Ion emission from arc discharge plasmas

Results are presented from studies of the ion-emission properties of the anode plasmas of low-pressure contracted arc discharges and vacuum arcs. It is shown that creating a longitudinal magnetic field in the anode region of a discharge changes the plasma parameters significantly and facilitates a large increase in the ion current. Space charge limited ion current in a vacuum arc leads to a reduction in the noise level of the total ion current and of its components with charges of up to +3, while creating Penning discharge conditions ensures that ions of different gases can be generated in this discharge system at fractions as high as 90%, depending on the type of plasma forming gas.Institute of High-Current Electronics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vuzov, Fizika, No. 3, pp. 24–33, March, 1994.     

Predictions of arc temperature profiles using approximate emission coefficients for radiation losses

Coefficients are calculated for the net emission of radiation per cc from the center of cylindrical isothermal plasmas of various temperatures and radii. Coefficients are given as a function of temperature and radius for the D lines of sodium vapor at 250 torr and for the continuum radiation of air at 1 and 30 atm. These coefficients are used in the Elenbaas Heller equation to derive temperature profiles of wall stabilized arcs e.g. for 4·5A with a radius of 0·35 cm in sodium vapor at 250 torr and 10,000 and 20,000A with a radius of 1 cm for air at 30 atm. Comparisons are made with calculated temperature profiles where self absorption effects and the radiation transfer are treated exactly, but still assuming thermodynamic equilibrium. It is found that the approximation using the net coefficients yields central arc temperatures and electric field strengths for a given arc current accurate to 10 per cent. Computation time for the approximate calculation is more than an order of magnitude less than for the calculation where radiation transfer is treated exactly. Thus the approximation facilitiates an account of line radiation for arc plasmas having complex spectra.     

Reconstruction of emission coefficients for a non-axisymmetric coupling arc by algebraic reconstruction technique

A preliminary investigation of tomographic reconstruction of an asymmetric arc plasma has been carried out. The objective of this work aims at reconstructing emission coefficients of a non-axisymmetric coupling arc from measured intensities by means of an algebraic reconstruction technique (ART). In order to define the optimal experimental scheme for good quality with limited views, the dependence of the reconstruction quality on three configurations (four, eight, ten projection angles) are presented and discussed via a displaced Gaussian model. Then, the emission coefficients of a free burning arc are reconstructed by the ART with the ten-view configuration and an Abel inversion, respectively, and good agreement is obtained. Finally, the emission coefficient profiles of the coupling arc are successfully achieved with the ten-view configuration. The results show that the distribution of emission coefficient for the coupling arc is different from centrosymmetric shape. The ART is perfectly suitable for reconstructing emission coefficients of the coupling arc with the ten-view configuration, proving the feasibility and utility of the ART to characterize an asymmetric arc.     

Empirical analysis of picosecond breakdown in sulfur hexafluoride

Using state of the art equipment and multiple simultaneous data acquisition systems, breakdown in sulfur hexafluoride (SF₆) is examined in high resolution. Recorded risetimes can be as fast as 50 ps. Influential parameters of breakdown are identified, recorded, and categorized. Methods for removing the impact of the measurement system are implemented in efforts to distinguish the physical phenomenon from influential external factors. Observed waveforms and breakdown characteristics are categorized into three types. Each type is particular to a specific parameter range – i.e. electric field E/p or the product of pressure and distance pd.     

Quasi-monochromatic measurements of homogeneous arc plasmas

The refined diagnostic information obtainable by high-order spectrometry is illustrated by the results of quantitative measurements of a few rotational lines of OH in the ultraviolet spectrum of water-vapor plasmas generated in a wall-stabilized arc. Because of the high spectral and spatial resolution achieved in end-on measurements, the emission and also the absorption coefficients pertaining to homogeneous arc regions were obtained directly from measured line spectra—although the absorption was not measured explicitly—leading to the occupation of the upper and the lower state for the transition. The gas temperature was determined from the halfwidth of the Doppler-broadened rotational lines. The measured resolving power of the spectrometer was of the order of 400,000 in these measurements.     

Electron impact excitation coefficients for laboratory and astrophysical plasmas

Electron impact excitation rate coefficients have been obtained for a number of transitions in highly ionized ions of interest to astrophysical and laboratory plasmas. The calculations were done using the method of distorted waves. Results are presented for various transitions in highly ionized Ne, Na, Al, Si, A, Ca, Ni and Fe.     

Renormalized diffusion coefficients for electron-hole plasmas in crossed fields

The diffusion and drift of an excess plasma in a semiconductor is described with magnetohydrodynamic two-fluid equations including the fluctuating electric field produced by the equilibrium plasma in the sample. Using the weak coupling limit an equation of motion for the mean density of the excess plasma is established with renormalized drift and diffusion coefficients. With the aid of the fluctuation dissipation theorem these coefficients are expressed in terms of the dielectric function and discussed in detail for stable systems. The renormalized diffusion coefficient differs from the bare one by an additional term with thet ^–3/2-long time dependence. It is shown that this term in addition represents an anomalous diffusion rate proportionalB ^–1 which overweights the classical ambipolar diffusion for sufficiently strong fields, but decreases with increasing external electric field. The results are compared with experimental data.     

