Method and apparatus for generating electromagnetic radiation
United States Patent 3911318

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.

Spero, Donald M. (Bethesda, MD)
Eastlund, Bernard J. (Rockville, MD)
Urv, Michael G. (Lanham, MD)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
250/493.1, 250/504R, 376/123, 376/140, 422/186.05, 422/186.29, 422/186.3, 422/906
International Classes:
B01J19/12; H01J65/04; H05H1/46; (IPC1-7): H01J7/46; H01J19/80
Field of Search:
315/39,111,111.2 25
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US Patent References:

Primary Examiner:
Chatmon Jr., Saxfield
Attorney, Agent or Firm:
Browne, Beveridge, Degrandi & Kline
Parent Case Data:

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.
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:

2. means for generating microwave energy, and

3. 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.

4. The light source of claim 1 wherein said mesh is made of copper.

5. 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.

6. 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.

7. 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.

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° and 600,000°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 ω 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:

ωce ≤ ω ≤ ω1 } (1)


ωci ≤ ω ≤ ω2 }

where ωce and ωci are the electron and ion cyclotron frequencies respectively, and are defined by ##EQU1## and where ##EQU2## in which definitions ωp is the electron plasma frequency, defined by ##EQU3## and α is the angle between the magnetic field and the gradient in density (and it is assumed that ##EQU4## and finally ωu is the upper hybrid frequency, defined by:

ωu =√ωce2 + ωp2

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 ω 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< ω< ωpe 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, ω , of the incident microwaves is equal to the electron cyclotron frequency, ωce , in the plasma, or to a harmonic of it. In the present process, no such restrictions on ω 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, ω=ωce 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 α 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%O2 ; 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 O2 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 10-5 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, Bo 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, Bo, 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.