Microwave Plasma Generation With Resonance Frequency Tracking and Power Regulation

This paper presents a palm-sized power-providing module for microwave plasma generation with resonance-frequency tracking and output-power-regulation schemes. A 2.4-GHz coaxial transmission line resonator is used as a microwave plasma generator. The power module covers the frequency range from 2.3 to 2.5 GHz with the maximum power capability of 24 W. The power module adaptively sets the output frequency so that the forward power to the resonator is minimally reflected in response to the resonance frequency shift due to plasma formation. At a forward input power of 2 W, the resonance frequency shift by plasma formation is 41 MHz. With the proposed adaptive frequency tracking scheme, the power-transfer efficiency improves from 56 to 88 %. In addition, production of $\cdot $ OH radical, which is one of the key species in biomedical applications, increases by 33 % at 2 W. The power regulation function is verified by measuring the plasma gas temperature, which is an important indicator of safety for biomedical applications. With a feedback loop control, the power module stably generates 1 W and maintains a constant plasma gas temperature of 40 °C for 300 s. The measurements reveal that the proposed schemes can improve the power efficiency and controllability of microwave plasma generation, and thereby increase the feasibility of producing a portable low-power microwave bio-plasma device.