FFT for Ar double arc plasma

For ensuring the accuracy of the results and preventing the lose of data, we use all of data to do Fast Fourier Transform for Ar plasma double arc as shown in Figure 1, 2.

                                                    Figure 1: FFT for electrical signal U1

                                                        Figure 2: FFT for electrical signal U2

In the two figures, the electrical signals do not have high frequency components (1-15 kHz). This is the main characteristics which is different from Fast Fourier Transform for Ar-N2 double arc plasma jet. However, the high frequency component is caused by the arc root motion on the anode wall or the large-scale shunting of the plasma jet. Therefore, the graphs which do not have high frequency components indicate this special design torch can reduce the high frequency fluctuations for argon plasma successfully. If we want to observe the characteristics of low frequency of pure argon plasma, the two figures need to be zoomed in as shown Figure 3, 4.

                                                 Figure 3: Zoom in the low frequency region for U1

                                               Figure 4: Zoom in the low frequency region for U2

As shown in the last figure, the prominent peak is located at 150 Hz, so the corresponding period is 6.67 ms. And its harmonics are around 300 and 600 Hz. 150 Hz is the power source frequency, so the frequencies only depend on fluctuation of rectified power supply, it is not related to the current intensity and gas flow rate. In other words, undulation of input voltages causes the non-stationary of output plasma jet.

In fact, the Y-axis does not represent the real value of amplitude, it is relative amplitude.