U.S. patent number 3,872,349 [Application Number 05/390,659] was granted by the patent office on 1975-03-18 for apparatus and method for generating radiation.
This patent grant is currently assigned to Fusion Systems Corporation. Invention is credited to Bernard J. Eastlund, Donald M. Spero, Michael G. Ury.
United States Patent |
3,872,349 |
Spero , et al. |
March 18, 1975 |
Apparatus and method for generating radiation
Abstract
An improved structure for a microwave generated plasma light
source for emitting radiation in the ultraviolet and visible
portions of the spectrum. Microwave energy generated by a microwave
source is coupled to a plasma forming medium which is confined in a
longitudinally extending tube. The tube is surrounded along its
length by a microwave chamber, a portion of which comprises a means
for reflecting the emitted radiation and a portion of which
comprises a mesh-like member which is substantially transparent to
the emitted radiation, but which is relatively opaque to the
microwave energy. The microwave energy may be fed to the microwave
chamber either from the end thereof, or from the top or sides if
the microwave sources is housed in a waveguide located on the top
or sides of the chamber. The plasma forming medium is confined at a
relatively high pressure and the microwave energy is coupled
thereto at a high enough power density to create electron densities
in the plasma in excess of the cutoff density. Electrons are
excited by the transformation of waves and wave absorption and
collide with the heavy particles of the plasma which emit
ultraviolet and visible radiation upon de-excitation.
Inventors: |
Spero; Donald M. (Bethesda,
MD), Eastlund; Bernard J. (Rockville, MD), Ury; Michael
G. (Bethesda, MD) |
Assignee: |
Fusion Systems Corporation
(Rockville, MD)
|
Family
ID: |
26932325 |
Appl.
No.: |
05/390,659 |
Filed: |
August 22, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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239149 |
Mar 29, 1973 |
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Current U.S.
Class: |
315/39;
422/186.3; 313/231.01; 422/186.29; 422/906; 422/186.05 |
Current CPC
Class: |
H01J
65/044 (20130101); Y10S 422/906 (20130101) |
Current International
Class: |
H01J
65/04 (20060101); H01j 007/46 (); H01j
019/80 () |
Field of
Search: |
;313/63,231 ;315/39,111
;250/542 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"High Power Microwave Discharge as an Excitation Source for
Spectroscopic Experiments," by S. Hattori et al., Journal of
Physics E.; Scientific Instruments, 1971, Vol. 4, Printed in Great
Britain..
|
Primary Examiner: Smith; Alfred E.
Assistant Examiner: Chatmon, Jr.; Saxfield
Parent Case Text
This application is a continuation-in-part of U. S. Pat.
application Ser. No. 239,149 filed Mar. 29, 1973, which copending
application is incorporated herein by reference.
Claims
1. A microwave generated plasma light source for emitting a
longitudinally extending sheet of light of arbitrary length
comprising
means for generating microwave energy,
means for coupling said generated microwave energy to a discharge
unit,
said discharge unit comprising a longitudinally extending
non-resonant microwave chamber of an arbitrary length which
encloses a sealed plasma-forming medium containing envelope which
extends in said longitudinal direction in said chamber,
said chamber being comprised of a first longitudinally extending
elliptically shaped reflecting member which is opaque to microwaves
but is light reflective on its inside surface for reflecting light
emitted by said envelope and a second longitudinally extending
plane member joined to said elliptically shaped reflecting member
along the bottom of said reflecting member to form said
chamber,
said second member being substantially opaque to microwaves but
substantially transparent to said emitted light, whereby light
emitted along the entire length of said envelope is reflected as a
sheet of light by said elliptical reflecting member through said
transparent member and
2. The light source of claim 1 wherein said longitudinally
extending envelope is located approximately along the focus of said
elliptical
3. A microwave generated plasma light source for emitting a
longitudinally extending sheet of light of arbitrary length
comprising means for generating microwave energy, means for
coupling said generated microwave energy to a discharge unit, said
discharge unit comprising a longitudinally extending nonresonant
microwave chamber of an arbitrary length which encloses a sealed
plasma-forming medium containing envelope which extends in said
longitudinal direction in said chamber, said chamber being
comprised of a first longitudinally extending shaped reflecting
member which is opaque to microwaves but is light reflective on its
inside surface for reflecting light emitted by said envelope and a
second longitudinally extending member joined to said shaped
reflecting member along the bottom of said reflecting member to
form said chamber, said second member being substantially opaque to
microwaves but substantially transparent to said emitted light,
whereby light emitted along the entire length of said envelope is
reflected as a sheet of light by said reflecting member through
said transparent member and out of said chamber, said means for
coupling comprising at least a microwave guiding enclosure in which
said microwave energy generating means is located, one wall of said
enclosure comprising a portion of said reflecting member, said
portion having at least a slot antenna therein for coupling
microwave
4. The light source of claim 3 wherein said enclosure comprises a
waveguide
5. The light source of claim 4 wherein said microwave chamber is
secured to an annular flange at each longitudinal end thereof to
form an enclosure which prevents the escape of microwaves, and at
least a pair of support
6. The light source of claim 5 wherein holding brackets for
supporting said longitudinally extending envelope are secured in
holes in said flanges, said envelope being formed with mounting
extensions at each end thereof which extend parallel to said axis
of said envelope and which are
7. The light source of claim 4 wherein said energy generating means
is located near one end of said waveguide and said at least a slot
antenna is
8. The light source of claim 7 wherein said at least a slot
antenna
9. The light source of claim 7 wherein said waveguide is situated
so that the long dimension thereof is parallel to the longitudinal
direction of
10. The light source of claim 9 wherein three sides of said
waveguide are comprised of three longitudinally extending members
at right angles to each other, and the fourth side is comprised of
said curved portion of
11. The light source of claim 10 wherein said at least a waveguide
comprises two of said waveguides which are situated parallel to
each other at different portions across the periphery of said
reflecting member, the microwave energy generating means in one of
said waveguides being located near the opposite end from the end
that the microwave energy generating
12. The light source of claim 7 wherein said waveguide is situated
so that the long dimension thereof is perpendicular to the
longitudinal direction
13. The light source of claim 12 wherein the cross-section of
said
14. A microwave generated plasma light source for emitting a
longitudinally extending sheet of light of arbitrary length
comprising means for generating microwave energy, means for
coupling said generated microwave energy to a discharge unit, said
discharge unit comprising a longitudinally extending non-resonant
microwave chamber of an arbitrary length which encloses a sealed
plasma-forming medium containing envelope which extends in said
longitudinal direction in said chamber, said chamber being
comprised of a first longitudinally extending shaped reflecting
member which is opaque to microwaves but is light reflective on its
inside surface for reflecting light emitted by said envelope and a
second longitudinally extending member joined to said shaped
reflecting member along the bottom of said reflecting member to
form said chamber, said second member being substantially opaque to
microwaves but substantially transparent to said emitted light,
whereby light emitted along the entire length of said envelope is
reflected as a sheet of light by said reflecting member through
said transparent member and out of said chamber, said means for
coupling comprising a rectangular to circular waveguide transition
section, the output of said generating means being coupled to the
rectangular portion of the section and the circular portion being
connected to an end of said microwave chamber for coupling energy
into
15. The light source of claim 4 wherein said reflecting member is
elliptically shaped and said second member is a plane mesh-like
member.
