This paper presents a joint calibration scheme for voltage (V) and current (I) probes that helps accurately resolve the voltage–current phase differences even when the difference is very close to 90°. The latter has been a major issue with V–I probes when used with miniature RF plasma devices such as the atmospheric pressure plasma jet (APPJ). Since the impedance of such miniature devices is predominantly capacitive, the phase difference between the voltage and current signals is very nearly 90°. It turns out, however, that when V–I probes are used with such devices without joint calibration, these frequently yield phase shifts over 90°. Also, since the power absorption is proportional to the resistive part of the impedance, it becomes very sensitive to the phase difference when it is close to [Formula: see text]. Thus, it is important to be able to accurately resolve the phases. Post-calibration, V–I probes would be indispensable for the electrical characterization of APPJs for determining the average RF power P av, plasma impedance Z p, etc. Typical post-calibration V–I data yield Z p [Formula: see text] 93.6 − j 1139 Ω (81.5 − j 1173 Ω) at P av [Formula: see text] ([Formula: see text] for helium (argon) gas.

The article reports the excitation of a helical argon atmospheric pressure plasma jet using a pulse-modulated 13.56 MHz radio frequency (RF) power source. This helical structure is observed in open ambient air, which is far different from the conventional conical shape. This helical structure originates due to the periodic pressure variation in the discharge region caused by pulse-modulated RF (2 kHz modulation frequency) and propagates downstream into the ambient air. The geometrical characteristics of the observed structure are explored using optical imaging. Moreover, the influence of various input parameters, viz., duty cycle, gas flow rate, and RF power, of the modulated pulse on the formation of a helical structure are studied. These helical structures have an implication on the plasma jet chemical features (enhancement of reactive oxygen and nitrogen species) as these are involved in an increase in air entrainment into the ionization region desired for various plasma applications.

This work investigates the influence of pulse modulation frequency ranging from 50 Hz–10 kHz on the helium RF atmospheric pressure plasma jet's fundamental characteristics. The impact of modulation frequency on plasma jet discharge behaviour, geometrical variation, reactive species emission, and plasma parameters (gas temperature Tg, electron excitation temperature Texc, and electron density (ne)) are studied using various diagnostics such as optical imaging, emission spectra, and thermal diagnostic. From the experiments, it is observed that operating the plasma jet at low pulse modulation frequencies (around 50 Hz) provides enhanced plasma dimensions, higher electron densities and greater optical emission from reactive species (viz., He I, O, OH, N2+, etc.) as compared to the higher modulation frequencies. Besides the low power consumption, three times less gas temperature of the modulated plasma jet than the continuous wave mode makes it more advantageous for the applications. Moreover, the influence of duty cycle (D) and applied RF power (P) on the plasma jet characteristics are also discussed. It is found that 10–40% duty cycle operation provides the most favourable attributes. More importantly, the concern of shorter plasma length in RF plasma jets is overcome by operating at 10–20% duty cycle with increased applied power. This work thoroughly characterizes helium atmospheric pressure RF plasma jet with a wide range of pulse mode operating parameters, which could help to select appropriate operating conditions for various industrial and biomedical applications.

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