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This section of the Website is about 39 pages in length and 18.5 Mb in size. One may go directly to topics of particular interest by clicking on the appropriate heading in the Outline below. This material is intended to serve those with a general interest in our activities at the University of Tennessee’s Plasma Science Laboratory. Archival information in more scholarly and technical depth, including references to the literature and our own published papers, may be found in the April, 2007 issue of the IEEE Transactions on Plasma Science. Our past papers on the topics summarized below can be found in the “Publications” section of this Website, where many are available for downloading.
OUTLINE
A.) THE ONE ATMOSPHERE UNIFORM GLOW DISCHARGE PLASMA (OAUGDP®) 1.) Historical Development of the OAUGDP® 2.) Research and Development of the OAUGDP® at the University of Tennessee 3.) Physics and Phenomenology of the OAUGDP® a) Uniformity of Appearance of the OAUGDP® b) Uniformity of Effect of the OAUGDP® B.) POTENTIAL APPLICATIONS OF THE OAUGDP® 1.) Increasing the Surface Energy of Fabrics, Films, and Solid Materials by exposure to a OAUGDP® a) Contact Angle Measurements of Small, Static Samples b) The MOD VIII Roll-to-Roll OAUGDP® Reactor c) Surface Energy Enhancement by the MOD VIII Roll-to-Roll Reactor 2.) OAUGDP® Etching at One Atmosphere of Surfaces of Microelectronic Interest a) Micro- and Nano-Scale Etching of Polymeric Surfaces b) Etching of Photoresist 4.) Decontaminating Surfaces Compromised by Chemical and Biological Warfare Agents 5.) A Sterilizable Air Filter to Deal with the Sick Building Syndrome 6.) Removal of Soot and Volatile Organic Compounds from Diesel Engine Exhaust 7.) A Mercury-Free Atmospheric Pressure Fluorescent Lamp 9.) Plasma-Chemical Vapor Deposition (PCVD) at Atmospheric Pressure using a OAUGDP® 10.) Atmospheric Plasma Diagnostics by Microwave Interactions with a OAUGDP®11.) Public Policy Issues and Advanced Concepts for Fusion Energy
THE ONE ATMOSPHERE UNIFORM GLOW DISCHARGE PLASMA (OAUGDP®) 1.) Historical Development of the OAUGDP® In 1933, von Engle, et al. appear to have been the first to report the operation of a DC normal glow discharge in air at one atmosphere. However, their procedure required initiation of the discharge under vacuum, followed by a gradual increase in pressure to one atmosphere. It also required aggressive cooling of the cathode to suppress the glow-to-arc transition. This discharge was not stable with respect to the glow-to-arc transition at one atmosphere, and it found few if any applications. The cathode heating responsible for the glow-to-arc instability of the von Engle discharge arises from ion bombardment and heating of the cathode. The “ion trapping mechanism” was conceived at the UT Plasma Sciences Laboratory to avoid cathode heating, and is achieved by applying RF to the electrodes at a frequency such that ions, but not electrons, are trapped between the electrodes. A dielectric plate on at least one of the electrodes is used to further suppress the glow-to-arc transition. This transformation of the von Engle discharge from DC to RF greatly reduces cathode heating by ion bombardment, reduces sputtering and erosion of the electrodes, reduces contamination of the plasma, stabilizes the plasma against the glow-to-arc transition, and provides a form of electrodynamic trapping that increases the ion and electron number density available for Lorentzian collisions and active species production. This type of discharge was called the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP®). Numerical simulations of the OAUGDP® by Ben Gadri and measurements of axial luminous intensity by Massines et al. in helium gas, showed that the OAUGDP® has the classic structures of the DC normal glow discharge, and that these reverse with each half-cycle of the RF. These structures are indicated in Figure 1. The identification of the OAUGDP® as a normal glow discharge is valuable from a plasma-physical and a phenomenological point of view, because it brings into play nearly two hundred years of accumulated observations and understanding from electrical discharge research. An important example is knowledge that the OAUGDP®, like all normal glow discharges, operates at the Stoletow point. This provides assurance that the generation of ion-electron pairs at one atmosphere in the OAUGDP® cannot be done more efficiently, an important consideration in many applications. For air, this minimum energy cost is 81 electron volts per ion-electron pair formed in the plasma. In plasma torches, this energy cost can be of the order of one keV/ion-electron pair; in arcs, it can range from 10 to 50 keV/ion-electron pair.
Figure 1. Upper graph: Computational simulation of plasma parameters in a parallel plate, One Atmosphere Uniform Glow Discharge Plasma (OAUGDP®) in helium by ben Gadri. The lower diagram shows the corresponding regions of a normal glow discharge: 1. Cathode region, including negative glow; 2. Faraday dark space; 3.positive column; and 4. Anode dark space. 2.)Research and Development of the OAUGDP®at the University of Tennessee
We will describe below the physics, phenomenology, and exploratory applications
of the One Atmosphere Uniform Glow Discharge Plasma OAUGDP, developed at the
University of Tennessee’s Plasma Sciences Laboratory. Exploratory investigations
of prospective industrial applications of the OAUGDP, have included increasing
the surface energy and wettability of fabrics, films, and solid surfaces;
sterilizing surfaces for healthcare and food processing; decontaminating
surfaces compromised by chemical or biological warfare agents; a sterilizable
air filter to deal with the sick building syndrome; removal of soot and volatile
organic compounds from Diesel engine exhaust; mercury-free atmospheric pressure
fluorescent lamps; stripping of photoresist and directional etching of possible
microelectronic relevance, and Plasma Chemical Vapor Deposition (PCVD) at one
atmosphere. In addition to these applications, plasma aerodynamic flow control
and flow attachment to airfoils have been reported in air at one atmosphere. 3.)Physics and Phenomenology of the OAUGDP® The electric fields employed to create the One Atmosphere Uniform Glow Discharge Plasma are normally less than ten kilovolts per centimeter in air and 2-3 kilovolts/cm in argon and helium, values too low to achieve DC electrical breakdown (sparking) of the operating gas at atmospheric pressure. In Section 12.5.2 of Industrial Plasma Engineering by J. R. Roth, it was shown that the critical RF driving frequency no, above which the uniform glow discharge should build up in the plasma volume as the result of ion trapping, is a function of the electrode gap spacing, rms voltage, and RF frequency. This trapping frequency is determined by the mobility drift of the ions in the oscillating RF electric field that traps them between the electrodes during an RF cycle. During ion trapping, the electrons are free to travel to the dielectric covering on the electrodes, where they re-combine, or build up a surface charge that carries the discharge through the RF current zero into the next half-cycle. If the RF driving frequency is above the ion trapping frequency, and so high that it also traps electrons as well as ions, then the plasma becomes polarized, and undergoes an instability that results in the formation of numerous coarse filaments. This instability may be related to the Negative Corona Instability. a) Uniformity of Appearance of the OAUGDP: - In this section we discuss the visible uniformity of the OAUGDP and in the next section we discuss its uniformity of effect down to the nanoscale by reference to Scanning Electron Micrographs (SEMs) of surfaces used as witness plates for filamentary discharge activity. The visual uniformity of a OAUGDP operating in a parallel plate configuration is illustrated in Figure 2, which shows three images taken of the MOD I OAUGDP Reactor, operating in helium at one atmosphere with a dielectric barrier coating on both electrodes, and an applied RF voltage of 4.5 kV rms at three frequencies. As the RF frequency was raised from below 1 kHz, the discharge first appeared as a few coarse filaments, then became brighter and more uniform, to the point where it was uniform to the eye at about 2-3 kHz. As the frequency was further raised, it remained uniform to the eye until 30 kHz, as shown in Figure 2a.
