U.S. patent number 3,814,983 [Application Number 05/224,038] was granted by the patent office on 1974-06-04 for apparatus and method for plasma generation and material treatment with electromagnetic radiation.
Invention is credited to Renato G. Bosisio, Carl F. Weissfloch, Michael R. Wertheimer.
United States Patent |
3,814,983 |
Weissfloch , et al. |
June 4, 1974 |
APPARATUS AND METHOD FOR PLASMA GENERATION AND MATERIAL TREATMENT
WITH ELECTROMAGNETIC RADIATION
Abstract
Apparatus for generating plasmas using electromagnetic energy in
the microwave frequency range, having a source of microwave energy,
a slow wave structure, conveying means for conveying microwave
energy from the source to the slow wave structure, a plasma
container and means for maintaining conditions of pressure and gas
flow in the container. There is also provided a novel transparent
radiation shield for use with such an apparatus. In another
embodiment, there is also provided a slow wave structure and
microwave energization means so that a region adjacent the
structure will contain a predominance of degenerate .pi./2 mode or
near degenerate .pi./2 mode electric field energy. Methods are also
disclosed for treating various types of material to alter their
properties using the above-described apparatus.
Inventors: |
Weissfloch; Carl F. (Montreal
456, Quebec, CA), Wertheimer; Michael R. (Montreal
215, Quebec, CA), Bosisio; Renato G. (Longueuil,
Quebec, CA) |
Family
ID: |
22839048 |
Appl.
No.: |
05/224,038 |
Filed: |
February 7, 1972 |
Current U.S.
Class: |
315/39; 315/3.5;
331/126; 204/164; 315/111.21; 331/78 |
Current CPC
Class: |
H01J
65/044 (20130101); H05H 1/46 (20130101); H01J
37/32192 (20130101); H01J 37/32211 (20130101); G21K
5/10 (20130101); H05H 1/4622 (20210501) |
Current International
Class: |
G21K
5/10 (20060101); H01J 65/04 (20060101); H01J
37/32 (20060101); H05H 1/24 (20060101); H05H
1/46 (20060101); H01j 007/46 (); H01j 019/80 () |
Field of
Search: |
;315/3,3.5,39,111
;331/78,126 ;313/231 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Chatmon, Jr.; Saxfield
Attorney, Agent or Firm: Schaap; Robert J. Fincham; Ian
Claims
We claim:
1. An apparatus for generating a plasma using electromagnetic
energy in the microwave frequency range comprising a source of
microwave energy, at least one slow wave structure having an input
end for receiving microwave energy from the source thereof, means
for conveying said microwave energy from said source of said
microwave energy to the input end of said slow wave structure to
create a region adjacent said slow wave structure containing
electromagnetic energy, a plasma container spaced from and located
in proximity to said slow wave structure in the region containing
electromagnetic energy, said plasma container being adapted to
receive an ionizable gaseous fluid so that the microwave energy is
applied to said gaseous fluid to generate a plasma therefrom, means
for maintaining conditions of pressure and flow of said ionizable
gaseous fluid in said plasma container, and said slow wave
structure having an output end for discharging certain of the
microwave energy traveling along said slow wave structure.
2. An apparatus, as defined in claim 1, said apparatus including a
dummy load connected to the output end of said slow wave
structure.
3. An apparatus, as defined in claim 1, said apparatus including
means for measuring forward, reflected and transmitted microwave
power.
4. An apparatus, as defined in claim 1, said apparatus including a
transparent microwave radiation shield adapted for unobstructed
observation of a plasma, said radiation shield surrounding said
slow wave structure and said plasma container.
5. An apparatus, as defined in claim 1, wherein said slow wave
structure is a non-resonant semi-radiant strapped bar type.
6. An apparatus, as defined in claim 4, wherein said radiation
shield comprises an inner wall made of a transparent dielectric
material, an outer wall made of a transparent dielectric material,
and a transparent liquid which absorbs microwave energy filling the
space between said inner and outer walls.
7. A slow-wave microwave applicator apparatus suitable for use for
heating, drying, curing, ionization of gases, or other treatment of
workpieces by microwave energy, and which gases or workpieces
contain lossy dielectric material, in solid, liquid or gaseous
form, said apparatus comprising a slow wave microwave structure,
and microwave energization means connected to the said microwave
structure adapted to energize said microwave structure so that a
region adjacent said structure will contain a predominance of
degenerate .pi./2 mode or near degenerate .pi./2 mode primarily
backward wave electric field energy to enable band edge operation
at the .pi./2 mode, the backward waves of said electric field
energy housing, a phase velocity approaching zero and propagating
their energy in one direction and their phase fronts in an opposite
direction, the operation bandpass of said slow wave structure being
defined by a frequency spectrum F.sub.1 -F.sub.0 near the mode
bandedge.
8. A slow-wave microwave applicator apparatus suitable for use for
heating, drying, curing, ionization of gases, or other treatment of
workpieces by microwave energy, and which gases or workpieces
contain lossy dielectric material, in solid, liquid or gaseous
form, said apparatus comprising:
a. a slow wave microwave structure,
b. said slow wave microwave structure comprising:
1. a first set of parallel conducting rods in spaced relation,
2. a second set of parallel conducting rods in spaced relation and
interleaved alternately with the first set,
3. a first strap-bar in the form of an extended conducting plate
making contact with each of the first set of parallel conducting
rods,
4. a second strap-bar generally parallel to the first strap-bar and
in the form of an extended conducting plate making contact with
each of the second set of parallel conducting rods,
c. and microwave energization means operatively attached to said
first and second strap-bars of said microwave structure adapted to
energize said microwave structure so that a region adjacent said
structure will contain a predominance of degenerate .pi./2 mode or
near degenerate .pi./2 mode electric field energy.
9. An apparatus, as defined in claim 1, wherein said source of
microwave energy is designed so that said apparatus is capable of
providing radiation in the wave length ranging from the vacuum
ultraviolet to the far infared.
10. An apparatus for generating a plasma as defined in Claim 1,
wherein said apparatus includes a transparent microwave radiation
shield surrounding said slow wave structure and said plasma
container and being adapted to provide an unobstructed observation
of a plasma generated from said ionizable gaseous fluid, said
radiation shield comprising:
a. an inner wall made of a transparent dielectric material,
b. an outer wall made of a transparent dielectric material,
c. and a transparent liquid which absorbs microwave energy filling
the space between said inner and outer walls.
11. An apparatus for generating a plasma using electromagnetic
energy in the microwave frequency range comprising a source of
microwave energy, at least one slow wave structure having an input
end for receiving microwave energy from the source thereof and
forming a region adjacent to said slow wave structure which
contains a predominance of degenerate .pi./2 or near-degenerate
.pi./2 mode primarily backward wave electromagnetic energy to
enable band edge operation at the .pi./2 mode, the backward waves
of said electromagnetic field energy housing a phase velocity
approaching zero and propagating their energy in one direction and
their phase fronts in the opposite direction, the operation
bandpass of said slow wave structure being defined by a frequency
spectrum F.sub.1 -F.sub.0 near the .pi./2 mode bandedge, said
electromagnetic energy having a normal attenuation constant
.beta..sub.n, and said slow wave structure having a longitudinal
attenuation constant .beta..sub.L, and the electromagnetic wave
inside of the plasma container having a linear attenuation constant
.beta..sub.g which are relatively constant, such that the electric
field vector of the backward wave electromagnetic energy is
relatively constant, means for conveying said microwave energy from
said source of said microwave energy to the input end of said slow
wave structure, a plasma container located with respect to said
region and adapted to contain a ionizable gaseous fluid for
receiving and applying said microwave energy to said ionizable
gaseous fluid to generate a plasma therefrom.
