U.S. patent application number 11/121803 was filed with the patent office on 2005-09-22 for method and apparatus for stabilizing of the glow plasma discharges.
Invention is credited to Becker, Kurt, Kunhardt, Erich.
Application Number | 20050206290 11/121803 |
Document ID | / |
Family ID | 32312092 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050206290 |
Kind Code |
A1 |
Kunhardt, Erich ; et
al. |
September 22, 2005 |
Method and apparatus for stabilizing of the glow plasma
discharges
Abstract
A method and apparatus for stabilizing glow plasma discharges by
suppressing the transition from glow-to-arc includes a perforated
dielectric plate having an upper surface and a lower surface and a
plurality of holes extending therethrough. The perforated
dielectric plate is positioned over the cathode. Each of the holes
acts as a separate active current limiting micro-channel that
prevents the overall current density from increasing above the
threshold for the glow-to-arc transition. This allows for a stable
glow discharge to be maintained for a wide range of operating
pressures (up to atmospheric pressures) and in a wide range of
electric fields include DC and RF fields of varying strength.
Inventors: |
Kunhardt, Erich; (Hoboken,
NJ) ; Becker, Kurt; (New York, NY) |
Correspondence
Address: |
MCCARTER & ENGLISH, LLP
FOUR GATEWAY CENTER
100 MULBERRY STREET
NEWARK
NJ
07102
US
|
Family ID: |
32312092 |
Appl. No.: |
11/121803 |
Filed: |
May 4, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11121803 |
May 4, 2005 |
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10288313 |
Nov 5, 2002 |
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6900592 |
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10288313 |
Nov 5, 2002 |
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09381328 |
Dec 27, 1999 |
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6879103 |
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09381328 |
Dec 27, 1999 |
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08820013 |
Mar 18, 1997 |
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5872426 |
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Current U.S.
Class: |
313/231.01 ;
313/231.31 |
Current CPC
Class: |
H01J 37/32623 20130101;
H01J 37/32018 20130101; H01J 37/32036 20130101; H01J 37/32559
20130101; H05H 1/2406 20130101; H01J 17/04 20130101; H01J 2237/0206
20130101 |
Class at
Publication: |
313/231.01 ;
313/231.31 |
International
Class: |
H01J 007/24; H01J
017/26; H01J 061/28 |
Goverment Interests
[0002] The present invention has been made under a government
contract and the government may have certain rights to the subject
invention.
Claims
1-10. (canceled)
11. An apparatus for generating and maintaining a glow plasma
discharge at atmospheric pressure, comprising: a pair of electrodes
positioned in facing relation having a space therebetween; a
dielectric layer having a bottom side and a plurality of recesses
positioned over one of the electrodes, each of the recesses defined
by a bottom wall and side walls, wherein the dielectric layer has a
thickness between the bottom walls of each of the plurality of the
plurality of recesses and the bottom side of the dielectric layer
on the order of microns for regulating energy available to the glow
plasma discharge; and an electric field generated between the
electrodes.
12. The apparatus of claim 11 wherein each of the plurality of
recesses are of micron dimension.
13. The apparatus of claim 11, wherein each of the plurality of
recesses limits current density from increasing above a glow-to-arc
transition threshold.
14. The apparatus of claim 11, further comprising a second
dielectric layer positioned over the other of the electrodes.
15. The apparatus of claim 14, wherein the second dielectric layer
includes a plurality of recesses, each of the plurality of recesses
defined by a bottom wall and side walls, wherein the second
dielectric layer has a thickness between the bottom walls of each
of the plurality of the plurality of recesses and the bottom side
of the second dielectric layer on the order of microns for
regulating energy available to the glow plasma discharge.
16. The apparatus of claim 15, wherein each of the each of the
plurality of recesses of the second dielectric layer are of micron
dimension.
17. The apparatus of claim 15, wherein each of the plurality of
recesses of the second dielectric layer limits current density from
increasing above a glow-to-arc transition threshold.
18. The apparatus of claim 11, further comprising a retaining
collar for retaining the dielectric layer to one of the
electrodes.
