CONTEXT

Plasma projection is a well known technic for surface coating; powders, liquids or supensions are injected in a plasma nozzle where there are fused and drops collapes on the surface where they adhere  very well. One major drawback of such process lies in the instabilities of the torch (tension variation) whih implies inhomogeneity of the coating.

TECHNICAL DESCRIPTION

The invention proposes a solution to these instabilities by the use in the direct current plasma torch of an external acoustic resonator connected with the cathodic cavity. This resonator is closed with a piston which position can be modified in order to change the depth of the resonator cavity. The resonator cavity and the cathodic cavity of the torch are connected through an aperture adjusted in diameter and length. The piston position can be slaved to the arc voltage by the use of a feedback control loop.

BENEFITS

The usage of the external acoustic resonator permits to significantly stabilize the arc voltage and electrical power supplied to plasma gases. The position of the piston is adjusted to enhance the coupling between pressure waves inside the cathodic cavity and the arc voltage, so that an anti-resonance effect is produced to damp the plasma instabilities. The acoustic resonator can be easily adapted on existing plasma torches. In addition to the reduction of the amplitude of instabilities, the mean value of the arc voltage and electrical power are slightly increased.

The reduction of plasma flow instabilities is useful to regulate the transient heat treatment of embedded materials particularly in plasma spraying or suspension plasma spraying.

INDUSTRIAL APPLICATIONS

Used in thermal spray industry for coating deposition and for plasma torch manufacturers.

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