An experimental study of the dependence on the current intensity of the erosion of thoriated tungsten cathodes in plasma arcs

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

“…The plasma gas was argon with 2vol.% of hydrogen. The small amount of hydrogen was added to avoid oxidation of the cathode surface by atmospheric oxygen [9]. The gas flux was 20 slm.…”

Section: Methodsmentioning

confidence: 99%

mentioning

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

“…Their discovery was explained by non-equilibrium effects in the near electrode arc regions, which led to possible recombination of metal ions affecting the neutral vapour density. Casado et al [5] explored the eroded thoriated tungsten cathode surface by a scanning electron microscope and an energy dispersive xray spectrometer. They discovered that the cathode erosion rate became a maximum value at a critical current intensity of 50 A.…”

Section: Introductionmentioning

confidence: 99%

Experimental study on copper cathode erosion rate and rotational velocity of magnetically driven arcs in a well-type cathode non-transferred plasma torch operating in air

Chau

Hsu

Lin

et al. 2007

The cathode erosion rate, arc root velocity and output power of a well-type cathode (WTC), non-transferred plasma torch operating in air are studied experimentally in this paper. An external solenoid to generate a magnetically driven arc and a circular swirler to produce a vortex flow structure are equipped in the studied torch system, which is designed to reduce the erosion rate at the cathode. A least square technique is applied to correlate the system parameters, i.e. current, axial magnetic field and mass flow rate, with the cathode erosion rate, arc root velocity and system power output. In the studied WTC torch system, the cathode erosion has a major thermal erosion component and a minor component due to the ion-bombardment effect. The cathode erosion increases with the increase of current due to the enhancement in both Joule heating and ion bombardment. The axial magnetic field can significantly reduce the cathode erosion by reducing the thermal loading of cathode materials at the arc root and improving the heat transfer to gas near the cathode. But, the rise in the mass flow rate leads to the deterioration of erosion, since the ion-bombardment effect prevails over the convective cooling at the cathode. The most dominant system parameter to influence the arc root velocity is the axial magnetic field, which is mainly contributed to the magnetic force driving the arc. The growth in current has a negative impact on increasing the arc root velocity, because the friction force acting at the spot due to a severe molten condition becomes the dominant component counteracting the magnetic force. The mass flow rate also suppresses the arc root velocity, as a result of which the arc root moves in the direction against that of the swirled working gas. All system parameters such as current, magnetic field and gas flow rate increase with the increase in the torch output power. The experimental evidences suggest that the axial magnetic field is the most important parameter to operate the straight-polarity WTC plasma torch at high output power with a limited cathode erosion rate. This emphasizes the importance of an external magnetic field on a WTC torch system for reducing the erosion at the cathode.

“…Since the ring appeared on the three types of cathodes, it indicated that a reaction of tungsten oxidation was taking place, possibly with the formation of the brown oxide WO 2 having a boiling point of 2273 K [15]. The formation of an oxide ring on the lateral surface of the cathode in an area away from the spot was previously observed in tungsten cathodes operated in free atmosphere in arcs with a copper anode [14] and with a graphite anode [16].…”

Section: Methodsmentioning

confidence: 53%

“…Cathode erosion is a problem that must be solved to allow a greater use of plasma torches. This problem is met by two basic means: the determination of the optimum working regime of the cathodes that allow for diminished erosion [1][2][3][4][5][6] and the search for new, more resistant cathode materials [7][8][9][10].…”

Section: Introductionmentioning

confidence: 99%

An experimental comparison of the erosion in tungsten cathodes doped with different rare-earth elements

Casado

Colomer

Munoz-Serrano

et al. 2002

Three different types of tungsten-based cathodes (thoriated, lanthanized and ceriated) were used in plasma torches operating at different current intensities ranging from 30 to 210 A. For each one of the current intensities used, the erosion rate was measured for each of the cathode types, thereby obtaining for them the curves describing the dependence of the erosion on the current intensity. The effects of erosion on the cathode surface were explored by scanning electron microscope and energy-dispersive x-ray spectrometer. On each of the types of cathodes studied, the cathodic erosion had a maximum for a critical value I* of the current intensity. The value of this critical current intensity depended on the dopant used. The following values were obtained: ITh* = 50 A for thoriated tungsten, ICe* = 80 A for ceriated tungsten and ILa* = 110 A for lanthanized tungsten.