A cold atmospheric pressure plasma jet controlled with spatially separated dual-frequency excitations

A dual-frequency cold atmospheric pressure plasma (CAP) jet is studied as a possible route to separate control of basic plasma parameters particularly plasma density, plasma plume length and gas temperature. With spatially separate application of two excitation frequencies, one at 5.5 MHz and the other at 30 kHz, plasma dynamics exhibit interaction between influences by the two individual excitation frequencies. However, this interaction is well controlled as manifested in the voltage dependence of plasma density and gas temperature. The dual-frequency CAP jet is shown to increase its optical emission intensity by at least three times, but without much increase in its gas temperature, compared with the maximum emission intensity of its single-frequency counterparts. Its plume length is also longer, realized at the applied voltages well below the minimum value necessary for single-frequency jets to form. It is shown that the upperstream discharge at 5.5 MHz feeds abundant electrons to the downstream plasma plume sustained at 30 kHz for the latter to acquire high plasma density and long plume length. Dual-frequency cold atmospheric pressure plasma jets offer a step change in capability, characters and possibly underpinning physics from their single-frequency counterparts.