Microplasmas were diagnosed by spatially resolved diode laser absorption using the Ar 801.4 nm transition (1s5-2p8). A 900 MHz microstrip split ring resonator was used to excite the microplasma which was operated between 100–760 Torr (13–101 kPa). The gas temperatures and the Ar 1s5 line-integrated densities were obtained from the atomic absorption lineshape. Spatially resolved data were obtained by focusing the laser to a 30 μm spot and translating the laser path through the plasma with an xyz microdrive. At 1 atm, the microplasma has a warm core (850 K) that spans 0.2 mm and a steep gradient to room temperature at the edge of the discharge. At lower pressure, the gas temperature decreases and the spatial profiles become more diffuse.

Abstract. Atmospheric argon microplasmas driven by 1.0 GHz power were studied by microwave circuit analyses and spatially-resolved optical diagnostics. These studies illuminate the mechanisms responsible for microplasma stability. A split-ring resonator (SRR) microplasma source is demonstrated to reflect excess microwave power, preventing the ionization overheating instability while limiting electron density to approximately 1 × 10 14 cm −3 and OH rotational temperature to 760 K at 0.76 W. Providing the SRR microplasma with an electrical path to ground, however, allows the microplasma to transition from the SRR mode to the so-called transmission line mode (T-line). This transition is due to matching of the microplasma and transmission line impedances. The higher power T-line mode supports a more intense microplasma with electron density of 1 × 10 15 cm −3 and OH rotational temperature of 1480 K with 15 W absorbed power. While the SRR mode is optimized for ignition and sustaining a stable nonequilibrium plasma, and T-line mode is better suited for driving a hot, high density microplasma. The estimated microwave discharge voltages were 15 V and 35 V in SRR mode and T-line mode, respectively, and the voltages are rather independent of input power. Microplasma stability is due to a combination of impedance mismatching and direct control of power, both inherent to microwave circuitry.

Argon microplasmas generated at 0.9 GHz by a split-ring resonator form a single filament-shaped nonequilibrium glow discharge. The internal structure of these microplasmas is probed using laser diode absorption and imaging emission spectrometry. These two-dimensional diagnostics are then Abel inverted to reveal that the core of the filament has a strongly depleted central metastable argon population. The microdischarges are approximately 0.2 mm in width at 1 atm and expand with increasing input power between 0.05–1.5 W. The relative electron density and the Ar(4p) density are estimated from the emission detected by a CCD camera through various bandpass filters. Absolute Ar(4s) densities, on the other hand, are determined by Ar 801.4 nm absorption. The Ar(4s) profile transitions from center-peaked at low power (0.05 W) to center-depleted above 0.25 W, saturating at 1019 m-3. The electron density profile within the microplasma, however, remains center-peaked regardless of the power. The spatially-resolved gas temperature was estimated from the broadening of the Ar 801.4 nm absorption profile. The error in this gas temperature measurement due to the depletion of the metastable atoms is corrected using numerical heat transfer models and shown to be 1650 K if the plasma power is 1.2 W. Differences between the peak and average temperatures based on the nitrogen rotational spectrum are also explained using nitrogen emission imaging.

No abstract

No abstract