Plasma-substrate interaction in a dual frequency APPJ

To achieve a uniform argon plasma in an atmospheric pressure jet, we explored the possibility of using dual-frequency excitation in a coaxial dielectric barrier discharge device. Two separate ring-shaped electrodes outside an alumina tube generate the two frequencies. The upstream electrode is powered at low frequency (LF, 17kHz), while the downstream electrode is powered at radio frequency (RF, 27MHz). To assess the interaction with the substrate, a grounded electrode, covered by a glass substrate, is placed 5mm from the outlet of the alumina tube. We analyze the device with a fluid model and compare the results with experimental electrical and optical characterization. As the plasma is ignited by the LF, positive streamers develop from the LF electrode and reach the substrate in a few hundred nanoseconds. At this stage, the substrate is charged and a surface discharge propagates on the glass; however, no additional ionization occurs in the jet. As RF is added, the plasma is sustained by modulation in the Omega regime. At the same time, the LF polarization propagates through the plasma, influencing the sheaths on the facing dielectric walls. When the voltage drop in the sheaths exceeds a threshold level, an additional gamma mode originates due to the secondary electrons emitted by the surfaces, leading to an increase in power dissipation. This coexistence of the two regimes is observed in the simulation, and it is validated experimentally by time-resolved photoemission measurements. As a result, the dual-frequency plasma exhibits a filamentary structure similar to that of an LF-driven jet. However, RF excitation caused diffused pre-ionization of the gas, which reduced the charge density gradients, resulting in wider microdischarge channels and a lower average electric field. Streamers propagation is therefore limited, while an ion flow to the substrate is maintained and controlled by the LF polarization.