Abstract:
A steady-state, glow discharge plasma is generated at one atmosphere of pressure within the volume between a pair of insulated metal plate electrodes spaced up to 5 cm apart and R.F. energized with an rms potential of 1 to 5 KV at 1 to 100 KHz. Space between the electrodes is occupied by air, nitrous oxide, a noble gas such as helium, neon, argon, etc. or mixtures thereof. The electrodes are charged by an impedance matching network adjusted to produce the most stable, uniform glow discharge.

Description:
LICENSE RIGHTS 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. AFOSR 89-0319 awarded by The U.S. Air Force. 
    
    
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a Continuation-In-Part of U.S. patent application Ser. No. 08/068,508 filed May 28, 1993. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to methods and apparatus for generating low power density glow discharge plasmas at atmospheric pressure. 
     2. Description of the Prior Art 
     In the discipline of physics, the term &#34;plasma&#34; describes a partially ionized gas composed of ions, electrons and neutral species. This state of matter may be produced by the action of either very high temperatures, strong constant electric fields or radio frequency (R.F.) electromagnetic fields. High temperature or &#34;hot&#34; plasmas are represented by celestial light bodies, nuclear explosions and electric arcs. Glow discharge plasmas are produced by free electrons which are energized by imposed direct current (DC) or R.F. electric fields, and then collide with neutral molecules. These neutral molecule collisions transfer energy to the molecules and form a variety of active species which may include photons, metastables, atomic species, free radicals, molecular fragments, monomers, electrons and ions. These active species are chemically active and/or physically modify the surface of materials and may therefore serve as the basis of new surface properties of chemical compounds and property modifications of existing compounds. 
     Very low power plasmas known as dark discharge coronas have been widely used in the surface treatment of thermally sensitive materials such as paper, wool and synthetic polymers such as polyethylene, polypropylene, polyolefin, nylon and poly(ethylene terephthalate). Because of their relatively low energy content, corona discharge plasmas can alter the properties of a material surface without damaging the surface. 
     Glow discharge plasmas represent another type of relatively low power density plasma useful for non-destructive material surface modification. These glow discharge plasmas can produce useful amounts of visible and ultraviolet radiation. Glow discharge plasmas have the additional advantage therefore of producing visible and UV radiation in the simultaneous presence of active species. However, glow discharge plasmas have heretofore been successfully generated typically in low pressure or partial vacuum environments below 10 torr, necessitating batch processing and the use of expensive vacuum systems. 
     Conversely, the generation of low power density plasmas at one atmosphere is not entirely new. Filamentary discharges between parallel plates in air at one atmosphere have been used in Europe to generate ozone in large quantities for the treatment of public water supplies since the late 19th century. Filamentary discharge plasmas are dramatically distinguished from uniform glow discharge plasmas. Such filamentary discharges, while useful for ozone production, are of limited utility for the surface treatment of materials, since the plasma filaments tend to puncture or treat the surface unevenly. 
     It is, therefore, an object of the present invention to teach the construction and operating parameters of a continuous glow discharge plasma sustained at a gas pressure of about one atmosphere or slightly greater. 
     INVENTION SUMMARY 
     This and other objects of the invention to be subsequently explained or made apparent are accomplished with an apparatus based upon a pair of electrically insulated metallic plate electrodes. These plates are mounted in face-to-face parallel or uniformly spaced alignment with means for reciprocatory position adjustment up to at least 5 cm of separation. Preferably, the plates are water cooled and covered with a dielectric insulation. 
     To broaden the range of operating frequency and other parameters over which the desirable uniform (as opposed to filamentary) glow discharge is observed, an impedance matching network is added to the circuit for charging the electrodes. The parameters of such a matching network are adjusted for the most stable, uniform operation of the glow discharge. This condition can occur when the reactive power of the plasma reactor is minimized. 
     A radio frequency power amplifier connected to both plates delivers up to several hundred watts of power at a working voltage of 1 to at least 5 KV rms and at 1 to 100 KHz. To assist the starting of the plasma, the electrical discharge from a standard commercial Tesla coil may be briefly applied to the volume between the R.F. energized plates. An electric field established between the metallic plate electrodes must be strong enough to electrically break down the gas used, and is much lower for helium and argon than for atmospheric air. The RF frequency must be in the right range, discussed below, since if it is too low, the discharge will not initiate, and if it is too high, the plasma forms filamentary discharges between the plates. Only in a relatively limited frequency band will the atmospheric glow discharge plasma reactor form a uniform plasma without filamentary discharges. 
     To stabilize the plasma and discourage the formation of plasma filaments, an inductor may be connected from the median plane to ground. 
     At least in the volume between the plates wherein the glow discharge plasma is established, a one atmosphere charge of helium, argon or other gas or mixture of gases is established and maintained. 
     The median plane between two parallel electrodes may or may not contain an insulating or electrically conductive screen or plate which may be used to support exposed materials in the glow discharge plasma environment. If electrically conductive, the median screen or plate may be electrically floating or grounded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Relative to the drawings wherein like reference characters designate like or similar elements throughout the several figures of the drawings: 
     FIG. 1 is a schematic of the present invention component assembly. 
     FIG. 2 represents a preferred embodiment of the electrical circuit, including an impedance matching network and a grounded midplane screen connected to ground through an inductor. 
     FIGS. 3, 4 and 5 represent alternative power supply output stage circuits. 
     FIG. 6 schematically represents the upper chamber of a one atmosphere glow discharge plasma reactor having a median grid plate. 
     FIG. 7 represents a graph of voltage, current and power waveforms for a uniform glow discharge plasma. 
     FIG. 8 represents a graph of voltage, current and power waveforms for a filamentary discharge plasma. 
     FIG. 9 is a log-log graph of total and plasma power density in milliwatts per cubic centimeter as a function of RMS voltage applied to the electrodes. 
     FIG. 10 is a log-log graph of total and plasma power density in milliwatts per cubic centimeter, as a function of R.F. frequency. 