Electrical characterization of atmospheric pressure arc plasmas

The properties of atmospheric pressure arcs are investigated by means of electric exploration of plasma column and anode region. For the electrostatic probe technique, where the level of collisionality distorts the characteristic curve, data interpretation is difficult because no comprehensive underlying theory exists for the non-homogeneous electric arcs used in industry. Results are presented from an extended study of Langmuir probes applied to short, point-plane arcs. A multi-wire apparatus, operating for arc currents in the range 50-200 A is described and ion current densities and temperature maps are shown. The reduction of the probe determined temperature with respect to emission spectroscopy values is discussed and the cooling is ascribed to ion-electron recombination within the perturbation region formed around the probe. This region is investigated by means of emission spectroscopy and the extension found agrees both with numerical estimations and fast speed camera photographs. Diamond Like Carbon (DLC) partially coated wires can address data inversion problems and the role of arc flow directionality on charge capture and preliminary observations are shown. Charge capture and anode fall structure can be investigated using a split-anode technique. A prototype of a modified apparatus is described and preliminary results on the collected current are given.Received: 20 August 2003PACS: 52. Physics of plasmas and electric discharges - 52.80.Mg Arcs; sparks; lightning; atmospheric electricity - 52.70.-m Plasma diagnostic techniques and instrumentation     

New developments in processing cathodic arc plasmas

Filtering of plasmas by curved solenoidal ducts is well established as a method of removing macroparticles. By analyzing the interactions of planar probes with the drifting plasma of the cathodic arc, new insights have been obtained into the operation of these ducts. Theoretical modeling of these interactions suggests, and experiment confirms, that the use of a separate biased electrode on the inside of the duct gives enhanced transmission without drawing excessive electron current. Theoretical modeling of a negatively biased planar electrode lying parallel to the drift velocity as well as experiment both show that ions are captured effectively onto the electrode producing a macroparticle free film at good deposition rates. The application of pulsed high voltage to the substrate placed at the exit of the duct is treated theoretically, and a model is proposed which gives a good agreement with the experimental concentration profile for a silicon surface coated and simultaneously implanted with titanium     

Macroparticle filtering of high-current vacuum arc plasmas

The transport of vacuum arc plasmas through a 90° curved magnetic macroparticle filter was investigated using a high-current pulsed arc source with a carbon cathode. The peak arc current was in the kiloampere range, exceeding considerably the level of what has been reported in the literature. The main question investigated was whether magnetic macroparticle filters could be scaled up while maintaining the transport efficiency of small filters. In front of the cathode, we found that arc current dependent total ion saturation currents were in the range from 10% to 23% of the arc current. The best relative transmission was 25% (time integrated output/time integrated input) at a duct wall bias of 12.5 V and at an axial magnetic field of about 100 mT. The measured relative transmission of the used high-current arrangement is comparable to what has been observed with other low-current filters. The absolute measurable ion saturation currents at the filter exit reached 70 A at an arc current of about 1000 A     

A hydrodynamic model of vacuum arc plasmas

The properties of plasmas expanding from cathode spots of vacuum arcs are calculated with a one-dimensional two-fluid model. The system of simplified hydrodynamic equations can be solved under stationary conditions using asymptotic power series. Although necessarily only an approximation, such analytical solutions prove to be advantageous compared with numerical integrations. All the plasma parameters are functions of (I/r)^2/5 (current, I: distance, r). The three forces accelerating the ions to high kinetic energies are quantitatively calculable: the electric field, the ion pressure gradient, and the electron-ion friction. The potential is decreasing towards the anode, and the residence of the plasma is negative. The ion temperature reaches only about 35% of the electron temperature. Although only asymptotic, the solution is suited to describe the arc plasma in a sufficient manner all over the expansion region     

Two-dimensional models of expanding vacuum arc plasmas

Multifluid equations describing the plasma of vacuum arcs expanding anisotropically from a cathode spot are given and discussed. Some first and preliminary results from an approximate analytical integration of such a system of equations, based on the representation of all plasma parameters by asymptotic power series, are presented and evaluated. Though the dependence of the plasma parameters on the direction (angle ϑ) is weak, the inclusion of angular terms and angular equations changes some results significantly, compared with the experiences from usually 1-D models. While the basic radial dependence (r) of the terms ~r^-2k/5, k=0,1,2..., remains the same as in 1-D theories, we now get an angular dependence ~ϑ² of the parameters in a first approximation, with positive factors, i.e., a flattening of the whole plasma distribution. However, with analytical solutions of higher flexibility, cos-like distributions are possible as well. The final kinetic energy of the plasma ions in the case of Cu arc spots is caused by the electric field within the plasma (contributing approximately 27%), the ion pressure gradient (~24%), and the electron-ion friction (plasma resistivity, ~49%) in directions near to the discharge axis. Again, similar to the results from 1-D models. With an approach to the cathode surface (ϑ→π/2), the plasma flow becomes ambipolar. Finally, some further possibilities which are available with the help of this model are discussed     

Electrical breakdown of transformer oil with sulfur hexafluoride and air bubbles

Technical Physics - The influence of gas bubbles on the breakdown voltage of transformer oil is experimentally studied. The influence of the oil flow on the electrical characteristics of breakdown...     

Transport coefficients of helium and argon-helium plasmas

Calculated values of the viscosity, thermal conductivity, and electrical conductivity of helium, and mixtures of argon and helium, at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the mixtures. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30000 K. The results are compared with those of previously published studies. Significant discrepancies are found; these are attributed to the improved values of the collision integrals used here in calculating the transport coefficients