U.S. patent number 3,911,318 [Application Number 05/439,173] was granted by the patent office on 1975-10-07 for method and apparatus for generating electromagnetic radiation.
This patent grant is currently assigned to Fusion Systems Corporation. Invention is credited to Bernard J. Eastlund, Donald M. Spero, Michael G. Urv.
United States Patent |
3,911,318 |
Spero , et al. |
October 7, 1975 |
Method and apparatus for generating electromagnetic radiation
Abstract
There is disclosed a novel method and apparatus for generating
high power electromagnetic radiation in the ultraviolet and visible
regions of the electromagnetic spectrum wherein high energy
electrons, produced by directing a lower frequency (radio to
microwave) electromagnetic energy wave into a plasma producing
medium (gas, vapor and mixtures thereof), are caused to collide
with heavy particles of the medium to thereby cause same to be
collisionally excited and subsequently emit electromagnetic
radiation. Particular means are disclosed for coupling the lower
frequency electromagnetic wave energy to the medium. In a preferred
embodiment the medium is confined in a closed vessel or chamber at
a selected pressure to which a magnetic field is applied to guide
the high energy electrons to collision with said heavy
particles.
Inventors: |
Spero; Donald M. (Bethesda,
MD), Eastlund; Bernard J. (Rockville, MD), Urv; Michael
G. (Lanham, MD) |
Assignee: |
Fusion Systems Corporation
(Rockville, MD)
|
Family
ID: |
26932324 |
Appl.
No.: |
05/439,173 |
Filed: |
February 4, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
239149 |
Mar 29, 1972 |
|
|
|
|
Current U.S.
Class: |
315/39;
250/493.1; 376/123; 422/186.05; 422/186.3; 250/504R; 376/140;
422/186.29; 422/906 |
Current CPC
Class: |
B01J
19/124 (20130101); B01J 19/126 (20130101); B01J
19/122 (20130101); B01J 19/129 (20130101); H01J
65/046 (20130101); H05H 1/46 (20130101); H01J
65/044 (20130101); B01J 2219/1227 (20130101); B01J
2219/0894 (20130101); Y10S 422/906 (20130101) |
Current International
Class: |
B01J
19/12 (20060101); H01J 65/04 (20060101); H05H
1/46 (20060101); H01J 007/46 (); H01J 019/80 () |
Field of
Search: |
;315/39,111,111.2
;250/493,504,542 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chatmon, Jr.; Saxfield
Attorney, Agent or Firm: Browne, Beveridge, Degrandi &
Kline
Parent Case Text
This is a division of application Ser. No. 239,149, filed Mar. 29,
1972, now abandoned. su
Attention is directed to the disclosure of the following references
for an exposition of background technology leading to a better
understanding of the present invention:
1. K. S. Golovanivskii and V.D. Dugar-Zhabon, "High Frequency Low
Pressure Discharge at the Electron Cyclotron Resonance," Sov.
Shys.--Tech. Phys. 16, 75 (July, 1971).
2. M.V. Krivosheev, "Production of a High-Density Plasma by
Microwaves in a Magnetic Field," Sov. Phys.--Tech Phys. 15, 1805
(May, 1971).
3. V.E. Golant and M.V. Krivosheev, "Anomalous Absorption of
Microwave Power by a Plasma at Supercritical Electron Densities,"
Sov. Phys.--Tech. Phys. 14, 719 (November, 1969).
4. V.E. Golant, M.V. Krivosheev, and V.I. Fedorov, "Linear Wave
Transformation and Absorption in a Plasma of Small Transverse
Dimensions," Sov. Phys.--Tech. Phys. 15, 282 (August, 1970).
5. A.I. Anisimov, N.I. Vinogradov, V. Ye Golant, S.I. Nanobashvili,
and L.P. Pakhomov, "Super-High Frequency Generation of a Strongly
Ionized Argon Plasma," AEC-tr-7056 of A.F. IOFFE Physico-Technical
Institute Report No. 113, 1968.
6. V.N. Budnikov, N.I. Vinogradov, V.E. Golant, and A.A. Obukhov,
"Investigation of a Steady-State Microwave Discharge in a Magnetic
Field," Sov. Phys.--Tech. Phys. 13,19 (July, 1968).
7. A. I. Anisimov, N.I. Vinogradov, V.E. Golant, S.I. Nanobashvili.
and L.P. Pakhomov, "Determination of Plasma Parameters Produced by
Super High-Frequency Power in a Magnetic Field in a Steady-State
Condition," AEC-tr-7052 of IOFFE Institute Report, 1968.
8. A. I. Anisimov, N.I. Vinogradov, V.E. Golant, and L.P. Pakhomov,
"Microwave Production of a Plasma in a Trap," Sov. Phys.--Tech.
Phys. 12, 486 (October, 1967).
9. V.N. Budnikov, N.I. Vinogradov, V.E. Golant, and A.A. Obukhov,
"Plasma Produced by Electron Cyclotron Resonance I. Absorption of
Microwave Power," Sov. Phys.--Tech. Phys. 12, (November, 1967).