19. A method for stabilizing a glow-to-arc transition for a
discharge plasma at atmospheric pressures, comprising the steps of:
positioning electrodes in a facing relation; providing a dielectric
having a plurality of recesses, each of the recesses defined by a
bottom wall and side walls, wherein the dielectric layer has a
thickness between the bottom walls of each of the plurality of the
plurality of recesses and the bottom side of the dielectric layer
on the order of microns for regulating energy available to the glow
plasma discharge; covering one of the electrodes with the
dielectric; and generating an electric field between the
electrodes.
20. The method of claim 19, wherein the step of providing the
dielectric comprises providing a dielectric having a plurality of
recesses of micron dimension.
21. The method of claim 19, further comprising providing a second
dielectric having a plurality of recesses, each of the recesses
defined by a bottom wall and side walls, wherein the second
dielectric layer has a thickness between the bottom walls of each
of the plurality of the plurality of recesses and the bottom side
of the second dielectric layer on the order of microns for
regulating energy available to the glow plasma discharge.
22. The method of claim 21, wherein the step of providing the
second dielectric comprises providing a second dielectric having a
plurality of recesses of micron dimension.
23. The method of claim 21, further comprising covering the other
of the electrodes with the second dielectric.
24. The method of claim 19, further comprising retaining the
dielectric to one of the electrodes with a retaining collar.
Description
BACKGROUND OF THE INVENTION
RELATED APPLICATION
[0001] This application is a continuation-in-part application of
U.S. patent application Ser. No. 09/381,328, filed Dec. 27, 1999 by
Erich Kunhardt and Kurt Becker, entitled Method and Apparatus for
Suppression of the Glow-to-Arc Transition in Glow Discharges, now
U.S. Pat. No. ______, issued ______, which is a
continuation-in-part application of U.S. patent application Ser.
No. 08/820,013 filed Mar. 18, 1997, now U.S. Pat. No. 5,872,426,
issued Feb. 16, 1999, the entire disclosures of which are expressly
incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention generally relates to a method and apparatus
for stabilizing glow plasma discharges, and more specifically to a
cathode configuration having a dielectric with a plurality of
recesses for stabilizing glow plasma discharges.
RELATED ART
[0004] A "plasma" is a partially ionized gas composed of ions,
electrons, and neutral species. This state of matter is produced by
high temperatures or strong electric fields created by constant or
pulsed DC current, AC current or time varying (e.g., R.F. or
microwave) electromagnetic fields. Discharge plasmas are produced
when free electrons are energized by electric fields in a
background of neutral atoms/molecules. These electrons cause
electron--atom/molecule collisions which transfer energy to the
atoms/molecules and form a variety of species which may include
photons, metastables, atomic excited states, free radicals,
molecular fragments, monomers, electrons, and ions. The neutral gas
becomes partially (or fully) ionized and is able to conduct
currents. The plasma species are chemically active and/or can
physically modify the surface of materials and may therefore serve
as the basis of new chemical compounds and may be used to modify
existing compounds. Discharge plasmas can also produce useful
amounts of optical radiation and can therefore be used in lighting.
There are additionally many other uses for such plasmas. Glow
discharges and arc discharges produce a class of plasmas known as
current-maintained plasmas, since they are maintained by the
passage of current therethrough. Such plasmas conduct only because
current is passed therethrough and the conductivity falls off
quickly if the source of energy to the charge carriers is
removed.
[0005] Transition points exist at which the various attributes of
the discharge and discharge plasma change from the characteristics
of a glow discharge to the characteristics of an arc discharge. The
characteristics that distinguish arc from glow are a high gas
temperature and a low cathode fall potential, though it is also
possible to have a high gas temperature associated with a high
cathode fall and vice versa.
[0006] The transition from glow to arc passes through a series of
stable or quasi-stable states. However, the final step from
abnormal glow to arc is very often an unstable change, since a very
large potential drop in the series resistance would be required to
make it stable. If there is no series resistance, the transition
may take place very rapidly, without equilibrium being achieved in
any intermediate stage. This transition becomes more rapid as the
pressure of the background neutral gas increases towards
atmospheric pressure.
[0007] In the past, there have been efforts to stabilize glow
plasma discharges in various ways such as the use of source
frequencies over 1 kHz, insertion of a dielectric plate (or plates)
between two metal electrodes and by using helium dilution gas.