16. The light source of claim 4 wherein said reflecting member is
parabolically shaped, the cross-section of said envelope is
semi-circular
17. The light source of claim 4 wherein said reflecting member is
semi-circular in shape, said second member is a mesh-like member
which is semi-circular in shape and the cross-section of said
envelope is circular.
18. A microwave generated plasma light source of arbitrary length
comprising means for generating microwave energy, means for
coupling said generated microwave energy to a discharge unit, said
discharge unit comprising a longitudinally extending non-resonant
microwave chamber of an arbitrary length which encloses a sealed
plasma-forming medium containing envelope which extends in said
longitudinal direction in said chamber, said chamber being
comprised of a first longitudinally extending cylindrical member
which is opaque to microwaves but is light reflective on its inside
surface for reflecting light emitted by said envelope having an
annular cross-section whereby emitted light is reflected into the
annulus of said envelope, said means for coupling comprising at
least a microwave guiding enclosure in which said microwave energy
generating means located one wall of said enclosure comprising a
portion of said reflecting member, said portion having at least a
slot antenna therein for
19. A microwave generated plasma light source of arbitrary length
comprising means for generating microwave energy, means for
coupling and generated energy to a discharge unit, said discharge
unit comprising a microwave chamber which encloses a sealed plasma
forming medium-containing envelope, said chamber being comprised of
a first reflecting member having the shape of a parabolic surface
of revolution, and a second plane mesh-like member which contacts
said first member, said first member being opaque to microwaves but
being reflective on its inside surface for reflecting said emitted
light and said mesh-like member being substantially opaque to
microwaves but substantially transparent to said emitted light,
said sealed container being located at or near the focus of said
parabolic surface and the longitudinal axis of said sealed envelope
being parallel to the axis of revolution of said parabolic
reflector, said means for coupling comprising at least a microwave
guiding enclosure in which said energy generating means is located,
one wall of said enclosure comprising a portion of said reflecting
member, said portion having at least a slot antenna therein for
coupling energy to the interior of said
20. The light source of claim 4 wherein said reflecting member has
a
21. The light source of claim 4 wherein said envelope has mounting
extensions formed thereon oriented in a direction perpendicular to
said longitudinal direction, said reflecting member having mounting
holes therein and said extensions being secured in said holes to
effect the
22. The light source of claim 4 wherein said reflecting member is a
curved reflecting member and said second member is made of
mesh-like material which is substantially opaque to microwaves and
substantially transparent to said emitted radiation, said chamber
further being sealed at each end thereof by a member of said
mesh-like material, whereby the bottom and ends of said chamber are
microwave opaque and emitted light transparent to
23. A plurality of light sources as recited in claim 22 placed with
the mesh-like material at the right longitudinal end of one light
source flat against the mesh-like material at the left longitudinal
end of the adjacent light source to form a continuous straight row
of light sources.
24. A plurality of light sources as recited in claim 23 wherein the
mesh-like material at each longitudinal end forms a plane member
and wherein said plane members of each light source are at an angle
of other than 90.degree. with respect to the longitudinal axis of
the light
25. A high efficiency high power microwave generated plasma light
source for emitting radiation in the ultraviolet range comprising a
plasma-forming medium confined in a longitudinally extending
cylindrical container having a diameter much greater than twice the
classical skin depth at a pressure of from 1 mm Hg to 2 atm.,
microwave energy source means for generating energy in the
microwave range, means for coupling said microwave energy to said
medium in said container so that the power density existing in said
medium is at least 50 watts/cm.sup.3 whereby electrons having
densities far in excess of the cut-off density are created in said
medium and anomalous absorption of said energy occurs at skin
depths greater than C/.omega..sub.p where C is the speed of light
and .omega..sub.p is the plasma frequency, which electrons collide
with heavy particles of said medium to cause said heavy particles
to become collisionally excited and to emit said radiation.
Description
The present invention relates to an improved structure for a
microwave generated plasma light source and to an improved method
and apparatus for efficiently generating high power ultraviolet and
visible radiation.
