O. A. Sinkevich

Investigation of low-temperature plasma generator with divergent channel of the output electrode and some applications of this generator

A review is made of experimental and theoretical investigations of processes occurring in low-temperature plasma generators (LTPG) with divergent channel of the output electrode, and the possibilities of utilizing these generators in new plasma technologies are analyzed. Comparison is made of the characteristics of discharge (including the current-voltage characteristic) in a divergent channel and in a cylindrical channel of uniform cross section. The effect of divergent channel of the output electrode and of its expansion ratio on the pattern of physical processes in LTPGs of different designs is studied. Investigations are performed of the distribution of electric current and heat flux density along a channel with a segmented output electrode. The voltaic equivalents of heat fluxes to cathode and anode are determined. The process of “shunting” of discharge is investigated, which causes fluctuations of electric arc-burning voltage. The investigations involving an LTPG with divergent channel reveal that the voltage amplitude in the case of shunting decreases with increasing current strength and, at high currents of argon arc, does not exceed 1–2 V. Results are given of spectral and visual investigations of LTPG. It is demonstrated that, in an LTPG with divergent channel, the plasma temperature in the region of energy input at currents of 300 A and higher exceeds 30 000 K. The significant part is found which is played by vacuum ultraviolet radiation in the process of closing the arc to anode. The mechanisms of erosion of the tungsten cathode tip are investigated, which play an important part in increasing the cathode service life by way of recirculation of tungsten atoms because of their ionization in the discharge gap. Results are given of using an LTPG with divergent channel of the output electrode in plasma technologies of surface hardening, cutting, and hard-facing of metals. The technology of plasma hardening of wheel pairs, adopted by the RZhD (Russian Railroads) Joint-Stock Company, provides for increasing the service life of railroad wheels by a factor of 1.5–2.

The corona discharge in nuclear excited plasma as a way of obtaining ordered dust particle structures

A model of a corona discharge in a nuclear excited dust plasma at the pressures of 1–100 atm is proposed. The distributions of the electric field and current-voltage characteristics of a corona discharge in a nuclear-excited dust plasma are found for a particular cylindrical geometry case at different methods of corona-producing electrode positioning. The conditions for the existence of a stationary corona discharge are obtained. A mathematical model describing the behavior of dust particles in a nuclear plasma that allows taking into account the key physical processes occurring in a nuclear-excited dust plasma is considered. The analyzed plasma properties are as follows: (1) shielding of the Coulomb forces of the interaction between dust particles, (2) the energy exchange and the stochastic character of the dust particle interaction with a buffer gas and ambient plasma, and (3) strong spatial inhomogeneity of the nuclear-excited plasma. The use of a corona discharge in a nuclear-excited plasma will make it possible to ensure stability of plasma-dust structures and more efficient conversion of the nuclear energy into laser radiation.

Study of a low temperature plasma generator with a divergent output electrode channel

Results of experimental studies of a plasma generator with a divergent output electrode channel are presented, and a comparative analysis is performed of operation of this plasma generator and plasma generator with cylindrical channel of constant cross section. Studies of sectioned output electrodes are performed along with plasma generators with solid electrodes. The influence of sectioning on the parameters of electric discharge is studied. It is shown that the optimal degree of divergence of the output electrode channel is a complex function of different parameters.

A Study into Development of Instability and Collapse of Vapor Layer on a Heated Solid Hemispherical Surface

A study is performed into the processes of development of instability and collapse of a vapor film on a solid hemispherical surface. Optical-fiber sensors of pressure and vapor-film thickness are used in measurements; the process is videofilmed with subsequent computer processing of images. A physical model of the phenomenon is developed, which includes superheating and explosive boiling of water in recesses of wavy vapor film. The predictions are in qualitative agreement with the experimental results.

Thermal fluxes in a generator of low temperature plasma with a divergent channel of the outlet electrode

A short review is presented of experimental results concerning distribution of heat fluxes over the length of a generator of low temperature plasma (plasmatron). Comparative analysis is given of the operation of plasma generators with a divergent channel of the outlet electrode against plasma generators with a cylindrical channel of constant cross section. Experimental studies were performed on a specially created facility consisting of four plasma generators with sectioned and continuous outlet electrodes and automated measuring system.

Investigation of the energy characteristics of a plasma torch with an expanding channel for the output electrode

The results of experimental and theoretical investigations of low-temperature plasma generators with an expanding channel of the output electrode are presented. Experimental equipment with a sectioned output electrode has been constructed. The distribution of electric current in the section for different values of the consumption of gas and current is determined.