10. V.N. Budnikov, N.I. Vinogradov, V.E. Golant, "Plasma Produced
by Electron Cyclotron Resonance, II Charged Particle Balance," Sov.
Phys.--Tech. Phys. 12, (November, 1967).
11. V.N. Budnikov, V.E. Golant, and A.A. Obukhov, "Absorption of
Microwave Power by a Plasma at Magnetic Fields Above the Cyclotron
Frequency," Sov. Phys.--Tech. Phys. 15, 97 (July, 1970).
12. A. I. Anisimov, V.N. Budnikov, N.I. Vinogradov, V.E. Golant,
S.I. Nanobashvili, A.A. Obukhov, A.P. Pakhomov, A.D. Piliya, and
V.I. Federov, "Ultra-High Frequency Plasma Heating in a Magnetic
Field," IAEA Conference on Plasma Physics and Controlled Fusion
Research, Novosibirsk (August, 1968), paper CN-24/J-3.
13. v.n. budnikov, V.P. Gorelik, V.V. D'yachenko, K.M. Novik, and
A. A. Obukhov, "Microwave Discharges at Harmonics of the Electron
Cyclotron Frequency," Sov. Phys.-Tech. Phys. 16, 404 (September,
1971).
14. B. V. Calaktionov, V.V. D'yachenko, and O.N. Shcherbinin, "High
Frequency Plasma Heating Near the Lower Hybrid Frequency," Sov.
Phys.-Tech. Phys. 15, 1809 (May, 1971).
15. B.V. Galaktionov, V.E. Golant, A.D. Piliya, and O.N.
Shcherbinin, "Plasma Absorption of RF Energy Near the Lower Hybrid
Frequency," Sov. Phys.-Tech. Phys. 14, 721 (November, 1969).
16. A.D. Piliya and V.I. Fedorov, "Linear Wave Conversion in an
Inhomogeneous Magnetoactive Plasma," Soviet Physics JETP 30, 653
(April, 1970).
17. V.I. Arkhipenko, A.B. Berezin, V.N. Budnikov, V. Ye Golant, K.
M. Novik, A.A. Obukhov, A.D. Piliya, V.I. Fedorov, and K.G.
Shakovets, "Studies of the Transformation and Adsorption of
High-Frequency Waves in a Plasma for the Purpose of Developing
Plasma Heating Techniques," IAEA Conference on Plasma Physics and
Controlled Nuclear Fusion Research, Madison, Wisconsin, June, 1971,
paper CN-28/L-4.
18. a.i. anisimov, N.I. Vinogradov, V.E. Golant, and L.P. Pakhomov,
"Absorption of Electromagnetic Waves in a Plasma at Frequencies
Near Harmonics of the Electron Cyclotron Frequency," Sov.
Phys.-Tech. Phys. 12, 141 (July, 1967).
19. V. N. Budnikov, V.E. Golant, and A.A. Obuchov, "The Study of
Microwave Absorption by a Plasma in a Magnetic Field," 31A, 76
(Jan. 26, 1970).
20. B.V. Galaktionov, V.E. Golant, V.V D'yachenko, and O.N.
Shcherbinin, "Determination of the Limiting Frequency for Plasma
Absortpion of High Frequency Waves at Frequencies between the
Electron Cyctron Frequency and the Lower Hybrid Frequency," Sov.
Phys.-Tech. Phys. 15, 1813 (May, 1971).
21. V.E. Golant and A.D. Piliya, "Linear Transformation and
Absorption of Waves in Plasma", Uspekhi Fiz. Nauk 74, No. 3
(1971).
22. G. Lisitano, M, Fontanesi, and E. Sindoni, Applied Physics
Letters, 122, Feb. 1, 1970.
23. G. Lisitano, P. Caldirola, N, Barassi, M. Fontanesi, and E.
Sindoni, IAEA Conference on Plasma Physics and Controlled
Thermonuclear Research, Novosibirsk, USSR, 1968.
24. schlag, E.W. and Comes, F.J., "Intense Light Sources for the
Vacuum Ultraviolet II", J. Opt. Sci. Am., Vol. 50, No. 9, p.
866.
25. Wilkinson, P.G., "New Krypton Light Source for the Vacuum
Ultraviolet", J. Opt. Sci. Am., Vol. 45, No. 12, p. 1044.
26. Wilkinson, P.G. and Tanaka, Y., "New Xenon-Light Source for the
Vacuum Ultraviolet" J. Opt. Soc. Am., Vol 45, No. 5, p. 344.
27. Dieke G.H. and Cunningham, S.P., "A New Type of Hydrogen
Discharge Tube", J. Opt. Sci. Am., Vol. 42, No. 3, p. 187.
28. Okabe, H., "Intense Resonance Line Sources for Photochemical
Work in the Vacuum Ultraviolet Region", J. Opt. Sci. Am., Vol. 54,
No. 4, p. 478.