12. An apparatus, as defined in claim 11, wherein said source of
microwave energy has a frequency in the range of from about 100 MHz
to about 30,000 MHz.
13. An apparatus, as defined in claim 11, wherein said source of
microwave energy has a frequency in the range of from about 915 MHz
to about 5,800 MHz.
14. An apparatus, as defined in claim 11, wherein said slow wave
structure is a non-resonant, semi-radiant strapped bar type.
15. An apparatus for generating a plasma using electromagnetic
energy in the microwave frequency range, said apparatus comprising
a plasma container capable of having an ionizable gaseous fluid
introducible thereinto, said container being formed with a wall
capable of permitting microwave energy to be coupled into the
gaseous fluid, at least one slow wave structure having an input end
for applying microwave energy to said ionizable gaseous fluid to
generate a plasma therefrom, means for conveying said microwave
energy from said microwave energy generating means to the input end
of said slow wave structure, the longitudinal axis of said plasma
container being angularly located with respect to the longitudinal
axis of said slow wave structure at an angle .theta. so that
electromagnetic waves propagating along the length of said slow
wave structure will provide a relatively constant energy transfer
profile along the length of the plasma container.
16. An apparatus, as defined in claim 15, wherein said angle
.theta. is between about 1.degree. to about 30.degree..
17. An apparatus, as defined in claim 15, wherein the angle .theta.
is established to obtain a constant energy density transfer through
the plasma container based on the normal attenuation constant of
the electric field strength which produces the electromagnetic
energy, the longitudinal attenuation constant of the slow wave
structure, and the linear attenuation constant of the
electromagnetic waves in the plasma container.
18. An apparatus, as defined in claim 15, wherein the angle .theta.
is established to obtain a constant energy density transfer through
the plasma container based on the ratio of the normal attenuation
constant of the electric field strength, which produces the
electromagnetic energy, divided by the longitudinal attenuation
constant of the slow wave structure, plus the linear attenuation
constant of the electromagnetic waves in the plasma container.
19. An apparatus, as defined in claim 15, wherein said slow wave
structure is a non-resonant, semi-radiant strapped bar type of
structure.
20. An apparatus, as defined in claim 15, wherein said slow wave
structure is a wide-band, semi-radiant strapped bar type of
structure, and said apparatus is adapted to be operated so that the
region adjacent to said slow wave structure contains a predominance
of degenerate .pi./2, or near-degenerate .pi./2 mode
electromagnetic energy.
21. An apparatus for generating a plasma using electromagnetic
energy in the microwave frequency range, said apparatus comprising
a plasma container capable of having an ionizable gaseous fluid
introducible thereinto, said container being formed of a wall
relatively transparent to microwave energy and capable of
permitting microwave energy to be coupled into the gaseous fluid,
microwave energy generating means for generating a source of
microwave energy, a transmission line adapted to transmit microwave
energy and being operatively connected to said microwave energy
generating means for receiving and transmitting the microwave
energy thus generated, tuning means operatively connected to said
transmission line to obtain minimum energy-return reflection to
said generating means, a slow wave structure having an input end
for applying microwave energy to an ionizable gaseous fluid in said
plasma container to generate a plasma therefrom, transition means
operatively connected to said tuning means and said input end of
said slow wave structure for delivering the microwave energy to
said slow wave structure, said plasma container being located
proximate to said slow wave structure to receive the microwave
energy carried by said slow wave structure, means for providing a
source of ionizable gaseous fluid to the plasma container, and
means operatively interposed between said last-named means and said
plasma container.
22. An apparatus, as defined in claim 21, said apparatus comprising
a dummy load operatively connected to an output end of said slow
wave structure.
23. An apparatus, as defined in claim 21, said apparatus comprising
monitoring means for measuring forward, reflected and transmitted
microwave power.
24. A slow-mode microwave applicator suitable for heating, drying,
curing, ionization of gases, or other treatment by microwave energy
of workpieces containing lossy dielectric material, and which
dielectric material may be in solid, liquid or gaseous form, said
microwave applicator comprising:
a. a slow wave microwave structure, and said slow wave microwave
structure comprising:
1. a transmission line member having at least one open end,
2. a pair of spaced-apart microwave energy conductors located
within said transmission line member and having terminal end
portions spaced inwardly from said one open end of said
transmission line member,
3. a pair of transmission members extending into said one open end
of said transmission line member and each one of said pair of
transmission members being connected at one of their ends thereof
to a respective one of said pair of spaced-apart conductors,
4. each of said conductors being tapered inwardly from one of their
ends to the other of their ends which are spaced inwardly from said
one open end of said transmission line member and which ends are
connected to said transmission members,
5. strapped bar conductive means operatively connected to each of
said transmission members at the other ends thereof,
b. and microwave energy generating means operatively connected to
said slow wave microwave structure for applying microwave energy to
said slow wave structure, said energy generating means applying
energy of such frequency and said conductors being constructed to
generate a region adjacent said structure which will contain a
predominance of degenerate .pi./2 of near degenerate .pi./2 mode
electric field energy.
25. A slow-mode microwave applicator, as defined in claim 24,
wherein said strapped bar conductive means comprises:
a. a first set of parallel conducting rods in spaced relation,
b. a second set of parallel conducting rods in spaced relation and
interleaved alternately with the first set,
c. a first strap-bar in the form of an extended conducting plate
making contact with each of the first set of parallel conducting
rods and one of said transmission members, and
d. a second strap-bar generally parallel to the first strap bar and
in the form of an extended conducting plate making contact with
each of the second set of parallel conducting rods and the other of
said transmission members.
Description
This invention relates to a process for introducing high-frequency
electromagnetic energy into a gaseous system so as to partially
ionize the gas and to maintain a stable discharge or plasma. More
particularly, this invention relates to an apparatus by means of
which a large-volume plasma can be generated, and to numerous
processes by which the large plasma volume can be used to impart
desirable changes in properties to different materials treated by
the plasma.
There exist many applications where it is desired to create a
plasma, that is, to excite a gas to a higher level of internal
energy or partial ionization. Some of these will be described
further on in the present disclosure. A particularly convenient
means for creating a plasma is by the use of electrical energy, and
of the various forms of electrical energy which can be used, very
high frequency energy, specifically in the microwave range, is
particularly advantageous. The reason for this will become apparent
from the present disclosure.
There has, however, so far existed a major obstacle to the
wide-spread use of microwave energy for generating plasmas, namely
the lack of suitable applicators. All previously available
applicators of microwave energy which could be used to generate
plasmas were limited to relatively very small plasma volumes, hence
to the treatment of small amounts of material only.
One object of the present invention is to increase the volume of a
microwave plasma so that it can be used for treating large
quantities of material.
