Heating of refractory cathodes by high-pressure arc plasmas: I

Benilov, M. S.; Cunha, M D

doi:10.1088/0022-3727/35/14/314

Cited by 88 publications

(232 citation statements)

References 29 publications

(78 reference statements)

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“…The cathode layer forms a potential barrier of voltage drop U s that enhances electron emission, accelerates the emitted electrons and the incoming ions, and decelerates the back diffusing electrons. The number of collisions is small since λ D < λ c ; this sub-layer is thus at the transition between being collisional and collisionless [49], and a continuum approach does not apply. The cathode sheath model combined with either a LTE-plasma in [25,26], or a FNE-plasma in [11][12][13][14], is based on the following assumptions:…”

Section: Cathode Sheath (Or Space Charge Layer)mentioning

confidence: 99%

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Gas tungsten arc models including the physics of the cathode layer: remaining issues

Choquet

2017

Weld World

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A recent review pointed out that the existing models for gas tungsten arc coupling the electrode (a cathode) and the plasma are not yet complete enough. Their strength is to predict with good accuracy either the electric potential or the temperature field in the region delimited by the electrode and the workpiece. Their weakness is their poor ability to predict with good accuracy these two fields at once. However, both of these fields are important since they govern the heat flux to the workpiece through current density and temperature gradient. New developments have been made since then. They mainly concern the approaches addressing the electrode sheath (or space charge layer) that suffered from an underestimation of the arc temperature. These new developments are summarized and discussed, the modelling assumptions are examined, and important modelling issues that remain unexplored are underlined.

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Section: Cathode Sheath (Or Space Charge Layer)mentioning

confidence: 99%

“…The validity of assumption (A8) was investigated by Benilov [49] and by Pekker and Hussary [19]. It can be seen in [19] that the ion collisional factor in the sheath decreases when the current density increases.…”

Section: Cathode Sheath (Or Space Charge Layer)mentioning

confidence: 99%

Gas tungsten arc models including the physics of the cathode layer: remaining issues

Choquet

2017

Weld World

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“…j j j = − (6) Substitution of (4), (5) and (6) into (3) allows the evaluation of ϕ as a function of j and T, with e T and c U as parameters. We used in (3) the values 15.76 X = eV (argon ionization potential), e 20000 T = K and c 22 U = V. The electron temperature in plasmas generated by the plasma torch we used takes values between 18000 K and 23000 K [7,18]. The difference between the actual value of e T in the plasma and the assumed value e 20000 T = K causes a variation in ϕ obtained as solution of (3) of less than 1%.…”

Section: Effective Work Functionmentioning

confidence: 99%

An experimental study of thoriated tungsten cathodes operating at different current intensities in an atmospheric-pressure plasma torch

Sillero¹,

Ortega²,

Munoz-Serrano³

et al. 2010

J. Phys. D: Appl. Phys.

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Abstract.Thoriated tungsten cathodes operating in an open-air plasma torch at current intensities between 30 and 200 A were experimentally studied. The morphology and composition of the cathode tip after arcing were investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Three relevant zones were found on the cathode tip (spot zone, thorium-depleted zone and thorium-enriched zone), and their dimensions were measured. For each current intensity, the spot temperature T during arcing was measured by one-colour pyrometry, and the spot current density was determined. Exponential growth was seen for the current density as the spot temperature increased. From the experimental data and the energy balance in the presheath, the dependence of the cathode effective work function ϕ on the arc current intensity and its dependence on T were determined. It was found that ϕ increased from ϕ = 2.64 eV when T 2900 K to ϕ = 3.06 eV when T 3700 K. These values are consistent with the presence in the spot of a layer of thorium atoms on the tungsten matrix during arcing. The atomic surface density of the layer determines the value of ϕ. IntroductionThe investigation of cathodes is important in order to improve the performance of plasma torches due to the essential role of cathodes as electron emitters and the severe erosion of cathodes caused by the high temperatures attained on the cathode surface. Binary cathodes, formed by a tungsten matrix doped with rare-earth oxides, are of particular interest, as they combine the refractory properties of tungsten with the good electron emission characteristics of the dopant. The processes that take place in such cathodes during arcing are complex due to the interaction between the dopant and the tungsten matrix and between both the dopant and the tungsten with the plasma arc. In spite of the progress achieved in the understanding of binary cathodes, there are still fundamental aspects that are not well understood and require further experimental investigation. For instance, the dependence of the cathode work function on the spot temperature has not yet been satisfactorily established.Tungsten cathodes doped with thoria are commonly used in industrial applications of plasma torches. Our goal in the present paper is to investigate the behaviour of these cathodes over a wide range of arc current intensities. Thoriated tungsten cathodes have been experimentally studied by different authors [1][2][3][4][5][6][7][8][9][10][11], but in most of the studies, the cathodes were operated with only a few current intensities; therefore, the dependences of the cathode parameters on the arc current intensity I were not found.In the present paper, we discuss our experimental investigation of thoriated tungsten cathodes operating in the actual working conditions of an industrial plasma torch. We used arc current intensities ranging from 30 A to 200 A, which allows the investigation of cathode

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“…with noble gases or mercury. It is on one hand an outcome of intense theoretical studies and on the other hand of experimental investigations, mainly at a model lamp, within the last years [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The description of the dc operation of anodes in HID lamps is less advanced, but at least a qualitative understanding has been achieved for cold and to some extent also for hot HID anodes [16,17,18,19].…”

Section: Introductionmentioning

confidence: 99%

Investigating the dependence of the temperature of high-intensity discharge (HID) lamp electrodes on the operating frequency by pyrometric measurements

Reinelt¹,

Westermeier²,

Ruhrmann³

et al. 2011

J. Phys. D: Appl. Phys.

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Abstract. Phase resolved temperature distributions are determined along a rod shaped tungsten electrode, by which an ac arc is operated within a model lamp filled with argon. Switched dc and sinusoidal currents are applied with amplitudes of several amps and operating frequencies being varied between 10 Hz and 10 kHz. The temperature is deduced from the grey body radiation of the electrode being recorded with a spectroscopic measuring system. Phase resolved values of the electrode tip temperature T tip and of the power input P in are determined comparing the measured temperature distributions with the integral of the one dimensional heat balance with these parameters as integration constants. They are supplemented by phase resolved measurements of the sum of cathode and anode fall called electrode sheath voltage. If a switched dc current is applied it is found that both quantities are within the cathodic phase only marginally higher than for a cathode being operated with a dc current. T tip and P in start to decrease for low currents and to increase for high currents at the beginning of the anodic phase. But with increasing operating frequency the deviations from the cathodic phase are reduced until they cannot be resolved for frequencies of several kHz. A more pronounced modulation but the same tendencies are observed with a sinusoidal current waveform. For 10 kHz a diffuse arc attachment with an almost phase independent electrode tip temperature, which deviates only marginally from that of a dc cathode, and an electrode sheath voltage proportional to the arc current is established with both current waveforms.

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Heating of refractory cathodes by high-pressure arc plasmas: I

Cited by 88 publications

References 29 publications

Gas tungsten arc models including the physics of the cathode layer: remaining issues

Gas tungsten arc models including the physics of the cathode layer: remaining issues

An experimental study of thoriated tungsten cathodes operating at different current intensities in an atmospheric-pressure plasma torch

Investigating the dependence of the temperature of high-intensity discharge (HID) lamp electrodes on the operating frequency by pyrometric measurements

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