a) Uniform 20 kHz
b) 40 kHz
c) 60 kHz Figure 2. Uniform and filamentary operation of the OAUGDP® in helium gas at one atmosphere at RF frequencies of a) 20 kHz; 40 kHz; and c.) 60 kHz. Above this frequency, however, the appearance of the discharge became progressively less uniform, with filament formation, increasing spacing between filaments and more coarse filaments, as shown in Figures 2b and 2c. These filaments were as much as 2mm in diameter, much larger than the sub-micron scale microfilaments associated with avalanching in Dielectric Barrier Discharges (DBDs). In helium and argon, the plasma initiated at electric fields from 2 to 3 kV/cm; in air, an electric field at least 8.5 kV/cm was required, and the frequency range over which the plasma was uniform was much more limited. Coarse filamentation associated with the OAUGDP® occurs at high electric fields and/or at high frequencies when the electrons, as well as the ions, are trapped between the electrodes. Plots of power versus frequency or voltage for the OAUGDP show no discontinuity at the uniform-to-filamentary boundary. Filaments may be anchored to fine points or asperities, and may range from 0.05 to 2mm in diameter. They are undesirable, and lead to non-uniformity of effect, pin-holing, cratering of the surface, and other forms of surface damage that are illustrated below. The coarse filaments of the OAUGDP®appear to be related to the “Negative Corona Instability”, first described by Peek in 1915, and later discussed by Cobine. A generally acceptable theory of the Negative Corona Instability that explains the equal spacing of the corona/filament discharges, or the physical processes responsible for the quantization/filamentation of the discharge, is not yet available. The coarse filaments of the OAUGDP are not to be confused with the microfilaments of the Dielectric Barrier Discharge. The DBD features higher electric fields than the OAUGDP, and its charge carriers are created by electron avalanche. The DBD plasma also features non-uniform, microfilaments, 10 to 500 nm in diameter, which occur in random locations. These microfilaments have short lifetimes, 1 to 50 ns, too short for many chained plasma-chemical reactions that produce important active species in longer-lived discharges, such as the OAUGDP. Filamentation in parallel-plate OAUGDP reactors may be avoided by staying within the operating envelope of voltage and frequency that produces a visibly uniform plasma in the gas used. Helium and argon are easy to make uniform: air, and especially humid air, is much more difficult. In air, the relative humidity should be kept below 14% by using dry air, or re-circulating the air through the plasma until dry. Negative ion formation, particularly OH-, should be avoided by keeping the humidity at low levels. Electrophilic species (those that attach electrons) should be avoided, as they appear to promote filamentation, and reduce the operating envelope in which a uniform plasma can be generated. One should avoid asperities on the electrode or dielectric, rough edges around the electrode edges, and dielectric configurations that lead to electric field concentrations. The edges of the electrodes should be rounded off so that the surface electric field at the edge is less than the electric field between parallel electrode plates. In air plasmas, gas flow across the plasma (normal to the electric field) tends to suppress filamentation. Also in air plasmas, it becomes progressively more difficult to prevent filamentation as the electrode gap increases in width, and almost impossible to do so for gaps larger than 5mm. To put most of these phenomena on a scientific basis, research on the Negative Corona Instability is needed. b) Uniformity of Effect of the OAUGDP® : The image in Figure 2a reveals only the apparent, visible luminosity of the plasma. For industrial applications, one would like, in addition, to show that the OAUGDP is uniform in its effect on workpieces at the microscopic scale. Such data were produced in the course of research programs conducted with other objectives. In one such program, we exposed several 20 cm diameter silicon wafers, coated with a proprietary photoresist, to an air OAUGDP in the MOD VI Reactor of Figure 3 for various durations, for visibly uniform OAUGDP conditions, and under other conditions for which the plasma was filamentary.
Figure 3. The MOD VI OAUGDP reactor operating in air at one atmosphere of pressure. Recirculating air flows from left to right. The MOD VI OAUGDP® reactor operating with a uniform visual appearance in air at one atmosphere is shown in Figure 3. This reactor was designed to expose samples to the OAUGDP for biological (sterilization) or surface energy testing in two principal modes of operation: direct exposure, in which the workpiece is placed on an electrode and directly exposed to the plasma; and recirculating remote exposure, in which the workpiece is located outside the plasma volume, and active species from the plasma are convected over its surface by a gas flow that re-circulates through the OAUGDP multiple times. During etching tests on photoresist, the 15 cm diameter lower electrode of the MOD VI Reactor was covered by a 20 cm diameter silicon wafer with the photoresist directly exposed to the plasma. After adjustment of the flow velocity, flow geometry, RF frequency, and rms voltage, we obtained the result shown in Figure 4, a topographical representation of the average depth of photoresist removed during five minutes of OAUGDP exposure. The macroscopic uniformity of stripping across the 15cm diameter of the plasma was no greater than a commercially acceptable 5%, and the stripping rate was 270 nanometers/minute.