In recent years, sources of ultraviolet and visible radiation
having emission wavelengths below 5000.degree. A. have found
widespread use in industry in applications such as curing paints
and inks, other coating and surface treatment processes and in the
industrial synthesis of certain chemicals by photochemical
reaction. The light sources of the prior art which have been used
in these processes have generally been limited by their low
efficiencies and unwanted radiation by-products or by their limited
output powers. As discussed in the Specification of co-pending
application Ser. No. 239,149 the prior art light sources are
generally either of the relatively high gas-pressure, DC or low
frequency excited plasma type which may generate power at adequate
levels but which are inefficient in that substantial portions of
the power is in the unusable visible and infra-red portions of the
spectrum, or of the relatively low gas-pressure, low frequency
(approximately 10 Mhz) or microwave (approximately 1 Ghz) excited
plasma type which may produce ultraviolet radiation fairly
efficiently but which are limited as to their operating power
densities and therefore as to their output power. Also the prior
art light sources with electrodes have limited operating lifetimes.
In contradistinction to the prior art light sources co-pending
application Ser. No. 239,149 discloses a microwave generated plasma
light source which produces ultraviolet radiation both highly
efficiently and at a high output power.
In the industrial processes mentioned above, it is desirable to
direct as much of the emitted ultraviolet radiation to the object
being irradiated as possible. From the point of view of economy and
compactness of structure, it is further desirable for the light
source apparatus to be comprised of as few discrete components such
as reflectors, lenses, etc. as possible. According to the present
invention an improved structure for a microwave generated plasma
light source is provided in which a portion of the microwave
chamber also comprises a reflector means for the emitted radiation.
In several embodiments of the invention, a further portion of the
microwave chamber comprises an ultraviolet and visible light
transparent mesh-like member or screen through which the reflected
radiation can be transmitted. The mesh-like member or screen, while
being transparent to the emitted radiation is relatively opaque to
the microwave radiation and is therefore effective to keep the
microwave energy confined in the microwave chamber. Hence a
structure is provided which efficiently focuses the emitted
radiation onto the surface to be irradiated and which does so
without the necessity of employing a separate reflector member.
Additionally, the novel microwave chamber of the invention provides
its additional reflection function without compromising its
properties as a microwave coupling means.
The microwave energy generated by the microwave source can be
coupled to the microwave chamber either by a rectangular to
circular waveguide transition section as disclosed in co-pending
application Ser. No. 239,149, by other means, or according to a
further aspect of the invention, by waveguide means mounted on top
of the microwave chamber. Feeding the microwave power in from the
top as opposed to from the end allows several light sources to be
placed end to end to form a composite light source of a selected
length.
According to a still further aspect of the invention a method and
apparatus for efficiently generating a plasma with microwave energy
to emit ultraviolet radiation at high power levels is provided
which operates without the presence of a magnetic field.
Economic and operational advantages are obtained in the present
source by eliminating the magnetic field and the apparatus for
producing it required by many prior art microwave generated plasma
light sources while retaining the high power and high efficiency
characteristics of the discharge. The microwave energy which is at
a high power density in the medium causes electrons to be generated
in densities exceeding the cut-off density. The electrons are
generated by processes including the collisionless and collisional
transformation of waves and normal and non-linear wave absorption.
The energetic electrons collide with the heavy particles of the
plasma thereby exciting them and the heavy particles emit the
desired radiation upon de-excitation.
It is therefore an object of the invention to provide an improved
structure for a microwave generated plasma light source.
It is a further object of the invention to provide a structure for
a microwave generated plasma light source in which a portion of the
microwave chamber also comprises a means for reflecting the emitted
radiation.
It is still a further object of the invention to provide a
microwave generated plasma light source wherein the microwave
energy is coupled to the plasma forming medium by waveguide means
mounted on the top or side of a microwave chamber which encloses
the container in which the plasma-forming medium is confined.
It is still a further object of the invention to provide a
longitudinally extending microwave generated plasma light source
having a structure which allows the source to be placed end to end
with similar light sources on the right and left ends thereof and
it is an object to provide a plurality of such light sources
positioned end to end to provide a composite light source with a
selected length of continuous spatial extent of ultraviolet
emission.
It is still a further object of the invention to provide a
microwave generated plasma light source which can efficiently
generate high power ultraviolet and visible radiation without the
use of a magnetic field.
It is still a further object of the invention to provide a method
and apparatus for efficiently generating high power ultraviolet and
visible radiation by coupling high power density microwave energy
into plasma forming medium to create electron densities in excess
of the cut-off density by processes including the transformation of
waves and non-linear wave absorption.
The invention will be better understood by referring to the
detailed description below when taken in conjunction with the
drawings in which:
FIG. 1 is a perspective view of an embodiment of a microwave
generated plasma light source according to the invention.
FIG. 2 is an exploded perspective view of the light source of FIG.
1.
FIG. 3 is a side view with portions cut away of the light source of
FIG. 1, additionally illustrating the sample to be irradiated by
the light source.
FIG. 4 is an end view taken at plane A--A of FIG. 3.
FIG. 5 is a schematic drawing of the light source of FIG. 1,
additionally illustrating a cooling opening located in the top of
the microwave chamber.
FIGS. 6 to 10 are schematic drawings of further embodiments of
light sources according to the invention having different
geometrical configurations.
FIGS. 11 and 12 are end and side views respectively of a light
source according to the invention showing how the plasma container
may be mounted.
FIG. 13 is a side-view of a power head which may be used instead of
the microwave source and rectangular to circular waveguide
transition section of FIG. 1.
FIG. 14 is an end view of the power head of FIG. 13 taken through
plane 14--14 of FIG. 13.
FIG. 15 shows an embodiment of the light source shown in FIGS. 19
and 20.
FIG. 16 is a side view of the embodiment of FIG. 15 at plane
16--16.
FIG. 17 shows a further embodiment of the light source shown in
FIGS. 19 and 20.
FIG. 18 is a side view of the embodiment of FIG. 17 at plane
18--18.
FIG. 19 is an end view with portions broken away of an embodiment
of a light source according to the invention.
FIG. 20 is a side view with portions broken away of the light
source of FIG. 19.
FIG. 21 shows a composite light source built of modular units such
as shown in FIGS. 19 and 20 and FIGS. 15 to 18.
FIG. 22 shows another embodiment of a composite light source.
FIG. 23 is a block diagram depicting the steps involved in the
generation of a plasma according to the invention.
