St Kolev

A 2D model for a gliding arc discharge

In this study we report on a 2D fluid model of a gliding arc discharge in argon. Despite the 3D nature of the discharge, 2D models are found to be capable of providing very useful information about the operation of the discharge. We employ two models—an axisymmetric and a Cartesian one. We show that for the considered experiment and the conditions of a low current arc (around 30 mA) in argon, there is no significant heating of the cathode surface and the discharge is sustained by field electron emission from the cathode accompanied by the formation of a cathode spot. The obtained discharge power and voltage are relatively sensitive to the surface properties and particularly to the surface roughness, causing effectively an amplification of the normal electric field. The arc body and anode region are not influenced by this and depend mainly on the current value. The gliding of the arc is modelled by means of a 2D Cartesian model. The arc–electrode contact points are analysed and the gliding mechanism along the electrode surface is discussed. Following experimental observations, the cathode spot is simulated as jumping from one point to another. A complete arc cycle is modelled from initial ignition to arc decay. The results show that there is no interaction between the successive gliding arcs.

Physics of a magnetic barrier in low-temperature bounded plasmas: insight from particle-in-cell simulations

The use of magnetic fields is quite common in low-pressure, low-temperature, gas-discharge devices for industrial applications. However, transport in such devices is still not very well clarified, mainly due to the presence of walls playing a crucial role and to the variety of configurations studied. The latter often obstruct the underlying basic physical phenomena and make the different studies valid only for very specific configurations. This work presents a numerical study of particle transport in low-pressure (0.3Pa) plasmas across a localized transverse magnetic field (magnetic barrier) by means of the 2D3V particle-in-cell with Monte Carlo collisions method. The problem is treated as generally as possible while trying to reveal the basic physics, using very simplified chemistry and considering a simple rectangular configuration. The conditions chosen for the magnetic field are common to many applications—magnetized electrons and almost unmagnetized ions. Two basic configurations with different magnetic field directions are analyzed in detail: magnetic field perpendicular to the simulation plane and along the simulation plane. An extensive parametric study is carried out in order to obtain the main trends and scaling laws for particle transport with respect to different parameters: plasma density, magnetic barrier size and magnetic field magnitude. The total current of electrons crossing the barrier is found to scale linearly with the plasma density, which extends the validity of the obtained results to a wide range of plasma density values. (Some figures may appear in colour only in the online journal)

Magnetic filter operation in hydrogen plasmas

A fluid-plasma model description of the operation of a magnetic filter for electron cooling in gas-discharge plasmas is presented in the study. Directed to the use of weak magnetic fields in the sources of negative hydrogen ion beams for additional heating of fusion plasmas, hydrogen discharges have been considered. The numerical results obtained within a 2D-model are stressed. The 1D-model presented aims at showing the main trends whereas the results obtained within the 3D-model, also developed, confirm the 2D-model description. The models outline the importance of the transport phenomena: electron-energy and charged-particle fluxes. Reduction of the thermal flux across the magnetic field together with thermal diffusion and diffusion, acting in combination, is the basis of the electron cooling and of the spatial distribution of the electron density. Effects due to the (E × B)-drift and the diamagnetic drift form a fine spatial structure of the plasma-parameter variations.

Particle-in-cell with Monte Carlo collision modeling of the electron and negative hydrogen ion transport across a localized transverse magnetic field

The control of the electron temperature and charged particle transport in negative hydrogen ion sources has a crucial role for the performance of the system. It is usually achieved by the use of a magnetic filter—localized transverse magnetic field, which reduces the electron temperature and enhances the negative ion yield. There are several works in literature on modeling of the magnetic filter effects based on fluid and kinetic modeling, which, however, suggest rather different mechanisms responsible for the electron cooling and particle transport through the filter. Here a kinetic modeling of the problem based on the particle-in-cell with Monte Carlo collisions method is presented. The charged particle transport across a magnetic filter is studied in hydrogen plasmas with and without including volume production of negative ions, in a one-dimensional Cartesian geometry. The simulation shows a classical (collisional) electron diffusion across the magnetic filter with reduction in the electron temperature...

Plasma behaviour affected by a magnetic filter

Operation of magnetic filters is studied experimentally regarding their use for electron cooling in the sources of the negative hydrogen ion beams. Axial profiles of the plasma parameters-electron temperature and density-are measured by probe diagnostics in the expansion plasma region of an inductively driven tandem type of a plasma source. The obtained drop of the electron temperature down to values of about and below 1 eV required for efficient production of negative hydrogen ions is the result proving the proper operation of the filter. The mechanism of electron cooling is discussed based on thermal conductivity effects.

