Sergey A. Barengolts

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.

Initiation of ecton processes by interaction of a plasma with a microprotrusion on a metal surface

Evolution of rapid (∼10 ns) Ohmic overheating of a microprotrusion on a surface in contact with a plasma by emission current is studied taking into account the energy carried by plasma ions and electrons, as well as Ohmic heating, emissive source of energy release (Nottingham effect), and heat removal due to heat conduction. Plasma parameters were considered in the range of n = 1014−1020 cm−3 and Te = 0.1 eV−10 keV. The threshold value of energy transferred to the surface from the plasma is found to be 200 MW/cm2; above this value, heating becomes explosive (namely, an increase in the temperature growth rate (δ2T/δt2 > 0) and in passing current (δJ/δt > 0) is observed in the final stage at T ∼ 104 K and j ∼ 108 A/cm2). In spite of the fact that Ohmic heating does not play any significant role for plasmas with a density lower than 10 18 cm−3 because the current is limited by the space charge of electrons, rapid overheating of top of microprotrusion is observed much sooner (over a time period of ∼1 ns) when the threshold is exceeded. In this case, intense ionization of vapor of the wall material leads to an increase in the plasma density at the surface, and the heating becomes of the Ohmic explosion type. Such conditions for the formation of a micrĭxplosion on the surface and of an ecton accompanying it can be created during the interaction of a plasma with the cathode, anode, or an insulated wall and may lead to the formation of cathode and anode spots, as well as unipolar arcs.

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.

The ecton mechanism of unipolar arcing in magnetic confinement fusion devices

It has been shown that the source of current and erosion plasma in a unipolar arc is explosive electron emission, which occurs as ejection of individual portions of electrons named ectons. This phenomenon is responsible for the numerous microcraters left by unipolar arcs on metal surfaces. An arc of this type is self-sustained due to the interaction of the erosion plasma with the electrode surface. The duration of an arc is determined by the conditions of its initiation: the higher the arc current, and hence, the number of cells in the spot formed on arc initiation, the longer the arc 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.

Explosive Electron Emission Ignition at the “W-Fuzz” Surface Under Plasma Power Load

The essentially nonstationary explosivelike character of erosion-emission processes was revealed by analyzing the plasma action onto a nanostructured W-fuzz surface. It was found that such a fine structure of the surface promotes a local ecton-process ignition by plasma action. The dense W-plasma production time is much less than that of a typical fusion-device first-wall transient-event edge localized mode (ELM) power load. The W-plasma density substantially exceeds that of the incident plasma and leads to intense growth of the electric field and strong emission buildup. Therefore, at the ELM power load, the electric field and emission current density (in the W-plasma-wall sheath) can achieve tens of megavolts per centimeter and 100 MA/cm2 or higher due to collective processes driven by an intense emission beam. The strong volume heating by Joule energy release occurs due to the intense emission current density that leads to electric explosion of other W-fuzz nanowire layers.

A simplified model of the formation of a deep potential well in a vacuum diode

The nonstationary problem of the formation of a virtual cathode in a diode with an accelerating electric field and a high-current electron beam entering the diode has been solved numerically. As a result, the possibility of the formation of a deep nonstationary potential well in the presence of an electric field in the diode gap has been shown, and a model for the current passage and formation of such a well in an explosive-electron-emission vacuum diode has been proposed.

Modeling of Cathode Plasma Flare Expansion

A kinetic 1-D model of cathode plasma flare expansion to the interelectrode vacuum gap was developed. The kinetic model is a model of 1D3V particle-in-cell and direct simulation Monte Carlo type. The model takes into account the main types of elastic and inelastic collisions of particles in the plasma as well as evaporation and thermofield electron emission from the cathode. The model treats the plasma flare expansion and the electron emission from the outer plasma boundary of the flare self-consistently. The plasma characteristics of the cathode flare obtained with the model are in a reasonable agreement with known experimental results.

The effect of cathode deuteration on the parameters of vacuum-arc plasma

We propose a model for determining the influence of the relative content of deuterium in a zirconium cathode on the properties of vacuum-arc plasma. It is shown that the occlusion of deuterium in the cathode leads to an additional energy consumption for its ionization and to the related decrease in the average charge of cathode material ions in the discharge plasma. Deuterium in the cathode spot is fully ionized, and the drift velocity of deuterium ions almost coincides with that of ions of the cathode material.

Model of Collective Acceleration of Ions in Spark Stage of Vacuum Discharge

A new mechanism of the collective acceleration of ions at the spark stage of a vacuum discharge is proposed. It has been show n that this acceleration can take place in the presence of a plasma cloud in the electrode gap with strong electronic instability developing in the plasma. The appearance of accelerated ions of the interelectrode plasma is accompanied by a jump in the diode current.