Another aspect of the invention is to use the large-volume
microwave plasma to carry out certain specific treatments of
materials, for example, to obtain efficient dissociation of
molecular gases, such as oxygen, nitrogen, hydrogen, etc., to
obtain increased bond strength of fibres and films of natural or
synthetic polymer materials or combination thereof, and to effect
chemical reaction of gaseous organic or inorganic compounds to form
other compounds, either within the plasma zone or by combination
with other material outside the plasma zone. Other applications for
which an embodiment of the present invention, the so-called "Large
Volume Microwave Plasma Generator" (LMP) is particularly well
suited, and has certain unique advantages over other devices, are
to "pump" a gas laser, and to generate electromagnetic radiation in
certain parts of the spectrum, for example, in the ultraviolet, the
visible or the infrared ranges of wavelengths.
One aspect of the invention common to all the features and
applications already mentioned involves the use of a slow wave
electromagnetic structure as the means for applying microwave
energy to the gas plasma. Another aspect involves a convenient form
of container for the gas and plasma (and other material to be
treated), this container being made of a microwave-transparent
material such as quartz or polytetrafluoroethylene, and means for
maintaining the gas at a certain desired pressure inside the
container and causing it to flow through the plasma zone at a
desired rate, should this be required.
Still another aspect of the invention involves the use of a double
walled, transparent plastic radiation shield where the space
between the double walls is filled with a microwave-absorbing,
transparent substance such as water. This shield surrounds the
microwave applicator and plasma container, and protects the
operator from any stray microwave radiation while permitting him an
unobstructed view of all parts of the apparatus for optimum control
of the process.
More particularly, in accordance with one embodiment of the present
invention, there is provided an apparatus for generating a plasma
using electromagnetic energy in the microwave frequency range
comprising a source of microwave energy, at least one slow wave
structure having an input end for applying said microwave energy to
a plasma, means for conveying said microwave energy from said
source of said microwave energy to the input end of said slow wave
structure, a plasma container and means for maintaining conditions
of pressure and gas flow in said plasma container. Preferably, such
apparatus includes a dummy load connected to an output end of said
slow wave structure; as well as including means for measuring
forward, reflected and transmitted microwave power. Still further,
the apparatus also preferably includes means for adjusting the
input of microwave energy into the plasma, as well as the
transparent microwave radiation shield adapted for unobstructed
observation of the plasma, said radiation shield surrounding said
slow wave structure and said plasma container.
The source of microwave energy is preferably one which has a
frequency in the range of 100 MHz to 30,000 MHz. In a preferred
embodiment, the plasma container includes vacuum locks adapted to
permit the continuous passage of material to be treated from the
atmosphere into and through said plasma container; the plasma
container may be vertically disposed to permit material to be
treated to pass through the plasma under the force of gravity.
In accordance with a further embodiment of the present invention,
there is also provided a slow-mode microwave applicator suitable
for use for heating, drying, curing, ionization of gases, or other
treatment by microwave energy of workpieces containing lossy
dielectric material, in solid, liquid or gaseous form, comprising a
slow wave microwave structure, and microwave energization means
connected to the said structure adapted to energize said microwave
structure whereby a region adjacent said structure will contain a
predominance of degenerate .pi./2 mode or near degenerate .pi./2
mode electric field energy. This apparatus preferably includes a
transparent microwave radiation shield adapted for unobstructed
observation of a plasma or workpieces, said radiation shield
surrounding said slow wave structure. A particularly preferred form
of the last embodiment comprises
a. a first set of parallel conducting rods in spaced relation,
b. a second set of parallel conducting rods in spaced relation and
interleaved alternately with the first set,
c. a first strap-bar in the form of an extended conducting plate
making contact with each of the first set of parallel conducting
rods,
d. a second strap-bar generally parallel to the first and in the
form of an extended conducting plate making contact with each of
the second set of parallel conducting rods,
e. microwave energization means attached to said first and second
strap-bars to energize said bars and engender an electric field
region adjacent said bars, said field containing a predominance of
degenerate .pi./2 mode or near degenerate .pi./2 mode electric
field energy.
In accordance with a still further embodiment of this invention,
there is provided an apparatus suitable for generating a plasma,
the improvement comprising a slow wave structure having an input
end adapted to apply microwave energy to a plasma, means for
conveying said microwave energy from a source thereof to the input
end of said slow wave structure, and a source of microwave energy;
preferably such apparatus also includes a transparent microwave
radiation shield adapted to provide an unobstructed observation of
a plasma, said shield surrounding said slow wave structure and
being also adapted to surround a plasma container.
In a still further embodiment of this invention, there is provided
a method of treating material to alter the properties of such
material comprising providing an apparatus for generating a plasma
using electromagnetic energy in the microwave frequency range, said
apparatus having a source of microwave energy, at least one slow
wave structure having an input end for applying said microwave
energy to a plasma, means for conveying said microwave energy from
said source of said microwave energy to the input end of said slow
wave structure, a plasma container and means for maintaining
conditions of pressure and gas flow in said plasma container, and
exposing said material to said plasma.
The slow wave structure may be of the "semi-radiating" type, such
as a strapped-bar structure, and may be terminated in a dummy load
or other type of dissipative termination. In accordance with a
preferred embodiment of the invention, the slow wave structure is
placed in close proximity to the gas plasma container so that
efficient "coupling," or transfer of power, can occur from the
electromagnetic field of the structure to the plasma within the
container. Under conditions where there is no plasma but a
non-ionized gas of high electrical impedance, and where the slow
wave structure is still being energized, this microwave energy will
not be radiated from the structure, but will traverse it with
little loss, and will be dissipated in the dummy load.
In accordance with a further feature of the invention, the slow
wave structure may be arranged to provide substantially constant
energy transfer along its length by increasing the coupling between
the slow wave structure and the plasma at points farther from the
source of microwave energy. This can be accomplished by mounting
the slow wave structure at an angle e.g. between 1.degree. and
30.degree., with respect to the plasma container, or by changing
the electrical characteristics of the structure along its length.
It can also be accomplished by "sandwiching" the plasma container
between two slow wave structures which are energized from opposite
ends.
In an electrical discharge, electrons produced by partial
ionization of the gas gain energy from the electric field, and
transfer some of this energy to molecules via collisions, thereby
creating more electrons and positive ions; the energetic electrons
are also capable of dissociating molecules, or of raising them to
higher states of excitation. The resulting species include, among
others, neutral atoms, free radicals, rotationally or vibrationally
excited molecules, etc. Many of these species are chemically highly
reactive, and tend to recombine to form the starting material, if
only one kind of atom is present, or various compounds if several
different kinds of atoms are present.
In addition to their usefulness for effecting chemical changes, the
excited species which constitute a plasma also emit radiation which
can be used for various purposes, for example, for laser action,
for spectral sources, for identifying trace impurities in the gas,
for generating light in various parts of the spectrum, etc. The
physical principles involved are well known to those skilled in the
art, and need not be discussed in detail here. Suffice it to say
that an efficient plasma generator such as the one embodied by the
present invention can have many and varied applications, some of
which constitute a particular aspect of the present invention.