Additionally, other attempts to stabilize the glow plasma discharge
include placement of an insulated plate on the lower electrode, use
of a brush-style upper electrode, and the use of a metal upper
plate in combination with an insulating plate on the bottom
thereof. However, there are certain drawbacks with these
requirements in that, e.g. helium is expensive and there are
physical limitations based on the structure of the electrodes and
the insulated plates.
[0008] Past work in this area include a series of articles by
Okazaki, Satiko, et al., starting back in 1989 with the article by
Kanazaw, S., et al., entitled, "Glow Plasma Treatment at
Atmospheric Pressure for Surface Modification and Film Deposition,"
Nuclear Instruments and Methods in Physics Research (1989) Elsevier
Science Publishers, B.V. (North-Holland Physics Publishing
Division), which disclosed a plasma treatment at atmospheric
pressure to stabilize glow plasma by treatment in a gas which
includes carbon-tetrafluoride (CF.sub.4), using helium as the
dilute gas and using an insulating plate on a lower electrode, and
using a brush style electrode for the upper electrode to create a
stable discharge at 3,000 Hz.
[0009] Yokoyama, T., et al., "The improvement of the
atmospheric-pressure glow plasma method and the deposition of
organic films," Journal of Physics (1990) IOP Publishing, Ltd.,
discloses an improved atmospheric pressure glow discharge plasma
method for treating metallic substrates wherein the middle plate
upper electrode is improved by use of an insulating plate set on
its bottom.
[0010] Yokoyama, T. et al., "The mechanism of the stabilization of
glow plasma at atmospheric pressure," Journal of Physics (1990) IOP
Publishing, Ltd., discloses stabilization of a glow discharge of
atmospheric pressure by controlling three conditions, namely, the
use of a high frequency source, the use of helium gas for dilution,
and the insertion of a dielectric plate between electrodes.
[0011] Okazaki, Satiko, et al., "Appearance of stable glow
discharge in air, argon, oxygen, and nitrogen at atmospheric
pressure using a 50 Hz source," Journal of Physics, (1993) IOP
Publishing, Ltd., discloses a method and apparatus for stabilizing
glow discharge by making the discharge occur in the early stages of
the Kekez curve, and at a lower discharge breakdown voltage, by use
of a metal wire mesh electrode.
[0012] Kogoma, Masuhiro, et al., "Raising of ozone formation
efficiency in a homogeneous glow discharge plasma at atmospheric
pressure," Journal of Physics (1994) IOP Publishing, Ltd.,
discloses an ozone formation apparatus for increasing the
efficiencies of ozone formation by use of a homogenous glow
discharge at atmospheric pressure to create ozone efficiencies
increased to about 10% in air to a maximum of 15% in oxygen over
conventional filamentary current discharges in gas. The increase is
attributed to better collision efficiency among electrons and
molecules and to a lower increase in temperature than in discharge
filaments of a silent electric discharge.
[0013] Other work in this area includes U.S. Pat. No. 4,498,551 to
Hoag, entitled, "Discharge Electrode for a Gas Discharge Device,"
which uses pin-shaped electrodes which are effectively cooled in
the glass flow and which promote a stable glow-discharge.
[0014] U.S. Pat. No. 5,387,842 dated Feb. 7, 1995 to Roth, et al.,
entitled, "Steady-State, Glow Discharge Plasma," and U.S. Pat. No.
5,414,324 dated May 9, 1995 to Roth, et al., entitled "One
Atmosphere, Uniform Glow Discharge Plasma," both disclose a steady
state glow discharge plasma generated between a pair of insulated
metal plate electrodes spaced up to five centimeters apart and
energized with a RMS potential of 1 to 5 KV at 1 to 100 Khz. The
space between the electrodes is occupied by a noble gas such as
helium, neon, argon, etc., and it may also include air. The radio
frequency amplifier means for generating and maintaining a glow
discharge plasma includes an impedance matching network. The arc of
electric field is high enough to trap the positive ions of the
plasma between the electrodes, but not so high that the electrons
of the plasma are also trapped during a half cycle of the RF
voltage.
[0015] None of these previous efforts disclose all of the benefits
of the present invention, nor does the prior art teach or suggest
all of the elements of the present invention.
OBJECTS AND SUMMARY OF THE INVENTION
[0016] It is a primary object of the present invention to provide a
method and apparatus for stabilizing glow discharge plasmas.
