Three distinct phases of electron heating in an rf-driven atmospheric-pressure plasma jet

Cold plasma jets represent one of the most attractive and versatile plasmas, and active studies on plasma properties such as electron density (n (e)) and temperature (T (e)) have facilitated the development and implementation of plasma jets. Here we demonstrate the temporal evolution of n (e) and T (e) in a radio-frequency (rf)-driven argon plasma jet operating at atmospheric pressure. The plasma jet is driven by 5 MHz sinusoidal rf power modulated by a 50 kHz square pulse with a 75% duty ratio. A 532 nm Nd:YAG laser is applied to the plasma, and laser Thomson scattering measurements are performed using a triple-grating spectrometer coupled to an intensified CCD camera. From this investigation, we find that n (e) and T (e) vary during the pulse repetition period of 20 mu s in the ranges of (2-12) x 10(18) m(-3) and 0.3-6.0 eV, respectively. With respect to the 5 MHz period (200 ns), T (e) varies with the rf oscillating field while n (e) remains constant. Special attention is given to three distinct electron characteristics depending on the pulse phase in the plasma jet-ionizing (on-pulse), stationary and recombining (off-pulse) states. Our measurements will be valuable for related experimental and numerical plasma research and provide further insights into the effect of rf pulsing on the electron kinetics in atmospheric-pressure plasmas.