  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 behavior, 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 diagnostics. 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, the 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 favorable 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 select appropriate operating
conditions for various industrial and biomedical applications.
