A. A. Belevtsev

Emission spectroscopy of highly ionized high-temperature plasma jets

This paper deals with advanced studies on the optical emission spectroscopy of atmospheric pressure highly ionized high-temperature argon and nitrogen plasma jets generated by a powerful arc plasmatron. The emission spectra are taken in the 200?1000?nm range with a spectral resolution of ~0.01?0.02?nm. The exposure times are 6 ? 10?6?2 ? 10?2?s, the spatial resolution is 0.02?0.03?mm. The recorded jet spectra are abundant in spectral lines originating from different ionization stages. In nitrogen plasmas, tens of vibronic bands are also observed. To interpret and process these spectra such that plasma characteristics can be derived, a purpose-developed automated processing system is applied. The use of a CCD camera at the spectrograph output allows a simultaneous recording of the spectral and chord intensity distributions of spectral lines, which can yet belong to the overlapped spectra of the first and second orders of interference. The modern optical diagnostic means and methods used permit the determination of spatial distributions of electron number densities and temperatures and evaluation of rotational temperatures. The radial profiles of the irradiating plasma components can also be obtained. Special attention is given to the method of deriving rotational temperatures using vibronic bands with an incompletely identified rotational structure.

Spectroscopic Analysis of Spatial Distribution of the Electron Temperature and Concentration in High-Enthalpy Flows of Argon and Nitrogen Plasma

The paper deals with the spectral determination of the temperature and number density of electrons in high-enthalpy flows of highly ionized plasma of argon and nitrogen at atmospheric pressure. Radial distributions of these parameters in different cross sections of the plasma jet are obtained. An automatic system of data acquisition and processing is used to investigate the distribution of atoms and ions of argon and nitrogen over excited states. Different methods of spectral determination of ne and Te are compared with one another in application to the plasma being investigated.

Analysis of Metrological Potentialities of a High-Current Arc in Plasma Generators with a Diverging Channel

The paper is concerned with substantiating a high metrological “quality” of the cathode section of a stationary electric arc in plasma generators which have a current density of over 2 kA/cm2, a diverging anode channel, and vortex stabilization. An automated spectroscopic system and a system for fast visualization of plasma filament are used to demonstrate the high reproducibility and stability of the electrical parameters of the arc and of the space and time radiating characteristics of highly ionized plasma of the cathode region of the discharge. The results of analysis of the thermodynamic state of plasma in the cathode region of the discharge point to the validity of the conditions of partial local thermodynamic equilibrium in this region. The use of the developed system of automatic acquisition and processing of spectroscopic information enables one to obtain unique (as regards the scope and accuracy) data on the radiating characteristics of lines of atoms and ions of different multiplicity.

Electronic transport coefficients and electric breakdown in condensed helium

The kinetic Boltzmann equation is applied to determine the electron energy distribution function in condensed helium over a wide range of pressures and temperatures. Account is taken of all the effects influencing the electron kinetics in dense media. The electron multiplication rates and basic electron transport coefficients in condensed helium at atmospheric and elevated pressures have been determined. Starting from the streamer theory, the limiting breakdown characteristics of condensed helium are calculated. The theoretical findings are compared with available experimental data. For liquid helium the long-standing fundamental problem is first solved of the intrinsic dielectric strength determined exclusively by the electron processes in a liquid itself rather than in gaseous bubbles formed at the prebreakdown stage. Comparison with experimental data allows also answering the question of how the voltage application duration and contaminants affect the breakdown characteristics of liquid helium.

Experimental study of the near-electrode plasma-tungsten cathode system in high-current atmospheric-pressure nitrogen arcs

Results of spectroscopic and pyrometric studies of the near-electrode plasma-tungsten cathode system are presented. The 2D-spectra of radiation of the multiply ionized nitrogen plasma of electric-arc discharge under atmospheric pressure are obtained at different current values. The axial electron temperature in this plasma is determined depending on the distance from the cathode tip, and the temperature distribution over the current-collecting surface of the cathode is found. The high-speed visualization of the cathode-near-cathode plasma region made it possible to observe phase transitions on the cathode surface and processes of crack formation and detachment of its fragments. The power released at the cathode is analyzed including the contribution of the arc column resonance emission, and the peculiarities of phase transitions on the cathode surface are revealed. The densities of the electron emission current at the cathode are estimated theoretically and compared with the data obtained in the experiment. Possible causes of the difference between the theoretical values of the emission current density and those found experimentally are discussed.

Charge kinetics in weakly ionized plasma of electronegative gases

This work is devoted to investigation of specific features for processes of ionization and recombination in a weakly ionized plasma of electronegative gases in an external electric field in the presence of ion-molecular reactions. By analogy with the problem of the shift of the energy level of the ground state of the quantum-mechanical Fermi system, a simple analytic expression for the calculation of the effective ionization rate under these conditions was found. For the first time, a criterion of the dielectric strength for electronegative gases, which takes into account all the ion-molecular reactions with the participation of negative ions, is formulated. The recombination process in the ion plasma of electronegative gases is considered both in the presence of and without these reactions. Cases permitting the analytic solution of a set of differential balance equation for components of the decaying ion plasma are selected.

Development of electron impact excitation and ionization in noble gases and liquids in strong electric fields

This paper reports on establishing the electron energy distribution function and electron impact excitation and ionization rates in noble gases and liquids in strong electric fields. The method of the unsteady electron kinetic Boltzmann equation is used allowing for all the dense effects typical of the electron scattering at spatially correlated heavy particles. Account is also taken of the influence of the electron number density growth on the relaxation process. Some special features of the excitation and ionization processes in noble media are revealed. The respective electron impact rates have been calculated and the delay times for each the process determined. A paradoxical conclusion has been drawn about the electron multiplication rates in the media with extremely sharp increase in the momentum transfer frequency.

Lifetime of the thermoemission cathodes of nitrogen plasma generators

Implementation of plasma technologies widely introduced into modern industrial processes requires plasma generators with a high efficiency, great economy, and long lifetime. The last requirement is caused by the necessity of an increase in the lifetime of the most easily worn plasmatron elements, i.e., the electrodes and, primarily, their cathode units. Thermoemission cathodes made of tungsten, the most refractory metal with the melting point Tm = 3695 K, are widely exploited in high-current (I = 300–1000 A) plasmatrons operating in oxygen-free media (inert gases like nitrogen and hydrogen). By the middle of the 1980s, due to the discovery and application of the phenomenon of the recirculation of electrode material in the cathode zone, erosion of the cooled thermocathode was successfully reduced to G = 10–10 g/C. The present study performed by means of a plasmatron with a self-adjusting arc length concerns important problems, such as measurement of the emission current, determination of cathode material erosion, and analysis of means of enhancement of the thermoemission cathode lifetime. Through the investigations performed and the resource tests of the above-mentioned plasmatron type with currents of 300–500 A, with nitrogen as the working gas, the requirements for the cathode and the plasmatron operating regimes providing for its low erosion losses, G ≤ 10–10 g/C, at a cathode surface temperature close to its melting point are determined. The experimentally obtained densities of the electron emission currents exceed, by an order of magnitude, those calculated according to the Richardson–Dushman theory with the Schottky correction and with account for the photoemission on the cathode under the action of the resonance emission generated by the positive column of the arc. In that connection, attention is paid to the mechanism of the anomaly electron emission proposed by S.V. Lebedev and caused by occurrence of the Frenkel defect accompanied by crystal lattice deformation, Fermi energy increase, and the respective decrease in the electron-from-metal work function. The obtained estimates of the Frenkel defect concentration in tungsten at the premelting temperatures are a cogent argument in favor of the anomaly electron emission concept.