Enhanced Power Coupling in a Microwave Plasma via Electrode-Tube Gap Adjustment

Resonant-type microwave electrodes have attractive advantages for various applications owing to their efficiency in generating plasmas with high densities of electrons and radical species [1]. They are particularly useful when low breakdown power is desired at plasma ignition. However, after ignition, the added plasma load can cause a severe impedance mismatch [2], reducing power coupling and restricting the plasma to a small volume. Launchers like surfatron [3] are designed to generate surface-wave plasmas (SWPs) in dielectric tubes with high efficiency, achieving impedance matching through adjustable internal components (e.g., a movable coupler and plunger).

This study demonstrates enhanced power coupling and expanded plasma volume in a coaxial transmission line resonator (CTLR) combined with a dielectric tube, by controlling the electrode-to-tube distance. The experiment utilizes a 2.45 GHz CTLR and a low-pressure argon-filled quartz tube. By adjusting the electrode-tube gap in the range of 0–4 mm, the plasma column length increases by about a factor of 10, from 12 mm to 105 mm. Concurrently, the power coupling improves from approximately 37% to 90% at an optimal gap of around 1.5 mm. COMSOL 3D electromagnetic simulations show similar trends on the electrode-tube gap. When the gap exceeds the optimum, both plasma length and power coupling decrease.

This work provides a geometric optimization approach to design compact, low-power (< 10 W), surface-wave CTLR (SW-CTLR) plasma generators. Our scheme achieves inherently high-power coupling efficiency, potentially reducing or eliminating the need for external matching components.

[1] SY Jeong et al. 2023 Plasma Sources Sci. Tech. 32 015003

[2] J Lee and GS Yun 2021 AIP Advances 11 105014

[3] M Moisan et al 1979 J. Phys. D: Appl. Phys. 12 129