Of the various forms of electrical energy which can be used to
generate gaseous plasmas (for example, d.c., low frequency a.c.,
"corona," radio frequency, microwave) microwave energy is
particularly advantageous in many cases. Some of the advantages are
the following:
a. There are no electrodes, which frequently constitute a major
source of contamination.
b. The yield of active species is much higher (ten times or more)
than in other forms of discharges, that is, the energy density is
higher.
c. Most of the energy in the plasma resides in the "electron gas,"
that is, the "electron temperature," Te, is much higher than the
gas temperature, Tg.
d. It is possible to sustain a microwave plasma at higher pressures
than most other forms of electrical discharges, at a given applied
power level.
e. Practically all the available microwave energy can be coupled
into the plasma.
f. The state of the art in the technology of generating microwave
power is advanced.
In spite of these advantages, the use of microwave plasmas has been
severely limited by the small size of plasma volumes achievable
with conventional microwave applicators, namely antennas,
waveguides, and cavity resonators. Such applicators are described,
for example, by Fehsenfeld et al. (Review of Scientific
Instruments, Volume 36, page 294, 1965), by Shaw (Formation and
Trapping of Free Radicals, A.M. Bass and H.P. Broida, Eds.,
Academic Press 1960, page 57), and by McTaggart (Plasma Chemistry
in Electrical Discharges, Elsevier, London, 1967, Chapter 4).
Processing a large volume of gas as required, for example, in a
chemical plant would require a multiplicity of such applicators
which would render the capital outlay for equipment prohibitive.
One object of this invention, namely the use of a slow wave
structure for applying the microwave energy to the plasma,
completely solves this problem: for example, by using a 36
inch-long "semi-radiant" slow wave applicator operating in the
degenerate .pi./2 mode, as an object of the present invention, it
is possible to generate microwave plasmas in a volume of 1,000
cubic centimeters or more; this is a factor of about 100 greater
than that which can be achieved with a typical, commercially
available cavity resonator. It should be emphasized that this ratio
can easily be increased even more.
Slow wave structures as microwave applicators, well know to those
skilled in the art, can be divided into two general types: resonant
slow wave structures, and traveling slow wave structures. As
examples of the former and latter there may be mentioned,
respectively, the disclosures of C.M. Loring (U.S. Pat. No.
3,532,848 issued Oct. 6, 1970), and J.E. Gerling (U.S. Pat. No.
3,472,200 issued Oct. 14, 1969).
In addition to slow wave applicators, there exists a "fast-wave"
applicator, the so-called "zero-mode" applicator for which a patent
application has been filed separately (U.S. Pat. Ser. No. 117,538).
All the prior disclosures listed above deal specifically with the
use of microwave energy for heating, drying or curing large areas
or moving sheets of material; none specifically mention, or are
suitable for, gaseous discharges as embodied by the present
invention. On the other hand, the wide band, traveling slow wave
structure operating at or near the degenerate .pi./2 mode, an
object of the present invention, is particularly advantageous for
generating large volume microwave plasmas in the context of the
present invention. The particular reasons for this will become
clear in the course of the present disclosure. It is to be
expressly understood, however, that this slow wave structure,
although very well suited for plasma generation, is used here by
way of illustration only, and that it is not intended as a
definition of the limits of the invention. It is also understood
that the beneficial properties of this applicator make it very
useful for applications other than plasma generation, such as
drying, heating or curing as taught by the prior art referred to
above.
The novel features which are believed to be characteristic of the
invention, both as to its organization and method of operation,
together with further objects and advantages thereof, will be
better understood from the following description considered in
connection with the accompanying drawings in which an illustrative
system embodying the principles of the invention is illustrated by
way of example. It is to be expressly understood, however, that the
drawings are for the purpose of illustration and description only,
and are not intended as a definition of the limits of the
invention.
In the drawings:
FIG. 1 is a front view of a large volume microwave plasma generator
(LMP) apparatus embodying several aspects of this invention.
FIG. 2 is a side view of said LMP apparatus.
FIG. 3 is a perspective view, partly in section, of the transparent
radiation shield portion of the apparatus in FIG. 1.
FIG. 4 is a detailed frontal view of a part of the linear slow
electromagnetic wave structure of the strapped bar type, as
incorporated on the apparatus of FIG. 1.
FIG. 5 is a cross sectional view taken along the line 5--5 of FIG.
4.
FIG. 6 is a plan view of the slow wave structure taken along the
line 6--6 of FIG. 4.
FIG. 7 is a sectional view, perpendicular to the axis, through a
slow wave electromagnetic structure of the strapped bar type in
circular geometry.
FIG. 8 is a cross sectional view taken along line 8--8 of FIG.
7.
FIG. 9 is a Brillouin diagram of a slow wave structure of the type
shown in FIGS. 4 to 6.
FIG. 10 is a schematic frontal view of a slow wave structure and
plasma container.
FIG. 11 is a schematic diagram depicting two graphs of average
electric field strength squared measured lengthwise along a slow
wave structure.
FIG. 12 is a cross sectional view taken through a slow wave
structure of the type shown in FIGS. 4 to 6 and a plasma container,
for the case where said plasma container is of the transverse flow
type.
FIGS. 13 and 14 are plan views of particular types of
transverse-flow plasma containers.
FIG. 15 is a schematic frontal view of a plasma container
sandwiched between two slow wave structures to which microwave
energy is applied from opposite directions.
FIG. 16 is a schematic plan view of a comb-type of slow wave
structure.
FIG. 17 is a schematic plan view of a ladder-type of slow wave
structure.
FIG. 18 is a schematic plan view of a zig-zag line type of slow
wave structure.
FIG. 19 is a schematic plan view of a helix-type of slow wave
structure.
FIG. 20 is a semi-logarithmic chart depicting the yield of oxygen
atoms in a flow of oxygen gas which is ionized by different amounts
of microwave power, where the latter is applied in one case by a
cavity resonator and in the other case by a slow wave structure of
the type shown in FIGS. 4 to 6.
FIG. 21 is a chart depicting the maximum yield of nitrogen atoms in
a flow of nitrogen gas which is ionized by microwave power, in one
case applied with a cavity resonator and in the other with a slow
wave structure of the type shown in FIGS. 4 to 6.
FIG. 22 is a schematic frontal section through a microwave plasma
apparatus for the treatment of material in web, sheet or plate
form.
FIG. 23 is a sectional view taken along line 23--23 in FIG. 22.
FIG. 24 is a schematic sectional side view of an alternate
microwave plasma apparatus for the treatment of material in web,
sheet or plate form.
FIG. 25 is a sectional view taken along line 25--25 in FIG. 24.
FIG. 26 is a schematic frontal sectional view of a microwave plasma
apparatus for the treatment of material in granular, fibrous, or
other form.
FIG. 27 is a schematic frontal view of a laser apparatus pumped by
microwave energy.
Referring more particularly to the drawings, the large volume
microwave plasma generator apparatus of FIGS. 1 and 2 consists of a
cabinet 19 which houses all the components. Microwave energy from a
power source 1 is fed to the slow wave structure 6 via conventional
rectangular waveguide 2, the two being coupled by a transition
section 5. Power monitors 3 and 10 allow one to measure the
forward, reflected, and transmitted microwave power, respectively.
