Mikhail M. Tsventoukh

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.

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.

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.

Arc spot grouping: An entanglement of arc spot cells

In recent experiments, clear transitions in velocity and trail width of an arc spot initiated on nanostructured tungsten were observed on the boundary of the thick and thin nanostructured layer regions. The velocity of arc spot was significantly decreased on the thick nanostructured region. It was suggested that the grouping decreased the velocity of arc spot. In this study, we try to explain the phenomena using a simple random walk model that has properties of directionality and self-avoidance. And grouping feature was added by installing an attractive force between spot cells with dealing with multi-spots. It was revealed that an entanglement of arc spot cells decreased the spot velocity, and spot cells tend to stamp at the same location many times.

Retrograde motion of cathode spots of the first type in a tangential magnetic field

We consider the dynamics of dense plasma of an explosive-electron-emission center—a cell of cathode spot of a vacuum arc in an applied magnetic field. It is established that the explosive expansion of plasma in a transverse magnetic field induces an electric field and an associated current. It is shown that this current initiates the cells of cathode spots of the first type in the direction opposite to the Ampere force action. In this case, the ignition time of a new spot does not exceed a few nanoseconds at a magnetic field of a few kilogauss.

On plasma jet formation in vacuum arc with composite cathode

This paper deals with the computer modeling of vacuum arc with composite multicomponent cathode. This arc is typical for certain kind of ion sources, plasma generator and vacuum interrupters. The described hybrid model treats the electrons as an inertialess fluid and ions as macroparticles. The macroparticle dynamic is calculated with the use of particle-incell method. Ion-ion Coulomb collision is considered with the use of Monte Carlo method. The model can simulate vacuum arc as a whole including separate cathode plasma jets, mixing zone, and common plasma column. The dependence of ion angular current distribution on the cathode composition reproduced with the help of developed model agrees well with experimental results.

Internal plasma pressure peaking in low-shear open and closed magnetic confinement systems

The plasma convective (flute-interchange) stability for low magnetic shear systems, with a low collisionality and a low beta, is considered in terms of the necessary and sufficient collisionless kinetic criterion. The magnetic confinement systems under consideration are axially symmetric mirrors equipped with outer divertors and inner field reversing rings (cusps, internal rings, high-beta cells) and closed multimirror traps. A simple approach is proposed for plasma stabilization resulting in a substantial steepening of the critical pressure profile. The essence is the combination of the plasma stabilization by a strong and an alternating-sign field-line curvature. Axially symmetric tandem mirror systems composed of a mirror device having an outer divertor and an inner field reversing ring are shown to have an internal radially peaked stable pressure profile. Bumpy tori are also shown to have an internally peaked stable pressure profile.

Plasma dynamics of an explosive emission center - ecton of a cathode spot in external tangential magnetic field

The plasma dynamics of an explosive-emission center demonstrates the plasma polarization in applied external magnetic field, induced by dense plasma spherical expansion across the magnetic field. The additional positive space charge appears at the retrograde side of the dense plasma due to the polarization current j<inf>ϕ</inf>=σBv/c ≈105BT [A/cm²]. Probability of initiation of a new explosive emission center increases at the retrograde side and decreases at the amperian side due to this polarization. Velocity of a cathode spot retrograde motion (for considered copper cathode) vretr was estimated as v<inf>retr</inf>∼0.3·v<inf>random</inf>·BT. For a clean surface v<inf>random</inf> ∼10⁴cm/s, hence v<inf>retr</inf> ∼0.3B<inf>Gs</inf> [cm/s]. The “Robson angle” function was derived from simple geometrical reasons of the spot-cell (explosive-emission-center) plasma polarization, i.e. ζ<inf>R</inf>=arctan(A·tanθ<inf>B</inf>), where A≡tanα<inf>pl</inf>/cosα<inf>pl</inf> and α<inf>pl</inf> = 20°–40° - plasma-jet-expansion parameter. Therefore, considering of a plasma dynamics of the explosive-emission center can give an explanation of the vacuum-arc cathode-spot retrograde motion in frames of the ecton model.

Ignition and sustainment of the explosive electron emission cyclic pulses — Ectons by plasma-surface interaction

The ignition and sustainment of the explosive electron emission cycles - ectons has been considered from point of view plasma-surface interactions. Intense plasma action onto the surface (in form of transient events such as ELMs) as well as fine structure of surface (such as W-fuzz layers and other film-like structures) have been found to promote ignition and sustainment of the explosive electron emission cycles - ectons. Easy erosion of such a film structures and the readiness of the explosive electron emission on them, in turn, indicates probably a lower specific erosion due to the arcing (e.g. less droplets) in comparison with the solid targets.

Magnetic field influence on the ecton processes ignition and sustainment

The dynamics of the dense pulsed plasma of the explosive emission center has been considered with taking into account applied magnetic field influence on the ignition and sustainment of the ecton processes. The transient spark stage and arc stage of a vacuum discharge have been analyzed from point of view non-stationary explosive-like behavior of the ecton cycles. It has been proposed that for a high-energy ion flux during the spark stage an intense sputtering and erosion plasma production could be responsible for the new ectons ignition under the cathode flares action. It has been found that the polarization current jφ = σvplB/c induced by explosive expanding of the ecton plasma (vpt~10 km/s) across the magnetic field give rise to the additional positive current density at the retrograde side of the ecton plasma. Taking into account a critical value of the positive current density from the plasma to surface, jcr of about 0.1-1 MA/cm2 one results in roughly linear dependence of the average displacement, δx, from the magnetic field. The corresponding retrograde drift velocity, vdrift/B, is about 40 m/s/T (for jcr = 1 MA/cm2) and 1300-1900 m/s/T (foryc,= 0.1 MA/cm2).