[0017] It is another object of the present invention to provide a
method and apparatus for suppressing the glow-to-arc transition in
glow discharges.
[0018] It is an additional object of the present invention to
provide a method and apparatus to stabilize glow discharge plasmas
in a constant electric field.
[0019] It is even an additional object of the present invention to
provide a method and apparatus to stabilize glow discharge plasmas
in time varying electric fields.
[0020] It is another object of the invention to provide a cathode
configuration for stabilizing the cathode fall in a glow
discharge.
[0021] It is an additional object of the present invention to
provide a method and apparatus for suppressing the glow-to-arc
transition for a wide range of operating conditions and a wide
range of operating pressures.
[0022] It is also an object of the present invention to provide a
method and apparatus for suppressing the glow-to-arc transition for
a wide range of electric field strengths.
[0023] It is an additional object of the present invention to
reduce the complexity and costs of plasma processing of
materials.
[0024] These and other objects are achieved by the method and
apparatus of the present invention for stabilizing glow plasma
discharges by suppressing the transition from glow-to-arc. A
dielectric having an upper surface and a lower surface and a
plurality of recesses extending partially therethrough is
positioned over the cathode. Each of the holes acts as a separate
active current limiting micro-channel that prevents the overall
current density from increasing above the threshold for the
glow-to-arc transition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Other important objects and features of the invention will
be apparent from the following Detailed Description of the
Invention when read in context with the accompanying drawings in
which:
[0026] FIG. 1 is an exploded perspective view of the perforated
dielectric covering a cathode of a DC embodiment of the present
invention.
[0027] FIG. 2 is a schematic view of a circuit configuration for
use with the present invention.
[0028] FIG. 3 is a graph of voltage v. current for applied voltage,
glow voltage, and arc voltage in Argon at 40 Torr.
[0029] FIG. 4 is a graph of voltage v. current for applied voltage,
glow voltage, and arc voltage in Argon at 20 Torr.
[0030] FIGS. 5a and 5b are graphs of applied voltage, glow voltage
and arc voltage with and without the perforated dielectric of the
present invention.
[0031] FIG. 6a is a photograph showing an arc discharge and FIG. 6b
is a photograph showing a glow discharge.
[0032] FIG. 7 is a side plan view of another embodiment of the
present invention for an RF field wherein perforated dielectrics
are positioned over both electrodes.
[0033] FIG. 8 is a partial view of a dielectric of an alternate
embodiment having blind apertures.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present invention is directed to a method and apparatus
for stabilizing plasma glow discharges by suppressing the
glow-to-arc transition in DC, RF electric field, pulsed DC, AC
current or any other glow discharges which evolves from the cathode
fall region. Referring to FIG. 1, it can been seen that a new
cathode configuration has been developed to stabilize the cathode
fall and suppress the glow-to-arc transition for a wide range of
operating conditions. Accordingly, a stable glow discharge can be
maintained with the cathode configuration of the present invention
for a very wide range of operating pressures (up to atmospheric
pressures) and in a wide range of electric field strengths.
[0035] Referring to FIG. 1, which shows a DC embodiment, the
cathode of the present invention, generally indicated at 10,
comprises a metal cathode 20 (aluminum, stainless steel, etc.),
covered with a perforated dielectric 30 positioned to face an upper
electrode 40. The perforated dielectric 30 may be retained on the
cathode 20 by a collar 35 that fits over cathode 20 and has an
aperture 36 therethrough, or may be formed as part of a cap or
cover for the cathode 20, or may be positioned thereon and held in
place thereon in any other manner known in the art.
[0036] Importantly, the perforated dielectric can be formed of any
desired dielectric type substance such as quartz, silicon nitride,
silicon carbide, etc., even glass. The dielectric is preferably
formed of a material that can withstand high temperatures.
Essentially, a perforated dielectric comprises a sieve of holes of
micron dimensions. The center to center distance of the holes is of
the same level of dimension. Hole dimensions are critical for
particular applications. In trials discussed hereinafter, a
dielectric having 10 micron holes with a center to center distance
between the holes of 12 microns was used. Hole dimensions can vary
from 5 to 200 .mu.m for the hole diameter and from between. 100
.mu.m to 2 mm for the hole length (thickness of the dielectric).