A triple-stub tuner 4 assures minimal reflection of power back to
the source 1, and any transmitted power is absorbed in the dummy
load 11. The direction of microwave energy flow is indicated by
heavy arrows 16. Lengthwise and in close proximity to the slow wave
structure 6 is a gas plasma container 8, typically made of
microwave-transparent material such as quartz; it contains the
ionized gas plasma 7 which is created when the gas inside the
container absorbs microwave power. Desired conditions of gas flow
rate and pressure are maintained by adjusting the gas feed rate
from the storage vessel 14, by means of a flow and pressure
regulator 15, and by means of the pumping rate of the vacuum pump
13. Details of the plasma and the gas flow system depend upon the
particular application, so that the latter is merely indicated
schematically by the gas flow conduit 12 and arrows 17 which depict
the direction of gas flow. The plasma container 8 is cooled from
the outside by a forced flow of air from a fan 18.
The radiation shield 9, shown in detail in FIG. 3, permits an
operator to observe the microwave plasma zone at close range, and
in the absence of exposure from any stray microwave radiation. The
shield consists of a rectangular box, open at the bottom, which is
made for example of 1/8 inch to 1/4 inch thick plexiglass plate 23.
Double walls enclose a leak-tight space, typically 1/2 inch wide,
filled via a filler or drainage plug 21 with a water solution 22
which strongly absorbs microwave radiation. Oval openings 20
provide a passage for the gas plasma container; the bottom of the
shield rests on the cabinet 19, which completes the radiation-tight
enclosure.
In the context of this invention, the strapped bar type of slow
wave structure depicted in FIGS. 4 to 6 is a particularly
advantageous applicator of microwave energy, as will be seen below.
The transition from the rectangular waveguide 2 to the slow wave
structure consists of a doubly tapered inner conductor 24 suitably
situated at a length L.sub.1 from the end of the rectangular
transmission line section. The inner conductor 24 is followed by a
tapered parallel plane transmission line 25 whose length L.sub.2,
best determined experimentally, is about .mu./4 where .mu. is the
free space wavelength at the operating frequency. The angle
.alpha., also determined by experiment, is about 30.degree..
A number of metal conducting bars or tubes 26 are placed in a plane
and electrically connected near their midplane by parallel straps
27 and 28 in an alternating manner as shown in FIG. 4, and the bars
26 which form the periodic structure are terminated at each end by
a shorting plane 29. The separation L.sub.3 between the
strap-conductors 27 and 28, and the distance L.sub.4 between the
end shorting planes determine the operating frequency of the
structure. As will be shown, it is advantageous to operate the
present structure in the so-called .pi./2 mode in which case
L.sub.3 and L.sub.4, best determined experimentally, are typically
.mu./10 and .mu./2 in length, respectively. When operated in this
manner, the region occupied by the plasma container contains the
strong electric field 30 of the .pi./2 mode, which couples into the
plasma, leading to a transfer of the microwave energy to the
plasma. Any unused microwave energy propagates along the slow wave
structure to the outlet transition, which is identical to that at
the inlet, and is transmitted via a rectangular waveguide to a
dummy load where it is converted to heat energy.
FIGS. 7 and 8 show two views of a strapped bartype of slow wave
structure having a cylindrical geometry, which is electrically
identical to the linear structure depicted in FIGS. 4 to 6. Here
again the conducting bars 31, arranged around the perimeter of a
circle, are connected alternately by the strap conductors 32 and
33, and are shorted on both ends by shorting planes 34. The strong
electric fields inside the cylinder may couple with a plasma, when
a plasma container is placed along the axis of the energized
structure.
A periodic structure is sometimes best described by a Brillouin
diagram such as the one shown in FIG. 9. Information regarding its
pass bands, stop bands, phase velocity and group velocity are
contained in such a diagram, and the frequencies at which the
degenerate .pi./2 mode occurs can be identified. The frequency
separation f.sub.1 - f.sub.o of the two degenerate .pi./2 modes 35
and 36 is dependent on the uniformity of the cells composing the
structure: theoretically the narrow stop band f.sub.1 - f.sub.o is
completely removed when the cells are identical, and the modes
lying on the two branches indicated by dashed lines 37 and 38 are
no longer existant. In actual practice an absolute symmetry is not
desirable if one wishes to operate at or near the degenerate .pi./2
mode.
It is known to those versed in periodic microwave structures that
operation near a band edge, that is, either in the degenerate
.pi./2 or in the .pi. mode, leads to particularly strong electric
fields near the slow wave structure; the reason for this is that
the electric field strength is inversely proportional to V.sub.g,
the group wave velocity, which is very small near a band edge.
Furthermore, as the electric field drops off as
.epsilon..sub.-.sup..beta.n.sup.y with distance y normal to the
plane of the slow wave structure, it extends out particularly far
in the case of the degenerate .pi./2 mode (for which .beta..sub.n
is a factor of 2 smaller than that of the .pi. mode, for example).
In order to create a gaseous plasma, the applied electric field
strength must exceed the dielectric breakdown strength of the gas;
as the breakdown strength increases with increasing gas pressure,
strong electric fields are necessary for plasmas at elevated
pressures. Clearly then, the main features associated with
operating the strapped bar structure of FIGS. 4 to 6 in the .pi./2
mode, namely high field strength extending over a considerable
distance from the slow wave structure, are very advantageous for
the production of large volume microwave plasmas at elevated
pressures.
For example, the experimental data shown in Table I below were
obtained with a 36 inch long strapped bar slow wave structure as
shown in FIGS. 4 to 6, operating in the .pi./2 mode at 2,450 MHz
frequency. Some further characteristics of this device were as
follows: the dimensions L.sub.3 and L.sub.4 were, respectively, 1/4
inch and 2.5 inches, and during normal operation substantially 90
percent or more of the microwave power was coupled into the plasma.
At elevated gas pressures there was a limit as to how much power
could be coupled into the plasma, the excess power being
transmitted to the dummy load. (Or, conversely, at a given power
level, there was a maximum pressure at which substantially all this
power would couple into the plasma). Further increases in pressure
eventually resulted in extinction of the plasma as shown in Table
I.
---------------------------------------------------------------------------
TABLE I
Appoximate Typical GAS Pressure Extinction Maximum Gas for Maximum
Power Pressure Power Flow Absorption Wm Rate po (torr) pe (torr)
(watts) (l./min)
__________________________________________________________________________
Argon 250 495 750 1 to 20 300 580 1500 Do. 500 980 2500 Do.
Nitrogen 40 80 750 1 to 15 60 110 1500 Do. 160 310 2500 Do.
__________________________________________________________________________
The plasma container in this case was a quartz tube having an
inside diameter of 19 millimeters and a total volume of 260 cubic
centimeters which was completely filled by the plasma. Similar
experiments have been carried out in substantially larger plasma
volumes.