Figure 4. Isometric contour plot of the etched surface of a uniform coating of photoresist on a 20 cm diameter silicon wafer (outer circle) directly exposed to an air OAUGDP®for five minutes in the MOD VI Reactor of Figure 2. The uniformity of depth across the 15 cm etched diameter is less than 5%; and the etching rate was 270 nm/min. [Wafer and topographic analysis courtesy of Eaton Corporation]. An additional requirement for microelectronic stripping is that the process not deposit dust or contaminants on the wafer. This is normally assured by maintaining clean room conditions, a requirement not met in our laboratory. Figure 5 is a scanning electron micrograph (SEM) of the etched surface depicted in Figure 4 under uniform OAUGDP conditions, which reveals small (≈50 nanometer) grains of dust that settled on the surface of the photoresist. The corrugated, parallel structures supporting the bottom of the particles are aligned with the gas flow in the MOD VI Reactor. It should be noted that under these uniform OAUGDP conditions, the surface of the wafer is free of the pitting or pin-holing associated with filamentation and/or DBD operation. When the MOD VI reactor was operating in a visibly filamentary/DBD mode, SEMs of the surface revealed the features shown in the images of Figure 6. These features are consistent with surface damage at the root of microfilaments in the DBD mode of operation, where the local energy flux is highest. The pitting and pin-holing evident in Figure 6 are not apparent in the SEM images taken in the uniform plasma, Figures 2a, 3 and 5. B.) POTENTIAL APPLICATIONS OF THE OAUGDP Since 1992, we at the University of Tennessee’s Plasma Sciences Laboratory have conducted a series of exploratory research and development tasks with various One Atmosphere Uniform Glow Discharge Plasma (OAUGDP) reactor configurations. These tasks have been designed to test the atmospheric glow discharge hypothesis: “Any plasma processing task possible with a glow discharge in vacuum can also be performed by a glow discharge at one atmosphere, provided that long mean free paths are not required.”
(a) Large scale topography
(b) Small scale topography Figure 5 Scanning Electron Micrographs (SEMs) of photoresist etched for five minutes in the presence of surface dust in the MOD VI Reactor operating in the OAUGDP mode with air flow from left to right. The approximate distance across the width of the above images is a) 7.9 microns, b) 1.1 microns. Note the absence of microdischarge pits and the presence of vertical etching. The reactor configurations used in this research and development include the parallel-plate MOD I Reactor, illustrated in Figure 2, the MOD VI Parallel-Plate Reactor illustrated in Figure 3, the MOD VIII Roll-to-Roll Reactor illustrated in Figures 10 and 11.
(a) Surface damage at filament root
b) Pitting/pin-holing at filament root Figure 6 Scanning Electron Micrographs (SEMs) of photoresist etched in the MOD VI Reactor operating in the filamentary OAUGDP®mode with air flow from left to right. The approximate distance across the width of the above images is a) 25.8 microns, and b) 2.4 microns. Note the pits formed by the roots of microdischarges and/or filaments. 1.)Increasing the Surface Energy of Fabrics, Films, and Solid Materials by Exposure to a OAUGDP® Paired comparisons were made of direct exposure and recirculating remote exposure of workpieces to the OAUGDP® active species in the MOD VI reactor of Figure 3, and it was found that direct exposure is much more effective in increasing the surface energy of materials than single or multiple pass remote exposure. In contrast, it was found that sterilization of surfaces by remote exposure remains highly effective by comparison with direct exposure to the plasma, and that microbial killing/sterilization is more effective for multiple-pass re-circulating remote exposure than for single-pass remote exposure. The active species responsible for sterilization build up (or at least become more effective) upon recirculation of the working gas through the plasma. It was found in static sample tests that direct exposure to an air OAUGDP® is an effective way to increase the surface energy and wettability of fabrics, films, fibers, and such solid materials as metals, polymers, paper, and plastics from values of 30-40 dynes/cm up to values of 70 dynes/cm. Contact angles below ten degrees (surface energies higher than circa 70 dynes/cm) can be achieved with less than one second of direct exposure to an air plasma in the MOD VI OAUGDP® reactor shown in Figure 3. The high surface energies are not durable in most cases. When samples are left exposed to the laboratory environment, their surface energy decreases to 50 dynes/cm over periods of days to weeks. If oxygen or other polar groups are added to polymeric materials, their wettability can be durable.
Figure 7. Static sample test in the parallel-plate exploratory testing OAUGDP reactor configuration for direct exposure of a bare metal workpiece as the upper electrode.
a) Contact Angle Measurements of Small, Static Samples: The contact angle of polyethylene teraphthalate (PET) film is shown in Figure 8 as a function of the duration of direct exposure to an air OAUGDP in the MOD VI Reactor of Figure 3, for selected times after exposure (the ageing effect). Durable contact angles below 25 degrees for periods of more than a year have also been observed in OAUGDP treated meltblown polyurethane (PU) fabrics.
Figure 8. Static sample test showing the decrease of contact angle (increase of surface energy) of polyethylene teraphthalate (PET) film as a function of duration of direct exposure to an air OAUGDP in the MOD VI Reactor of Figure 3, and the aging effect for selected times after exposure. The physical process responsible for increasing the surface energy is believed to be the removal of the last few tightly-adsorbed monolayers of surface contaminants by direct plasma exposure. After this point, the substrate material is etched by the active species. The etching of the surface of individual fibers in a meltblown polypropylene (PP) fabric exposed to a CO2 plasma in the MOD I Reactor is illustrated in Figure 9. Particularly interesting in the image of Fig. 9b is the absence of shadowing. These and similar SEM images of plasma etched fibers show that etching takes place uniformly around the circumference of each fiber, an important factor when the role of the active species is to sterilize the fabric, prepare it for dyeing, or for use in a composite material. This surface etching mechanism is consistent with the observation that iron alloys directly exposed to an air OAUGDP in the Exploratory Testing Reactor of Figure 7 form a coating of rust within a few minutes of exposure, presumably because removal of the oils/contaminants that protect the surface allow oxidation of the iron within this abnormally short duration. During our past studies, Teflon has been made wettable after a five minute exposure to an air OAUGDP, with a water contact angle of 20 degrees. The low contact angles (high surface energies) imparted by direct exposure to the OAUGDP can make it possible to use water-based inks for printing, to improve the adhesion of paints on plastics, to increase the adhesion of electroplated layers to metals, and to make fabrics wettable and wickable. Applications to the improvement of composite materials such as automobile tires, Fiberglas, and aircraft fuselages should be possible.
a) Untreated b) 30-second plasma exposure Figure 9. Scanning electron micrographs (SEM) of polypropylene (PP) fibers. (a) untreated, and (b) fibers exposed for 30 seconds to an OAUGDP CO2 plasma. Note three-micron fiduciary scale at the lower right. b) The MOD VIII Roll-to-Roll OAUGDP Reactor: Previous surface treatment studies conducted at the UT Plasma Sciences Laboratory consisted of static sample studies in batches, or as single workpieces. In order to move on to more industrially relevant roll-to-roll web treatment technology, we designed and constructed the MOD VIII OAUGDP Roll-to-Roll Reactor, a schematic of which is shown in Figure 10. This reactor allows continuous OAUGDPÒ treatment of webs or films at one atmosphere pressure with air and other gases.
Figure 10. Roll-to-roll web path and location of drive belts on the MOD VIII Reactor. The fabric feed system is a dual motor design, allowing separate control of web speed and tension.