FIGS. 1 to 4 show an embodiment of a microwave generated plasma
light source having an improved structure according to the
invention. The essence of the novel structure is a microwave
chamber which functions both as a means for coupling microwave
energy to to the plasma forming medium and as a reflector means for
the emitted ultraviolet and visible radiation.
Referring to the exploded view of FIG. 2 the novel microwave
chamber is comprised of longitudinally extending geometrically
shaped member 17 and mesh, mesh-like surface or screen 22. In the
embodiment shown, member 17 comprises a member of elliptical
cross-section but as discussed in conjunction with FIGS. 6 to 11
the shape of member 17 as well as member 22 may be varied to suit
the particular application. The chamber is constructed so that
member 17 is sealed electromagnetically at the bottom thereof by
mesh 22 thus forming a longitudinally extending chamber which is
opaque to microwave energy and which therefore can be used as a
microwave coupling chamber. Member 17 is arranged to be reflective
on its inside surface, for instance by polishing the inside surface
and mesh 22 is arranged to be substantially light transmissive by
control of the mesh spacing. Hence light emitted as lamp container
19 which is mounted inside the microwave chamber will be reflected
by the inside surface of member 17 and will be transmitted through
mesh 22 to the sample to be irradiated. The member 17 may be made
of aluminum sheet which is polished and anodized on its inner
surface for maximum ultraviolet reflectivity. A polished, anodized
aluminum sold under the trademark Alzak may be used. Mesh sizes up
to one-eighth inch spacing with 0.011 inch copper wire have been
used. However, larger mesh sizes may result in unacceptably high
microwave leakage and optimum mesh size appears to be 1/40 to 1/50
inch using 0.001 inch tungsten wire. This gives excellent microwave
shielding and allows transmission of about 90 percent of the lamp
radiation in the ultraviolet, visible and infrared regions. The
wire mesh has the added advantage of helping to prevent unwanted
materials, such as ink droplets or paper fragments from hitting the
lamp in production line environments.
As shown in FIG. 4, 22 may be somewhat wider than member 17 at the
bottom thereof and the two members may be secured to each other by
conventional means such as soldering or welding. In the embodiment
shown for structural rigidity mesh 22 is secured to side support
bars 9 and 10 and the side support bars as well as top support bar
8 are secured at each end to flanges 5 and 11. As will be apparent
to those skilled in the art, other mechanical means to ensure
structural rigidity may be utilized. The end surfaces of microwave
chamber 17, 22 are adhered to flanges 5 and 11 in a manner which
prevents escape of microwaves, for instance by soldering, welding,
conductive epoxy, or various mechanical means employing slots, lips
or the like.
Tube 19 is formed with mounting extensions 20 and 11 which are
supported by holding brackets 7 and 14 as shown in FIGS. 3 and 4.
Brackets 7 and 14 are inserted in vertically extending slots (not
shown) in flanges 5 and 11 respectively and are retained in
position by collars 24 and 26 respectively which may for instance
be of the set screw type. In the alternative lamp bulb 19 may be
supported in the microwave chamber of the invention by any
conventional support means apparent to those skilled in the art.
Lamp bulb 19 is filled with an appropriate mixture of gasses, is
sealed and is provided with extensions to form supports 20 and 21
which are unfilled and unsealed. The lamp bulb as well as the
holding brackets may be made of quartz. The ends of bulb 19 may be
flat as shown or tapered as described in copending U.S. application
Ser. No. 239,149.
Flange 11 having holes 23 therein is secured to end flange 13 by
bolts extending through holes 23 inflange 11 and holes 13 in flange
12. End flange 12 has a flange collar 15 thereon which forms the
termination for the microwave chamber. Flange collar 15 is designed
with a sufficiently large length to diameter ratio as known to
those skilled in the art to attenuate the escape of microwave
energy and has an inside flange taper 16 to reduce field
enhancement which would otherwise encourage arcing. Additionally it
is noted that flange collar 15 is mounted somewhat above the middle
of flange 12 so that its axis coincides with that of lamp bulb
19.
The plasma light source thus far described actually is a coaxial
microwave in which the outer conductor is the microwave chamber and
the inner conductor is a lossy conductor consisting of the lampbulb
and its contents. One way of coupling microwave energy to the
microwave cavity for interacting with the plasma forming medium is
shown in FIGS. 1 to 3. Microwave energy generator 2 which may be a
magnetron, klystron, other known microwave generator or a standard
microwave power pack including power meters and a tuner, is
provided. In the preferred embodiment of the invention a microwave
frequency of 2450 MHZ is used (915 MHZ can be used if cavity
dimensions are increased) and the generator output is a series of
pulses at a rate of 120 per second. The output of microwave
generator 2 is coupled to a rectangular to circular S-Band
waveguide transition section 3 which terminates in a flange 4.
Flange 4 is secured to flange 5 of the lamp assembly by bolts 25
which are passed through holes 18 and 6 of flanges 4 and 5
respectively. To ensure proper mating flanges, 4 and 5 are arranged
so that the radius of curvature of the top of the opening in flange
5 is approximately the same as the radius of curvature of the top
of the opening of flange 4 and further that the top surfaces of
these openings are positioned next to each other to provide a
smooth transition into the microwave chamber.
As shown in FIGS. 3 and 4 object 28 which is to be irradiated is
moved past the source in a direction perpendicular to the plane of
the paper, for instance by conveying means 29, motion 34 is as
shown. The light source would be mounted on a frame or other
support for instance by securing flanges 5 and 11 thereto. If
member 17 is elliptical in cross-section as shown in embodiment of
FIG. 1 then lamp bulb 19 would be located at or near one focus of
the ellipse while the object 28 to be irradiated would be located
at the other focus. Ultraviolet and visible radiation emitted by
lamp bulb 19 is reflected by the inside reflector surface of member
17 downwardly through ultraviolet and visible light transmissive
mesh 22 and to the surface 28 to be irradiated.
