Dielectric barrier discharge

The process normally uses high voltage alternating current, ranging from lower RF to microwave frequencies. However, other methods were developed to extend the frequency range all the way down to the DC. One method was to use a high resistivity layer to cover one of the electrodes. This is known as the resistive barrier discharge. Another technique using a semiconductor layer of gallium arsenide (GaAs) to replace the dielectric layer, enables these devices to be driven by a DC voltage between 580 V and 740 V. DBD devices can be made in many configurations, typically planar, using parallel plates separated by a dielectric or cylindrical, using coaxial plates with a dielectric tube between them. In a common coaxial configuration, the dielectric is shaped in the same form as common fluorescent tubing. It is filled at atmospheric pressure with either a rare gas or rare gas-halide mix, with the glass walls acting as the dielectric barrier. Due to the atmospheric pressure level, such processes require high energy levels to sustain. Common dielectric materials include glass, quartz, ceramics and polymers. The gap distance between electrodes varies considerably, from less than 0.1 mm in plasma displays, several millimetres in ozone generators and up to several centimetres in CO2 lasers. Depending on the geometry, DBD can be generated in a volume (VDBD) or on a surface (SDBD). For VDBD the plasma is generated between two electrodes, for example between two parallel plates with a dielectric in between. At SDBD the microdischarges are generated on the surface of a dielectric, which results in a more homogeneous plasma than can be achieved using the VDBD configuration At SDBD the microdischarges are limited to the surface, therefore their density is higher compared to the VDBD. The plasma is generated on top of the surface of an SDBD plate. To easily ignite VDBD and obtain a uniformly distributed discharge in the gap, a pre-ionization DBD can be used. A particular compact and economic DBD plasma generator can be built based on the principles of the piezoelectric direct discharge. In this technique, the high voltage is generated with a piezo-transformer, the secondary circuit of which acts also as the high voltage electrode. Since the transformer material is a dielectric, the produced electric discharge resembles properties of the dielectric barrier discharge. A multitude of random arcs form in operation gap exceeding 1.5 mm between the two electrodes during discharges in gases at the atmospheric pressure . As the charges collect on the surface of the dielectric, they discharge in microseconds (millionths of a second), leading to their reformation elsewhere on the surface. Similar to other electrical discharge methods, the contained plasma is sustained if the continuous energy source provides the required degree of ionization, overcoming the recombination process leading to the extinction of the discharge plasma. Such recombinations are directly proportional to the collisions between the molecules and in turn to the pressure of the gas, as explained by Paschen's Law. The discharge process causes the emission of an energetic photon, the frequency and energy of which corresponds to the type of gas used to fill the discharge gap. The electrical diagram of the DBD device at the absence of discharge can be presented in the form shown in Fig. 1 where C 1 {displaystyle C_{1}} is capacitance of dielectric adjacent to one of two electrodes and C 2 {displaystyle C_{2}} is capacitance of the air (or gas) gap between the dielectric within the adjacent electrode footprint and the ground electrode. C p {displaystyle C_{p}} and R p {displaystyle R_{p}} are capacity and resistance modeling electric response of plasma. If a switch S {displaystyle S} connects the capacitors C 1 {displaystyle C_{1}} and C 2 {displaystyle C_{2}} shown in Fig. 1 (there is no electrical breakdown), the voltage generator is connected to a circuit comprising two capacitors C 1 {displaystyle C_{1}} and C 2 {displaystyle C_{2}} connected in a series circuit. A capacitance of this circuit can be expressed as C s = C 1 C 2 C 1 + C 2 {displaystyle C_{s}={frac {C_{1}C_{2}}{C_{1}+C_{2}}}} , (1) and the electric current I ( t ) {displaystyle I(t)} through this circuit can be expressed in the form