     FIG. 11 is a graph of amplifier frequency and corresponding breakdown current phase angles respective to a particular operating example of the invention. 
     FIG. 12 is a graph of amplifier frequency and corresponding total reactor power consumption respective to a particular operating example of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the invention schematic illustrated by FIG. 1, the electrodes 10 are fabricated of copper plate having a representative square plan dimension of 21.6 cm×21.6 cm. Silver soldered to the plates 10 are closed loops 11 of 0.95 cm copper tubing having hose nipples 12 and 13 connected therewith on opposite sides of the closed tubing loop. The edges of the electrode plates should have a radius of curvature comparable to the separation between the electrode plates and median plate to discourage electrical breakdown at the edge of the electrodes. Not shown are fluid flow conduits connected to the inlet nipples 12 for delivering coolant fluid to the loop 11 and to the outlet nipples 13 for recovering such coolant fluid. 
     The integral metallic units comprising plates 10 and tubing 11 are covered with a high dielectric insulation material 14 on all sides to discourage electrical arcing from the edges or back side of the electrode plates. 
     Preferably, some mechanism should be provided for adjusting the distance s between plates 10 up to at least 5 cm separation while maintaining relative parallelism. Such a mechanism is represented schematically in FIG. 1 by the rod adjusters 15 secured to the upper and lower plates 10. This arrangement anticipates a positionally fixed median screen or plate 30. Although parallelism is used here in the context of parallel planes, it should be understood that the term also comprises non-planar surfaces that are substantially equidistant. Also included are the geometry characteristics of a cylinder having an axis parallel to another cylinder or to a plate. 
     Energizing the plates 10 is a low impedance, high voltage, R.F. power amplifier 20 having independently variable voltage and frequency capacities over the respective ranges of 1 to at least 9 KV and 1 to 100 KHz. An impedance matching network 31 is connected between the R.F. power amplifier and the electrode plates. 
     Surrounding the plate assembly is an environmental isolation barrier 21 such as a structural enclosure suitable for maintaining a controlled gas atmosphere in the projected plan volume between the plates 10. Inlet port 22 is provided to receive an appropriate gas such as air, helium or argon, mixtures of either with air or a mixture of argon with helium. Ambient air may also be used. In any case, gas pressure within the isolation barrier 21 is substantially ambient thereby obviating or reducing the need for gas tight seals. Normally, it is sufficient to maintain a low flow rate of the modified one atmosphere gas through the inlet port 22 that is sufficient to equal the leakage rate. Since the pressure within the isolation barrier 21 is essentially the same as that outside the barrier, no great pressure differential drives the leakage rate. A vent conduit 28 controlled by valve 29 is provided as an air escape channel during initial flushing of the enclosure. Thereafter, the valve 29 may be closed for normal operation. 
     To broaden the range of operating frequency and other parameters over which the desirable uniform (as opposed to filamentary) glow discharge occurs, an impedance matching network, one embodiment of which is illustrated schematically by FIG. 2, is added to the power circuit for charging the electrodes 10. The parameters of this impedance matching network are adjusted for the most stable, uniform operation of the glow discharge. This condition can occur when the reactive power of the plasma reactor is minimized. 
     FIGS. 3 through 5 represent alternative power supply options having respective attractions. FIG. 3 corresponds to a configuration wherein the bottom electrode terminal T 1  is connected to ground potential and the top terminal T 2  is charged at the full working potential. FIGS. 4 and 5 are electrical equivalents wherein the T 1  and T 2  voltages are 180° out of phase but at only half the maximum potential. FIG. 4 represents a grounded center tap transformer whereas FIG. 5 represents a solid state power circuit embodiment. 
     Electric fields employed in a one atmosphere, uniform glow discharge plasma reactor are only a few kilovolts per centimeter, values which if D.C. would usually be too low to electrically break down the background gas. Gases such as helium and air will break down under such low electric fields, however, if the positive ion population is trapped between the two parallel or uniformly spaced electrodes, thus greatly increasing their lifetime in the plasma while at the same time the electrons are free to travel to the insulated electrode plates where they recombine or build up a surface charge. The most desirable uniform one atmosphere glow discharge plasma is therefore created when the applied frequency of the RF electric field is high enough to trap the ions between the median screen and an electrode plate, but not so high that the electrons are also trapped during a half cycle of the R.F. voltage. The electrons may be trapped by bipolar electrostatic forces. 
     If the RF frequency is so low that both the ions and the electrons can reach the boundaries and recombine, the particle lifetimes will be short and the plasma will either not initiate or form a few coarse filamentary discharges between the plates. If the applied frequency is in a narrow band in which the ions oscillate between the median screen and an electrode plate, they do not have time to reach either boundary during a half period of oscillation and are carried for long times. If the more mobile electrons are still able to leave the plasma volume and impinge on the boundary surfaces, then the desirable uniform plasma is produced. If the applied RF frequency is still higher so that both electrons and ions are trapped in the discharge, then the discharge forms a filamentary plasma. 
     Without limiting our invention to any particular theory, we are disposed to a relationship between the electrode spacing, the RMS electrode voltage, and the applied frequency which results in trapping ions but not electrons between the two plates, and produces the desired uniform one atmosphere glow discharge plasma. On FIG. 6 is a schematic of the upper chamber of the one atmosphere glow discharge plasma reactor. The lower boundary of this space is the midplane screen or plate, the floating potential of which should remain near ground if the RF power supply output is connected as a push-pull circuit to the two electrodes with a grounded center tap. In the data reported herein, the median screen was grounded through an inductive current choke. In the configuration of FIG. 6, a Cartesian coordinate system is applied as shown, with the applied electric field in the x-direction. The maximum amplitude of the electric field between the grounded median screen and the upper electrode is E o , and the separation of the screen from the electrodes is the distance d. The median screen, with an exposed sample on it, is assumed not to allow ions through the median plane from the upper chamber to the lower, or vice-versa. 
     The electric field between the electrodes shown on FIG. 6 is given by 
     
