The reverse-polarity plasma torch (RPT) is a promising plasma source for material processing, e.g. plasma atomization for spherical powder and plasma synthesis for nanostructured carbon, due to its high temperature plasma jet and thermal efficiency. However, its performance highly depends on the working stability of the RPT, which is undermined by the large-scale shunting. To improve the working stability of the RPT, a novel method for predicting the occurrence of the large-scale shunting is proposed for optimizing the RPT’s internal structure and operation conditions. The method is based on the thermal non-equilibrium modelling of the RPT to obtain the thickness of the cold boundary layer (CBL) and the breakdown voltage. Then, the occurrence of the large-scale shunting is predicted by comparing the breakdown voltage with the voltage difference between the electrode surface and the arc column. Based on the proposed prediction method, three different shapes of the front electrode (cathode) corresponding to different thickness of the cold boundary layer (CBL) were manufactured. Experimental and numerical study on the effect of the electrode geometry, the arc current and the gas flow rate on the working stability of the RPT and the thickness of the CBL were conducted. Results showed the quantitative correlations between the CBL thickness and the instability of the RPT and verified that the proposed numerical method is useful for optimizing the design and operation of the plasma torch with minimizing the large-scale shunting instabilities.

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