29. Minnhagen, L., "Review of Methods for the Excitation of Atomic
and Ionic Spectra by Means of High-Frequency Discharges and Sliding
Sparks", J. Res. Natl. Bur. Std. (U.S.), Vol. 68C, No. 4, p.
237.
30. Warneck, P., "A Microwave-Powered Hydrogen Lamp for Vacuum
Ultraviolet Photochemical Research", Appln. Opt., Vol. 1, No. 6, p.
721.
31 McCarroll, B., "An Improved Microwave Discharge Cavity for 2450
MHz", Rev. Sci. Instr., 41, 279, (70).
32. Worden, E.F., Gutmacher, R.G. and Conway, J.G., "Use of
Electrodeless Discharge Lamps in the Analysis of Atomic Spectra",
Appl. Opt., Vol. 2, No. 7, p. 707.
33. Fehsenfeld, F.C., Evenson, K.M. and Broida, H.P., "Microwave
Discharge Cavities Operating at 2450 MHZ ", Rev. Sci. Instr., Vol.
36, No. 3, p. 294.
34. Tuma, D.T., "A Quiet Uniform Microwave Gas Discharge for
Lasers", Rev. Sci. Instr., Vol. 41, No. 10, p. 1519.
35. Gleason, W.S. and Pertel, R., "High Stability Electrodeless
Discharge Lamps", Rev. Sci. Instr., Vol. 42, No. 11, p. 1638.
36. Dodo, Taro, et al., "Electron Cyclotron Resonance Heating
Device", U.S. Pat. No. 3,431,461, Mar. 4, 1969.
37. Omura, Itiro, et. al., "Microwave Plasma Light Source", U.S.
Pat. No. 3,541,372, Nov. 17, 1970.
This invention relates to a novel method and apparatus for
efficiently generating high power ultraviolet and visible radiation
with a high degree of control over the range of frequencies
produced.
Sources of ultraviolet and visible radiation, having wavelengths of
less than 5,000 A, are extensively used in industry for curing
paints and inks, in other coating and surface treatment processes,
and in the industrial synthesis of certain chemicals by
photochemical reactions. Present sources of such radiation are
generally limited by their low efficiencies and unwanted radiation
by-products, or by their limited output powers. Existing large
industrial ultraviolet sources are based on plasmas produced by DC
or low frequency electrical discharges. The plasmas generated are
at relatively high gas pressures (about 1 mm of mercury to about 1
atmosphere), and low plasma temperatures (about 5,000.degree. to
10,000.degree.K). These sources may produce several hundred watts
of ultraviolet radiation but large fractions of their radiation are
in the visible and infrared portions of the spectrum. The
considerable power which is radiated in the visible and infrared
regions represents an inefficiency for the ultraviolet source and
also is often deleterious to the materials whose treatment by
ultraviolet radiation is desired. Moreover, in such cool dense
plasmas the electromagnetic energy is converted into kinetic energy
of the atoms and ions as well as the electrons, whereas it is
principally the energetic electrons which are responsible for the
production of ultraviolet radiation. Hence, the kinetic energy
unavoidably invested in ion and neutral atom motion represents
another limitation to the efficiency of such devices as ultraviolet
sources. Finally, such devices provide limited control over the
frequencies of radiation produced.
Another category of commonly used ultraviolet sources operates at
lower gas pressures and may be based on low frequency (.about.10
MHz) or microwave (.about.1 GHz) discharges (Refs. 2437). They
operate at relatively higher electron energies and may produce
ultraviolet radiation fairly efficiently, either directly by
emission or through the use of fluorescent materials. However,
because of the nature of the discharge plasma employed, such
devices are severely restricted as to their operating power
densities (i.e., the average number of watts of input
electromagnetic power which can be absorbed in a given volume of
plasma). This limits the total power of ultraviolet radiation that
can be obtained from a source of a given size. In addition, the
size and hence the power of these devices may be rigidly limited by
the methods, such as resonant cavity and waveguide structures.
employed for coupling microwave energy into the plasma.
Furthermore, the microwave coupling structures often constrain
useful viewing geometrics of such devices. Because of these
limitations in ultraviolet power and geometry, this class of
devices is not used extensively in industrial applications,
although they are employed in analytical chemistry.
The method and apparatus disclosed herein will permit the efficient
production of selected ranges of ultraviolet radiation (for
example, between 1800 and 2500 Angstroms in wavelength) at high
power levels which are of great importance in industrial processes
such as photon induced crosslinking of polymers, free radical
formation, and chemical synthesis by photochemical reactions.
Accordingly, it is an object of this invention to provide an
apparatus and method for producing ultraviolet and visible
radiation using a microwave generated plasma.
Another object of this invention is to provide apparatus and method
for efficiently producing ultraviolet radiation at very high power
densities and extracting the radiation in suitable geometric
configurations.
A further object of this invention is to provide an apparatus and
method for producing ultraviolet radiation in selected wavelength
regions without generating comparable powers of visible and
infrared radiation.
