The challenges and benefits of microwave-induced microdischarges are reviewed. Transmission lines, resonators and surface wave launchers may be used for coupling microwave power to very small plasmas. Fortunately, microplasmas are typically much smaller than the wavelength of microwaves, and the electromagnetic problem may be treated electrostatically within the plasma. It is possible to trap electrons within small discharge gaps if the amplitude of electron oscillation is smaller than the plasma size. Typically occurring above 0.3 GHz, this condition results in lower breakdown fields than are required by direct current or radio frequency systems. Trapping of electrons also decreases the electrode potential to only tens of volts and makes the plasma density invariant in time. The steady-state microplasma produces electron densities of up to 10 15 cm −3 in argon but the electrons are not in equilibrium with the low gas temperatures (500-1000 K). Microwave discharges are compared with other forms of microplasma and guidelines for device selection are recommended. Scale-up of microplasmas using array concepts are presented followed by some exciting new applications.

We have performed measurements of the force induced by both single (one electrode insulated) and double ( both electrodes insulated) dielectric barrier discharge plasma actuators in quiescent air. We have shown that, for single barrier actuators, as the electrode diameter decreased below those values previously studied the induced Force increases exponentially rather than linearly. This behavior has been experimentally verified using two different measurement techniques: stagnation probe measurements of the induced flow velocity and direct measurement of the force using an electronic balance. In addition, we have shown the the induced force is independent of the material used for the exposed electrode. The same techniques have shown that the induced force of a double barrier actuator increases with decreasing narrow electrode diameter.

The optically pumped rare-gas metastable laser is a chemically inert analogue to three-state optically pumped alkali laser systems. The concept requires efficient generation of electronically excited metastable atoms in a continuous-wave (CW) electric discharge in flowing gas mixtures near atmospheric pressure. We have observed CW optical gain and laser oscillation at 912.3 nm using a linear micro-discharge array to generate metastable Ar(4s, 1s(5)) atoms at atmospheric pressure. We observed the optical excitation of the 1s(5) → 2p(9) transition at 811.5 nm and the corresponding fluorescence, optical gain and laser oscillation on the 2p(10) ↔ 1s(5) transition at 912.3 nm, following 2p(9)→2p(10) collisional energy transfer. A steady-state kinetics model indicates efficient collisional coupling within the Ar(4s) manifold.

Microwave resonators are used to generate microplasmas in atmospheric-pressure argon. We present spectroscopic and electrical measurements of these microplasmas for both a single resonator and a five-element resonator array with dc voltage-switchable power distribution. These measurements include gas temperatures from fits to rotational emission spectra and electron densities from Stark broadening, both resolved in two spatial dimensions. Peak gas temperatures are found to be near 900 K in the centre of the microplasmas, while electron densities peak near 3 × 10 14 cm −3 . Spectroscopically derived plasma densities are validated by comparison with electrical measurements of the complex plasma impedances. The plasma impedances shift the resonant frequencies and quality factors of the individual resonators, which in turn influence power distribution to the resonators. Data suggest that this feedback loop reinforces the electrical switching mechanism.

Surface dielectric barrier discharges (DBDs) used as plasma actuators can induce significant time-averaged forces in nearby neutral gases. For single-barrier actuators (one electrode insulated) these forces are dependent on the geometry of the exposed electrode. We demonstrate that using thin cylindrical exposed electrodes can increase the induced force by several hundred percent compared with an actuator with a rectangular exposed electrode of the same thickness. This difference is due almost exclusively to the extent of the exposed electrode in the same direction as the gap between the two electrodes, which tends to be much longer for actuators constructed with rectangular exposed electrodes. The exact shape of the electrode cross-section plays no role. In addition, using an intensified digital camera we observed a new filament-free plasma that occurred only in discharges with exposed electrodes smaller than approximately 0.15 mm in diameter. These discharges spent an increasing fraction of the applied voltage period in this mode as we reduced the exposed electrode diameter. The mode shared several characteristics with a positive corona, and was partially responsible for a decrease in the electrical power used by these discharges.