         E=(E.sub.o sin ωt,O,O).                              (1) 
    
     It is assumed that the one atmosphere glow discharge operates in a magnetic field free plasma. The equation of motion for the ions or electrons between the two plates is given by a Lorentzian model, in which the electrons and ions collide only with the neutral background gas and, on each collision, give up all the energy they acquired from the RF electric field since the last collision to the neutral gas. The equation of motion for the ions or electrons in the Lorentzian model is given by 
     
         F=ma=-mv.sub.c v-eE,                                       (b 2) 
    
     where the first term on the right hand side is the Lorentzian collision term, according to which the momentum mv is lost with each collision that occurs with a collision frequency v c . The x component of Eq. 2 is given by ##EQU1## where the electric field E from Eq. 1 has been substituted into the right hand side of Eq. 2. The general solution to Eq. 3 is 
     
         x=C.sub.1 sin ωt=C.sub.2 cos ωt,               (4) 
    
     where the constants C 1  and C 2  are given by ##EQU2## 
     The one atmosphere helium glow discharge is operated at frequencies between ω/2π=1 and 30 KHz, where, for helium at one atmosphere, 
     
         v.sub.ci ≈6.8×10.sup.9 ion collisions/sec.,  (7a) 
    
     and 
     
         v.sub.ce ≈1.8×10.sup.12 electron coll./sec.  (7b) 
    