The lamp is ignited by turning on the microwave power. In some
cases, a momentary discharge from a high voltage tesla coil is
required to initiate break-down by means of a wire inserted through
the flange collar 15. In order in flange prevent the wire from
conducting large levels of microwave energy out of the chamber a
resistor in the form of a graphite rod or other material of similar
conductivity may be used. The resistor is inserted into the chamber
but insulated from the chamber walls so its end is near the lamp
bulb. When the starting pulse is applied the resistor transmits the
high voltage, low current pulse to the lamp causing the required
breakdown for starting. Subsequently, the resistor appears as an
insulator to the microwave field and therefore causes no radiation
leakage from the chamber.
Within several minutes after starting the lamp warms up to the
operating wall temperature, which in the case of mercury fills is
approximately 400.degree. C. Once the operating temperature is
reached, if the microwave power pack is properly adjusted, the
reflected microwave power falls to a minimum value and the lamp
operates steadily for as long as it is required. The lamp may be
turned off and then instantly restarted, usually with the aid of
another high voltage spark, as long as its temperature does not
fall so low as to re-condense the mercury.
During operation of the lamp, water cooling is provided for end
flange 12 for the rectangular to circular waveguide transition
section 3 and if desired for reflector member 17. The lamp itself
may be air-cooled by: (1) air fed in through the transition section
and blowing from left to right in the chamber; (2) a direct air
stream blowing from beneath the chamber upward through the wire
mesh and onto the lamp, and/or (3) a suction provided through the
flange collar 15. Depending on the power level of operation, one,
two, or three of the foregoing cooling sources may be used to
prevent the lamp bulb from overheating and melting. Additionally,
or instead of the air cooling described above as is shown in FIG. 5
a cooling slot may be added to chamber member 17. The cooling slot
runs longitudinally along the length of the chamber at the top
thereof. Air 32 may be drawn out of or forced into the slot which
causes air flow around the lamp as shown by the arrows to effect
cooling. The advantage of providing the cooling slot is that
uniform cooling along the entire lamp length is effected.
Additionally, this type of cooling has the advantage of removing
harmful ozone from the vicinity of the lamp which may have been
generated during operation. Alternatively, it provides a convenient
means for blowing inert gas into the lamp chamber and out over the
irradiated surface 28, a desirable procedure in some applications.
A wire mesh 30 choke collar 31 are provided at the slot to prevent
the escape of microwave energy. The choke collar should have a
sufficiently large height to width ratio to prevent the escape of
microwave energy.
FIGS. 6 to 10 are schematic illustrations of variations of the
light source of FIG. 1 in which the cross-sectional areas of the
lampo tube and reflector member are of different geometrical
configurations. While the embodiment of FIG. 1 illustrates a
relatively focused system, in applications requiring large total
ultraviolet radiation doses but not requiring locally high energy
density the unfocused systems shown in FIGS. 6 to 8 are more
appropriate. FIG. 6 shows a reflector 40 having a thickness of
parabolic cross-section which encloses a lamp bulb 41 of
semicircular cross-section. The microwave chamber is completed by
plane mesh member 42 and the sample to be irradiated would pass
underneath and parallel to mesh 42.
FIG. 7 illustrates a further embodiment in which the thicknesses of
both reflector 43 and mesh 45 are of semi-circular cross-section,
together forming a microwave cavity of circular cross-section. The
lamp bulb 44 is also of circular cross-section and has a diameter
nearly equal to the diameter of the cross-section of the chamber.
In the embodiment of FIG. 7, the sample to be irradiated would pass
underneath mesh 45.
FIG. 8 shows a further embodiment utilizing a lamp bulb 47 of
annular cross-section and a reflector 46 having a thickness of
circular cross-section. In this embodiment, the sample to be
irradiated is passed within the cut-out portion 48 of the annular
lamp bulb. Further, in this embodiment to prevent the electrical
properties of the material being treated from affecting lamp
operation it may be desirable to add an inner cylindrical mesh
inside of and concentric with the inner lamp wall.
FIGS. 9 and 10 show an embodiment for producing a parallel
collimated beam of radiation. Parabolic point-source lamp 50 shown
in side view in FIG. 9 and in end view in FIG. 10 is located inside
of reflector 49 having inside and outside surfaces which are
parabolic surfaces of revolution and which is sealed by mesh 51.
Light emitted by bulb 50 is reflected in parallel fashion from
reflector 49 and through mesh 51. It should be understood that
different reflector/lamp bulb geometries may be used depending on
the particular application and are considered to be within the
scope of the invention.
A further mounting technique for the lamp bulb is shown in FIGS. 11
and 12. In the embodiments of these FIGS., lamp bulb 53 is formed
with extensions 54 extending in the radial direction, one of which
is shown in the drawings. Extensions 54 are mounted to reflector
member 52 by set screw means 55, 56 and at least two such
extensions would be provided for mounting the lamp bulb. The
advantage of the mounting configuration illustrated in FIGS. 11 and
12 is that it allows the lamp bulb to extend very nearly to the end
of the microwave chamber at both ends. This is of particular
importance in the embodiment shown in FIG. 21 discussed below.
Additionally, a support means comprising three dielectric rods
extending from three different points on the periphery of the
reflector may be used to support the lamp. The rods would be of
equal length and each rod would terminate in a support bracket for
the lamp at its inner end.
Instead of coupling the microwave energy to the microwave chamber
by a rectangular to circular transition section as shown in FIG. 1,
it may be desirable to mount the microwave power tube in a
microwave power head which bolts directly to the lamp chamber. FIG.