Importantly, the ratio of the hole diameter to the dielectric
thickness is an important factor and something that can be
controlled depending upon the application. One example of such a
ratio could be 10 to 1, the hole diameter being one-tenth of the
thickness of the dielectric.
[0037] The perforated dielectric can be made by laser ablation.
Blanks for dielectric plates made by Norton International can be
used, and function in a desirable matter (a dielectric having a
hole diameter of 10 .mu.m, and a hole length of 0.6 mm). The hole
diameter, hole lengths, hole density, and material can be varied to
optimize the invention for a particular application. Any silicon
carbide wafer can be perforated by laser ablation to form a
perforated dielectric for use in connection with the present
invention.
[0038] Referring to FIG. 2, shows a circuit that has been used to
conduct trials of the present invention that will hereinafter be
discussed, which can be used with the cathode configuration of the
present invention to effect a stable DC glow plasma discharge. The
circuit is governed by equation (1): 1 V s = I 1 ( R 1 + R 2 ) = 1
R 1 ( R 1 + R 2 ) V Equation ( 1 )
[0039] where
I.sub.2=4I
[0040] 2 R E R 3 4 ( for R 1 R 3 )
V.sub.g=V.sub.s-I.sub.gR.sub.3
V.sub.d=V.sub.s-I.sub.dR.sub.3
[0041] In this way, by measuring voltage V across resistor R, and
current i through resistor R.sub.1, we can calculate the voltage
and the current across the cathode 10.
[0042] The present invention allows DC glow discharges, which have
a well known instability that limits the operating range, to
operate at much higher pressure up to atmospheric pressures.
Accordingly, this stabilization allows for applications in many
aspects of material processing, pollution remediation, novel
lighting devices, and discharge-enhanced combustion.
[0043] The perforated dielectric covering the metal cathode
stabilizes the cathode fall region of the DC discharge by breaking
the discharge up into a large number of separate micro-channels.
Each of the holes comprising the perforated dielectric acts as a
separate, active current-limiting micro-channel. Particle losses
due to wall effects and the finite volume of each channel place an
upper limit on the electrical conductivity of each channel, and
therefore place an upper limit on the current density that it can
carry. This prevents the current density from increasing above the
threshold for the glow-to-arc transition.
[0044] Additionally, it should be noted that a dielectric material
could be directly deposited in a proper geometry directly onto a
cathode by a vapor deposition or other process to apply the
dielectric directly to the cathode. In this way, the cathode itself
becomes an active current-limiting device.
[0045] A prototype DC glow discharge apparatus was set up using a
parallel plate electrode arrangement in an Argon atmosphere of
between 10-100 Torr, to illustrate the present invention. At these
pressures, the phases of the glow-to-arc transition can be readily
shown because the transition is sufficiently slow. The transition
at atmospheric pressures occurs very rapidly and is difficult to
observe. However, it should be pointed out that the present
invention is designed to be used at pressures up to atmospheric
pressures. Current voltage characteristics were recorded for a
variety of operating conditions using a standard metal (Al)
cathode. The measured curves show the well-known first transition
corresponding to the breakdown of gas in the formation of a stable
glow discharge, followed by a prominent second transition
characteristic of the transition from the glow regime to an arc
which creates a filamentary (high current density) channel.
Subsequently, the conventional cathode was replaced by the new
cathode design and the same current-voltage curves were recorded.
All curves showed only the first transition to the stable
high-current glow. No indications of the previously observed
glow-to-arc transition were found under any operating conditions.
The spatial distribution of the discharge is also diffuse.
[0046] Referring now to FIG. 3, a graph of voltage vs. current for
applied voltage (VG), glow voltage (Vg) and arc voltage (Vd) is
shown for Argon at 40 Torr. FIG. 4 shows a graph of voltage vs.
current for applied voltage (VG), glow voltage (Vg), and arc
voltage (Vd) in Argon at 20 Torr.
[0047] FIGS. 5a and 5b are graphs of the applied voltage and
glow-to-arc voltage with and without the perforated dielectric of
the present invention. These figures show the stabilization of the
glow plasma discharge. In a first Stage A, there is no current. In
the second Stage B, voltage is applied, but current stays at zero.
In a third Stage C, a glow discharge is achieved. As seen in FIG.