In this reactor, a moving substrate on the rotating drum passes through the plasma region, allowing motional averaging to improve uniformity of effect in the direction of web motion. This motion also helps prevent the formation of filamentation-induced pinholes, and addresses real world issues of high-speed industrial processing of webs. The independent variables being optimized are web/film feed speed, web/film tension, workpiece temperature, plasma uniformity, applied RF voltage, RF frequency, minimum gap distance, and gas flow uniformity, angle, and rate. Diagnostic measurements include time-dependent current and voltage analysis, visual inspection of the plasma volume, and “witness plate” methods of assessing surface treatment effects, particularly pin-holing.
Figure 11. Digital image front view of the MOD VIII roll-to-roll web treatment reactor based on the (OAUGDPÒ). Note rotating drum and gas supply and exhaust manifolds. The schematic of the MOD VIII reactor in Figure 10 shows the roll-to-roll web path and the location of the drive belt. The fabric feed system is a dual motor design, allowing separate control of web speed and tension. The primary treatment drum turns counter-clockwise, is grounded, is polished stainless steel, and is 20 cm in diameter. The quartz plate and RF electrode are shown to the left of this drum. A digital image of the MOD VIII reactor is shown in Figure 11. A non-uniform electric field exists between the quartz plate and the rotating drum, and a uniform plasma exists along the axial length of the rotating drum, resulting in a power density gradient in the azimuthal direction. The gas feed system blows air or other feed gases from the top, parallel to the fabric motion, into the plasma volume. The minimum gap between the grounded rotating drum and the 1.0 mm thick quartz plate is d = 2 mm. In Figure 12a at the left-hand side is shown a filamentary OAUGDPÒ having suboptimal plasma density, and with the grounded drum bare. In Figure 12b on the right-hand side, a uniform OAUGDPÒ with optimal plasma density is shown, with the grounded drum covered by a thin PET film. The grounded drum rotates counter-clockwise, and the air flows from the top to the bottom of the plasma. c) Surface Energy Enhancement by the MOD VIII Roll-to-Roll Reactor: The sessile liquid drop test was used to measure the contact angle of water drops on the surface of PolyEthylene Terephthalate (PET) film, a measure of wettability directly related to surface energy. Contact angles were measured from photographic enlargements of water drops such as those illustrated in Figure 13, on the film surface after plasma treatment. A contact angle less than 20 degrees represents a very wettable surface. Figure 13 shows two characteristic photographs of water droplets on a PET surface before and after plasma exposure.
Figure 12. Plasma appearance in the plasma treatment region with a 290 cm2 electrode behind a 1.2 mm thick quartz dielectric sheet. The rotating cylinder is grounded. Shown on the left is a filamentary OAUGDP with bare rotating drum; on the right, a uniform OAUGDP with the grounded drum covered by a PET film.
a) Untreated PET film. 70° b) 10 sec exposure 3 kHz, 9 kV Figure 13. Digital image of water contact angle as PolyEthylene Terpethalate (PET) film is exposed to the OAUGDP in the MOD VIII reactor. a) Unexposed sample; b) Ten second exposure to 3 kHz, 9 kV plasma. The contact angle, exposure time, and fabric speed from some early experiments with the OAUGDP® under un-optimized conditions are shown in Figure 14, at an RMS voltage of 7.5 kV and an RF frequency of 3 kHz. Clearly, the web speed required to produce the lower contact angles is too low for immediate industrial use, but we expect to see major improvements as the plasma operating conditions are optimized. In previous work on static samples, exposure times less than one second were routinely observed.
Figure 14. PET exposed to an air plasma for various durations in the MOD VIII Roll-to-Roll Reactor. Operating conditions: air, quartz dielectric plate, 7.5 kV, 3 kHz, and circa 500 W of power in the plasma volume. Surface treatment of polyethylene terephthalate (PET) and high density polyethylene (HDPE) are being used to optimize performance. The main challenge of generating a uniform, homogenously distributed discharge was achieved with a plane–curved electrode geometry. Initial experiments show uniformity of effect on multi-pass treatment with no pin-holing. Plasma operating conditions need to be optimized to support higher web speeds. 2.)OAUGDP®Etching at One Atmosphere of Surfaces of Microelectronic Interest In the work of Section 1 above, we demonstrated the increasew of surface energy and the removal of adsorbed surface monolayers of contaminants from such materials as polymers, plastics, metals, papers, fabrics, and glass. If the surface of these materials is exposed to the OAUGDP® for minutes rather than seconds, the active species of the OAUGDP® etch away the surface of the substrate after removing the surface contaminants. a) Micro- and Nano-Scale Etching of Polymeric Surfaces: In the course of research aimed at increasing the surface energy of polymeric films, we saw evidence of vertical etching in a poly(ethylene terephthalate) (PET) film exposed to a uniform OAUGDP® operating in air. This film is made commercially as the white plastic covering of baby diapers, and the white color is imparted by fine grains of titanium dioxide (TiO2) pigment. The titanium dioxide is much harder and more resistant to etching than the PET that surrounds it, and thus serves as a mask for etching the PET below it.
(a)
15. Scanning electron micrographs (SEMs) of a) untreated and b) plasma etched PET containing scattered grains of titanium dioxide pigment. Note the five micron fiduciary scale in the upper border and the sub-micron spires under titanium dioxide grains. The PET was directly exposed to a OAUGDPÒ in still air for five minutes at 5kHz and 12 kV rms. Figure 15b shows a scanning electron micrograph (SEM) of a piece of the white PET film made after direct exposure for five minutes to an air OAUGDPÒ operating in the MOD IV Reactor at 5 kHz and 12 kV rms, with single pass airflow. Close inspection of Figure 15b reveals numerous spires that feature a grain of titanium dioxide at their top, some with diameters less than 200 nanometers, and length/diameter ratios as high as 20. The etching rate of PET implied by the tallest spires (those with the TiO2 grains originally nearest the surface) over the five minute exposure period is about 500 nanometers/minute. There is no indication in the field of view of the SEM of pitting or melt damage similar to that in Figure 6. It is also likely that the spire structures would have served as “lightning rods” for the formation of avalanches and filamentation, and been damaged or destroyed. The fact that no such damage is evident is another indication that the OAUGDPÒ can be made uniform in effect. b) Etching of Photoresist - With the assistance of the Eaton Corporation of Rockville, MD, we determined that an air OAUGDP could strip photoresist at the rate of 270 nanometers/minute at room temperature and pressure, under relatively low power and low density OAUGDP operating conditions, and do so with a uniform stripping rate variation of about 5% across the diameter (15 cm) of the etching plasma [See Section A 3.b above]. These results are illustrated in the topographic profile of the OAUGDP-etched photoresist coated silicon wafer in Figure 4. Problem areas with OAUGDP etching of microelectronic structures are evident in the SEM images of the wafer surface in Figures 16 and 17. Figure 16 is a series of “zoom” shots of the etched photoresist, 16a at a fractured edge of the wafer, with a silicon substrate and a layer of photoresist on top. These images feature linear structures along the gas flow direction from upper left to lower right across the wafer, and “mesas” or “spires” underneath what are presumably dust grains occasioned by the absence of clean room conditions in our laboratory. Figure 17 shows surface damage that results from operating the MOD VI Reactor in the DBD/filamentary mode of operation. This takes the form of pin-holing (which in this case did not penetrate through the thick wafer) in Figures 17a and 17b, and surface melting at a filament root, in Figure 17c. When the reactor was operated in the uniform OAUGDP mode, as it was in Figure 16, such surface damage is not present.