13 is a schematic illustration of such a microwave power head which
it is seen is comprised of circular waveguide section 65, flange 68
for connection to the lamp chamber flange, magnetron 66 mounted
inside the power head, magnetron anode, filament, cooling, and
magnet structures 61, and connection cables 62 for connection to a
remotely located high voltage and filament power supply. Also
provided is a quarter wave tuning stub 63 with the direction of
motion indicated by the arrow which may be used instead or in
addition to a quarter wave fixed reflector 67. Flange 68 would be
secured to flange 5 in FIGS. 1 to 4 to attach the power head to the
microwave chamber. The waveguide 65 may be varied in size and shape
for convenience in mating to the lamp chamber and for optimum
microwave tuning. Instead of utilizing a magnetron in the power
head, a klystron or other microwave power tube may be utilized.
For increased lamp power, a power head can be mounted at both ends
of the lamp chamber. In this case, for improved power tube
lifetime, it is preferable to arrange the outputs of the 60 cycle
power tube power supplies so that the output microwave power also
at 60 cycle repetition rates occurs on alternate half-cycles for
each of the two tubes. It is also possible to increase the power by
mounting a second power tube in the same power head at a different
axial location or on top of the waveguide section pointing
downward. In this case also, running the power tubes on alternate
60 cycle pulses will improve their lifetimes.
Additionally, increased lamp power and a more unifrom light output
(in time) can be obtained by mounting two or three power heads on
one chamber. If each power supply is hooked to different phases of
a three phase power supply (120 electrical degrees out of phase)
and a full wave rectification system is used for each magnetron
power supply (120 microwave pulses per second, each pulse 60
electrical degrees out of phase with an adjacent pulse) then a
smoothing effect will take place. In addition, it appears that a
lag of several m sec occurs in ionizing the plasma at the onset of
each microwave pulse. If microwave pulses are only 60 electrical
degrees apart then this lag should be substantially reduced and the
net system efficiency increased.
FIGS. 19 and 20 illustrate a further embodiment of the invention in
which the microwave power is coupled into the top of the microwave
chamber as opposed to into an end thereof. An advantage of this
mode of microwave coupling is that a plurality of light sources can
be positioned in end-to-end fashion to form a composite light
source of desired length. Referring to FIGS. 19 and 20 lamp bulb
100 is located within a microwave chamber comprised of reflecting
member 101 of elliptical cross-section and mesh 102 which form a
chamber similar to that illustrated in FIGS. 1 to 4. The chamber of
FIGS. 19 and 20, however, unlike the chamber of FIGS. 1 to 4 is
terminated at each end thereof by a mesh surface such as 120 shown
at the left end. Hence, the reflector 101 is sealed
electromagentically by mesh surfaces at the bottom and at each end
thereof. Lamp bulb 100 is mounted in the chamber preferably by the
method illustrated in FIGS. 11 and 12 so that the lamp bulb can
extend nearly to the end of the chamber at each end.
Cooling slot 108, mesh, 109, and cut-off collar 110 run
longitudinally along the top of reflecting member 101 as described
in conjunction with FIG. 5. A waveguide section 102 having three
longitudinally extending sides at right angles to each other is
mounted on top of the reflector member. The fourth side of the
waveguide is the reflector member itself which has coupling slot or
slots 116 therein to allow microwave energy to enter the chamber.
The waveguide section contains the magnetron dome 105 and possibly
quarterwave tuning plate 104. The position, size and shape of the
coupling slots in the reflector may be varied to obtain maximum
coupling of microwave power into the lamp. Additionally, a tuning
capability can be obtained by providing a mechanical means such as
a sliding slot cover for adjusting the slot sizes. The magnetron
body 106 containing anode, filament, magnets and cooling structures
is mounted above the waveguide. The most compact form of magnetron
cooling is water cooling, but air cooling can be used if necessary.
Magnetron power and cooling connections 114 are fed to external
power supply and controls. Lamp bulb cooling is provided by air
flow 115 through duct 113. In FIGS. 19 and 20 slot 108 is made
accessible to the cooling air flow by constructing portions of
waveguide section 103 out of screen mesh or otherwise ventilating
the waveguide. The entire module is enclosed by an outer metal
casing 107 which facilitates cooling by suction or blowing, offers
mechanical protection, and provides microwave shielding for the
module. Where the module is to be used as one module in a composite
light source as shown in FIG. 21 the housing at the ends of each
module would cover the entire end except for the end meshes which
would be left uncovered so that the emitted radiation could pass
therethrough. The embodiment may also include pre-heater elements
112 which may be nichrome wires through which current is passed
when the lamp is in the quiescent OFF state so that the lamp will
start instantly. In the operation of the device, microwave power
generated by magnetron 105, 106 is coupled through coupling slot
116 to the microwave chamber 101, 102, where it interacts with the
plasma forming medium contained in lamp bulb 100 to cause the
medium to emit radiation.
Two high power embodiments of the light source of FIGS. 19 and 20
are shown in FIGS. 15 and 16, and 17 and 18 respectively. In FIGS.
15 and 16, dual waveguide structures 73 and 78 are positioned
parallel to the lamp axis at different azimuthal positions on
reflector member 70. In FIGS. 15 to 18 the magnetron bodies such as
106 in FIG. 19 which would be mounted on top of the waveguide
sections are not shown. Waveguide 73 contains magnetron dome 74 and
the energy generated thereby coupled to the chamber through slots
75 and 76 in the reflector member shown in FIG. 16. Waveguide 78
includes magnetron dome 79 and the energy generated thereby is
coupled to the chamber through slots 80 and 81. As may be seen in
FIG. 16, magnetron dome 74 is located in a relatively forward
position of the waveguide and its associated coupling slots 75 and
76 are located in a relatively rearward position of the reflector
member while magnetron dome 79 is located in a relatively rearward
position of its waveguide and associated coupling slots 80 and 81
are located in a relatively forward position of the reflector
member. The distance between the magnetron dome and its associated
coupling slots tends to be roughly half a wavelength of the
microwave radiation. Similarly in FIGS. 18 and 20.