     The collision frequency for ions and electrons given by Eqs. 7a and 7b is much greater than the RF frequency, V c  &gt;&gt;ω. The relation v c  &gt;&gt;ω for ions and electrons, implies that C 2  is much greater than the constant C 1 , or ##EQU3## The time dependent position of an ion or an electron in the electric field between the plates is given by substituting Eq. 8 into Eq. 4, to obtain ##EQU4## The RMS displacement of the ion or electron during a half cycle is given by ##EQU5## If V o  is the driving frequency, in Hertz, then the radian RF frequency is given by 
     
         ω=2πv.sub.o,                                      (11) 
    
     and the maximum electric field between the plates can be approximated by the maximum voltage V o  appearing between them, ##EQU6## If the charge in question moves across the discharge width from the median plane to one of the electrode plates during one full cycle, then we may write ##EQU7## Equation 13 states that the RMS displacement of the particle has to be less than half the clear spacing in order to have a buildup of charge between the plates. In the geometry shown in FIG. 6, the distance d is identified with the distance between the grounded median screen and the energized electrode. Substituting Eqs. 11 to 13 into Eq. 10 yields the relationship ##EQU8## If we now solve for the critical frequency v o  above which charge buildup should occur in the plasma volume, we have ##EQU9## In Eq. 15, the collision frequency v c  is approximately given by Eqs. 7a or 7b for ions or electrons, respectively, at one atmosphere, and the RMS voltage is that which bounds the upper and lower limit of the uniform discharge regime. 
     The range of parameters over which we have operated a one atmosphere, uniform glow discharge plasma reactor is given in Table I. The nominal pressure at which this discharge has been operated is one atmosphere. The variation of several torr shown in Table I is not intended to represent the day-to-day fluctuations of barometric pressure, but the pressure differential across the midplane screen which is intended to drive active species from the upper plasma through the fabric being exposed. The RMS power shown in Table I is the net power delivered to the plasma, less the reactive power which does not appear in the plasma. The total volume of plasma between the two electrode plates is given by 
     
         S=0.93 d(cm)liters,                                        (16) 
    
     where d is the separation of a plate from the median screen in centimeters. 
     The power densities shown in Table I are far below those of electrical arcs or plasma torches, but also are several orders of magnitude higher than the power densities associated with some other forms of plasma treatment such as corona discharges. The power densities of the one atmosphere glow discharge plasma are generally low enough not to damage exposed fabrics, but are also enough higher than coronal plasmas used for surface treatment that they should provide far more active species than the latter. The plasma parameters, such as electron kinetic temperature and number density are somewhat speculative at this early stage in the development of our invention. A few results from probing the plasma midplane with a floating Langmuir probe indicates that the plasma, without grounding the midplane screen, will float to positive potentials of several hundred volts. The ion kinetic temperatures are very likely close to that of the room temperature atoms with which they frequently collide at these high pressures; the electrons apparently remain numerous and energetic enough to excite the neutral background atoms, hence making this a glow discharge. The existence of excited states which emit visible photons implies that the electron population has a kinetic temperature of at least an electron volt. The diagnostic difficulties of measuring plasma parameters at this high pressure are very severe, since ordinary Langmuir probing technique cannot be applied due to the short mean free paths of the electrons compared to a Debye distance. Electron number densities, however, may be measured by microwave interferometric techniques. 
     
                       TABLE I______________________________________OPERATING CHARACTERISTICS OF THE ONEATMOSPHERE GLOW DISCHARGE PLASMA REACTOR______________________________________working gas = H.sub.e, H.sub.e + 1-7% O.sub.2, Ar, Ar + H.sub.e,Ar + 1-7% O.sub.2, N.sub.2 O and atmospheric airfrequency = 1 KHz to 100 KHzvoltage = 1.5-9.5 kV.sub.rms plate to plateelectrode gap d = 0.8-3.2 cmpressure = 760 +15, -5 torrRMS power = 10 watts to 150 wattspower density = 4-120 mW/cm.sup.3plasma volume = 0.7-3.1 liters______________________________________ 
    