c) 4.6 microns total image width d) 1.7 microns total image width Figure 16. “Zoom” Scanning Electron Micrograph (SEM) images of photoresist etched for 5 minutes by OAUGDPÒ air plasma without filamentation. Lineations were produced by gas flow across wafer from upper left to lower right. Fiduciary scales are in the lower right. The total horizontal widths of the images are a) 37 microns, b) 7.8 microns, c) 4.6 microns, and d) 1.7 microns. SEM images courtesy of the Eaton Corporation. a) 1.2 microns total image width b) 2.3 microns total image width
c) 23 microns total image width Figure 17. Scanning Electron Micrograph (SEM) images of photoresist surface damage after 5 minutes exposure to an OAUGDP air plasma with filamentation. Fiduciary scales are in the lower right. The total horizontal widths of the images are a) 1.2 microns, b) 2.3 microns, and c) 23 microns. SEM images courtesy of the Eaton Corporation. Sterilizing Surfaces for Healthcare, Medical, and Food Processing Applications by Exposure to a OAUGDP The etching of surfaces by OAUGDP® active species illustrated by Figure 9 strongly suggests that exposure to the OAUGDP® should be a powerful sterilizing agent. Clearly, if active species are capable of removing surface contaminants and etching the surface without shadowing, no microorganism on the surface should long survive conditions sufficient to etch a much more robust, crystalline or polymeric substrate such as photoresist. In June 1995, a research program was initiated to determine whether exposure to the OAUGDP® was effective in sterilizing and decontaminating fabrics and other surfaces. Preliminary results with an air plasma were promising, and led to contracts with the NIH, the EPA, and the DoD to further develop the technology. In the year 2000, this technology advanced to the point that a spin-off company (Atmospheric Glow Technologies, AGT) was formed (see. www.atmosphericglow.com). Prior to the formation of AGT, research at the UT Plasma Sciences Laboratory demonstrated the complete sterilization at room temperature of samples contaminated with as many as 100 million microorganisms in times ranging from 5 seconds to 5 minutes. Samples exposed in the MOD VI Reactor of Figure 3 consisted of fabrics, solid surfaces, agar medium, and filter paper. A characteristic example of such data are shown in the E. Coli survival curve of Figure 18. The first 20 seconds of exposure showed the
Figure 18. Characteristic survival curve for a polypropylene sample containing 6 x 106 E. coli cells directly exposed for the times shown to an air plasma in the OAUGDP Reactor at conditions 10 kV rms and 7 kHz. expected exponential decrease due to chemical/environmental stress on the microorganisms by active species of the plasma. At 20 seconds, however, there is a knee in the curve, after which the numbers of microorganisms drop rapidly to zero as the result of rupture of the cell wall, as is clearly indicated in the SEM images in Figure 19. This cellular destruction assures that there will be no survivors with resistance to the sterilizing agent to carry this characteristic to future generations. Sterilization was observed using air as the working gas in an OAUGDP; for direct and remote exposure of samples to the plasma; for sealed spore and sample strips; and for samples in sealed medical sterilization bags. The killing mechanism is believed to be toxic stress due to OAUGDP active species during the first phase of the survival curve, followed by structural damage to the cell walls during the latter phase of exposure (shown in the SEM image in Figure 19). The increase in sample temperature due to direct OAUGDP exposure seldom exceeded 20 °C, which eliminated thermal stress as a significant killing mechanism. The active species most responsible for sterilization and decontamination appears to be atomic oxygen, which has a very high chemical rate constant for oxidation reactions at room temperature, and one of the smallest atomic radii of any element in the Periodic Table. This latter characteristic allows atomic oxygen to diffuse rapidly through biofilms or other membranes, and to travel through small crevices to reach contaminants or microorganisms.
a) an unexposed b) 30 seconds of exposure Figure 19. Scanning Electron Micrograph (SEM) images of E. coli a) before and b) after 30 seconds exposure to the OAUGDP operating at 10 kV rms and 7.1 kHz. Note burst cell walls and release of cell contents into surrounding medium. We have exposed more than a dozen microorganisms to the OAUGDP®; and all have been killed in a few tens of seconds and at room temperature. We have sterilized bacteria, fungi, spores, and virus-like agents. We also have sterilized microorganisms that are “index microorganisms” for the health care and food processing industries. Some of these have an extraordinary ability to resist ionizing radiation, high temperatures, or desiccation. Our survival curves characteristically have the bi-phasic structure illustrated in Figure 18: a first, shallow slope due to the normal toxic stress of the OAUGDP® active species, and a second much steeper phase associated with atomic oxygen-induced physical damage. Photomicrographs (such as Figure 19) and spectroscopic measures of lipid leakage confirm massive physical damage to the microorganisms in the second phase of the survival curve: there are no survivors. Decontaminating Surfaces Compromised by Chemical and Biological Warfare Agents The UT Plasma Sciences Laboratory has worked on several contracts that address the use of the OAUGDP® for the decontamination of surfaces compromised by (simulants of) chemical and biological warfare agents. Oil of wintergreen, a chemically stable simulant of chemical warfare agents, has been oxidized and denatured by a 5-minute exposure to the OAUGDP®. Since oil of wintergreen is inert to ozone exposure, atomic oxygen is the only active species likely to produce this and other effects (i. e. sterilization). A wide range of biological warfare simulant microorganisms have been killed at room temperature and within a few tens of seconds on a variety of surfaces, including glass, metals, fabrics, and microorganisms imbedded in agar. Figure 20: Schematic of the MOD V OAUGDP Remote Exposure Reactor with recirculating gas flow capability. Since the decontamination of surfaces must be done by remote exposure, the MOD V Remote Exposure Reactor shown in Figure 20 was developed. In it, a OAUGDP in air is created on flat panels, as illustrated in Figure 21, and the active species are entrained in a serpentine gas flow that exits the plasma generating volume and passes over a workpiece in the remote exposure chamber below. The gas flow, with its entrained active species, can either be directed on a surface to be decontaminated, or re-circulated to build up concentrations to treat a workpiece in the remote exposure chamber. The MOD V reactor, operating with an air plasma, can reduce the number of microorganisms on a sample in the remote exposure chamber by a factor of one million (i.e. achieve sterilization) after a few tens of seconds of exposure. In Figure 21 is shown a digital image of five energized panels of the MOD V Remote Exposure Reactor in operation. Plasma was created on both sides of each of the panels to increase the active species entrained in the air flow. In Figure 22 is shown the result of a test of the MOD V reactor with a remotely placed sample of E. Coli.