The shunting of current and the resultant variation of voltage in channels of plasmatrons with self-adjusting length of electric arc

The characteristics of pulsation processes in dc plasmatrons with the cathode located in the channel center and the anode in the form of a cylindrical or a conical channel wall are investigated in plasmas of argon and nitrogen at atmospheric pressure. The fast Fourier transform of signals and their subsequent computer processing are used to obtain the dependences of the frequency of fluctuations of the arcing voltage on the working parameters of the plasmatron, namely, the electric arc current, the flow rate of plasma-forming gas, and the channel diameter. For plasmatrons with the self-adjusting length of the electric arc, analysis is performed of the mechanism of reclosing of the anode region of the arc, i.e., of the electric arc shunting associated with the stretching of the current filament by a flow of gas and with the electrodynamic interaction of different filament regions. A formula is derived which defines the dependence of the characteristic frequency of fluctuations of the arcing voltage on the external parameters of the problem, namely, the arc current, the flow rate of the working gas, and the characteristic channel diameter. It is demonstrated that the pattern of the dependence of the frequency of voltage fluctuations on the gas flow rate may vary with the values of the parameter of magnetohydrodynamic interaction. The formula generalizes the experimental results of numerous researchers obtained in a wide range of variation of the external parameters.

Explosive regime of the development of instability leading to vapor-film collapse on a heated solid hemispherical surface

We investigated the local processes proceeding in the case of changing the boiling regime on a heated cylindrical rod with a hemispherical end immersed in water ( T = 285 K) at a depth equal to the radius of the hemisphere. The rod was heated in ambient air up to temperature T ≤ 1000 K. Vapor-film escape was observed visually using a microscope and two video cameras installed below and aside from the hemisphere immersed in the liquid. The pressure arising in the liquid and the vapor-film thickness were measured using fiber-optical sensors [1]. In our experiments, we used metallic rods made of steel, pure copper, and copper covered by PSr62 silver solder. After immersing the hemisphere in a liquid, freeconvection flow formed near the heated surface. Then, wave formations appeared on the vapor‐fluid interface. The amplitude and characteristic length of the surface waves were on the order of magnitude of a vapor-film thickness. The process of vapor-film escape and passing to bubble boiling proceeded following two different scenarios. According to one of them, the wave perturbances having the form of isolated solitons or wave trains were enhanced with time and enveloped the entire hemisphere immersed in the water. Afterwards, a vapor-film explosion occurred and the nucleate-boiling regime was established. In a number of cases, the explosive boiling of the film was accompanied by the formation of a jet flow directed from the lower end of the hemisphere into the fluid. The explosive escape of the vapor film accompanied by the formation of the jet flow can be repeated (up to 30 times) in intervals of 0.3 to 1 s. In the other regime, the wave perturbances, once arisen, attenuated gradually and passed relatively smoothly to the nucleate-boiling regime. Smooth passage to nucleate boiling was observed only at the first immersion in water for a new hemisphere or for that freshly cleaned from oxides. In repeated experiments with surfaces having the oxide film, the vapor escaped explosively. In Figs. 1‐3, we show photographs of a vapor film before explosion and with various types of its escape. The experiments were carried out under normal pressure, and the temperatures of the cooling water and heated surface were 293 and 795 K, respectively. The characteristic values of the vapor‐fluid jet velocity (Fig. 3) attained 0.3 m/s. Vibrations of the vapor-film surface were detected both at the second boiling crisis with quiet vapor-film escape and with vapor explosion. The characteristic longitudinal dimension of the wave structures attained

Corona Discharge in Nuclear Excited Dusty Plasma

We considered the possibility of using a corona discharge in a nuclear excited dusty plasma to provide the stability of well ordered dusty plasma structures from a fissionable material and to accomplish a more efficient conversion of nuclear energy into radiation.

Calculation of Laminar Flows of Plasma in the Channel of a Plasmatron with the Self-Adjusting Length of the Electric Arc

A simplified method is suggested for the calculation of the parameters of laminar flows of plasma in the channel of a plasmatron. A new analytical solution of the Elenbaas–Heller equation is derived, which generalizes the channel model of electric arc to the case when the volume radiation makes a significant contribution to the electric arc energy balance. Numerical calculations are performed in order to determine the electric field intensity, the longitudinal pressure gradient, and the heat transfer to the electric arc channel wall depending on the working parameters of the plasmatron in a laminar flow of gas in a stabilized section. In determining the arc length, it is assumed that the electric arc is shifted downstream of the flow and, at the same time, performs random walks over the channel cross section. The walks occur under the effect of vortexes whose characteristic size is of the order of the arc diameter.