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46822-GB6
Fundamental Studies of Plasmas Generated and Maintained in Spatially Confined Geometries: Capillary Plasma Electrode (CPE) Discharges

Jose Luis Lopez, Saint Peter's College

Non-equilibrium or cold plasmas at atmospheric pressure are susceptible to instabilities in particular arcing which quench the cold plasma and create a hot arc discharge. One of the most promising approaches pursued recently is based on the recognition that arc formation in high-pressure plasmas can be avoided and stable high-pressure plasmas can be generated and maintained when the plasmas are spatially constricted to dimensions of tens to hundreds of microns. Aptly referred to as microplasmas or microdischarges, these weakly-ionized plasmas represent a new and fascinating realm of plasma science.

The method for cold plasma generation investigated in this study was that of the capillary plasma electrode (CPE) discharge. Unlike other well-studied approaches of forming non-equilibrium, atmospheric pressure plasmas, the operating principles and basic properties of the CPE discharge are much less understood due to fewer experimental studies. This research project looked to remedy this gap in knowledge by performing intensive electrical and optical diagnostic studies with the goal of further increasing the understanding of the fundamental plasma physics and chemistry involved in the formation and sustainability of the CPE microplasma.  The capillary plasma electrode (CPE) discharge is a special case of the dielectric barrier discharge, where the filamentary mode is geometrically confined or localized to a fixed location. The process is achieved by making small holes in the dielectric covering at least one of the electrodes as seen in the figure 1. It is believed that the field inside the capillaries is higher than the field on the outside. This helps to initiate the discharge. Once the discharge ignites inside the capillary it feeds the discharge on the outside, which is now localized to that space right outside the capillary. As with the standard DBD both filamentary and glow modes can be achieved under the right conditions, however, what separates the CPE from the DBD is the ‘capillary jet mode’. When the frequency reaches a critical value (which depends strongly on the length to diameter value of the capillaries and the feed gas), the capillaries “turn on” and a bright, intense plasma jet emerges from the capillaries. When many capillaries are placed in close proximity to each other, the emerging plasma jets overlap and the discharge appears uniform.

Figure 1. Schematic of a CPE plasma reactor.
The capillary jet mode is characteristic to the CPE discharge and is the preferred stable mode of operation for this atmospheric pressure plasma. However, this ‘capillary jet mode’ mode has only been characterized in a few rudimentary investigations. The proposed research project investigated the capillary plasma electrode discharge. The research was initially commenced by concentrating on investigating one capillary. This was done on a specially constructed single cylindrical capillary reactor. A picture of the reactor is seen in figure 2. The main piece of the reactor is a solid quartz insert which consists of the capillary and a cylinder providing the dielectric barrier between the two electrodes. One electrode is a platinum pin (0.06 mm diameter) centered inside the capillary and with the tip at 1.5 mm from the opening of the capillary. The second electrode is a nickel-chromium washer, which fits tightly over the outer shell of the quartz insert. The pin electrode is connected internally to a high voltage coaxial connector, whereas the ring electrode is in direct contact with the outside shell of the reactor made out of the aluminum and grounded. The inside of the reactor is Teflon to prevent unwanted discharge and to provide a channel for the gas to the capillary. The ring electrode is also covered with a 4.0 mm Teflon washer.

 Figure 2. Single Capillary Reactor with the characteristic plasma jet.

       The construction of the single capillary CPE was divided between the electronics laboratory and the College machine shop which took most of the Fall 2007 semester. The CPE reactor was powered using an AC power supply that operates in the mid-kilohertz region. The air flow and frequency were varied by using a flow regulator. In each case, only one factor was altered while the other was kept constant. Readings were then taken by using optical emission spectromscopy and measuring the power (Watts). By specifying the region we wanted to examine we were able to look at certain peaks more clearly, namely the ones around the 250 to 500 nanometer (nm) region in Figure 3. The highest peak (337.2 nm) was used to compare its intensity to air flow and frequency as shown in Figure 4.

 
Figure 3. Optical emission spectra of CPE.

Figure 4. Air flow versus emission intensity at a steady power of 75 W..

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