LEED studies of thorium adsorption on tungsten

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

“…Finally, substitution of E into (8) [19]. Therefore, our results are consistent with the presence of a layer of thorium atoms on the tungsten substrate of the spot during arcing.…”

Section: Effective Work Functionsupporting

confidence: 86%

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

“…The value of ϕ min depends on the crystallographic characteristics of the tungsten substrate. It varies from ϕ min 3.0 eV for polycrystalline tungsten to ϕ min 3.4 eV for W(100) [9]. According to equation ( 1), a spot temperature T S ∼ 3500 K corresponds to these values of ϕ for emission current densities of 10 5 -10 4 A cm −2 , which agrees well with the experimental observations [11,12].…”

Section: The Modelsupporting

confidence: 84%

“…For values θ 1, the experimental dependence of the effective work function with the fractional coverage [9,10] can be described by the simple expression…”

Section: The Modelmentioning

confidence: 99%

See 1 more Smart Citation

The numerical modelling of Joule heating effects in thoriated tungsten cathodes of high-current plasma arcs

Casado

Colomer

2000

The influence of the Joule heating on the performance of thoriated tungsten cathodes working at current densities of 10 3 -10 4 A cm −2 is investigated. The variation of the degree of coverage of the cathode surface by thorium atoms is considered, together with the heat conduction and the diffusion of thorium in the cathode bulk. The nonlinear system of equations governing these processes is solved numerically and the time evolution of the spot temperature is obtained. The effects of different working conditions on the cathode lifetime are discussed. It is found that a critical current density exists for which the service time of the cathode has a minimum. It is shown that a small increment in the cathode cooling temperature significantly increases the cathode lifetime.

“…The degree of coverage is governed by the balance equation. Let N 0 be the number of sites available for adatoms per unit surface (N 0 = 4-5 × 10 14 cm −2 for a W-Th system [5]), and let N be the surface concentration of adatoms, so that θ = N/N 0 is the degree of coverage. When θ < 1, the surface is covered by less than a monolayer.…”

Section: Basic Equationsmentioning

confidence: 99%

Life time of a refractory cathode doped with a work-function-lowering dopant

Nemchinsky¹

1996

The diffusion of dopant atoms to the surface of a refractory cathode doped with a lowering work-function dopant is considered. The variations of the cathode temperature with time and with the coordinate along the cathode length are taken into account. It is shown that, depending on the value of , the work function at the monolayer coverage, there are two possible scenarios of cathode temperature development. At low , the cathode experiences instability which leads to cleaning of the surface from adatoms and to a sharp rise in temperature. At higher , the cathode temperature changes gradually. The cathode service time, as a period of time when the dopant surface concentration falls and the cathode temperature raises, is estimated. It is shown that a properly organized temperature distribution inside the cathode may increase its service life several times.