Fluctuation phenomena commonly exist in arc plasmas,limiting the application of this technology.In this paper,we report an investigation of fluctuations of arc plasmas in an arc plasma torch with multiple cathodes.Time-resolved images of the plasma column and anode arc roots are captured.Variations of the arc voltage,plasma column diameter,and pressure are also revealed.The results indicate that two well-separated fluctuations exist in the arc plasma torch.One is the high-frequency fluctuation (of several thousand Hz),which arises from transferring of the anode arc root.The other is the low-frequency fluctuation (of several hundred Hz),which may come from the pressure variation in the arc plasma torch.Initial analysis reveals that as the gas flow rate changes,the low-frequency fluctuation shows a similar variation trend with the Helmholtz oscillation.This oscillation leads to the shrinking and expanding of the plasma column.As a result,the arc voltage shows a sinusoidal fluctuation.