methods. It was shown that the length of what was removed during discharge part of specimens in most of the cases was several orders greater than that of an initial radius of the specimen apex. Experimental results obtained in this work indicate that electrical breakdown in field-ion microscopes is initiated by the impact of nanoparticles detached from a point anode in a high electric field. A new approach for explanation of an observed specimen failure in a high electric field was proposed, where the formation and expansion of the cathode plasma play the key role in the mechanical rupture and melting of field-ion tips. The expanding plasma behaves as a dynamic capacitor plate characterized by a high velocity and increasing field strength at the specimen tip. The field rise rate at a constant voltage in a dynamic plasma-to-tip capacitor is of the order of >10 20 V/m/s. The present experimental data along with the ecton model of the vacuum breakdown allow the results to be discussed as a mechanical sample failure under the action of the superhigh positive field pulse induced by the cathode plasma expansion.

Structure and time behavior of vacuum arc cathode spots

This paper reviews the state-of-the-art study of the physical processes in the cathode spot of a vacuum arc. The most important experimental data are explained in terms of the ecton model of a cathode spot. The finite lifetime of an ecton is responsible for the cyclic character of the processes occurring in a cathode spot. It has been shown that the arc plasma is generated by microexplosions occurring at the cathode surface heated by the Joule mechanism due to the high-explosive emission current density. Up to kiloampere currents, the charge state of the arc plasma and directed velocities of the ions are governed by the operation of a cathode spot cell-an ecton.

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.

Mechanism of ion flow generation in vacuum arcs

The ecton model of the cathode spot is used to analyze the main parameters of ion flow in vacuum arcs (ion erosion, mean charge, and velocity). It is shown that the arc plasma is formed as a result of microexplosions at the cathode surface, induced by the Joule heating by the high-density current of explosive electron emission. Ionization processes are localized in a narrow region of the order of a micrometer near the cathode and the ionization composition of the plasma subsequently remains unchanged. Under the action of the electron pressure gradient, ions acquire directional velocities of the order of 106 cm/s even over small distances of the order of several micrometers.

Ecton mechanism for the generation of ion flows in a vacuum arc

The principal characteristics of the process of generation of the cathode plasma in a vacuum arc (ion erosion, mean charge of ions) are considered in terms of the ecton model of the cathode spot of a vacuum arc. The estimates of the parameters of ions obtained for a unit cell of a cathode spot – an ecton – are in good qualitative and quantitative agreement with experimental data. The following mechanism for the generation of the cathode plasma of a vacuum arc is proposed. When a region of the cathode is destroyed in an explosive manner due to Joule heating, the material of the cathode sequentially goes over a series of states: the condensed state and the states of imperfect and ideal plasma. During this transition the charge state of the plasma is formed and the ions are accelerated under the action of the pressure gradient in the plasma whose density decreases by several orders of magnitude over distances of ∼10 μm from the cathode surface. The increase in current results in an increase in number of cells, and the principal parameters of the ions are formed as a result of the operation of a unit cell of the spot.

Spontaneous arc quenching in the ecton model

It is shown that the phenomenon of spontaneous quenching of a vacuum arc is determined by a finite ecton lifetime and the related cyclic character of processes occurring in a cathode spot of the arc. The main experimental results are explained within the framework of a statistical model based on the ecton model of the cathode spot operation.

The cathode spot of a high-current vacuum arc as a multiecton phenomenon

The operation of the cathode spot of a high-current vacuum arc is treated in terms of the ecton model. It has been demonstrated that, in this case, the cathode spot cells are grouped to adjoin one another. The existence of such,a collective spot is due to the cumulative effect of a number of microexplosions, resulting in that the current density in the spot increases by-almost an order of magnitude. Moreover, in such a spot, conditions are realized which are energetically profitable for the repetition of ecton processes, namely, an elevated temperature of the surface and an increased density of the near-cathode plasma. The parameters of a collective cathode spot, such as the crater diameter, the current density, and the lifetime, have been related to the arc current. The predicted relationships are in good agreement with experimental results. It has been shown that as the arc current increases, the current density in the cathode spot drops, resulting in division of the spot. An estimate of the limiting arc current per unit spot has been obtained.

Spontaneous extinguishing of a vacuum arc in terms of the ecton model

Based on the ecton model, a statistical method for studying the processes in the cathode spot of a vacuum arc has been proposed. It has been demonstrated that the spontaneous extinguishing of a DC arc and the effect of "current chopping" inherent in an AC arc are due to the cellular structure of a cathode spot and by the finite lifetime of an ecton. The principal characteristics that are responsible for the stability of the arc operation are the current per cell and the efficiency of the recovery mechanism. The average operative time of the discharge is longer for the materials showing lower currents per cell. The influence of the conditions of arc operation, such as the parameters of the external electric circuit, the cathode surface condition, the presence (or absence) of an external electric field, etc., is reduced in the main to a change in the efficiency of the recovery mechanism.