5a, the glow quickly goes to arc D, while in FIG. 5b, the
perforated dielectric suppresses the glow-to-arc transition and
stabilizes the glow discharge such that there is no arc.
[0048] FIG. 6a is a photograph showing an arc discharge which
creates a filamentary (high current density) channel. FIG. 6b is a
photograph showing a glow discharge characterized by a uniform glow
discharge.
[0049] FIG. 7 is a side plan view of an RF discharge embodiment of
the present invention wherein the perforated dielectric is
positioned over both electrodes. As can be seen electrodes 120 and
140 are both covered by a perforated dielectric 130. Because the
current reverses itself in a RF electric field, the dielectric 130
must be positioned over both electrodes 120 and 140, as both
electrodes alternately serve as cathodes. By this configuration,
the glow discharge can be sustained under broader operating
conditions. Such a configuration results in a frequency independent
and size independent device.
[0050] By applying the method and apparatus of the present
invention to large volume plasmas at atmospheric pressures, it is
possible to increase the energy released during combustion of fuels
to levels several times higher than the Heating Value of the fuel.
Efforts in this area in the past have failed because the
distribution of energy is required over a substantial volume and
cannot be concentrated in a small area. Because of the glow-to-arc
transition, there has been tendency to produce arcs of a very high
energy level with the rest of the volume remaining at a normal
combustion level. By suppressing the glow-to-arc transition and
stabilizing the plasma glow, the method and apparatus of the
present invention overcomes the limitations of the prior attempts
and results in an enhancement of the combustion process resulting
in much higher energy densities than could be previously
achieved.
[0051] Additional applications of the present invention may occur
in the field of air pollution remediation where stabilization of
the glow-to-arc transition may result in real time destruction of
constituents of air emissions from manufacturing operations in
remediation of soil and ground water pollution, in large volumes at
high pressures. By suppressing the glow-to-arc transition and
stabilizing the glow discharge, the present invention creates large
volume plasmas to destroy the polluting vapors at higher
efficiencies with reduced cost. There could be additional
applications relating to the destruction of combustion by-products
such as NO.sub.x and SO.sub.x which have heretofore been destroyed
by pulsed corona and barrier discharges.
[0052] The present invention is additionally applicable to the
cleaning of lithography sheet surfaces in atmospheric pressures.
Additionally, there may be possible utility for large area surface
cleaning at atmospheric pressure for curing polymer films. By being
able to operate at atmospheric pressure, a great advantage is
achieved over the high processing cost required in a vacuum
process. Additionally, the present invention can be used for
pretreatment of semi-conductors, glasses, and polymers which are to
be used for direct metal ion beam processing.
[0053] Additionally, an atmospheric pressure glow discharge plasma
can be used to sterilize biologically contaminated surfaces.
Current techniques in this area utilize high temperatures, strong
chemicals, and/or ultraviolet radiation to sterilize contaminated
items. However, there are problems with these approaches in that
the processes are time intensive and potentially hazardous and
result in the formation of potentially hazardous by-products. It
has been demonstrated that materials exposed to a one-atmosphere
pressure glow discharge plasma can be sterilized of biological
contaminants in under one minute.
[0054] In another embodiment as shown in FIG. 8, the dielectric 230
includes blind apertures 231. By "blind" what is meant is that the
aperture extends only partially through the dielectric, not
entirely through. Anotherwords, the dielectric 230 includes a
plurality of recesses 231 extending partially through the
dielectric. The recesses are defined by sidewalls 232 and bottom
wall 233. A solid component having a thickness x is defined between
bottom wall 233 of the open portion and bottom 234 of the
dielectric. Importantly, this solid component serves to regulate
the energy available to the discharge when an alternating current
is used. The dielectric portion provides for storage capacitance to
the device and serves to moderate the electric discharge to create
a glow plasma that can be tailored for specific applications. The
dielectric 230 can be made of any dielectric material known in the
art. The size of the recesses can vary from very small (on the
order of 5 .mu.m) to large (millimeters). The thickness of the
solid component can range from a few microns to hundreds of
microns.
[0055] Having thus described the invention in detail, it is to be
understood that the foregoing description is not intended to limit
the spirit and scope thereof. What is desired to be protected by
the Letters Patent is set forth in the appended claims.
* * * * *