M A Sargsyan

Nonequilibrium state of highly ionized helium plasma at atmospheric pressure

A spectral study of arc helium plasma of atmospheric pressure with the electron density ne ≤ 1017 cm−3 and the ionization degree of ∼0.25 is performed. The character of the population of HeI levels with the excitation energy of 20.96–24.04 eV, which indicates infeasibility of the Boltzmann law with the electron temperature Te for describing the relative population of excited HeI states, is experimentally determined. Due to the experimentally revealed unfitness of the local thermodynamic equilibrium (LTE) model for estimating Te, mixed contours of the HeI 1083 line were divided into the Lorentz and Gaussian components. As a result, both the Stark effect constant that is poorly studied for this line and the temperature of heavy particles Tg in the plasma were estimated based on the Doppler component of its Gaussian half-width. As the arc current varied from 200 to 400 A, Tg increased from 20000 to 25 500 K, while the increase in the ionization temperature was, in this case, negligible (from 20000 to 21000 K).

One more study of argon arc binding to pure tungsten cathode

Pyrometric and spectroscopic investigations of pure tungsten cathode in argon arc plasma discharge at atmospheric pressure are reported. The distribution of surface cathode temperature and the radial distribution of plasma temperature at different distance from the cathode tip were measured. We conducted a comparison between our work and other studies on arc discharges where cathodes from lanthanated (W-2% La 2 O 3 ) and thoriated (W-2% ThO 2 ) tungsten were used.

Investigation of argon arc binding to the hafnium cathode

Spectroscopic and thermometric measurements were performed on the hafnium cathode in argon plasma at atmospheric pressure in a stable burning mode of the arc. The distribution of temperatures on the cathode and distribution of electron temperatures in plasma near the cathode are presented. It was shown that the obtained theoretical range of values for the voltage drop in the volume charge layer is in good agreement with experimental results. Based on the numerical simulations, a range of values were obtained for the current densities of ions and electrons and electric field strength on the cathode tip.

Spatial-temporal diagnostics of the system of a plasma stream interacting with a surface of heat resistant material

In an automated measuring complex using optical and spectral methods the spatial and temporal changes in the parameters and composition of nitrogen plasma jet were investigated. The plasma jet was flowing out of the nozzle of the plasma torch with 10–12 kK temperature and acting on the sample of MPG-6 graphite. Due to the heating of the sample to the temperatures of 2.5–3 kK the influence of the sublimating material of the sample on the plasma composition and temperature in the near-surface region of the sample was investigated. An original method based on the analysis of movement of optical inhomogeneities in the plasma flow was used to estimate the plasma jet velocity in the region where it interacts with the sample. The combined analysis of the results of two-positioning video recordings opens up the possibility of determining spatial-temporal distributions of the plasma jet velocities, in medium and high pressure environments, in the ranges from few to thousands of m/s and 3–15 kK temperatures.

Generator of the low-temperature heterogeneous plasma flow

A generator of low-temperature dc plasma with an expanding channel of an output electrode for gas-thermal spraying was designed and constructed. The delivery of the sprayed powder into the cathode and anode arc-binding zones or into the plasma jet below the anode binding was realized. The electrophysical characteristics of both the plasma torch and the heterogeneous plasma flow with Al2O3 powder are studied. It is shown that the current–voltage characteristic (CVC) of a plasma torch depends on the gas flow rate. If the flow rate varies from 1 to 3 g/s, the falling CVC becomes gradually increasing. The speed and temperature of the sprayed powder are determined.

Megawatt low-temperature DC plasma generator with divergent channels of gas-discharge tract

We have developed and studied a new effective megawatt double-unit generator of low-temperature argon plasma, which belongs to the class of dc plasmatrons and comprises the cathode and anode units with divergent gas-discharge channels. The generator has an efficiency of about 80–85% and ensures a long working life at operating currents up to 4000 A.

Comprehensive Study of the Effect of Plasma Stream on Heat-Resistant Materials

An experimental set-up with a unified system of synchronized control signals was constructed to study the interaction of high-enthalpy plasma jet with the surface of heat-resistant materials. The analysis of air–plasma stream interaction with technical (isotropic) graphite is performed. The air–plasma jet has a temperature of 7000–8000 K in the interaction zone, this interaction is accompanied by destruction of the graphite material and lasts for 100–200 s. Using methods like two-positioning visualization, micropyrometry and spectroscopy the following data are obtained: temperature fields on the graphite sample’s surface and their temporal and spatial changes, dynamics of the sample’s mass loss rate, changes in the parameters (temperature and concentration of electrons, heavy particles temperature) of the incoming plasma stream.

The effect of gas bubbles on electrical breakdown in transformer oil

To study the breakdown of transformer oil with gas bubbles an experimental setup was created that allows to determine electrical and optical properties of the discharge. Oil was sparged with air and sulfur hexafluoride gas. It was found that sparging oil with gas lowers the breakdown voltage of the oil. When a gas bubble is present between the electrodes at a considerable distance from the electrodes at first there is a spherically shape flash observed, resulting in the discharge gap overlapping by a conductive channel. These leads to discharges forming in the discharge gap with the frequency of hundreds Hz and higher. Despite the slightly lower breakdown voltage of oil sparged with sulfur hexafluoride the advantage of this medium to clean oil can serve as a two-phase medium damping properties, which may be sufficient to prevent the destruction of the body in the breakdown of oil-filled equipment.

Study of spatial distributions of highly ionized nonequilibrium helium plasma at atmospheric pressures

Experimental study of helium plasma in the state of quasistationary heating under atmospheric pressure was made. The plasma state is shown to fail to be described by Saha- Boltzmann approximation at high ionization levels α i = 0.5-0.9, temperatures 2.5-4.0 eV and electron concentrations about 1017 cm-3. The deviation from the equilibrium state of the plasma is caused by lack of spatial uniformity due to charged particles loss by ambipolar diffusion. In order to thoroughly study the temporal changes of plasma radiation characteristics, spectroscopic analysis was carried out with DFS-452 spectrometer and high-speed CMOS camera Andor iStar attached to its output. The system yields the spatial resolution of 30-50 μm and temporal resolution of 5-50 μs. Electron concentration ne was measured from the half-width of the local Hei spectrum line contours having dominant quadruple Stark effect with well-known constants. In order to determine the temperature of heavy particles, Doppler component of HeI line triplet at 1083 nm was studied. The temporal evolution of the following important characteristics has been determined for helium plasma during pulsed heating: current power, intensities of a number of HeI and HeII spectral lines, electron temperatures and concentrations.

Generator of chemically active low-temperature plasma

A new generator of high enthalpy (H 0 > 40 kJ/g), chemically active nitrogen and air plasmas was designed and constructed. Main feature of the generator is an expanding channel of an output electrode; the generator belongs to the class of DC plasma torches with thermionic cathode with an efficiency of 80%. The generator ensures the formation of a slightly divergent plasma jet (2α = 12°) with a diameter of D = 10-12 mm, an electric arc maximum power of 20-50 kW, plasma forming gas flow rate 1.0-2.0 g/s, and the average plasma temperature at an outlet of 8000-11000 K.