Claims
What is claimed is:
1. A microwave generated light source for both producing and
emitting light in the visible to ultraviolet region along a
substantial length of a longitudinally extending plasma forming
medium containing envelope, comprising; a sealed, longitudinally
extending plasma forming medium containing envelope, means for
exciting said plasma forming medium in said envelope to generate a
plasma and produce said light, said means for exciting
including:
1. means for generating microwave energy, and
2. microwave chamber means for coupling said generated microwave
energy to said plasma forming medium, a part of said chamber means
extending in the longitudinal direction of said envelope and
surrounding said envelope at least along a substantial portion of
the length of the envelope, and at least part of said chamber means
being made of a metallic mesh whereby said microwave energy is
retained in said chamber means while said light produced along a
length of said envelope is emitted out of said chamber means
through said mesh.
2. The light source of claim 1 wherein said mesh is made of
copper.
3. The light source of claim 1 wherein said mesh is surrounded by a
shield of solid, ultraviolet and visible transmissive material to
prevent cooling gas which may be circulated around said envelope in
the vicinity of said mesh from escaping.
4. The light source of claim 1 wherein said medium is of the type
which may be collisionlessly excited and said means for exciting
comprises means for collisionlessly exciting said medium.
5. The light source of claim 4 wherein said envelope is tapered at
each longitudinal end thereof, further including microwave energy
dividing means for dividing said generated microwave energy into
two parts, said chamber means coupling each part to a respective
longitudinal end of said envelope.
Description
The above and other objects, advantages and distinguishing features
of the invention will become apparent from the following
specification, when considered with the following drawings,
wherein:
FIG. 1 is a schematic block diagram illustrating the method
disclosed herein,
FIG. 2 is a diagramatic structural illustration of a preferred
embodiment of the invention for generating ultraviolet
radiation,
FIG. 3 illustrates the location and relative intensity of the
magnetic fields with respect to the heated plasma,
FIG. 4 is a diagramatic illustration of a modification of the
invention incorporating power coupled to the plasma tube or vessel
from two ends and a UV reflector for treating a surface or
material,
FIG. 5 is a diagramatic illustration in cross section of a further
modification wherein microwave power is coupled to an annular
plasma tube through the center of which flows the material to be
irradiated by ultraviolet light,
FIG. 6a discloses an alternative structure for coupling microwave
energy with the plasma tube, and
FIG. 6b discloses a structure wherein the microwave energy is
coupled radially inward to the plasma tube and allows working
access to both ends of said vessel.
The present invention produces ultraviolet radiation by efficiently
generating a magnetized plasma, at relatively low gas densities but
high input power densities, in which the electron temperature may
be varied between 10,000.degree. and 600,000.degree.K. These highly
energetic electrons subsequently collide with cooler heavy
particles of the medium: atoms, molecules or ions. It is these
inelastic excitation collisions and the subsequent radiative
de-excitations of the heavier particles of the plasma producing
medium which produce the large amounts of ultraviolet radiation.
These heavy particles of the plasma producing medium include one or
more of the particle species of atoms, ions and molecules. By the
proper choice of operating parameters, as will be discussed below,
such plasma media continuously emit up to many hundreds of watts of
ultraviolet radiation without appreciable amounts of visible and
infrared radiation. Moreover, by appropriately changing the
operating conditions, such as the microwave power level and/or the
composition and/or pressure of the gas or gases from which the
plasma is formed, the radiation can be made to be emitted
elsewhwere in the spectrum from the visible to, in principle, the
x-ray region. However, in the preferred practice of this invention,
operation in the UV region of the spectrum is particularly
preferred.
In order to clarify the description of the present invention, it is
of interest to note some definitions, particularly as here
employed. By "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 subsequently
electrically neutral. The plasma utilized in the present invention
is generated by a technique known as "collisionless transformation
of waves". This term refers to the process by which the energy in
electromagnetic waves (which, in the present example, represents
power flowing from the microwave generator), is efficiently
transferred within some region of a magnetized plasma, into
electrostatic or longitudinal plasma waves. The energy of such
plasma waves, in turn, is rapidly transferred into the kinetic
energy of the plasma electrons by collective loss mechanisms such
as the well-known Landau damping or collisionless absorption
process. Theoretical and experimental analysis of collisionless
transformation of waves indicates that the process is efficient in
local regions of the plasma in which the index of refraction
becomes infinite for the incident electromagnetic waves. This
determines ranges of frequency .omega. for the incident
electromagnetic energy which will be effective at heating the
plasma electrons for a given set of local plasma parameters. The
frequency ranges (Refs. 6-21) are:
.omega.ce .ltoreq. .omega. .ltoreq. .omega..sub.1 } (1)
or
.omega.ci .ltoreq. .omega. .ltoreq. .omega..sub.2 }
where .omega.ce and .omega.ci are the electron and ion cyclotron
frequencies respectively, and are defined by ##EQU1## and where
##EQU2## in which definitions .omega..sub.p is the electron plasma
frequency, defined by ##EQU3## and .alpha. is the angle between the
magnetic field and the gradient in density (and it is assumed that
##EQU4## and finally .omega.u is the upper hybrid frequency,
defined by:
.omega.u =.sqroot..omega.ce.sup.2 + .omega.p.sup.2
In these expressions, n is the number density of electrons per
cubic centimeter; me and mi the electron and ion masses,
respectively; e the electron charge, C the speed of light, and B
the magnetic field strength. The units are CGS Gaussian.
