Reports: GB6
46822-GB6 Fundamental Studies of Plasmas Generated and Maintained in Spatially Confined Geometries: Capillary Plasma Electrode Discharges
In the final phase of this research project, we used the investigative techniques of electric and optical analysis characterization studies to better understand the operational behavior of the atmospheric pressure Capillary Plasma Electrode (CPE) discharge. The CPE is a type of non-equilibrium or cold plasma at atmospheric pressure. It’s a unique plasma since it is a stable high-pressure plasma generated and maintained by spatially constricting the plasma to micron dimensions. The CPE is further an example of a microplasma or microdischarge, which is weakly-ionized and allows for a fascinating new investigative realm for plasma science.
In this study, voltage-current (V-I) measurements, optical emission spectroscopic studies, and measurements of the rotational and vibrational temperatures of the plasma species were carried out to determine the gas temperature of the generated microplasma. In particular, we concentrated the studies on specifically one capillary discharge. These methods of investigation have given us much clearer understanding of the operation of the CPE and have allowed for the first comprehensive explanation of the plasma physics and chemistry of this microplasma source.
We now know that to achieve breakdown in the atmospheric pressure CPE discharge, an AC voltage applied to the opposing electrodes is necessary, where typically the applied voltage is in sinusoidal form. A complete cycle is described in the provided figure. At the beginning of the positive cycle the applied field Ea is established between the electrodes. At the same time the electric field inside the capillary Ec is stronger than the applied field due to the surrounding dielectric. The stronger field inside the capillary helps to initiate a streamer-like discharge. If the applied external electric field is strong enough the plasma initiated inside the capillary emerges in the form of a jet. Depending on the distance and geometry of the opposing electrode, the jet can be either collimated or not. The discharge persists until the charges accumulating on the dielectric surface of the opposing electrode cancel out the applied field.
During the negative half of the cycle the space charge contributes to the applied field and the back charges are formed, which neutralize the barrier charge. In order to further stimulate the breakdown inside the capillary a more non-uniform field was introduced as a pin electrode inside the capillary, which is particularly useful for discharges in air.
In conclusion, our investigative work has lead this project’s original objective of yielding the first comprehensive physical picture of the operational behavior of the Capillary Plasma Electrode (CPE) discharge.
