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X. Zhou - One of the best experts on this subject based on the ideXlab platform.
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an experimental investigation of factors affecting Arc Cathode erosion
Journal of Physics D, 1998Co-Authors: X. Zhou, J HeberleinAbstract:A specially designed thermal plasma reactor system for the investigation of Arc-Cathode erosion has been set up. By using an OMA-spectrometer system, emission spectroscopic measurements of electron temperature and electron number density in the Cathode region have been performed, together with single-colour and two-colour pyrometry of Cathode temperature distributions. Observation of Cathode spot behaviour has been carried out simultaneously by employing a telemicroscope and a high-speed vision system. Cathodes have been examined by SEM and EDX after Arcing. For pure tungsten Cathodes, the initial Cathode geometry has almost no effect on the Cathode spot's behaviour due to the molten state of the Cathode spot. The major erosion mechanism is the ejection of liquid droplets from the Cathode spot. However, the initial Cathode geometry has a certain influence on the Cathode's erosion for 2% thoriated tungsten Cathodes. A highly non-uniform erosion pattern will occur if the Cathode is overcooled, probably due to ion bombardment in the low-temperature regions of the Arc-attachment spot.
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Characterization of the Arc Cathode attachment by emission spectroscopy and comparison to theoretical predictions
Plasma Chemistry and Plasma Processing, 1995Co-Authors: X. Zhou, Joachim HeberleinAbstract:Emission spectroscopic diagnostics of electron temperature distribution and electron number density distribution in the Cathode region have been carried out employing an OMA/spectrometer system. Single color and two color pyrometry of Cathode temperature distributions have also been performed. The experimental results are compared with results of a theoretical model formulated previously to study the Arc Cathode interaction.
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Characterization of the Arc Cathode attachment by emission spectroscopy and comparison to theoretical predictions
Plasma Chemistry and Plasma Processing, 1995Co-Authors: X. Zhou, J HeberleinAbstract:Emission spectroscopic diagnostics of electron temperature distribution and electron number density distribution in the Cathode region have been carried out employing an OMA/spectrometer system. Single color and two color pyrometry of Cathode temperature distributions have also been performed. The experimental results are compared with results of a theoretical model formulated previously to study the Arc Cathode interaction. The results show that the plasma in the Cathode region strongly deviates from LTE. Thermionic cooling is the major cooling mechanism of Cathodes at high Arc currents. The work function of 2% thoriated tungsten Cathodes increases during Arcing due to fast evaporation of thorium from 2% thoriated tungsten Cathodes.
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Analysis of the Arc-Cathode interaction of free-burning Arcs
Plasma Sources Science and Technology, 1994Co-Authors: X. Zhou, Joachim HeberleinAbstract:A theofetical model has been formulated describing the influence of the Arc condition and the Cathode material and geometry on Arc Cathode erosion. To arrive at a self-consistent description for the entire Arc Cathode attachment region, a realistic onedimensional sheath model has been used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the Arc, and by a differential energy balance of the Cathode. For the case of a tungsten Cathode in an argon Arc, it has been shown that the ion current density is almost 50% of the total current density at low Arc currents, while it decreases to about 18% of the total cunent density and the thermionic electron current density increases to about 82% of the total current density at high currents. It has also been found that heat conduction within the Cathode and radiation from the Cathode surface control energy transport from the Cathode spot at low currents, and that dissipation by thermionic electron release dominates at high currents.