It should be noted that if the frequency .omega. of incident
electromagnetic energy is considered as fixed, the expressions (1)
define a range of magnetic field strengths B for which electron
heating will occur. The expressions predict that as B is reduced to
zero, a finite frequency range 0<.omega.<.omega.pe still
exists for collisionless transformation of waves. The frequency
ranges defined by (1) are calculated by a somewhat simplified, or
"linearized" theory. It is expected that nonlinear effects, such as
high electromagnetic power levels, may tend to broaden the
frequency range in which efficient transformation of waves may
occur.
If the conditions in some region or regions of the plasma are such
that the incident electromagnetic wave frequency lies in either of
the regions defined by (1) above, an efficient transformation of
energy from the electromagnetic waves into electrostatic or plasma
waves, can occur in that region. These plasma waves, in turn, will
carry the energy out of the regions and be themselves converted by
collective loss porcesses such as Landau damping and other
collisionless effects into energetic electrons throughout the
plasma. The energetic electrons then collide with cooler heavier
particles (atoms, molecules and ions), and transfer energy to their
internal excitation. Finally, this energy is released in the form
of deexcitation radiation. The distribution of this radiation among
the ultraviolet, visible, and other parts of the spectrum is
determined by the energies of the electrons in the plasma, as well
as by the types of heavy particles (atoms, ions and molecules)
present.
Several features of the processes described above will be
mentioned. First, the plasma heating is a collisionless process in
which collective plasma behavior is responsible for converting the
incident electromagnetic (microwave) energy to random kinetic
energy of the electrons. This is in distinction to conventional low
frequency or DC discharge lamps in which collisional heating of the
gas by an electric current produces the ultraviolet emission. It is
also distinguishable from non-magnetized electrodeless discharges
in which the directed energy imparted to the electrons by the
electromagnetic fields is converted to kinetic energy by
collisions. Thus, the present process is relatively insensitive to
initial gas density or temperature. Since the present technique is
not collision dependent it can also be used in plasmas which are
already highly ionized where collisional methods would be
ineffective. Thus unusually high power densities of incident
microwave radiation can be effectively absorbed.
Second, although this process commonly makes use of a magnetic
field in the plasma region, it can easily be distinguished from the
more conventional electron cyclotron resonance heated (ECRH)
discharges. In these latter plasmas, the magnetic field, B, and
external electromagnetic power generator must be adjusted so that
the frequency, .omega. , of the incident microwaves is equal to the
electron cyclotron frequency, .omega.ce , in the plasma, or to a
harmonic of it. In the present process, no such restrictions on
.omega. or B exist, provided the more general conditions of (1) are
met. Thus, for example, lower values of magnetic field than those
required by the cyclotron resonance heating condition,
.omega.=.omega.ce are sufficient for the present process, down to
and including B=0. Further, the desired value of magnetic field
need only be achieved over the local regions of heating, which
regions have dimensions much smaller than the wavelength of the
incident electromagnetic radiation (Ref. 17). These facts result in
a considerable simplification in the design of a practical plasma
radiation source. A third feature to be noted about the process of
the present invention is that it depends geometircally only on the
angle .alpha. between the magnetic field and the density gradient,
but not on the direction or polarization of the incident
electromagnetic waves. All three of the preceeding features are in
sharp contrast to the requirements of other plasma heating
techniques such as microwave cavity resonance or ECRH. These
features serve to differentiate the collisionless wave
transformation process and also permit flexibility in the design of
a radiation source.
The manner in which the plasma heating process described above is
employed to produce a unique ultraviolet source will become more
clear from a consideration of a flow or operational block diagram
in FIG. 1. The input electromagnetic energy, in the form of
microwave radiation, is generated by a conventional source 10 such
as a magnetron, and is transmitted by conventional waveguides or
coaxial transmission lines to the chamber in which the plasma is to
be made. The coupling of the microwave power to the gas in the
plasma chamber can be accomplished by a variety of techniques and
configurations.
A particularly useful configuration is shown in FIG. 2, which is a
schematic of a preferred embodiment. It uses a waveguide to
circular transition section 11 fitted over a tapered section 21 of
the plasma tube 22. The tapered section 21 provides a region of
rapidly changing plasma density in which the angle between the
density gradient (i.e., the direction of maximum rate of change of
density) and magnetic field can be adjusted so as to satisfy
equation (1) above. This then permits efficient transformation of
the microwave energy to plasma wave energy in that region. The
plasma waves propagate out of that region and rapidly convert their
energy into electron kinetic energy. The heated electrons then flow
throughout the plasma region causing collisional excitation of
atoms, ions or molecules (e.g., the heavy particles in the plasma
forming medium) and subsequent deexcitation by the emission of
ultraviolet radiation. In practice, the ultraviolet radiation is
accompanied by other radiation in the visible and infrared regions,
whose relative intensities are generally minimized by proper design
of the system. However, in cases where it is desirable these same
design principles may be employed to produce intense, efficient
light sources at particular wavelengths in the visible region, as
well as or instead of the ultraviolet.