Other important features of the invention can be seen from FIG. 10:
in order to obtain a uniform field intensity, hence a plasma of
uniform power density along the full length L of the slow wave
structure 6, it is necessary to incline the plasma container 8 at a
slight angle .theta. with respect to the former. The
electromagnetic wave propagating down the length of the slow wave
structure 6 may be expressed, for small values of .theta., by E =
Eo.epsilon..sub.-.sup..beta.n.sup.y
.epsilon..sub.-.sup.(.sup..beta. L .sub.+ .sup..beta.g.sup.)z
.sub.+ i.sup..beta. ez.epsilon..sup.j.sup..omega.t (1)
where .beta..sub.n is the normal attenuation constant of the
electric field strength, .beta..sub.L is the longitudinal
attenuation constant of the slow wave structure, and .beta..sub.g
is the linear attenuation constant of the electomagnetic wave
inside the plasma container. Under optimum conditions, the electric
field vector of the electromagnetic wave should be constant. From
(1) this signifies that
.beta..sub.n y - .beta..sub.L z - .beta..sub.g z = constant (2)
Differentiating equation (2) with respect to y,
dz/dy = -(.beta..sub.n /.beta..sub.L + .beta..sub.g) .apprxeq.
.theta. (3)
It is seen that a uniform power transfer, hence plasma density, can
be maintained throughout the full length of the plasma container 8,
by inclining the latter at an angle .theta. with respect to the
plane of the slow wave applicator 6 without in any way restricting
the length L of the applicator. This is shown graphically in FIG.
11 where the time-averaged relative electric field intensity
squared (E.sup.2), which is proportional to power transfer, is
plotted against longitudinal distance z along the slow wave
structure. Curve 39 shows the exponentially decreasing profile for
the case where the plasma tube is maintained at a constant distance
y from the plane of the slow wave structure; curve 40 shows the
constant power transfer profile obtained by proper choice of the
angle .theta.. In the case of a 36 inch long structure operating at
2,450 MHz frequency, .theta. is found to be about 3.degree. to
10.degree..
FIG. 12, a sectional view perpendicular to the z axis of a linear
strapped bar slow wave structure and adjacent plasma container 41,
depicts a transverse flow modification of the gas flow geometry,
which can be convenient under certain circumstances, notably where
short average residence time of a gas molecule in the plasma zone
is desired. FIGS. 13 and 14 show plan views of two possible
transverse flow plasma containers, where 42 is the plasma container
proper, 43 and 43' are the gas inlet and exit ports, respectively,
44 is the gas distribution manifold, and 45 is a porous diffuser
type of manifold. Arrows 17 show the overall gas flow direction and
arrows 46 show particular examples of gas flow paths.
FIG. 15 schematically depicts another possible variation of the
invention, namely one in which microwave power is simultaneously
applied to the plasma 7 in the plasma container 8 from two slow
wave structures 6 and 6' between which the plasma container is
"sandwiched." The plasma container makes an angle .theta. with each
of the slow wave structures 6 and 6', but the direction of flow of
the microwave power 16 in the case of 6' is opposite to that of 6.
The advantage of this arrangement is that a plasma can be
maintained in an even greater volume, that is, in a plasma
container of greater diameter, than in the case where microwave
power is applied from a single slow wave structure. Clearly, it
would be possible, if desired, to apply microwave power from more
than two slow wave structures by placing them around the plasma
container in a suitable manner.
Although the wide band, strapped bar type of slow wave structure
discussed so far is well suited for applying microwave power to a
plasma, it is possible to use other types of geometries, well known
to those familiar with slow wave structures, for the same purpose.
FIGS. 16 to 19 show schematically some examples of slow wave
structure geometries which could be suitably adapted to the
purposes of the present invention. FIG. 16 represents a plan view
of a comb-type of slow wave structure in which microwave energy
follows the direction of arrows 47 between the conductive side
walls 48 and the vanes 49. FIG. 17 is a plan view of a ladder-type
of slow wave structure consisting of conductive side walls 50 and
bars 51; FIG. 18 is a plan view of a zig-zag line type of slow wave
structure composed of a conductor 52, and FIG. 19 is a helical type
of slow wave structure consisting of a conductor 53. It is
understood that these or other types of slow wave structures not
mentioned here could be suitably adapted for the requirements of
the present invention, and that this is included in the spirit of
the present invention.
In FIGS. 20 and 21 experimental results obtained when two different
kinds of microwave applicators are compared; in one case an LMP
apparatus of the type depicted in FIGS. 1 and 2 was used, and in
the other case microwave power was applied by means of a cavity
resonator, the frequency in both cases being 2,450 MHz. In the case
of FIG. 20, a flow of oxygen gas was ionized by microwave power,
leading to partial dissociation of the molecules to atoms. The flow
rate of atoms is plotted against absorbed microwave power; it is
seen from curves 55 and 56 that the atom concentration reaches a
limiting value when power is applied with a cavity resonator, but
from curve 54 it is clear that much larger atom yields are possible
using a slow wave structure for applying the microwave power. In
FIG. 21 a similar comparison is made, this time for the case of
dissociation of nitrogen gas. The chart shows maximum yield of
nitrogen atoms plotted against nitrogen gas flow rate. Again the
percentage dissociation obtained with the LMP (curve 58) is seen to
be substantially higher than that obtained with a cavity resonator
(curve 57). It is well known that atoms are extremely reactive
chemically; there are numerous processes of industrial interest in
which high yields of atomic oxygen, nitrogen, or atoms of other
molecular gases can be put to good use. For example, it is possible
to form protective oxide or nitride layers on the surfaces of
metals or semiconductors, to synthesize useful organic or inorganic
molecules, and even to obtain laser action by so-called "chemical
pumping." Details of these processes, familiar to those skilled in
the art, need not be given here; suffice it to state that the LMP's
efficiency for producing atoms and other chemically active species
can be highly advantageous in these processes.
The remaining figures represent various possible industrial
applications using the principles of the present invention. It is
well known to persons familiar with plasma chemistry that desirable
characteristics can be imparted to various materials via a plasma
treatment: for example "cross-linking" can be achieved on the
surface of plastics when exposed to a gaseous plasma; plastic
films, e.g. polyethylene and the like treated in this way have
greatly improved bonding and printing characteristics. It is also
possible to graft various molecules to free radical sites created
by plasma treatment; in this manner the dyeability and washability
characteristics of certain textiles, e.g. polyester and other
synthetic materials can be greatly improved. Exposure to a plasma
also has been found to substantially reduce shrinkage of natural
fibres such as wool. Certain organic vapours can be made to form
solid polymer films in a plasma; when a substrate is passed through
the plasma, a layer of polymer which can be made very thin and free
of defects will tend to deposit on it. Such layers are very useful
for various industrial purposes such as encapsulation of electronic
components, protection of surfaces against corrosion, etc.
A most important application of plasmas, however, is to improve the
bonding characteristics of films or fibres of natural or synthetic
polymeric materials or combination thereof. Some experiments have
been carried out in which batches of various such materials were
exposed to microwave plasmas in an LMP apparatus of the type
illustrated in FIGS. 1 and 2. Some typical results of these
experiments are shown in Tables II to IV.
