Synergistic control of radical generation in a radio-frequency atmospheric-pressure plasma jet via voltage waveform tailoring and structured electrodes

Abstract The synergy between voltage waveform tailoring and structured electrodes is investigated in a radio -frequency atmospheric-pressure microplasma jet operated in helium with a 0.1% oxygen admixture. The device incorporates rectangular trenches in both electrodes and is driven by ‘Peaks’ and ‘Valleys’ waveforms synthesized from four harmonics (base frequency <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mtext>b</mml:mtext> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>13.56</mml:mn> </mml:mrow> </mml:math> MHz, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mtext>pp</mml:mtext> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>500</mml:mn> </mml:mrow> </mml:math> V, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>P</mml:mi> <mml:mo>=</mml:mo> </mml:mrow> </mml:math> 1.2 W). Two-dimensional plasma fluid simulations, together with spatially and temporally resolved optical diagnostics (phase-resolved optical emission spectroscopy and tunable diode laser absorption spectroscopy), are used to demonstrate that the combination of asymmetric voltage waveforms with electrode structuring results in the strong spatial localization of electron power absorption and radical generation. This synergy leads to a single pronounced maximum inside a trench at either the powered or grounded electrode, depending on the applied waveform, unlike a symmetric excitation, which produces a spatially symmetric enhancement at both electrodes. This effect is attributed to the interplay between waveform-induced sheath dynamics and geometric focusing provided by the trenches, enabling electrically reversible and selective enhancement of electron power absorption at a chosen location.