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In this article, the power- and spatial-dependent macroscopic electrical transmission and reflection properties of an argon microwave plasma are analyzed. A custom two-port coaxial prototype is built with a variable spatial gap between two electrodes. All measurements are based on the extension of the large-signal scattering parameter (Hot-S) measuring environment and are proof-of-concept for the determination of plasma properties without an external probe. The coaxial topology of the prototype has a fixed reference impedance so that the reference planes can be located at the tip of both electrodes. An equivalent lumped element circuit valid over a wide bandwidth is investigated to simulate the linear macroscopic electromagnetic reflection and transmission properties of the plasma. A series-resonance circuit is chosen since its representation is capable of covering the measurements from 10 to 40 W of input power and gaps from 1 to 3 mm. With this circuit, a continuous relationship between the impedance, power, and the spatial dimensions of the plasma is found. [ABSTRACT FROM AUTHOR]

Impedance matching plays a vital role in a microwave atmospheric pressure plasma jet (MW-APPJ). However, real-time tuning of MW-APPJ impedance matching has rarely been investigated. In this study, we have found that the dissipated microwave power and length of a MW-APPJ are effected by the application of a DC-bias magnetic field. By placing a MW-APPJ in a gap between two electromagnets, the consumed microwave power varied with the insertion position of the MW-APPJ. When the MW-APPJ was fixed at the center of the electromagnets, the consumed microwave power decreased as the DC magnetic field intensities increased, from 12 to 750 mT. The variation of consumed microwave power was much higher at lower gas velocities, which indicates that the DC magnetic field has a relatively strong effect on microwave plasma with lower gas velocity. The length of the plasma jet also decreased with increasing DC magnetic field, which was mainly due to the variation of the consumed microwave power. To explain these interesting phenomena, the effective permittivity of microwave plasma and the equivalent circuit model of a generator under DC-bias magnetic field have been analyzed. The effective permittivity of the plasma is related to its cyclotron frequency, which is a function of the DC-bias magnetic field. The variation of the effective permittivity changes the load impedance of the plasma generator. Therefore, the reflection coefficient of the plasma generator is adjusted and the length of the plasma jet varies with the change in consumed microwave power. [ABSTRACT FROM AUTHOR]

A novel method for measuring microwave plasma impedance is presented. Plasma impedance is measured in real-time using three coupling probes and a microwave detector, rather than using a vector network analyzer or directional coupler, spectrometer. The measured impedance of a plasma filament at atmospheric pressure demonstrates that the impedance changes significantly with power variation, whereas the influence of airflow is minor. This implies that the impedance matching of atmospheric pressure microwave plasmas should take into account the effects of microwave power variations, and the use of a fixed matcher is not recommended for impedance matching of plasma jets unless they are operating at a fixed power. This measurement method avoids the intervention of other microwave power or expensive instruments, which facilitates the development of microwave plasma models and the tuning of efficiency. [ABSTRACT FROM AUTHOR]

A circuit modeling approach for self-sustained capacitively coupled microwave plasmas is introduced and validated in this article. The model solves the particle balance equation using a novel circuit optimization scheme to calculate the plasma parameters like electron density, sheath thickness, and electron temperature for a given input power, operating frequency, gas type, and pressure. While the existing models capture the ohmic loss in the bulk plasma region, the proposed model also takes into account power required to sustain plasma which proves to be considerable for higher electron densities. The model is rigorously validated using experimental results from two cases—one resonant and one non-resonant—both with self-sustained microwave plasma. As the post-processing results, other discharge parameters like sheath and plasma voltages as well as plasma reduced field are calculated. The proposed modeling approach can substantially simplify design and analysis of capacitively coupled microwave plasmas. [ABSTRACT FROM AUTHOR]

Plasma-based microwave power limitation is experimentally investigated in a microstrip transmission line integrating a direct current (dc) plasma discharge for preionization. Steady state and transient high-power microwave measurements demonstrate that the preionization by the dc plasma discharge helps in reducing limiting power thresholds and time responses to about 30 dBm and tens to few microsecond, respectively. Besides, optical diagnostics also exhibit interesting behaviors of the plasma discharge such as diffusion and parasitic breakdowns. Finally, the effect of plasma confinement is discussed, and it is shown that reducing the volume devoted to plasma expansion leads to a decrease in the dynamic range of the microwave power limitation. [ABSTRACT FROM AUTHOR]

In this paper, the multiphysical and frequency-dependent characterization of a microwave argon plasma jet in the GHz range is presented. Three different frequency ranges around 1.3, 2.4, and 3.5GHz are compared using electrical, geometrical, optical, and thermal analyses for different power levels between 2 and 10 W at a constant gas flow. Based on measurements using the large signal S-parameter measuring technique, the plasma base point impedance and microwave conductivity are determined via circuit and finite-element method simulations. Major changes within a small frequency interval are exhibited. Optical measurements are dependent on the geometrical dimension changes of the microwave plasma. It is shown that the geometrical dimensions are strongly dependent on frequency and applied power. These investigated dependencies of the microwave plasma on operation conditions open up new degrees of freedom to match the frequency to the application needs. A description of different applications is given based on the different areas of the measurement results. [ABSTRACT FROM PUBLISHER]

A technique has been developed to generate stable Ar plasmas at atmospheric pressure inside of a quartz capillary tube. The microwave plasma in the capillary tube is produced by utilizing a single mode resonant circular cavity applicator with 2.45 GHz frequency excitation. The plasma can be confined or fully exposed outside the cavity as plasma jets depending on the operating conditions. The profile of the plasma front has been characterized as a conical tip or split branching jet plume. The measured discharge length and the power density of the plasma range from 6 to 10 cm and 35 to 75 W $\cdot $ cm ^{-3} , respectively. Addition of CH4 into the Ar plasma results in an increase in the abundances of excited CN and C2 carbon radicals and a concomitant decrease in the intensity of the Ar emission. [ABSTRACT FROM AUTHOR]

This paper introduces a novel microwave plasma source based on inductive coupling window—rectangular resonator. It consists of a WR430 waveguide, an inductive coupling window, a rectangular resonator, a closed cuboid quartz cavity, and a sliding short-circuit plunger. This microwave plasma source can produce a large-volume stable atmospheric air microwave plasma, and the plasma size is at the smallest 63 mm $\times\,$ (40–50) mm $\times\,$ 30 mm. The microwave reflectance ratio of the microwave plasma source exactly at higher power is roughly unrelated to the input microwave power before the plasma is produced. After the plasma is generated, the reflectance ratio increases with the increase in the input microwave power and the ratio at lower power is even smaller than that before the plasma is produced. The gas temperature of the atmospheric air microwave plasma is approximately changeless with the input microwave power. The atmospheric air microwave plasma cuts off the microwave with the frequency of 2450 MHz. [ABSTRACT FROM AUTHOR]