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Model predictions of Arc Cathode erosion rate dependence on plasma gas and on Cathode material
Proceedings of IEEE Holm Conference on Electrical Contacts, 1993Co-Authors: X. Zhou, J Heberlein, E. PfenderAbstract:We have previously reported results of a theoretical study which predict that high current Arc Cathode erosion is predominantly dependent on the work function and the vapor pressure of the Cathode material, and that the thermal design plays a secondary role. These results have been obtained with a newly developed self-consistent model of the Cathode region including a realistic one-dimensional sheath model. The results have been obtained for an argon Arc and a tungsten Cathode. The model has now been extended and results have been obtained for different Arc gases and different electrode materials. The Arc gas has a strong effect because it affects not only the temperature at the boundary between the Arc and the Cathode region, but also the electron density in the Cathode region and at the Cathode. The results of the calculations show that the Cathode material plays a dominant role in terms of the Cathode spot temperature and the associated mass loss rate by evaporation of Cathode material. Since the addition of thorium oxide to tungsten reduces the work function of the Cathode material, the Cathode spot temperature as well as the mass loss rate by evaporation are reduced. For the same Cathode material, hydrogen leads to the highest Cathode spot temperature and mass loss rate, followed by nitrogen and argon. The current density at the Cathode spot, the Cathode spot size, and the percentages of the energy fluxes removed from the Cathode spot are mainly determined by the plasma gas rather than by the Cathode material.
B. Jüttner - One of the best experts on this subject based on the ideXlab platform.
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the retrograde motion of Arc Cathode spots in vacuum
Journal of Physics D, 2000Co-Authors: B. Jüttner, Ingmar KlebergAbstract:Experiments are reported on the retrograde Arc spot motion on copper and tantalum Cathodes in vacuum in the presence of a magnetic field. The spots are imaged with time and space resolutions of <100?ns and <10??m, respectively. The magnetic flux density amounted to B = 0.4?T and the Arc currents to 2-100?A. For times <1??s random displacement occurs on a time scale <100?ns. At intervals of about 4??s, jumps of the spot are observed over distances of 50-300??m in the retrograde direction, thus yielding macroscopic velocities of about 50?m?s-1. The jumps are preceded by the ejection of plasma jets in the retrograde direction, having average velocities of about v = 5?km?s-1. New spots are formed exactly in the jet direction. The jets are explained by instabilities in the magnetically confined spot plasma, and the spot formation by electric fields = ? within the jets. The jets are ejected in periods of enhanced plasma production caused by the inner spot processes, i.e., by the dynamics of fragments and cells, having diameters of ?20 and ?10??m, respectively. No reversal of the motion has been observed at elevated temperatures up to 2100?K.
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The retrograde motion of Arc Cathode spots in vacuum
Journal of Physics D, 2000Co-Authors: B. Jüttner, Ingmar KlebergAbstract:Experiments are reported on the retrograde Arc spot motion on copper and tantalum Cathodes in vacuum in the presence of a magnetic field. The spots are imaged with time and space resolutions of
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nanosecond displacement times of Arc Cathode spots in vacuum
IEEE Transactions on Plasma Science, 1999Co-Authors: B. JüttnerAbstract:With a high speed camera consisting of a combination of framing and streak channels, Arc spots on a copper Cathode are imaged in the spectral range 200-800 nm with spatial and time resolution of to the observation time t of /t=(2.3/spl plusmn/0.6)/spl times/10/sup -3/m/sup 2//s. This holds down to t=100 ns. Thus, fragments and spots operate on nanosecond time scales. Prior to apparent spot splitting and after apparent fragment merging the spot brightness increases considerably. When analyzing time-integrated pictures, the stages of increased brightness lead to overestimating the average residence time. Because of the short formation time, the fragments do not reach a balance between surface heating and heat conduction into the bulk, i.e., there is no stationary evaporation. A further substructure of the fragments exists with size <5 /spl mu/m and timescale /spl les/10 ns.