TABLE II
Increase in the water-induced bonding of cellulose strips after
treatment
---------------------------------------------------------------------------
in a microwave plasma
Treatment Bond Gas Pressure time Power strength (torr) (sec)
(watts) (kg/cm.sup.2)
__________________________________________________________________________
Control -- -- -- 1 Argon 45 120 100 113 Argon + 5% Oxygen 20 30 200
80 Air 20 120 200 16
__________________________________________________________________________
TABLE III
Increase in the autohesion of polyethylene strips after treatment
in a
---------------------------------------------------------------------------
microwave plasma
Treatment Bond Gas Pressure time Power strength (torr) (sec)
(watts) (kg/cm.sup.2)
__________________________________________________________________________
Control -- -- -- 0.9 Argon 75 5 1500 1.6 Air 5 10 1500 1.6 Argon +
5% Oxygen 50 20 1500 1.3 Helium 50 1500 1.4 15 Helium + 5% Oxygen
17 90 2500 2.0
__________________________________________________________________________
TABLE IV
Increase in the strength of kraft pulp after treatment in a
microwave
---------------------------------------------------------------------------
plasma
Treatment Gas Pressure time Power Breaking length (torr) (sec)
(watts) (m)
__________________________________________________________________________
Control -- -- -- 880 Argon + 10% Oxygen 20 100 600 1720 Helium + 5%
Oxygen 17 90 25 1490 Air 20 120 200 950 Argon 45 120 100 950
__________________________________________________________________________
Clearly, the treatment of cellulose or polyethylene in the
microwave discharge of an LMP apparatus can produce marked
increases in the bonding ability of the surfaces. FIGS. 22 to 26
illustrate apparatus which can be used to carry out such treatments
on an industrial scale.
FIGS. 22 and 23 show schematically two views of an apparatus
utilizing the principles of the present invention, in which
material in web, film, foil, or plate form can be treated by any
one of the plasma processes mentioned above. FIG. 22 is a frontal
section and FIG. 23 a transverse section through the apparatus,
respectively taken along lines 22--22 and 23--23. Material in the
form of a film 64, say, unrolls continuously from discharge roll 69
to take-up roll 69', as indicated by arrows 71. The incoming
material first passes through a vacuum seal 68 into a lock which is
maintained at reduced pressure by continuous pumping through a tube
70. From the lock the film passes through another seal 68 into the
treatment chamber 66, where a uniform microwave plasma 65 and 65'
is maintained on both sides of the film by slow wave structures 63
and 63', respectively; these are inclined with respect to the plane
of the film, the direction of microwave power flow being indicated
by arrows 16. The linear speed with which the film travels through
the plasma zone is chosen in such a way that the time of exposure
to the plasma is sufficient to give the desired effect. After
treatment the film exits via vacuum seals 68' and another lock 67'
to atmosphere where it is received on roll 69'. The desired gas
purity and pressure inside the treatment chamber 66 is maintained
by a gas supply 59 and a vacuum pump 60 via gas feed lines 61, the
gas flow direction being indicated by arrows 62.
In FIGS. 24 and 25 a similar apparatus is illustrated, except that
here the costly vacuum treatment chamber has been replaced by
suitable plasma containers 73 and 73', of which the film to be
treated 78 makes up part of the vacuum-tight wall. As shown in FIG.
24, which represents a transverse section through the apparatus,
this is accomplished in the following manner: the film 78 is
pressed firmly against vacuum seals 76, which are part of the
plasma container 73, by a cylinder 75 rotating about an axis 79.
Thus the plasma 77 can impinge directly upon one side of the film
surface, which is continuously renewed as the film travels from
discharge roll 74 to take-up roll 74' in the direction indicated by
arrows 81. The other side of the film is treated by a similar unit
also shown. FIG. 25 represents a section along line 25--25 in FIG.
24, in which the film 78 is seen to be in contact with the plasma
77 over its entire width. As before, the plasma is maintained
uniformly by microwave power from the slow wave applicator 72, and
gas flow and pressure conditions in the plasma container can be
maintained as desired via gas flow conduits 80.
FIG. 26 represents an apparatus for the microwave plasma treatment
of material which is not in web or film form, hence not conducive
to treatment by apparatus of the type illustrated in FIGS. 22 to
25. Granular, fibrous, and other types of materials may be treated
in this apparatus which consists of a wide plasma container section
81, in this case mounted vertically, in which a uniform microwave
plasma 82 is maintained by two (or more) slow wave structures 83
and 83'. Again, the direction of microwave energy flow is indicated
by heavy arrows 16. The material to be treated 84 is placed inside
a hopper-vessel 85 which can be closed vacuum-tight by means of a
lid 86. During a treatment cycle, the material passes through the
plasma zone under the action of gravity, and is collected in the
vacuum-tight storage bin 87. Again the desired gas purity and
pressure is maintained in the apparatus by means of a gas supply
88, gas conduits 90, and a vacuum pump 89.
Another possible application of the present invention of industrial
interest is illustrated schematically in FIG. 27: the use of
microwave power for pumping a gas laser. As is well known to those
skilled in the art, numerous gases or mixtures of gases such as
nitrogen, argon, carbon dioxide, carbon monoxide and others, when
excited to a higher state of internal energy, for example by an
electrical discharge, can be made to emit coherent radiation at
particular wave-lengths. The CO.sub.2 system was given particular
attention while adapting the present invention to pumping a gas
laser; the reason was that the infrared emission at 10.6 microns
wave-length of the CO.sub.2 molecule is very useful industrially,
as is well known to those familiar with lasers. Several features of
the present invention are very advantageous from the point of view
of laser technology, particularly in the case of CO.sub.2
lasers:
1. The long length of the plasma column permits a high degree of
amplification.
2. The large plasma volume permits a considerable amount of energy
to be stored and released, and facilitates effective heat
exchange.
3. The high electron temperature to gas temperature ratio improves
the efficiency of the laser by minimizing losses from the system in
the form of low-grade thermal energy.
4. The absence of electrodes, hence absence of erosion problems and
contamination.
5. High stability and uniformity of the microwave plasma.
6. Relatively high operating pressure.
7. Choice of continuous or pulsed mode of operation.
Referring to FIG. 27, a plasma 95 is created inside the plasma
container 91, which is of the transverse flow variety as shown in
FIGS. 12 to 14, by applying microwave energy from a slow wave
structure 6, or from a multiplicity of slow wave structures, for
example as shown in FIG. 15. A suitable transverse flow of gas or
gas mixture is maintained through the plasma container so as to
minimize heating effects. (In the experiments which have been
carried out, a mixture consisting substantially of 80 percent
helium, 10 percent nitrogen and 10 percent carbon dioxide was
used). Laser action is brought about when properly aligned mirrors
are placed at either end of the plasma container: 92 is a fully
reflecting spherical mirror (polished stainless steel, in the
present case), and 93 is a partly reflecting, partly transmitting
plane mirror (coated germanium in the present case), through which
part of the radiation escapes in the form of a narrow laser beam
94. In the present case a forced flow of air was used to cool the
plasma container 91, but circulation of a suitable cooling fluid
could be used as well.
Further features and advantages of the present invention can be
pointed out by referring to the preceding drawings and their
descriptions.
a. The primary advantage of this invention is, of course, the large
plasma volume which allows one to treat materials in industrial
quantities. Although all the examples presented so far pertained to
the 2,450 MHz ISM band (Government-approved frequency for
industrial microwave applications) larger plasma volumes could be
achieved by operating at the 915 MHz ISM band. At this lower
frequency, the characteristic dimensions of the slow wave structure
are greater, and the electric field extends still further out from
the plane of the structure.
