On the Fluctuation Mode in a Plasma Jet

The periodic fluctuations present in arc voltage and luminous intensity of D. C. plasma jet have been studied. The D. C. plasma jet generator was a non-transferred, vortex and axial gas flow, constricted configuration, and taper type single port nozzles were prepared for Argon, Nitrogen and mixed gas. A P-P type valve voltmeter was used to measure the arc voltage and luminosity fluctuations were observed by a MS-9SY photomultiplier tube, which was positioned perpendicular to the jet axis at a nozzle exit. A SS-5102 synchroscope was used to determine the instantaneous waveforms of arc voltage and photocurrent. A saw-tooth fluctuation of the arc voltage has been produced in Argon plasma jet and similar in form to the fluctuation obtaind with Nitrogen. The frequency and amplitude of this fluctuation were dependent upon the arc current and the gas flow rate. There was a tendency for the frequency to increase with the gas flow rate, and for the amplitude to reduce gradually. An increase in the arc current to 60～80 A caused the frequency of the voltage fluctuation to a maximum value. A further increase in current over 100 A reduced the frequency and fluctuations were vanished. So the transition the fluctuating mode to the steady mode depens upon the value of the arc current. The typical value of the frequency in voltage fluctuation was of the order of 4～10kc/s in Argon and 10～20kc/s in Nitrogen for our experimental condition. It is suggested that the cause of voltage fluctuations is the movement of the anode spot. But the different forms in Argon and Nitrogen may be attributed to the different arc mechanism in these gases. The waveform of luminosity fluctuations in plasma jets was synchronized to the voltage fluctuation and the peak-to-peak intensity fluctuations was equal to the 17% about the mean output of a photomultiplier.