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Nanosecond displacement times of Arc Cathode spots in vacuum
IEEE Transactions on Plasma Science, 1999Co-Authors: B. JüttnerAbstract:With a high speed camera consisting of a combination of framing and streak channels, Arc spots on a copper Cathode are imaged in the spectral range 200-800 nm with spatial and time resolution of
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the dynamics of Arc Cathode spots in vacuum part iii measurements with improved resolution and uv radiation
Journal of Physics D, 1998Co-Authors: B. JüttnerAbstract:Arc Cathode spots in vacuum with Cu Cathodes have been registered by combining framing and streak channels of a fast image converter camera equipped with high-magnification optics. In a special version UV radiation could be used for imaging. In this way the internal fragment structure of the spots could be resolved down to values with exposure times of 10 ns. With light from the spectral range 200-800 nm the spot intensity is about ten times higher than when in the visible range 350-800 nm. The smallest observable fragment size is . It is demonstrated that the spot fragments can change their position within 10-50 ns, the corresponding velocities reaching . These positional changes are associated with splitting and merging of the fragments. Prior to splitting and after merging the fragments become very bright. Characteristic brightness peaks have been found in the nanosecond as well in the microsecond range. The average intervals between fluctuations or groups of fluctuations in each range amount to about 19 ns, 180 ns, and respectively. The fragment dynamics lead to random displacement of the spot as a whole. The streak pictures show this spot movement for times down to 100 ns. Within 100 ns, the spot velocity can reach about . Hence, spots and fragments are nanosecond phenomena. Apparent time constants of the fragments are explained by the nonlinear increase of the spot luminosity with increasing sojourn probability of the fragments in a given area. The effect leads to overestimating the phases of small displacements and of fragment merging when analysing time-integrated pictures.
J Heberlein - One of the best experts on this subject based on the ideXlab platform.
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an experimental investigation of factors affecting Arc Cathode erosion
Journal of Physics D, 1998Co-Authors: X. Zhou, J HeberleinAbstract:A specially designed thermal plasma reactor system for the investigation of Arc-Cathode erosion has been set up. By using an OMA-spectrometer system, emission spectroscopic measurements of electron temperature and electron number density in the Cathode region have been performed, together with single-colour and two-colour pyrometry of Cathode temperature distributions. Observation of Cathode spot behaviour has been carried out simultaneously by employing a telemicroscope and a high-speed vision system. Cathodes have been examined by SEM and EDX after Arcing. For pure tungsten Cathodes, the initial Cathode geometry has almost no effect on the Cathode spot's behaviour due to the molten state of the Cathode spot. The major erosion mechanism is the ejection of liquid droplets from the Cathode spot. However, the initial Cathode geometry has a certain influence on the Cathode's erosion for 2% thoriated tungsten Cathodes. A highly non-uniform erosion pattern will occur if the Cathode is overcooled, probably due to ion bombardment in the low-temperature regions of the Arc-attachment spot.
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Characterization of the Arc Cathode attachment by emission spectroscopy and comparison to theoretical predictions
Plasma Chemistry and Plasma Processing, 1995Co-Authors: X. Zhou, J HeberleinAbstract:Emission spectroscopic diagnostics of electron temperature distribution and electron number density distribution in the Cathode region have been carried out employing an OMA/spectrometer system. Single color and two color pyrometry of Cathode temperature distributions have also been performed. The experimental results are compared with results of a theoretical model formulated previously to study the Arc Cathode interaction. The results show that the plasma in the Cathode region strongly deviates from LTE. Thermionic cooling is the major cooling mechanism of Cathodes at high Arc currents. The work function of 2% thoriated tungsten Cathodes increases during Arcing due to fast evaporation of thorium from 2% thoriated tungsten Cathodes.
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Model predictions of Arc Cathode erosion rate dependence on plasma gas and on Cathode material
Proceedings of IEEE Holm Conference on Electrical Contacts, 1993Co-Authors: X. Zhou, J Heberlein, E. PfenderAbstract:We have previously reported results of a theoretical study which predict that high current Arc Cathode erosion is predominantly dependent on the work function and the vapor pressure of the Cathode material, and that the thermal design plays a secondary role. These results have been obtained with a newly developed self-consistent model of the Cathode region including a realistic one-dimensional sheath model. The results have been obtained for an argon Arc and a tungsten Cathode. The model has now been extended and results have been obtained for different Arc gases and different electrode materials. The Arc gas has a strong effect because it affects not only the temperature at the boundary between the Arc and the Cathode region, but also the electron density in the Cathode region and at the Cathode. The results of the calculations show that the Cathode material plays a dominant role in terms of the Cathode spot temperature and the associated mass loss rate by evaporation of Cathode material. Since the addition of thorium oxide to tungsten reduces the work function of the Cathode material, the Cathode spot temperature as well as the mass loss rate by evaporation are reduced. For the same Cathode material, hydrogen leads to the highest Cathode spot temperature and mass loss rate, followed by nitrogen and argon. The current density at the Cathode spot, the Cathode spot size, and the percentages of the energy fluxes removed from the Cathode spot are mainly determined by the plasma gas rather than by the Cathode material.