Figure 21. Five energized plasma panels in the MOD V Remote Exposure Reactor operating with an air OAUGDP
Figure 22. Destruction of remotely exposed E. coli in the MOD V Remote Exposure Reactor. A modification of the MOD V Reactor of Figure 20 designed to decontaminate large surface areas is the leaf-blower backpack unit shown in Figure 23. In such a unit, the active species of air can be generated either on flat panels or in parallel plate discharges, either in the backpack or in the application wand itself. The latter approach may be desirable if the active species recombine or become less potent as they leave the plasma volume.
Figure 23 Portable backpack decontamination wand based on OAUGDP active species generated by panels or between parallel plates. In June 1995, the Industrial Plasma Engineering Group of the UT Plasma Sciences Laboratory began collaborating with the Toxins Laboratory of the UT Department of Microbiology, and the UT Textiles and Nonwovens Development Center (TANDEC) to determine whether exposure to the OAUGDP® was effective in sterilizing and decontaminating fabrics and other surfaces. Preliminary results that year with an air plasma were very promising, and led to a series of contracts with the NIH, the EPA, and the DoD to further develop the technology. The results of these investigations were reported in publications listed elsewhere on this website. By the year 2000, this technology advanced to the point that formation of a spin-off company was judged to be appropriate. In August 2000, such a spin-off company, www.atmosphericglow.com was formed to develop commercially the use of active species generated by the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP®) to decontaminate and sterilize fabrics and solid surfaces, as well as for other healthcare and environmental remediation purposes. In these fields, AGT has, since its formation, functioned independently of the Plasma Sciences Laboratory. AGT received two Tibbetts Awards from the Federal Government, one in 2001 and one in 2006, as a promising client of the SBIR program, and two R&D 100 Awards, one in 2002 for its sterilizable air filter, and one in 2005 for its DNA Extractor, technologies that had their origin at the UT Plasma Sciences Laboratory. During the period prior to the formation of AGT, work done in the Plasma Sciences Laboratory demonstrated the complete sterilization at room temperature of samples contaminated with as many as 100 million microorganisms in times ranging from 5 seconds to 5 minutes. Samples tested during this early work consisted of fabrics, solid surfaces, agar medium, and filter paper. Sterilization was observed using air as the working gas in an OAUGDP®, for direct exposure of samples to the plasma, for sealed spore and sample strips, and for samples in sealed medical sterilization bags. The killing mechanism appeared to be toxic stress due to OAUGDP® active species during the first phase of the survival curve, followed by structural damage to the cell walls during the latter phase of exposure (shown in the SEM Image above). The active species most responsible for sterilization/decontamination appears to be atomic oxygen, which has a very high chemical rate constant for oxidation reactions, and one of the smallest atomic radii of any element in the Periodic Table. This latter characteristic allows atomic oxygen to diffuse rapidly and travel through small cervices to reach contaminants or microorganisms. The UT Plasma Sciences Laboratory and later AGT have worked on several contracts that address the use of the OAUGDP® for the decontamination and sterilization of simulants for chemical and biological warfare agents. AGT later showed the effectiveness of the OAUGDP® for the decontamination of surfaces compromised by actual CBW agents. Decontamination and sterilization applications of the OAUGDP® are intellectual property protected by current patents owned by the University of Tennessee Research Foundation. A Sterilizable Air Filter to Deal with the Sick Building Syndrome Another decontamination technology using the OAUGDP® is the sterilizable air filter shown in Figure 24. This filter utilizes a constant DC electric field across the filter material to greatly enhance its filtration efficiency, a proprietary technology of Atmospheric Glow Technologies, Inc. When viruses or other microorganisms are filtered from the air stream, a OAUGDP® plasma is generated by energizing the electrodes on either side of the filter, to form a surface plasma. The plasma active species decontaminate the filter after a few tens of seconds of operation, and this needs to be repeated as infrequently as once every 24 hours. In this way, harmful microorganisms can be filtered out of HVAC systems, and the “sick building syndrome” prevented. This sterilizable air filter received a R&D 100 award in 2002. In Figure 25 is a digital image of the energized sterilizable OAUGDP® air filter. When not being sterilized in this way, the plasma is turned off, and a DC electric field applied between the electrodes on opposite sides of the filter to enhance its filtration efficiency.
Figure 24 Three-dimensional schematic of the sterilizable OAUGDPÒ air filter that received a 2002 R&D 100 award.
Figure 25. Energized OAUGDP on sterilizable air filter This filter was tested in the MOD VII Filtration Test Assembly shown in Figure 26, in which a nebulized mist containing microorganisms was carried with the gas stream through the filter material. As a paired comparison, half the filter was fitted for OAUGDP® sterilization, and half was operated as a standard filter.
Figure 26. Experimental arrangement of MOD VII Reactor for testing of sterilizable air filter with controlled doses of microorganisms An example of early, un-optimized data from this sterilizable air filter is shown in Figure 27. The S. aureus and PhiX 174 (a virus-like bacteriophage) were individually introduced into the air stream by a nebulizer, filtered out, and then the OAUGDP® energized for the times noted on the abscissa. After optimizing the plasma operating parameters, the time required to achieve a million-fold reduction in the number of microorganisms was reduced to tens of seconds of exposure.
Figure 27. Early survival curve data of Staphylococcus aureus and the viral bacteriophage Phi X 174 exposed to the OAUGDPÒ active species on the sterilizable filter for the times on the abscissa. Removal of Soot and Volatile Organic Compounds from Diesel Engine Exhaust It has been found that the fine particles of soot emitted from Diesel engines are a serious threat to public health. As a result, the Environmental Protection Agency (EPA) mandated large reductions in such emissions. Removing such particles in an economically practicable way is difficult. Simply filtering the exhaust leads to rapid (on the order of tens of minutes) clogging of the filter by soot particles, and a need for filter replacement. The OAUGDP® soot filter shown schematically in Figure 28 was developed at the UT Plasma Sciences Laboratory. This device requires no more than a few hundred watts of RF power, and consists of an annular OAUGDP® surrounding an annular ceramic or metallic screen filter. Soot removed from the air stream is oxidized by active species from the plasma at least as fast as it accumulates on the filter surface. The volatile carbon oxides leave the device through the inner filter with the remainder of the combustion products. A digital image of this filter in operation is shown as Figure 29. The performance of this filter when connected to the exhaust of a 2 kw Diesel-electric generator is plotted in Figure 30, which shows the pressure drop across the filter as a function of time. When the plasma is on, the trapped soot is oxidized and the pressure drop deceases; when the plasma is turned off, the soot accumulates in the filter and the pressure drop increases. The concentration of volatile organic compounds (VOCs) were also monitored and, as a bonus, were decreased by a factor of approximately 3 times when the OAUGDP® was energized.
