FFT for Ar-N2 Plasma double arc electrical signal

We need to do fast Fourier transform for Ar-N2 plasma double arc electrical signal. There are three kinds of  data such as time, U1 and U2, however, there are 1000,000 values so that we cannot obtain FFT graph with using ORIGIN because there are too many values. Therefore, we need to extract one value from every ten values so that 100,000 values can be obtained for time, U1 and U2 respectively.

The following graph shows the FFT for electrical signal U1 and U2:

Figure 1: Fast Fourier Transform for U1

Figure 2: Fast Fourier Transform for U2

As shown in the above figures, the shapes of FFT for U1 and U2 is nearly same. The prominent peak is located at 150 Hz (period is 6.67 ms) representing the frequency of power source and its harmonics is at 300 and 600 Hz. The fluctuation of frequency is caused by the undulation of the rectified power supply, not depend on the current and gas flow rate.

Another peak is around 4100 Hz and its corresponding arc breakdown period is 0.244 ms. Its first harmonic is located at 8.2 Hz. The frequency is caused by the arc root motion on the anode surface. Also, the gas flow rates affects the position of this frequency. Recent investigation has proven that the current mainly affects the fluctuation of the plasma arc in the restrike mode.

The y-axis represents the relative amplitude of the electrical signals not real amplitude. For example, 150 Hz peak occupies a major proportion compared to the first one. And the amplitude of U1 is nearly twice amplitude of U2 at 150 Hz.