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Theoretical study of factors influencing Arc erosion of Cathode
Electrical Contacts - 1992 Proceedings of the Thirty-Eighth IEEE Holm Conference on Electrical Contacts, 1992Co-Authors: X. Zhou, J Heberlein, E. PfenderAbstract:A theoretical model describing the influence of the Arc condition and the Cathode material and geometry on Arc Cathode erosion has been formulated. To arrive at a self-consistent description for the entire Arc Cathode attachment region, a realistic, one-dimensional sheath model is used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the Arc, and by a differential energy balance of the Cathode. For the case of a tungsten Cathode, it is shown that Arc constriction has a strong effect on the current density at the Cathode spot but little influence on the Cathode temperature. Heat conduction within the Cathode and radiation from the Cathode surface control the energy transport from the Cathode spot at low currents, and dissipation by thermionic electron release dominates at high currents. For high-current applications, erosion can be minimized by using a Cathode material with a low vapor pressure. For these conditions, the thermal design of the Cathode plays a secondary role. For low currents, the erosion will be determined by the thermal characteristics of the Cathode material.
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theoretical study of factors influencing Arc erosion of Cathode
Holm Conference on Electrical Contacts, 1992Co-Authors: X. Zhou, J Heberlein, E. PfenderAbstract:A theoretical model has been formulated describing the influence of the Arc condition and the Cathode material and geometry on Arc Cathode erosion. To arrive at a self-consistent description for the entire Arc Cathode attachment region, a realistic, one-dimensional sheath model has been used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the Arc, and by a differential energy balance of the Cathode. For the case of a tungsten Cathode, it has been shown that Arc constriction has a strong effect on the current density at the Cathode spot but little influence on the Cathode temperature. It has also been found that heat conduction within the Cathode and radiation from the Cathode surface control the energy transport from the Cathode spot at low currents, and dissipation by thermionic electron release dominates at high currents. The conclusions are that for high-current applications erosion can be minimized by using a Cathode material with a low vapor pressure, and that for these conditions the thermal design of the Cathode plays a secondary role. In contrast, for low currents the erosion will be determined by the thermal characteristics of the Cathode material. >
J J Gonzalez - One of the best experts on this subject based on the ideXlab platform.
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two dimensional self consistent modelling of the Arc Cathode interaction
Journal of Physics D, 2009Co-Authors: François Cayla, Pierre Freton, J J Gonzalez, Philippe TeuletAbstract:A theory based on the literature works is developed to define a self-consistent model of the Arc?Cathode interaction in thermal plasma domain. The proposed model, applied in this paper in a two-dimensional coordinate system includes a physical description of the Arc?Cathode interaction, the Cathode bulk, the sheath, the pre-sheath and the local thermodynamic equilibrium plasma column.This model allows assuming the electric current conservation through a two-temperature treatment of the sheath and pre-sheath regions. This model has been applied in a free burning Arc configuration in an argon gas with a cylindrical tungsten Cathode and current intensity equal to 200?A. The orders of magnitude of the current density and of the heat flux are, respectively, 106?A?m?2 and 107?W?m?2.Three parametric studies are performed: on the ionization layer length, on the value of the secondary emission coefficient and on the formulation of the electrical conductivity in the pre-sheath. The size of the ionization layer and the chosen value of the secondary emission coefficient have a significant influence on the properties of the discharge.