Figure 28. Schematic of Diesel soot filter based on oxidation of soot in the engine exhaust by active species from an annular OAUGDPÒ.
Figure 29. Diesel soot filter in operation (angled mirror behind filter assembly). A Mercury-Free Atmospheric Pressure Fluorescent Lamp The mercury used in conventional fluorescent lamps is a serious contaminant of landfills and groundwater. The U. S. Environmental Protection Administration (EPA) therefore has encouraged the development of lighting devices as efficient as conventional fluorescent lamps, but without the mercury conventionally needed to provide UV radiation to excite their phosphors.
Figure 30. Pressure drop across filter with OAUGDP off and on. The concentration of volatile organic compounds (Cv) was monitored and decreases when the OAUGDP is energized. A fluorescent lamp based on the OAUGDP was developed at the UT Plasma Sciences Laboratory that requires neither a vacuum nor the use of mercury. This lamp consists of a OAUGDP® surface layer on the outside of a cylinder or on a flat panel. The electrodes and surface of the OAUGDP® Lamp can be covered with the same phosphors used in the interior of ordinary fluorescent lamps. These phosphors are excited by the ultraviolet radiation from the OAUGDP® layer above it, and emit visible radiation in a manner similar to that of a conventional fluorescent lamp. The surface layer of OAUGDP® can be generated by a flat panel or, as shown in Figure 31, by a pair of parallel wires wrapped around a cylinder, each connected to one polarity of the RF voltage. When energized, a surface plasma is generated, as is illustrated with helium in Figure 32a. When the wires are covered with a standard proprietary phosphor, the UV emission from the OAUGDP® excites the phosphor below it, as shown in Figure 32b, yielding the higher level of illumination evident at the bottom of the image.
Figure 31 Two parallel wires wound on a 5 cm diameter PVC cylinder.
(a)
b) Figure 32 Cylindrical plasma operating in helium gas at one atmosphere pressure. a) No phosphors on surface; b) with fluorescent phosphors on surface. EHD Plasma Actuators for Subsonic Plasma Aerodynamics, Including Flow Acceleration,Attachment, and Boundary Layer Control Applied to Airfoils, Jet Engines, and Wind Turbines The Industrial Plasma Engineering Group of the UT Plasma Sciences Laboratory has been engaged in research relevant to subsonic plasma aerodynamics and plasma actuators since early 1994. A complete listing of the publications on this subject may be found in the "Publications" section of this website, and many are available for downloading in their entirety. After some initial exploratory work supported by our University, a program of wind tunnel research was supported by NASA Grant NCC 1-223 from October 1, 1995 to December 31, 1998. The purpose of these exploratory investigations was to use plasma actuators based on paraelectric electrohydrodynamic (EHD) effects associated with the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP®) for aerodynamic boundary layer modification and flow control. These and related applications are described in U. S. Patent 5,669,583, "Method and Apparatus for Covering Bodies with a Uniform Glow Discharge Plasma and Applications Thereof", issued September 23, 1997, and U. S. Patent 6,200,539 B1, "Paraelectric Gas Flow Accelerator", issued March 13, 2001. Wind tunnel measurements for this research were taken in the 7 X 11 inch Low Speed Wind Tunnel of the NASA Langley Research Center's Fluid Modeling and Control Branch, Hampton, VA, with the collaboration of Mr. Stephen P. Wilkinson. A plasma actuator consists of two parallel electrode strips, one on each side of a dielectric panel, with a small displacement gap between them. An array of plasma actuators that accelerates neutral gas flow to the right is illustrated in Figure 33. Each actuator acts like a wall jet that adds momentum, but not mass, to the boundary layer flow. Plasma actuators can be based upon corona discharges, dielectric barrier discharges (DBDs), and the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP®). In our research, we have emphasized the physics and phenomenology of the OAUGDP®, because, as a normal glow discharge, it operates at the Stoletow point where the energy cost of producing an ion-electron pair is a minimum. An example of plasma actuators in action is shown in Figure 34, in which a series of eight plasma actuators have been mounted on a flexible panel attached to the upper surface of a NACA 0015 airfoil operated at an angle of attack of 12 degrees. The images of Figure 34 were taken in the NASA Langley 7 x 11 Low Speed Wind Tunnel at a free stream velocity of 2.6 meters/second. These actuators add momentum, but not mass, to the flow across the airfoil, which moves from left to right. The re-attachment of the flow is evident, as is the stabilization of the instability of the separated boundary, which otherwise grows to form vortices and increase drag.
Figure 33 An array of combined paraelectric and peristaltic plasma actuators on a flat panel that accelerate flow to the right. In our experiments so far, a wide range of boundary layer and flow control phenomena have been demonstrated. The boundary layer flow can be accelerated, slowed, stopped, and diverted; the boundary layer thickness can be increased or thinned; and we have seen suggestive evidence that the flow can not only be tripped into the turbulent regime by these EHD body forces, but also that the laminar regime can be extended by OAUGDP®-based EHD effects. As a flow control device, the OAUGDP® layer can accelerate a flow, thus acting as a plasma pump. We have increased the drag with an OAUGDP® layer in the laminar regime by a factor of at least ten, and have shown drag reductions (in the vicinity of 4 meters/second) of up to 50%. These early results from wind tunnel testing at NASA LaRC were published in AIAA Paper 98-0328 in January, 1998, and in the AIAA Journal, July 2000. Both are available from the "publications" section of this website for downloading in their entirety.