After being produced, the ultraviolet radiation passes out of the
discharge tube, and by means of reflectors or lenses is directed
onto the region to be irradiated. In this configuration, the
ultraviolet can be emitted from the length of the discharge vessel.
The intensity and spectral quality of the radiation is relatively
insensitive to the gas pressure in the vessel 22, the frequency of
power of the microwave radiation, or the magnitude of the magnetic
field over certain ranges of these parameters. However, particular
adjustments of these parameters within the allowed ranges can be
employed to optimize the intensity, efficiency and spectral
distribution of emitted ultraviolet radiation. The broad operating
range of the device is a decided advantage over conventional
methods of producing microwave plasma light sources, and in the
practical utilization of such devices.
The plasma vessel 22 or tube, is usually circularly shaped and made
out of quartz or fused silica to permit the ultraviolet radiation
to escape. Straight section 12 of length designated B is permitted
to protrude into the rectangular to circular microwave waveguide
transition section 11 having inside diameter, D, and may be any
size that will fit in waveguide transition section 11. Straight
section 12 of plasma tube 22 is followed by a conically tapered
section 21 of length designated A, protruding still further into
waveguide transition section 11. Conically tapered section 21 is a
convenient way of providing a region of density gradient in which
collisionless transformation of waves can occur. While a conical
section is disclosed herein specifically, any section providing a
region of density gradient in which collisionless transformations
can occur can be used, including rounded tips and other shapes. In
typical operation quartz tubing of 1 inch OD and 1 mm wall
thickness is used as the plasma tube, and the various lengths are
A=3 inches, B=6 inches, C=30 inches and diameter D=3 inches. The
plasma tube is filled with a preselected gas vapor, or mixture of
gases constituting the plasma forming medium and sealed off.
Typical fills are gases such as air, oxygen, and xenon, mixtures
such as 90%CO plus 10%O.sub.2 ; metal vapors such as mercury,
cadmium, zinc, and antimony, and other vapors such as phosphorus
and iodine. The metals may be introduced by using their inherent
vapor pressure (e.g., mercury) or their high vapor pressure
compounds, by using heated filaments coated with the desired metal,
or by placing a small quantity of the pure metal in the tube and
allowing it to be heated by a background plasma of hydrogen,
helium, neon, or some other gas. In the case where a pure gas is
used, the gas pressure is sufficient to sustain the microwave
plasma generation, and the radiation produced is characteristic of
the atoms, ions, or molecules present in the plasma. A mixture of
gas such as CO plus O.sub.2 may be used in order to create a
chemical equilibrium in the plasma which prevents material from
coating the walls of the tube during operation. Another
modification is to use majority gas with either a second minority
gas or a metal vapor which is present in much smaller quantity, in
which the minority gas or vapor is the principal source of
ultraviolet radiation, and the function of the majority gas is
principally to create the proper plasma conditions for excitation
of the minority atoms or ions.
Typical gas fill pressures range from 1 to 100 millitorr for
optimum performance, with limited operation possible throughout the
pressure range of 10.sup.-.sup.5 to 5 torr. The choice of gases and
pressures is determined by the spectral output that is desired. The
choice of plasma tube material is primarily governed by the
requirement that as much ultraviolet radiation as possible be
transmitted through the tube wall in the desired spectral region. A
second requirement is that the wall material exhibits minimal
dielectric loss for the incident microwave energy. The plasma tube
22 may have any diameter which fits inside the circular waveguide
diameter D. Large tube diameters are used in cases where the
maximum ultraviolet radiation intensity (in watts per inch of lamp
length) is desired.
The plasma tube is surrounded by a concentric copper mesh screen 25
or any conducting surface which acts as an extension of the
circular portion of the transition section 11. The copper mesh
waveguide extension 25 serves to prevent microwave radiation
leakage outside its cylindrical volume and to redistribute that
fraction of the incident microwave power which is not absorbed in
the vicinity of the tapered end 21 of the plasma tube 22. This
redistribution causes further microwave energy absorption along the
length of the plasma tube which improves the axial uniformity of
the emitted radiation. The waveguide transition section 11 is
separated from the concentric screen 25 by a waveguide flange plate
26 with a hole 27 to permit the plasma tube to fit through. The
size of the hole 27 and the thickness of the flange plate 26 may be
varied in order to vary the distribution of microwave power between
the transition section region and the remainder of the extended
concentric waveguide mesh or screen. A second waveguide flange
plate 28 is used to terminate the concentric screen waveguide 25.