A 3D model of a reverse vortex flow gliding arc reactor

In this computational study, a gliding arc plasma reactor with a reverse-vortex flow stabilization is modelled for the first time by a fluid plasma description. The plasma reactor operates with argon gas at atmospheric pressure. The gas flow is simulated using the k-e Reynolds-averaged Navier–Stokes turbulent model. A quasi-neutral fluid plasma model is used for computing the plasma properties. The plasma arc movement in the reactor is observed, and the results for the gas flow, electrical characteristics, plasma density, electron temperature, and gas temperature are analyzed.

Two-dimensional fluid model of a two-chamber plasma source

This study presents a two-dimensional fluid-plasma model developed for describing the cw regime operation of a tandem plasma source, consisting of a driver and an expansion plasma volume of different sizes. The moderate pressure range considered (tens to hundreds of milliTorr) allows a description within the drift‐diffusion approximation, as employed in the model. Argon discharges maintained in a metal gas-discharge vessel are treated. The discussions stress charged-particle and electron-energy fluxes as well as the spatial distribution of their components. The main conclusions are for (i) different electron and ion fluxes resulting in a net current in the discharge; (ii) a radial ion flux prevailing over the axial one and an axial electron flux prevailing over the radial one; (iii) ion motion determined by the dc electric field and drift‐diffusion electron motion influenced by thermal diffusion; (iv) plasma maintenance in the expansion plasma chamber due to charged-particle and electron-energy fluxes from the driver; (v) importance of the convective flux in the electron-energy balance; (vi) electron-energy losses for sustaining the dc electric field in the expansion plasma volume strongly predominating over the losses through collisions and (vii) electron cooling accompanied by a strong drop in the plasma density and in the potential of the dc electric field, due to the plasma expansion in a bigger volume. In general, the results show that the gas pressure range usually considered to be governed by ambipolar diffusion shows up in a different regime: a regime with a dc current, when the discharge is in a metal chamber with different dimensions in the transverse and longitudinal directions.

Coupled gas flow-plasma model for a gliding arc: investigations of the back-breakdown phenomenon and its effect on the gliding arc characteristics

We present a 3D and 2D Cartesian quasi-neutral plasma model for a low current argon gliding arc discharge, including strong interactions between the gas flow and arc plasma column. The 3D model is applied only for a short time of 0.2 ms due to its huge computational cost. It mainly serves to verify the reliability of the 2D model. As the results in 2D compare well with those in 3D, they can be used for a better understanding of the gliding arc basic characteristics. More specifically, we investigate the back-breakdown phenomenon induced by an artificially controlled plasma channel, and we discuss its effect on the gliding arc characteristics. The back-breakdown phenomenon, or backward-jump motion of the arc, as observed in the experiments, results in a drop of the gas temperature, as well as in a delay of the arc velocity with respect to the gas flow velocity, allowing more gas to pass through the arc, and thus increasing the efficiency of the gliding arc for gas treatment applications.

Expanding hydrogen plasmas: photodetachment-technique diagnostics

The laser photodetachment technique in its combination with probe measurements is applied for the determination of the electronegativity and the concentration of the negative hydrogen ions in the expansion plasma volume of an inductively driven two-chamber plasma source. Both the implementation of the method and the obtained results are discussed in the study. The variations of the gas pressure and the applied rf power in the experiments are in the ranges p = (0.3‐5)Pa and P = (200‐700)W, respectively. The results for an almostconstant—withthegaspressure—electronegativityanditsincreasewith the applied power are in agreement with theoretical estimations. The obtained nonmonotonicvariationoftheconcentrationofthenegativeionswithincreasing gas pressure correlates with the gas pressure dependence of the electron density in the expansion plasma region of the source. (Some figures in this article are in colour only in the electronic version)

Nonlocal conductivity effects in low-pressure cylindrical inductive discharges

The study is on nonlocal conductivity and stochastic heating in low-pressure inductive discharges with cylindrical coils. The rf current density derived by solving the Boltzmann equation in cylindrical coordinates, with accounting for both the high-frequency heating field and the dc field in the discharge, is coupled with the wave equation resulting into the spatial distribution-under the conditions of anomalous skin-of the electromagnetic field components, of the rf current density and the power deposition in the free-fall regime of maintenance of hydrogen discharges.