In the embodiment shown in FIGS. 17 and 18, the waveguide
structures 86 and 89 are positioned at different axial positions
and are curved azimulthally around reflector member 82. Waveguide
89 includes magnetron dome 90 and is associated with coupling slots
91 and 91' in one side of the reflector member while waveguide 86
includes magnetron dome 88 and is associated with coupling slots 87
and 87' in the other side of the reflector member. In the
embodiments of FIGS. 15 to 18, half-power lamp operation is
obtainable by turning off one of the two magnetrons. Higher power
operation is obtainable in any of the embodiments of FIGS. 19 and
20 and 15 to 18 by mounting more than one magnetron in a waveguide
structure.
According to a further aspect of the invention a plurality of lamps
of the type illustrated in FIGS. 19 and 20 and FIGS. 15 to 18 are
placed in end-to-end relationship with each other to form a light
source of a selected length. Each lamp is then a modular unit and
the length of the composite light source may be varied by
increasing or decreasing the number of modular units used. This
type of system is made possible by the fact that the microwave
energy in the embodiments of FIGS. 19 and 20 and FIGS. 15 to 18 is
fed to the lamp from the top of the microwave chamber as opposed to
from one of the ends and such a composite light source is
illustrated in FIG. 21. End mesh sections 151, 152 and 153 and 154
are placed in end to end relationship and the solid end plates 156,
157, 158 and 159 of the housings such as housing 107 in FIG. 19
extending above the mesh are bolted rigidly together to form a
single rigid light source in which it is possible for light emitted
by one lamp, for instance 160 to pass through the end mesh 151 of
that lamp into the chamber of the adjacent lamp and to possibly be
reflected by the reflecting member 161 of the adjacent lamp onto an
irradiated surface. The effect is thus similar to that obtained by
a single lamp bulb and a single reflector of a length equal to the
composite length of the modular units. Each modular unit would be
fed with power from a different power supply and all of the power
supplies would be mounted together in a single cabinet located
remotely from the lamp. Similarly, all of the modular units would
be operated simultaneously by a single master control unit.
FIG. 22 illustrates a further embodiment of a composite light
source in which end mesh sections 181, 182, 183 and 184 are angled
with respect to the axis of the bulb. The technique of canting the
end mesh sections as shown has the effect of eliminating regions of
diminished emitted ratiation which may tend to exist at the regions
of the end meshes in the embodiment of FIG. 21. The outer housings
186 of the modules in FIG. 22 need not be shaped as shown and the
end section 187 could be perpendicular to the lamp axis instead of
canted relative thereto. Other modifications of a composite light
source will be obvious, such as replacing multiple exhaust ducts
171, 172, 173 and 174 in FIG. 21 with a single duct, while also
replacing solid end plates 156, 157, 158 and 159 with screened
sections.
The advantage of using modular units as is shown in FIGS. 21 and 22
instead of a single long light source is that a substantial savings
in power supply component costs and engineering production costs
results. Thus, power supply components are much cheaper at the 700
to 3000 watt levels than at the 5 to 30 kilowatt levels and it has
been determined that a lamp system of approximately 3 kilowatts and
10 inches length may be optimum from the cost/power viewpoint.
Also, if 10 inch lamp modules are joined end to end to form useful
commercial lamp systems in lamps of 30 inches, 40 inches, 60
inches, etc., then the number of modules manufactured will be 3, 4,
6 etc. times the number of lamp systems manufactured and design and
engineering costs will be reduced because only one product, instead
of a number of products need be produced. Furthermore, production
costs will decrease because larger numbers of components will be
purchased from vendors. Also, the modular lamp units have the
advantage of having no end overhand of the lamp housing (space
between the end of the lamp bulb and the end of the chamber), which
facilitates installation of lamps on certain production lines.
Finally, a composite light source built up of modular units affords
quick and easy servicing by permitting the removal of a faulty
module with the lamp bulb inside and replacement of the entire unit
instead of having to utilize a procedure which involves handling of
lamp bulbs on the production line.
While the improved structure of the lamp disclosed in FIGS. 1 to 22
may be utilized with different plasmas, in a preferred embodiment
of the invention a novel and improved plasma resulting in the
production of high-power ultraviolet radiation not requiring a
magnetic field is utilized. The plasma is generated by interacting
high power density microwave energy with a mixture of gases
maintained at a relatively high pressure to create electron
densities in the plasma which are in excess of the cut-off
density.
The theory of operation of the lamp as it is presently understood
is described with reference to the block diagram of FIG. 23. As
used above, as well as in the discussion below, by the term
"plasma" is meant a partially or highly ionized gas composed of
atomic or molecular particles having one or more orbital electrons
removed and thus constituting ions, together with a sufficient
number of free electrons to balance the electrical charge of the
ions, so that the resultant plasma is substantially electrically
neutral. By the term "cut-off density" is meant the minimum density
of electrons which in "classical" microwave plasmas leads to
reflection of microwave power.
The choice of gases to be used as a fill for the lamp is determined
by the exact spectral output that is desired. In a preferred
embodiment of the invention, a mixture of mercury and a xenon
background fill gas is used. The mixture of gases is confined in
the lamp at a pressure of between 1 torr and 2 atm. This is a
relatively high range of pressures compared to these discussed in
copending application Ser. No. 239,149. A consequence of these high
operating pressures is the fact that collisional phenomena are
involved in generating the plasma and the electromagnetic
radiation. In the operation of the lamp, microwave energy is
coupled to the plasma forming medium by any of the modes heretofore
described. The dimensions of the lamp, the output power of the
microwave generator, and the microwave coupling mode are arranged
so that microwave energy at a power density of greater than 50
watts /cm.sup.3 is coupled to the plasma forming medium. The high
power density interacting with the high pressure gas creates
extremely high electron densities in a large volume of the gas. The
electron densities in portions of the gas actually exceed the
cut-off density by as much as 100 or 1000 times. Equivalently, the
frequency .omega. of the input microwave energy is less than the
plasma frequency .omega..sub.p where .omega..sub.p =
.sqroot.4.pi.ne.sup.2 /m and where n is the number density of
electrons per cubic centimeter and e and m are the electron charge
and mass, respectively. As n increases .omega..sub.p also
increases. The value of n at which .omega..sub.p becomes equal to
.omega. is termed the cut-off density, because for .omega..sub.p
>.omega., "classical" plasmas reflect all the incident microwave
energy, and consequently result in a limitation of the absorbed
power. In the present plasma, the extremely high electron densities
which result in the high ultraviolet power outputs are possible
because of two unique non-classical effects which are discussed
below in conjunction with the block diagram of FIG. 23.