To prevent microwave radiation from escaping at that end, the hole
29 in the plate 28 must be made only slightly larger than the tube
diameter and a short metal tube 30 is inserted over the discharge
tube 22 and through the flange plate hole 29. Typically, this metal
tube may be 1 inch ID and 2 inches long for a plasma tube of 1 inch
OD. Alternatively, this tube may have any larger ID which is
convenient and its end may be terminated with a metallic cap to
prevent microwave leakage.
In the single ended operation as in FIG. 2, plasma tube 22 is
supported by a collar clamped to the tube by set screws (not shown)
at the right end and cantilevered so as to pass through the center
of hole 27 in flange 26. Hole 27 is preferably much larger than the
outside diameter of plasma tube 22 to permit more of the microwave
energy to flow from transition section 11 to the extended, screen
mesh wave guide section 25, thereby improving the axial uniformity
of the emitted ultraviolet energy. The edges of hole 27 are rounded
as shown to minimize electric field enhancement and resultant
arcing. In cases where the screen mesh 25 is sufficiently opaque to
the incident microwave radiation, flange 26 may be entirely
dispensed with by increasing the diameter of hole 27 until it
equals dimension D of the transition section 11.
At microwave input power levels in excess of 200 watts certain
precautions may be needed to cool the system. Water cooling coils
(not shown) may be required on waveguide transition section 11, and
the second waveguide flange plate 28. It is sometimes necessary to
flow gas around the discharge tube 22 itself to prevent excessive
heating of the quartz wall. A way of effecting this is to install a
lucite, glass, or quartz shield 31, concentric with the main
discharge tube 22. It may also be used to hold the mesh screen 25
in place (and in some cases the mesh may be formed or plated on the
inner surface of shield 31). The shield material must be chosen to
transmit the desired wavelength regions if the lamp's radiation is
to be used outside the shield. A gas such as nitrogen may then be
flowed into the transition section 11 through the annular space
between the shield 25 and the discharge tube 22 and out the second
waveguide flange plate 28. The flow rate may be adjusted to provide
adequate cooling. Proper choice of gas and shield material may also
serve to control unwanted photochemical processes caused by the
lamp. For example, nitrogen gas prevents ozone formation around the
lamp. Other gases can be used to effect surface changes caused by
the lamp's radiation if the material to be irradiated is either
placed within the shield or passed beneath an opening cut in the
shield.
The electromagnetic power which excites the discharge is produced
by a conventional microwave generator 10 such as a commercial type
2450 MHz magnetron supply. Other microwave frequencies, such as 915
and 119 MHz, and radio frequencies, such as 19 MHz (Ref. 21) may
also be used. This applied electromagnetic power may be continuous
or pulsed. The microwave power is transmitted to the transition
section through suitable lengths of the waveguide 40 or high power
coaxial waveguide. The microwave power transmitted to the discharge
tube and reflected back toward the microwave generator may be
monitored by power meters (not shown). For efficient coupling into
the plasma, the reflected power must be minimized. For a particular
set of operating conditions this is most easily accomplished by
adjustment of the magnetic field. A microwave tuner (not shown) may
also be used to minimize the reflected power and an isolator (not
shown) may be used to limit the power reflected back toward the
microwave generator.
The axial magnetic field is produced by a pair of water cooled
Helmholtz coils 45 and 46 which are powered by a DC power supply
(not shown). The variation of the axial magnetic field with axial
position is shown in FIG. 3. The precise variation of the magnetic
field is not important for the operation of the lamp, nor is the
absolute magnitude of the peak magnetic field, provided only that
the conditions of equation (1) are met in some volume. Operation at
zero magnetic field is found in some cases to be possible, as
predicted by equation (1). However, of the wide range of magnetic
fields over which the lamp operates, the region of optimum power
coupling (i.e. minimum reflected microwave power) usually occurs
when, simultaneously with satisfying conditions (1), the microwave
frequency is approximately equal to the electron cyclotron
frequency somewhere in the lamp volume, and preferably inside the
waveguide transition section. The design of the magnetic field,
besides creating conditions for collisionless transformation of
waves, acts to guide the heated electrons throughout the quartz
plasma tube. For this reason, the peak in magnetic field, B.sub.o
should be in the near or to the left of the region of plasma
heating R, as indicated in FIG. 3, so that the heated plasma, which
tends to flow to regions of lower magnetic field, will move out
into the tube 22. The magnetic field also acts to minimize the loss
of electron kinetic energy to the walls of the plasma tube, thus
contributing to the ability for low temperature wall operation.
Other magnetic field configurations and means for producing same,
such as a single turn high current helical conductor extending the
length of the plasma tube may also be used. By changing the spatial
structure of the magnetic field, or fields, the spatial
distribution of emitted radiation from plasma tube 22 may be
changed to obtain optimum radiation distribution.
