| Microplasmas are sub-millimeter regions of ionized gas. This dissertation describes the measurement of the physical properties, the computational modeling, and a practical application of a microplasma generated by a microwave split-ring resonator. This novel device has shown broad potential applications including tracing hazardous gases and detecting airborne particles. Measurements show better performance (higher electron density and reduced sputter erosion) in atmospheric pressure gases than other microplasma sources. It is essential to measure the plasma characteristic parameters in order to improve the understanding of microplasma physics and optimize the performance of this microplasma device. This dissertation covers the experimental characterization of microplasma parameters, a transient three dimensional microplasma model to verify and explain the experimental results, and a new practical application of this microplasma.Electron densities of high pressure argon microplasma were measured from the Stark broadening of the Hbeta emission line. The results show that the electron density is 4-8x1013 cm -3 but increases sublinearly as a function of input power. It was found that the electron density with constant input power also increases as gas pressure increases. The results were compared with DC microplasma performance and show that the split-ring resonator driven at microwave frequency can generate a more intense microplasma than those driven by DC potentials. In addition, this work shows for the first time that the driving frequency has an important effect in determining the microplasma characteristics.How the electric field frequency effects microplasma was experimentally investigated by employing the plasma impedance analysis method. This is the first work that focused on evaluating microwave frequency effects on microplasmas. The split-ring resonator device enables one to evaluate the frequency effect over broader operational pressures (400 mTorr to 1 atmosphere), and a higher frequency range (400 MHz to 1800 MHz) compared with conventional plasma generators. The observations showed high excitation frequency plasmas have a lower plasma resistance, which indicates that high excitation frequency plasmas have higher electron density. Furthermore, high excitation frequency decreases the microwave electrode voltage. Therefore, the operating frequency becomes another "knob" to control microplasma properties.For validation of the experiment results and improved understanding of the discharge mechanisms, a transient three-dimensional argon microplasma simulation was carried out using the CFD-ACE+ modeling tool. The simulation results are consistent with experimental results: higher excitation frequency results in an increased electron density. Moreover, modeling has revealed a mechanism responsible for this frequency effect: high excitation frequency plasmas have thinner sheaths which decrease the reactive sheath impedance and, therefore, decrease the electrode voltage. This partitions more of the available power into ionization rather than ion acceleration toward the electrodes.Finally, the microplasma was applied in the charging and trapping of microparticles. Microparticles were charged by free electrons in the plasma and then suspended above an unbiased dielectric substrate by the strong potential gradient between the substrate and the microplasma. These particles were physically manipulated by expanding the plasma sheaths through the application of external voltage pulses. In these experiments the microplasma has demonstrated a realistic potential for the replacement of radioisotopic charging in the application of microparticle detection. |