The first of these effects is the collisional and collisionless
transformation of waves which is effective to transform the
electromagnetic waves in the plasma to electrostatic plasma waves,
which are in turn damped by collisional and collisionless processes
thereby exciting electrons. The second effect is "anomolous" or
"non-linear" wave absorption by which the electromagnetic waves are
directly damped resulting in the absorption of power levels not
possible in "classical" plasmas. The electrons which are energized
by the transformation of waves and wave absorption phenomena
collide with the heavier atoms and ions of the plasma thereby
exciting them and the heavy particles radiate the ultra-violet and
visible radiation in the process of de-excitation.
Describing the operation of the plasma system in greater detail,
electromagnetic energy is coupled into the plasma forming medium
and the waves flow into the less dense regions of the plasma until
they reach regions where the electron density is just below the
cut-off density for the microwaves. In these regions, the
transformation of waves phenomenon occurs and the electromagnetic
waves are transformed into electrostatic plasma waves which
propogate on the surface or through the volume of the plasma.
The transformation of waves phenomenon is discussed in co-pending
U.S. Pat. application Ser. No. 239,149 which application disclosed
the generation of a lower pressure plasma in the presence of a
magnetic field. Because of the lower pressure of the plasma in the
co-pending application, the transformation of waves in that
application was referred to as "collisionless transformation of
waves." In the present application, because of the higher pressures
involved, collisons will play a greater part in the generation and
energizing of electrons and the transformation of waves will no
longer be "collisionless" but as predicted by theory will be both
collisionless and collisional. It was shown in the co-pending
application that transformation of waves for the case B = 0, where
B is the magnetic field, would take place over the range 0 <
.omega. < .omega..sub.p, and in the present application where B
= 0 this equation is also valid notwithstanding the fact that
plasma operates at a higher pressure and collisions occur.
The plasma waves produced by the transformation of waves phenomenon
propagate further into the plasma and are slowed down and have
their energy absorbed through "collisionless Landau damping" as
discussed in the co-pending application, and because of the higher
pressures involved also through collisional damping. The effect of
either damping mechanism is to dissipate the energy introduced by
the high power electromagnetic waves into local regions of the
plasma in the form of excited electrons uniformly throughout the
plasma.
The second mechanism by which electrons are excited in the present
plasma system occurs simultaneously with the transformation of
waves and is referred to as normal and non-linear wave absorption.
These effects involve the direct collisional damping of the
electromagnetic waves in the plasma. This damping takes place in
the lower density regions of the plasma where the density is less
than the cut-off density or equivalently where the microwave
frequency is greater than the plasma frequency, and is effected by
electrons in these regions being set in motion by the
electromagnetic waves and obtaining thermal energy by randomizing
collisions with background gas atoms and ions.
"Normal" collisional wave absorption has heretofore been observed
and analysed. It occurs in regions of the plasma where the electron
density is equal to the cut-off density (or the microwave frequency
is equal to the plasma frequency) over depths within the plasma of
the order of C/.omega..sub.p , the plasma skin depth, where C is
the speed of light. In a plasma at high electron densities (so that
in most of the plasma volume, the density is greater than the
cut-off density) this means that the absorption occurs in a narrow
sheath or skin near the outside of the plasma column. A consequence
of this is that, in a given plasma system, as the average electron
density of neutral gas pressure is increased, the absorbed
microwave power will reach a maximum and then decrease because the
skin depth becomes smaller. The "normal" or "linear" wave
absorption has been observed at moderately high microwave powers
(greater than 10 (watts/cm.sup.3)) and these observations have
discouraged consideration of very high power densities of gas
pressures.
The present invention, however, by working at higher power density
levels than have generally been used in the prior art has harnessed
the effect of "anomalous" or "non-linear" wave absorption to result
in increased power absorption at depths greater than
C/.omega..sub.p. While all of the phenomenon involved in
"anomalous" wave absorption are not yet fully understood, the
physical origin of the absorption appears to be due to the fact
that at very high microwave power densities the incident
electromagnetic waves can themselves have a strong effect on the
plasma. Thus, for example, high power microwave energy can cause
additional ionization in a high pressure plasma. This in turn
increases the electron density at a given point and changes the way
that the plasma responds. In one theoretical analysis of such a
situation it was calculated that the skin depth for wave absorption
is increased from (C/.omega..sub.p) to C/.omega..sub.p .times.
(.nu./.omega.).sup.1/2 .times. (Teo/I).sup.1/3 where .nu. is the
gas collision frequency, Teo is the electron temperature of the
undisturbed plasma, and I is the ionization potential of the gas.
These factors can considerably enhance the skin depth and therefore
the plasma uniformity and the fraction of incident energy finally
absorbed by the electrons. This is particularly true when a gas of
relatively low ionization potential such as mercury is used.
The electrons in the plasma which are excited by both
transformation of waves and normal and non-linear wave absorption
collide with the heavy particles of the plasma including atoms and
ions and thereby excite the heavy particles. The desired
ultraviolet and visible radiation is then emitted by the heavy
particles during the process of de-excitation.
The resulting plasma thus results in the high efficiency production
of ultraviolet and visible radiation at high power levels without
the necessity of using a magnetic field.
While we have disclosed and described the preferred embodiments of
our invention, we wish it understood that we do not intend to be
restricted solely thereto, but that we do intend to include all
embodiments thereof which would be apparent to one skilled in the
art and which come within the spirit and scope of our
invention.
* * * * *