In the operation of the microwave plasma ultraviolet light source,
cooling water and gas flows are first turned on. The DC magnet
power supply is then turned on and the magnet coils 45 and 46 are
energized. The current from the power supply is adjusted to provide
the desired value of axial magnetic field. Typically, the power
supply will provide 350 amps (DC) at 20 volts (DC) to produce a
peak magnetic field, B.sub.o, on axis of 1000 gauss and a minimum
field of 500 gauss with two coils (45 and 46) of 24 inch separation
and 19 inch mean diameter. To ionize or excite the gas in plasma
tube 22, the microwave power source 10 is activated to supply
microwave radiation to the gas within the vessel by means of
waveguide 40 and transition section 11. Typically, the microwave
power source provides an average power output of 500 to 2000 watts
at a frequency of 2450 MHz (which is a standard FCC allowed
industrial frequency). In practice, the magnetic field strength may
be adjusted in order to ignite the plasma tube, or a Tesla coil or
other high voltage generator may be used to initiate the plasma.
The microwave power levels into and reflected from the plasma tube
may be measured by means of a dual directional coupler and
microwave power meters (not shown). The efficiency of operation of
the system can be adjusted by adjusting the position of the plasma
tube in the transition section (as indicated by arrow X in FIG. 2),
or by varying the magnetic field intensity or distribution, or
other parameters in such a way as to minimize the reflected
microwave power, as measured on the power meter, and maximize the
power transmitted to the plasma.
The magnetic field coils and plasma tube geometry are selected to
produce the desired length and shape of ultraviolet emitting tube.
Special materials may be required for the plasma tube, or for
windows in it, in order to permit ultraviolet radiation to secape
with minimal attenuation. Thus, for example, sapphire windows might
be used to allow the emission of 1700 A radiation. The emitted
radiation can be focused and directed with suitable optical
equipment such as mirrors, lenses, or reflectors 60 and 65 (FIG.
4). The specific form of the auxiliary optical system would depend
on the application as well as the desired wavelength. For example,
a system which seeks to utilize 1800 A radiation would have to be
evacuated or filled with a gas such as argon to prevent excessive
absorption of the radiation by gas along the optical path, between
the lamp and the material to be irradiated. Alternatively, the
material could be placed within the plasma tube itself, which
thereby serves directly as a reaction vessel for ultraviolet
radiation.
Although the invention has been described above with reference to a
preferred embodiment, it will be apparent that other modifications
may be made within the scope contemplated by the invention. For
instance, the microwave generator output may be split by means of a
power divider 61 (FIG. 4) and the power fed into the discharge tube
from waveguide transition sections 62, 63 at both ends of the tube
22. This would tend to provide a more uniform ultraviolet emission
from a long plasma tube. Such a modification is shown schematically
in FIG. 4 where a UV reflector 60 is also shown to illustrate one
way of concentrating the ultra-violet energy on a surface to be
irradiated. The reflector may also be constructed by depositing a
coating of high UV reflectivity on the inside of the screen
waveguide 25' around roughly half of its circumference. Another way
is to use a lens system 65.
A further modification utilizing microwave power coupled in from
two ends 76, 77 of a microwave chamber 74 is shown in FIG. 5. It is
applicable to a large annular plasma vessel which is useful for
photochemical applications. the material to be irradiated would be
caused to pass or flow along the axis of the inner annular vessel
72, typically of quartz, and thereby would be surrounded by
ultraviolet emitting plasma. The outside or outer vessel 78 could
have a coating of high UV reflectivity material on its outer
surface, or the inner surface 75 of the microwave chamber 74 could
carry the reflective coating. In the embodiment shown in FIG. 5 the
ends 70 and 71 of the plasma vessel are tapered to replace full
conical sections. The embodiment of FIG. 5 includes metallic tubes
79 of length l and inside diameter d at both ends of the microwave
chamber. The aspect ratio l/d must be sufficiently large and d
sufficiently small compared to the microwave wavelength that no
appreciable amounts of microwave energy leak out of the cavity.
As a final example, other techniques may be employed for coupling
the microwave energy into the plasma vessel. One particular
technique involves the use of a rectangular waveguide section 80
directly, as shown in FIG. 6a. In this approach a standard section
of rectangular waveguide is terminated (capped) at one end and a
circular hole is cut to accept the plasma tube. A limitation to
this approach is that the tube diameter cannot exceed the minimum
waveguide dimension, h. A further advantageous microwave coupling
technique uses an axially or helically slotted slow wave structure
85 which fits directly over the plasma tube such as the one
illustrated in FIG. 6b and described in detail in references 22 and
23. In this case, a shielded coaxial cable 86 is used to convey
microwave energy to the slotted coil 85. Alternatively, a
waveguide, properly terminated may be used to couple energy to
slotted coil 85. For low frequency electromagnetic input radiation
(2-200 MHz), a simple antenna such as a single turn copper strap
(Ref. 21) can be used to couple to the plasma tube. For these
elctromagnetic coupling schemes and others, such as that shown in
FIG. 5, which do not require termination of the plasma vessel at
its end, the plasma vessel may be bent around and joined one end
upon the other to form a closed ring or toroid.
The invention herein described is to be construed to be limited
only by the prior art when considered by the spirit and scope of
the appended claims.
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