There may be other advantages to operating at the next higher ISM
band, 5,800 MHz, such as the use of higher operating pressures. It
is understood, then, that the invention is in no way limited to a
particular microwave frequency range, except as governed by
practical considerations of size of the slow wave structure, which
becomes too large and cumbersome at frequencies substantially lower
than about 500 MHz, and too small at frequencies substantially
above the 5,800 MHz ISM band. Returning to the large volume feature
of the invention, in most chemical applications of microwave
plasmas, an important parameter is the average time, t, which it
takes for a gas molecule to pass through the plasma zone. This
"residence time" is given by
t = AL/F = V/F (4)
where A, L, and V are, respectively, the cross sectional area,
length and volume of the plasma zone, and F is the gas flow rate.
For a given value of t, it is seen that an increase in V permits a
corresponding increase in F, the amount of gas that can be
processed per unit of time.
b. In certain types of chemical syntheses, for example, in the
synthesis of unstable molecules such as hydrazine or acetylene, it
is important that t be kept as small as possible in order to
prevent decomposition by the plasma of the product formed; in such
cases it is preferable to use the transverse flow configuration
illustrated in FIG. 12. An additional advantage of the invention,
then, is the ability to control the residence time for a given gas
throughput.
c. As mentioned earlier, large uniform plasma volumes can be
obtained by operating the slow wave structure in the .pi./2 mode,
and by inclining the plasma tube at an angle .theta. with respect
to the structure, as shown in FIG. 10. Another advantage of this
arrangement is that standing wave patterns which normally arise due
to points of discontinuity along the transmission line are not
likely to be important on account of the efficient coupling.
d. The uniform electric field characteristic discussed in
connection with FIGS. 10 and 11, is not generally a property of
resonant cavity type applicators. In fact, the latter usually have
very inhomogeneous electric field distributions. A uniform field,
however, can be very important, for example, in chemical synthesis
reactions such as chemical conversion (cracking) of hydrocarbon
molecules where many reaction schemes are possible, each requiring
a different energy of activation. Now, the energy distribution of
free electrons, which are primarily responsible for effecting
chemical changes by rupturing chemical bonds, depends very strongly
upon the electric field intensity; if the latter is uniform, the
energy distribution of electrons tends to be uniform, hence the
type of reaction taking place will tend to be more specific, and
conditions can be chosen so as to give a greater yield of a desired
product.
e. Another important advantage of the traveling slow-wave structure
as opposed to resonant type applicators, described for example in
U.S. Pat. No. 3,532,848, is that changes in the dielectric constant
of the work load affect only the total phase change between the
input and the output points, but have little or no effect on the
field amplitude, due to the loose coupling between the slow-wave
structure and the work load (the plasma). In the case of the above
mentioned resonant-cavity applicators, however, where a tight
coupling is necessary between the applicator and the work load,
changes in the dielectric constant can cause considerable amounts
of microwave power to be reflected back to the generator.
f. On the other hand, a traveling wave type slow wave structure
such as the one described in U.S. Pat. No. 3,472,200 is a
"non-radiative," narrow band periodic structure which is normally
non-propagating; the reason for this is that adjacent cells of the
structure are not strongly coupled to each other, for example, by
irises or other coupling means known to those familiar with
microwave slow wave structures. In theory this periodic structure
only propagates energy when a conducting surface is placed close
by, where this surface increases the fringing field capacitance
between the cells; this, in turn, increases the pass-band of the
circuit sufficiently to allow propagation, hence energy transfer to
the work piece. These limitations do not exist in the case of the
broad band, "semi-radiant" traveling wave structure of the present
invention.
g. It is also well known to those versed in periodic structures
that correct functioning of narrow band structures such as the one
described in U.S. Pat. No. 3,472,200 depends very critically on
dimensions: each cell must have exactly the same cut-off resonant
frequency which must lie well within the narrow pass-band of the
structure. As a result, such structures can only conveniently be
manufactured in relatively short lengths, 2 feet or less. This
drawback does not exist in the case of the broad-band travelling
wave structure depicted in FIGS. 4 to 6, which can easily be
manufactured in lengths of 60 inches or more with conventional
machine tools and without the need for highly accurate machining
operations.
h. An inherent advantage of the present invention, embodiments of
which are shown in FIGS. 1 and 2, is the accessibility of the
plasma container 8 for cooling purposes: particularly at higher
pressures, and in the case of exothermic chemical reactions, the
plasma may create sufficient heat to necessitate cooling the plasma
container. This can be accomplished simply by a forced flow of air
from any direction. Usually a longitudinal air flow provided by a
fan or blower 18 is preferred, as the radiation shield 9 acts so as
to guide the flow of cooling air along the plasma container 8. An
alternate method of cooling could be the flow of a liquid having
low dielectric losses, such as Dow Corning Type 200 Dielectric
Fluid, or of a petroleum product known as BAYOL 35, through a
concentric enclosure around the plasma container.
i. A further advantage of the accessibility of the plasma container
8 is that it can be viewed from all sides. This is particularly
important when the plasma is used as a source of electromagnetic
radiation, another industrial use of the present invention which
has many possible applications.
As is well known to those skilled in the art, short life-times,
particularly at power levels in the kilowatt range, are a standard
feature of arc lamps, such as high pressure xenon arcs. The reason
for early failure resides in the presence of electrodes: not only
do the electrode seals with the quartz envelope tend to rupture,
but metal sputters off the electrode, darkens the envelope which in
turn increases heat absorption by the latter, leading to rapid
degradative failure. The present invention, illustrated
schematically by FIG. 10, can be used as an electrodeless source of
radiation; the absence of electrodes removes the problems mentioned
above, lending to this design a practically unlimited life.
Depending on the desired application, several parameters can be
changed: the size and shape of the plasma container (and of the
accompanying slow wave structure), and the gas in the plasma
container. For example, if a small, very bright source of "white"
light is required, one could choose a small slow wave structure
designed to give a very intense electric field, and a filling of
xenon gas in the plasma container. Other requirements could be for
sources having large surface areas and yielding relatively high
amounts of ultraviolet radiation in their spectrum; such sources
could be useful, for example, in photochemistry, for activating
large photo-emitting surfaces, or for other uses. In this case one
might prefer to operate at a lower frequency, for example 915 MHz,
so as to have the largest practically sized slow wave structure;
one might also want to choose a configuration of plasma container
having the largest possible useful surface area, for example a
flat, wide configuration; and one might select a gas, such as
carbon monoxide, or a mixture of gases, whose emission spectrum is
known to consist, to a substantial degree, of wavelengths in the
ultraviolet portion of the spectrum.
Yet another requirement for an electrodeless radiation source, as
embodied by the present invention, may exist in the field of
analytical chemistry. It is well known to those familiar with
spectroscopy that all substances, when excited to a high state of
internal energy as in a plasma, emit radiation of characteristic
wavelengths, by which they may be identified. This can be
particularly useful, for example, in the detection and
identification of trace amounts of impurities in gases, vapours, or
gas mixtures where the absence of possible contamination from
electrodes is particularly important.
It will be understood that various modifications can be made to the
above-described preferred embodiments without departing from the
spirit and scope of the invention.
* * * * *