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

Abstract

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.

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

“…After that, the thorium coverage suffers a slow but sustained decrease until it reaches a critical value, θ cr ∼ 0.64, in which instabilities in the working regime of the cathode are produced along with a fast clearing of thorium from the cathode surface. This behaviour is similar to that found for low current intensities [8]. It is associated to the S-shaped Langmuir curves [7] whose qualitative form is not altered by Joule heating.…”

“…The values of the ξ parameter were studied in [7,8] in two different ways. In [7] the value of ξ was estimated by considering the fundamental processes that occur in the cathode-plasma interface.…”

“…In [8] a more precise estimate of ξ is established based on the processes that occur in the thoriated tungsten cathode working at relatively low current intensities, in which the Joule effect is negligible. The calculations made in [8] show that the lifetime of these cathodes is of the order of magnitude of several t 0 , where t 0 is a typical time depending on ξ and ϕ min .The comparison of this result with the experimental values on the lifetime of the thoriated tungsten cathodes permits the establishing in [8] that ξ = 1.4 × 10 −2 if ϕ min = 3.0 eV. We take this value of ξ for our calculations.…”

“…This last hypothesis, even though valid in vacuum arcs, is inadequate in high-pressure plasma arcs in which the evaporation rate is decreased by a factor of 10 2 -10 3 , because the evaporated atoms are rapidly ionized and the ions are swept back to the cathode by the electric field, as was shown by Moizhes and Nemchinskii in [7], maintaining the hypothesis of a semiinfinite cathode at a constant temperature. Nemchinskii [8] substituted this hypothesis for that of a finite cathode with a linear variation of temperature between the spot and the cold side of the cathode and concluded that the temperature distribution along the cathode length has a strong influence on the cathode life. His hypothesis, although much more realistic, is only valid for low-current arcs in which the ohmic heating can be disregarded.…”

Effect of Electromagnetic Boundary Conditions on Reliability of Plasma Torch Models

“…The distribution of the rare-earth oxide changed and concentrated at some places determined by the cathode temperature distribution, modifying also the appearance of the cathode. Therefore, the definitive validation of the current density profile would require (1) to implement a model of dopant reaction with tungsten, diffusion in the cathode and evaporation and Ref 45,46,(2) to investigate actual cathode microstructure and composition after operation in given conditions and (3) to implement a cathode sheath model to calculate the voltage drop variation. This is beyond the scope of this study that aimed to investigate the effect of the electromagnetic boundary conditions of the model.…”