     On FIGS. 7 and 8 are shown two waveforms of voltage and current taken in helium at the same electrode separation and gas flow conditions, but at two different frequencies. FIG. 7 was taken in the uniform glow discharge regime at a frequency of 2.0 kHz, and FIG. 8 was taken in the filamentary discharge regime at a frequency above the uniform plasma operating band at 8.0 kHz. The high output impedance of our RF power supply results in a voltage waveform (trace B) that is very close to sinusoidal. The reactive current waveform (trace C) is interrupted by a breakdown of the plasma twice each cycle, once when the voltage is positive, and once when the voltage is negative. Trace A shows the reactive current waveform at the same voltage and operating conditions, but in air, rather than helium. There was no perceptible plasma present in air under these conditions, and the power is completely reactive. This purely reactive current of trace A was subtracted from the total plasma current in trace C, to yield trace D. The instantaneous power deposited in the plasma (trace E) is found by multiplying the excess plasma current above the reactive current (trace D) by the voltage at that point (trace B). The average power is found by integrating over the duration of the pulses shown, and dividing by this duration. It is in this manner that the power and power density into the plasma were calculated for these highly nonsinusoidal current waveforms. FIGS. 8 for the uniform discharge and 8 for the filamentary discharge show characteristically different power waveforms in trace E; this is a method of distinguishing the uniform from the filamentary discharge. 
     The plasma power is of interest because it is proportional to the production rate of active species in the plasma; the reactive power is significant because it determines the required power handling rating of the plasma power supply and associated equipment. The total power is the sum of plasma and reactive power. On FIG. 9 is shown a log-log plot of the plasma and total power density in milliwatts per cubic centimeter, as functions of the RMS voltage applied to the parallel plates. The active plasma volume in FIG. 9 was 1.63 liters, with a separation between the median screen and each plate of d=1.75 centimeters in a plasma of helium gas. On FIG. 10 is a similar presentation of the power density plotted on log-log coordinates as a function of the frequency. The approximate bound of the uniform plasma discharge regime is shown by the arrow. These data were taken in helium gas for the same plasma volume and electrode separation as FIG. 10. 
     EXAMPLE 1 
     In a first operational example of the invention, the above described physical apparatus sustained a glow discharge plasma in one atmosphere of helium at standard temperature with a separation distance s of 3.0 cm between plates 10. The plates were charged with a 4.4 KV working potential. Holding these parameters constant, the R.F. frequency was increased as an independent variable. As the dependent variable, FIG. 11 charts the corresponding breakdown current phase angle. Similarly, FIG. 12 charts the total power, reactive and plasma, used to sustain the plasma at the respective R.F. frequencies. 
     EXAMPLE 2 
     In a second operational example of the invention, the above described physical apparatus is used to sustain a glow discharge plasma in one atmosphere of helium at standard temperature with a separation distance s of 1.0 cm between plates 10. In this example, the R.F. frequency was held constant at 30 KHz while plate potential was manipulated as the independent variable and current breakdown phase angle, Θ, (Table 2) and total power, P, (Table 3) measured as dependent variables. 
     
                       TABLE 2______________________________________V(KV)      1    1.5       2  2.5     3   3.5θ(deg)     28    40       61  46     65  76.5______________________________________ 
    
     
                       TABLE 3______________________________________V(KV)     1     1.5       2  2.5     3   3.5P(W)      7     13       22  57     50  44.9______________________________________ 
    
     EXAMPLE 3 
     A third operational example of the invention included a one atmosphere environment of helium between a 1 cm separation distance s between plate electrodes 10 charged at 1.5 KV rms potential. R.F. frequency was manipulated as the independent variable. As a measured dependent variable, Table 4 reports the corresponding phase angle e of breakdown current. The measured dependent variable of Table 5 reports the corresponding total power consumption data, both reactive and plasma. 
     
                       TABLE 4______________________________________f(KHz) 10    20     30  40   50  60   70  80   90  100θ(deg)  43    32     43  52   54  61   60  56   45  22.5______________________________________ 
    
     
                       TABLE 5______________________________________f(KHz) 10    20     30  40   50  60   70  80   90  100P(W)    5     8     11  19   35  43   47  57   89  124______________________________________ 
    
     EXAMPLE 4 
     The largest volume helium plasma of 3.1 liters was achieved with the above described apparatus at a 3.2 cm plate separation having a 5 KV potential charged at 4 KHz. 
     It will be understood by those of ordinary skill in the art that the present invention is capable of numerous arrangements, modifications and substitutions of parts without departing from the scope of the invention. In particular, FIGS. 3, 4 and 5 represent respective power supply options having respective attractions. 
     FIGS. 3, 4 and 5 are electrical equivalents wherein the T 1  and T 2  voltages are 180° out of phase but at only half the maximum potential. FIG. 4 represents a grounded center top transformer whereas FIG. 5 represents a solid state power circuit embodiment with or without a provision for a grounded center tap of the output. 
     Although expansive data is not presently available, it is to be noted that a uniform glow discharge plasma has been sustained by the FIG. 1 apparatus with a one atmosphere ambient air environment and 8 KV/cm R.F. electric field. 
     Having fully disclosed our invention, the presently preferred embodiments and best modes of practice,