Normally,arc columns fluctuate continuously in the non-transferred DC(Direct.Current) plasma torches due to the complicated arc instabilities and the movement of arc spot position.As a result,arc voltages between electrodes change continuously depending on the variation of arc column length[1].In order to mitigate these fluctuations of the arc column and the arc voltages,for example,a stepped electrode has been introduced as an anode nozzle in the rod-nozzle type of non-transferred DC plasma torch.Since a step-structured nozzle can bring a fixed arc spot to the nozzle anode depending on the turbulence effect by the rapid expansion of arc flow at the step region,the amplitude of arc shunting can be suppressed[2].