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Arc Cathode interaction model
IEEE Transactions on Plasma Science, 2008Co-Authors: François Cayla, Pierre Freton, J J GonzalezAbstract:A 1-D model of the interaction between an electric Arc and a solid refractory Cathode has been developed. This model is based on the equilibrium of the charged particle fluxes in the Cathode layer by considering current density conservation, and balance of energy at the sheath/presheath and at the sheath/Cathode surface interfaces forming a closed system of equations. It allows the sheath and presheath to be described and the main physical quantities to be obtained by only using current density as input parameter. The calculations were performed for atmospheric argon discharge and a tungsten refractory Cathode. The results obtained, such as the Cathode sheath voltage drop and the power flux transmitted to the Cathode, are compared with those of the literature, and good agreement is observed. Moreover, our model can be used for a range of current densities (1 times 104-5 times 108 A ldr m-2) accurately describing attachment at low current. The heat flux deduced reaches a maximum of 6 times 107 W ldr m-2 at equilibrium between ionic heating and thermionic cooling. The thermionic electron emission current density is dominant for current densities higher than 5 times 106 A ldr m-2.
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3D modelling of Arc-Cathode interaction
2008 17th International Conference on Gas Discharges and Their Applications, 2008Co-Authors: François Cayla, Pierre Freton, J J Gonzalez, Ph. TeuletAbstract:The aim of this work is to propose a model which can describe the plasma bulk and its interaction with the Cathode using current density as input parameter. The pre-sheath is modelled through a two-temperature electrical conductivity estimation which takes into account the local state of the Cathode layer. Thanks to this model, the current continuity resolution change following the plasma and material characteristics. Results obtained with our model are presented in a 2D configuration. More precisely we show that with our approach, the Cathode sheath voltage drop profile at the Cathode surface is non constant and we found that the ¿average¿ Cathode sheath voltage drop is about 18V for a 200 Amps argon discharge with a cylindrical tungsten Cathode.
Joachim Heberlein - One of the best experts on this subject based on the ideXlab platform.
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Characterization of the Arc Cathode attachment by emission spectroscopy and comparison to theoretical predictions
Plasma Chemistry and Plasma Processing, 1995Co-Authors: X. Zhou, Joachim HeberleinAbstract:Emission spectroscopic diagnostics of electron temperature distribution and electron number density distribution in the Cathode region have been carried out employing an OMA/spectrometer system. Single color and two color pyrometry of Cathode temperature distributions have also been performed. The experimental results are compared with results of a theoretical model formulated previously to study the Arc Cathode interaction.
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Analysis of the Arc-Cathode interaction of free-burning Arcs
Plasma Sources Science and Technology, 1994Co-Authors: X. Zhou, Joachim HeberleinAbstract:A theofetical model has been formulated describing the influence of the Arc condition and the Cathode material and geometry on Arc Cathode erosion. To arrive at a self-consistent description for the entire Arc Cathode attachment region, a realistic onedimensional sheath model has been used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the Arc, and by a differential energy balance of the Cathode. For the case of a tungsten Cathode in an argon Arc, it has been shown that the ion current density is almost 50% of the total current density at low Arc currents, while it decreases to about 18% of the total cunent density and the thermionic electron current density increases to about 82% of the total current density at high currents. It has also been found that heat conduction within the Cathode and radiation from the Cathode surface control energy transport from the Cathode spot at low currents, and that dissipation by thermionic electron release dominates at high currents.
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Arc-Cathode interaction study
1992Co-Authors: X. Zhou, Joachim HeberleinAbstract:Insufficient electrode life and uncertainties in that life are major problems hampering the development in many plasma application areas which make use of plasma torches, Arc heaters, and Arc jet thrusters. In spite of a considerable amount of work published dealing with Arc-Cathode phenomena, our present understanding is still incomplete because different physical phenomena dominate for different combinations of experimental parameters. The objective of our present reseArch project is to gain a better understanding of the behavior of Arc-Cathode surface interaction over a wide range of parameters, and furthermore to develop guidelines for better thermal design of the electrode and the selection of materials. This report will present the reseArch results and progress obtained on the Arc-Cathode interaction studies at the University of Minnesota. It includes results which have been obtained under programs other than the NASA funded program. Some of the results have been submitted in an informal interim progress report, and all of the results have been presented in a seminar during a visit to the NASA Lewis ReseArch Center on October 16, 1992.