a) Plasma actuators off b) Plasma actuators on Figure 34. Flow re-attachment by plasma actuators on a NACA 0015 airfoil at an angle of attack of 12 degrees, actuator electrode voltage V = 3.6 kV, driving frequency f = 4.2 kHz, stream flow velocity 2.6 meters/sec. Other results include demonstration of flow re-attachment on the NACA 0015 airfoil illustrated in Figure 34; and demonstration that plasma actuators are best located at the leading edge of the airfoil, and not at the location of the flow separation bubble. Thus far, plasma actuators have produced EHD-induced flow velocities in still air of up to 10 meters/sec. Potential applications of EHD plasma actuators on aircraft include increasing or decreasing the lift of airfoils, increasing the stall angle of airfoils, altering the effective shape (camber) of airfoils, flow attachment for external aerodynamics on airfoils and fuselages, flow attachment for internal aerodynamics on turbine blades and ducts, and vortex suppression by active feedback control. In addition to these aircraft applications, plasma actuators may be useful in increasing the efficiency of wind turbines and improving the performance of helicopter blades. If induced velocities of 30-60 meters/sec can be achieved by plasma actuators, they might be used to replace the hydraulic-mechanical control surfaces used for takeoff and landing of aircraft. Ours is not a Magnetohydrodynamic (MHD) approach to plasma-aided aerodynamics, since it does not require strong magnetic fields or real currents in the plasma. It is an electrohydrodynamic (EHD) approach distinct from coronal and "ion wind" mechanisms, and is based on the paraelectric acceleration of the airflow by a region of low neutral gas pressure associated with high electrostatic pressure in a colliding, Lorentzian plasma. For conditions of interest to boundary layer or flow control, EHD body forces are from 1 to 4 orders of magnitude larger than MHD body forces for realistic plasma and engineering constraints. This mechanism operates on low frequency displacement currents and does not require current-carrying electrodes in contact with the plasma or the airflow. During the breakdown phase of the RF cycle, the OAUGDP® is a high pressure "DC" glow discharge with the structures and phenomenology of the classical low pressure normal glow discharge. As such, the OAUGDP® operates near the Stoletow limit for air, 81 eV/ion-electron pair. This relatively efficient form of plasma generation is helpful in aeronautical applications, where on-board power is at a premium. PLASMA CHEMICAL VAPOR DEPOSITION (PCVD) AT ATMOSPHERIC PRESSURE Our experience has shown that any plasma processing effect that that can be accomplished by low pressure plasmas below ten Torr can also be accomplished by the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP®), provided that long mean free paths are not required. This appears to apply to Plasma Chemical Vapour Deposition (PCVD) with a OAUGDP®, since the OAUGDP® has plasma, and presumably also active species concentrations, at least as great as the low pressure glow discharge plasma currently used for PCVD. Although this application is protected by patents pending, we have not extensively researched this application for lack of financial support and equipment for handling the working gases needed for PCVD. We have, however produced oxide coatings on metals, and SiOx coatings on polymeric films. Plasma chemical vapor deposition with a OAUGDP® has the very significant advantages of requiring neither expensive vacuum systems, nor batch processing of fabric webs or thin films used in packaging. ATMOSPHERIC PLASMA DIAGNOSTICS BY MICROWAVE INTERACTIONS A major effort of our group has been to develop a microwave-based diagnostic system for atmospheric plasmas. Our approach is to pass a beam of 15 GHz microwave radiation through a plasma, and measure both the attenuation and the slowing down (phase angle) of the beam with a microwave network analyzer. By applying Appleton's Equation, these two measured quantities can be used to calculate the electron number density and the electron collision frequency. This diagnostic approach provides the electron collision frequency as the result of a direct experimental measurement, without needing to know the electron kinetic temperature or the nature of the electron energy distribution function. As stated previously, these two data allow one to calculate the transport coefficients and model the plasma. Thus far, we have tested this diagnostic approach by measuring the electron number density and collision frequency as functions of time in a fluorescent light tube plasma, as shown on the accompanying graphs. We hope to have time-resolved data from a OAUGDP® as the result of continuing research.
A plasma is not considered to be well characterized unless its electron kinetic temperature, electron energy distribution function, electron number density and plasma potential are known. The electron energy distribution function and temperature are important primarily because they are needed to determine the electron collision frequency, a quantity required to calculate the plasma transport coefficients (diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity) and hence to model the plasma. The usual methods of measuring the electron kinetic temperature, electron energy distribution function, electron number density, and plasma potential involve the use of a Mach-Zender microwave interferometer (electron number density) or a Langmuir probe (all four quantities). At one atmosphere, however, the electron collision frequency is higher than the microwave frequency used in the interferometers, making interpretation of the data problematical; and the electron mean free path is smaller than the Debye distance that characterizes the Langmuir probe sheath thickness, so the electrons from the plasma lose all "memory" of their energy as they travel from the plasma to the probe surface. Because of the very limited options for plasma diagnostics at one atmosphere, some investigators have opted to obtain plasma parameters by spectroscopic methods, but these do not yield the electron number density or the electron collision frequency, and are suspect in any case because they assume a quasi-equilibrium electron energy distribution function, an assumption that is unknowable in all but high energy density thermal arcs. PUBLIC POLICY AND ADVANCED CONCEPTS FOR FUSION ENERGY Prof. Roth maintains an active interest in the public policy issues of fusion energy, which has included giving Congressional testimony, serving on the Organizing Committee of all seven of the International Symposia on Current Trends in Fusion Research, participating in NAS-NRC panels relating to advanced concepts and Space Applications of fusion energy, giving public lectures on the status of fusion energy, and responding to inquiries from the public and representatives of the media. Prof. Roth also maintains an active interest in “cold fusion”, advanced fuels, and advanced confinement concepts for fusion energy. Topics on which he has published in the past ten years include magnetoelectric toroidal confinement, ball lightning, the Bohr-van Leeuwen Theorem, Buckingham's Pi Theorem, space applications of fusion energy, safety and environmental implications of fusion energy for space propulsion, comparative evaluation of the safety and environmental impacts of fusion and fusion energy and a textbook, Introduction to Fusion Energy (also available in a Chinese edition). Based on the exploratory studies of plasma processing applications discussed above, the atmospheric glow discharge hypothesis appears to be valid, “Any plasma processing task possible with a glow discharge in vacuum can also be performed by a glow discharge at one atmosphere, provided that long mean free paths are not required.” The OAUGDP®, and possibly other atmospheric plasmas, should be capable of at least as wide a range of plasma processing tasks as vacuum glow discharges, provided that long mean free paths are not required in the application. Expensive vacuum systems and batch processing are not required for OAUGDP® processing, and this should open up industrial applications for which vacuum glow discharges are uneconomic. An unexpected outcome of this exploratory research on plasma surface treatment is that the OAUGDP® is capable of directional etching at room temperature and at one atmosphere. The physical processes responsible for such directional etching produce structures as small as 200 nm in diameter with height/diameter ratios as large as 20/1. This capability may lead to microelectronic and related etching applications conducted at one atmosphere. The ability to etch even photoresist can be used to also kill microorganisms, opening up many healthcare, food processing, and decontamination applications.
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Copyright ©2003 · Industrial Plasma
Engineering Group · Knoxville Tennessee 37996-2100 ·
Telephone: (865) 974-4446 · Fax: (865) 974-5483 · e-mail: jrr@utk.edu |
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