Modification of Helical Structures Observed in Pulsed RF Argon Plasma Jets by Alteration of Modulation Frequency and Helium Admixing

Recent experiments have successfully demonstrated production of unique helical shape in atmospheric pressure argon plasma jets excited by pulse modulated RF, at 2 kHz [1] . The present work attempts to broaden the scope of the latter study by expanding the range of modulation frequencies to ~ 500 Hz -10 kHz and considering the effect of admixing a diluting gas (He + Ar). The experimental observations show that the helical jets are initiated somewhat crudely at low frequencies (~ 500 Hz) and become most prominent at ~ 2 kHz; with increasing modulation frequency the plume transforms to the smooth conical shape. The latter observations corroborate proposed physical mechanism for formation of the helical shape, which relies on acoustic wave generation via pressure pulsations. It is also worth noting that a smooth conical shape with no signature of helical shape is observed with pure helium gas, under similar input pulse conditions used for argon. Additionally, formation of helical jet is gradually suppressed with increasing dilution of argon gas with helium. The latter results demonstrate that helical shape of plasma jet is exclusive to argon. Also, it seems at present that the best pulse conditions for observation of helical structure is about 2 kHz, which possibly is ideal for triggering the acoustic pressure pulses. Paper will discuss the experimental outcomes in detail.