Improved Plasma Spray Torch Stability Through Multi‐Electrode Design

Abstract

Coating production by thermal plasma spray is dependent on the residence time of particles in the plasma jet produced by the gas flow inside a plasma torch. To ensure a high fraction of well-molten particles to be accelerated towards the substrate a long reproducible residence time is needed. This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. While single electrode plasma torches need an unstable attachment of the anodic arc root in order to avoid excess erosion, which also causes an unstable plasma jet, multi-electrode torches allow operation with fixed anode attachments by subdividing the anode current by the number of electrodes used, and thereby thus reducing the power input for each separated arc root. Once the steady anode attachment has been obtained the produced plasma jet exhibits a steady characteristic, but also looses rotational symmetry. The separation can be achieved by using either multi anode or multi cathode geometry with appropriate electrical control. Both version have been produced with 3 electrodes each resulting in two systems known as the Delta Gun (3 anodes) and Triplex (3 cathodes).

Blown Arc Plasma Source for the Elaboration of Finely Structured Coatings

“…Heat power convected to the anode wall of length L is: 12The mean electric field of the arc column can also be written according to: (13) It comes: (14) Fig. (5) shows good agreement between predicted convective power lost at anode electrode wall and those measured for three different anode nozzle extensions, 7 mm in diameter and 2, 3 or 5 cm in length, electrically insulated from the anode. In the following, this model will be used to evaluate the plasma velocity at the nozzle exit which is modelled from a simplifed isentropic model for the plasma flow.…”

“…Moreover, strong arc current densities at the anode wall (> 10 9 A.m -2 ) cause progressive electrode erosion which leads to reduce the energetic efficiency of these plasma torches. To limit these problems of erosion and arc instabilities, multi-electrode dc plasma torches were developed [5]. They are consisted of three stick-type hot cathodes to equally distribute the total arc current and a "neutrode" can be inserted upstream of the anode to stretch the arc column.…”

Dynamic behaviour of alternating current arc discharge in a multi‐arc generator at atmospheric pressure

“…However, due to the discharge properties of multi‐phase arc, it is impossible for multiple arcs to appear at the same moment, which means that the distribution of discharge plasma in the arc chamber is relatively non‐uniform. To avoid this kind of intermittent discharge caused by multiple electrodes powered by a single power supply, a triple‐cathode plasma generator has been developed by three cathodes powered independently. Three independent arc channels were formed simultaneously instead of one arc channel, which dispersed the current density of cathode attachment, and, therefore, the volume of arc discharge increased and the lifetime of electrode was improved.…”

Velocity and texture of a plasma jet created in a plasma torch with fixed minimal arc length

“…Most operational plasma torches today have a conventional construction consisting of a cone shaped cathode and a hollow anode [1]. This design possesses a well-known limitation, namely, a short arc path with both high azimuthal and axial oscillations [2].…”