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Title: A study of spot evolution in hot refractory cathodes of high-pressure arcs

Abstract

In this paper the behavior of the cathode spot in atmospheric pressure arc is studied from a dynamic point of view and later applied to a specific case of a tungsten refractory cathode. For this, a model of the cathode region was developed. In this model the tungsten atoms proceeding from the cathode evaporation, which were all supposed to be ionized in the presheath, returned to the cathode surface as ions, provoking a heating of this surface. Therefore, the model allowed us to evaluate the importance of those atoms. Furthermore, different mechanisms of the electron emission from the cathode surface were considered as function of the spot temperature and the electric-field strength. This model of the cathode region allowed getting important parameters in the study of the dynamics of the cathode spot such as total current density, which is necessary for the calculation of Joule heating effect, and the total-energy flux density, which was incorporated as a boundary condition for the solution of the heat conduction equation in the cathode. The dependence of these parameters on the temperature of the cathode surface in contact with the plasma introduced nonlinearities in the equations. The model takes into account the different phasemore » changes that take place in the cathode as well as the thermal ablation of the melted cathode and the dependence on the temperature of the physical magnitudes that characterize the cathode material. In this way, it is possible to get the time evolution of the temperature distribution in the cathode and to study the spot dynamics on the cathode surface. Three different values of the initial cathode voltage drop were used, U{sub 0}=15, 20, and 25 V, which cover a wide range of working conditions. The cathode spot was assumed to be placed on a crater. For U{sub 0}=15 V, the crater radius increases in time until it reaches a critical value when the energy balance principle is broken. This leads to spot death and its jump to another position on the cathode surface. Nevertheless for higher-voltage drops, U{sub 0}=20 and 25 V, the spot reaches a stationary regime before arriving at that critical state and the spot remains fixed. This way, the maximum spot radius and the possibility of the spot moving or not appear naturally during the numerical development of the model. The results of this model show that the contribution of the evaporated and later ionized cathode atoms is not significant in comparison with the electrons and plasmagen ions contributions. In this sense these atoms need not be taken into account in the cathode region models for hot refractory cathodes in atmospheric pressure arcs.« less

Authors:
; ;  [1]
  1. Departamento de Fisica, Universidad de Cordoba, Edificio C2, Campus de Rabanales, 14071 Cordoba (Spain)
Publication Date:
OSTI Identifier:
20719651
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 98; Journal Issue: 9; Other Information: DOI: 10.1063/1.2121934; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ABLATION; ATMOSPHERIC PRESSURE; BOUNDARY CONDITIONS; CATHODES; CURRENT DENSITY; ELECTRIC FIELDS; ELECTRON EMISSION; ELECTRON TEMPERATURE; ENERGY BALANCE; FLUX DENSITY; JOULE HEATING; PLASMA; PLASMA PRESSURE; PLASMA SHEATH; REFRACTORIES; TEMPERATURE DISTRIBUTION; TUNGSTEN; VOLTAGE DROP; WALL EFFECTS; WORKING CONDITIONS

