An experimental investigation of factors affecting arc-cathode erosion

Abstract

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.

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

“…This phenomenon, discovered by Langmuir with tungsten filaments [44], was further investigated via studies on e.g. electrode erosion [18,35,[45][46][47]. It is known that the rare earth oxides do not all behave in the same way during their diffusion.…”

“…The effect of the rare earth oxides constituting the cathode surface layer in physical state (II) on the cathode sheath remains to be investigated. Also, after some operating time more physical states are known to be formed [18,35,[45][46][47]. Their modelling has not been addressed yet.…”

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

“…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%.…”

“…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…”

Non-equilibrium modelling of arc plasma torches

“…where r is the radial distance from the torch axis (r 2 = x 2 + y 2 ), and J cath0 , R c , and n c are parameters that specify the shape of the current density profile chosen to mimic the experimental measurements in [40]. The values of these parameters for the cases studied in Section 5 are: for 400 A, J cath0 = 2⋅10 8 Am -2 , R c = 0.81015 mm, n c = 4; and for 800 A, J cath0 = 3⋅10 8 A-m -2 , R c = 0.91306 mm, n c = 4.…”