Citation Formats

Munoz-Serrano, E., Colomer, V., and Casado, E. A study of spot evolution in hot refractory cathodes of high-pressure arcs. United States: N. p., 2005. Web. doi:10.1063/1.2121934.
Munoz-Serrano, E., Colomer, V., & Casado, E. A study of spot evolution in hot refractory cathodes of high-pressure arcs. United States. doi:10.1063/1.2121934.
Munoz-Serrano, E., Colomer, V., and Casado, E. Tue . "A study of spot evolution in hot refractory cathodes of high-pressure arcs". United States. doi:10.1063/1.2121934.
@article{osti_20719651,
title = {A study of spot evolution in hot refractory cathodes of high-pressure arcs},
author = {Munoz-Serrano, E. and Colomer, V. and Casado, E.},
abstractNote = {In this paper the behavior of the cathode spot in atmospheric pressure arc is studied from a dynamic point of view and later applied to a specific case of a tungsten refractory cathode. For this, a model of the cathode region was developed. In this model the tungsten atoms proceeding from the cathode evaporation, which were all supposed to be ionized in the presheath, returned to the cathode surface as ions, provoking a heating of this surface. Therefore, the model allowed us to evaluate the importance of those atoms. Furthermore, different mechanisms of the electron emission from the cathode surface were considered as function of the spot temperature and the electric-field strength. This model of the cathode region allowed getting important parameters in the study of the dynamics of the cathode spot such as total current density, which is necessary for the calculation of Joule heating effect, and the total-energy flux density, which was incorporated as a boundary condition for the solution of the heat conduction equation in the cathode. The dependence of these parameters on the temperature of the cathode surface in contact with the plasma introduced nonlinearities in the equations. The model takes into account the different phase changes that take place in the cathode as well as the thermal ablation of the melted cathode and the dependence on the temperature of the physical magnitudes that characterize the cathode material. In this way, it is possible to get the time evolution of the temperature distribution in the cathode and to study the spot dynamics on the cathode surface. Three different values of the initial cathode voltage drop were used, U{sub 0}=15, 20, and 25 V, which cover a wide range of working conditions. The cathode spot was assumed to be placed on a crater. For U{sub 0}=15 V, the crater radius increases in time until it reaches a critical value when the energy balance principle is broken. This leads to spot death and its jump to another position on the cathode surface. Nevertheless for higher-voltage drops, U{sub 0}=20 and 25 V, the spot reaches a stationary regime before arriving at that critical state and the spot remains fixed. This way, the maximum spot radius and the possibility of the spot moving or not appear naturally during the numerical development of the model. The results of this model show that the contribution of the evaporated and later ionized cathode atoms is not significant in comparison with the electrons and plasmagen ions contributions. In this sense these atoms need not be taken into account in the cathode region models for hot refractory cathodes in atmospheric pressure arcs.},
doi = {10.1063/1.2121934},
journal = {Journal of Applied Physics},
number = 9,
volume = 98,
place = {United States},
year = {Tue Nov 01 00:00:00 EST 2005},
month = {Tue Nov 01 00:00:00 EST 2005}
}
  • From a model recently developed for refractory cathodes [Munoz-Serrano et al., J. Appl. Phys.98, 093303 (2005)], the behavior of a graphite cathode spot in a dc plasma torch at atmospheric pressure was investigated. Furthermore, an experimental study of these cathodes was made guided by the results obtained from the model. The model includes the modeling of the cathode region, the solution of the heat conduction problem in the cathode, and the simulation of the cathode ablation process. As a result of the model, the values of the parameters which characterize the cathode region were obtained, and the evolution of themore » spot under different working conditions determined by the value of the initial voltage drop in the cathode region, U{sub 0}, was investigated. The results obtained show that the maximum spot radius diminishes when U{sub 0} increases. Furthermore, two qualitatively different conditions for the spot dynamics were found. For U{sub 0}{>=}31 V, the spot radius continually grows over time until reaching a maximum value r{sub m}, and the spot remains fixed on a point of the cathode surface. For values of U{sub 0} less than 31 V the spot radius continues growing over time until reaching a maximum value with which it is not possible to satisfy the energy balance equation. This leads to spot extinction and to its appearance at another point of the cathode surface. Several graphite cathodes were experimentally studied, each one using different interelectrode voltage drop values U{sub a-c}. Before and after arcing, the cathode surface was explored by an electron microscope, and the roughness profile of that surface was determined by a perthometer. This allowed measuring the average size of the craters produced on the cathode surface by the arc. The movement of the spot attachment for different values of interelectrode voltage was observed by a digital video camera. It was experimentally found that the average crater radius diminished when the U{sub a-c} potential increased. Furthermore, it was seen that for U{sub a-c} values less than 32 V, the spot moved over the cathode surface and that this movement became slower when the U{sub a-c} increased. The spot became immobile for U{sub a-c}=32 V. These experimental results corroborate the spot behavior obtained from the theoretical model.« less
  • The partial pressure of thallium in high-pressure Hg-TlI discharges with different mercury, thallium, and electron pressures has been measured by using the optically thin line Tl 655 nm and the self-reversed line Tl 535 nm. The partial pressure of the arc axis has been measured from the line Tl 655nm. The effective partial pressure has been measured from the self-reversed line Tl 535 nm on the basis of the multiparameter method, and it has been calculated from the known axis pressure of thallium and the calculation of its radial variation by taking into account the chemical reactions. The experimental resultsmore » confirm the dispersion character of the blue wing of the line Tl 535 nm. The systematic difference obtained between the measured and calculated effective pressure, particularly at the moment of minimum electron density, may be interpreted by deviations from the local thermodynamic equilibrium (LTE) caused by overpopulation of the upper level of the line Tl 535 nm.« less
  • Long Baseline Array imaging of the z = 0.663 broadline radio galaxy PKS 1421-490 reveals a 400 pc diameter high surface brightness hot spot at a projected distance of {approx}40 kpc from the active galactic nucleus. The isotropic X-ray luminosity of the hot spot, L {sub 2-10keV} = 3 x 10{sup 44} ergs s{sup -1}, is comparable to the isotropic X-ray luminosity of the entire X-ray jet of PKS 0637-752, and the peak radio surface brightness is hundreds of times greater than that of the brightest hot spot in Cygnus A. We model the radio to X-ray spectral energy distributionmore » using a one-zone synchrotron self-Compton model with a near equipartition magnetic field strength of 3 mG. There is a strong brightness asymmetry between the approaching and receding hotspots and the hot spot spectrum remains flat ({alpha} {approx} 0.5) well beyond the predicted cooling break for a 3 mG magnetic field, indicating that the hotspot emission may be Doppler beamed. A high plasma velocity beyond the terminal jet shock could be the result of a dynamically important magnetic field in the jet. There is a change in the slope of the hotspot radio spectrum at GHz frequencies, which we model by incorporating a cutoff in the electron energy distribution at {gamma}{sub min} {approx} 650, with higher values implied if the hotspot emission is Doppler beamed. We show that a sharp decrease in the electron number density below a Lorentz factor of 650 would arise from the dissipation of bulk kinetic energy in an electron/proton jet with a Lorentz factor {gamma}{sub jet} {approx}> 5.« less
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  • In this work, the dependence of the cathode erosion rate on vacuum arc spot velocities and cathode pore size are investigated for four types of graphite under an external variable magnetic field. Different graphite cathode properties show varying erosion rates, indicating that each graphite type should be treated as a different material. Increasing the spot velocity through an increase of the magnetic field intensity on a given cathode material decreases the erosion rate. At given magnetic field values, cathodes having higher arc velocities show an increase in the erosion rate. Other studies indicating that the increase in the arc spotmore » velocity leads to a reduction in macroparticles emission indicate a possibility of increasing the emission of the carbon ions component. A decrease in the pore size of the cathode also shows an increase in the cathode erosion rate. A good knowledge of the erosion properties of graphite when subjected to electric arcs is particularly important in the low pressure applications of arc ion plating, in which solid carbon particles have to be minimized, and of wall surface shields in fusion devices where a minimal mass loss of the graphite is needed.« less