US 3702973 A
Apparatus for and a method of producing controlled discharges substantially throughout a large volume of a gaseous medium by generating in an enclosure a controlled density of free electrons in the medium and controlling the electron temperature of the free electrons to a level preventing a substantial increase in their density by a self-regenerative ionization process so that for a wide range of uniformity of both the density and temperature of the medium, a stable and controlled discharge is produced in the medium suitable for the intended use of the medium.
Description (OCR text may contain errors)
United States Patent 11 1 3,702,973
Daugherty et al. Nov. 14, 1972  LASER OR OZONE GENERATOR IN 3,576,583 4/1971 Uno ..313/74 WHICH A BROAD ELECTRON BEAM 3,641,454 2/1972 Krawetz ..33 1/945 WITH A SUSTAINER FIELD PRODUCE A LARGE AREA, UNIFORM OTHER PUBLICATIONS DISCHARGE Dumanchin et al.: Comptes Rendus, vol. 269,
November, 1969, pp. 916- 917.
 Inventors: Jac g y, winChester; Beaulieu: DREV Memorandum M-2005/7O January,
Diarmaid H. Douglas-Hamilton, 1970 Boston; Richard M. Patrick,
Winch er; n g g- Primary Examiner-Ronald L. Wibert 311 Of Mass- Assistant ExaminerEdward S. Bauer  Assigneez Avco Corporation, Cincinnati Ohio AtzorneyCharles M. Hogan Melvin E. Frederick  Filed: Sept. 17, 1970  ABSTRACT  Appl. No.: 72,982 Apparatus for and a method of producing controlled discharges substantially throughout a large volume of 521 US. Cl ..331/945 PE, 204/176 204/313 a gasews medium by generating in enclmre controlled density of free electrons in the medium and controlling the electron temperature of the free electrons to a level reventing a substantial increase in 51 111 11. (:1 ..H0ls 3/00 their density by apselfqegenerative ionization prosess Field Of Search 3 1 3 1 l 1; so that for a range f uniformity of both the 204/ 316; 310/1 1 sity and temperature of the medium, a stable and controlled discharge is produced in the medium suitable 204/3l6, 310/11, 313/74, 3l5/lll, 331/945 G References Cited for the intended use of the medium.
UNITED STATES PATENTS Apparatus for and the method of producing a 2,030,492 2/1936 Applebaum ..313/74 X Charge n accordance w1th the 1nvent1on 1s useful for the production of lasting action, electrically con- 2,686,275 8/1954 Cohen ..313/74 (motive ionized as for use in ma etch dmd namic 3,403,353 9/1968 Lamb, Jr. et al ..331/945 (MHD) devicesg and the like, f to g or 3,555,451 1/1971 W1tte etal. ..331/94.5 facilitae carrying out chemic a1 processes Such as, for 2,333,842 1 1/1943 Casc1o et al. ..313/94 x example, the generation of ozone and the like. 2,373,661 4/1945 DePhillips ..313/74 2,429,217 10/1947 Brasch ..313/74 x 29 Claims, 6 Drawing Figures TO VACUUM PUMP GAS FLOW T0 FILAMENT POWER SUPPLY 30 T0 E SUSTAINER V cmcun 53 commuous R PULSED F LOW PATENTEDnnv 14 1972 3,702 973 SHEET 2 or 3 LASER OUTPUT commuous GAS OR FLOW "PuLsED FLOW I 63 LASER OUTPUT 5 mg E JACK 0. DAUGHER TY DIARMAID DOUGLAS-HAMILTON PM? RICHARD M. PATRICK SUS'ITEINER 5| EVAN CIRCUIT INVENTORS 4&4 s3 i 5 ATTORNEYS sum 3 or 3 T N E M A H F 0 T POWER SU PPLY JACK D. DAUGHERTY' DIARMAID DOUGLAS- HAMILTON RICHARD M. PATRICK EVAN R. PUGH LASER OUTPUT a F F F 5 F GAS FLOW ATTORNEYS LASER OR OZONE GENERATOR IN WHICH A BROAD ELECTRON BEAM WITH A SUSTAINER FIELD PRODUCE A LARGE AREA, UNIFORM DISCHARGE The present invention in its broadest sense is directed to the production of and apparatus for providing useful controlled discharges in a gas at pressure levels and volumes such that discharge stabilization by electron pair diffusion to confining walls is negligible, that is, the discharge is not wall dominated.
In one embodiment, the invention may comprise means for increasing if not providing the desired electrical conductivity of the gaseous working medium in MHD devicessuch as generators and accelerators. It is equally applicable to other devices and the like that require or use electrically conductive or ionized gas.
In another embodiment, the invention comprises means for producing ozone wherein the working medium may comprise oxygen or air which is passed through a discharge comprising an independent source of electrons and an electric field in accordance with the invention. Since the electric field is decoupled from the production of electrons optimum conditions for ozone formation are attainable without severe requirements on ballasting as present in the use of a Townsend discharge, or on electrode geometry as present in the use of corona discharge. Because uniform conditions are provided in the positive column, the overall energy efficiency is increased and heat dissipation involved in the process is reduced. In a still further and preferred embodiment, the invention comprises a high-power gas laser which is volumetric in character and that can be sealed in all three characteristic dimensions as well as in pressure level. A controlled discharge is created where electron-ion diffusion to the walls is negligible. 2
While the preferred embodiment of the present invention will be described in connection with an electrically excited nitrogen (N2), carbon dioxide (CO and helium (He) laser, it may, as noted above by way of example, be applied to other systems where a conducting ionized gas is required or useful and including, but not restricted to, gas constituents other than N,, CO, and He as well as other lasing systems. Since the discharge produced by this invention does not require ionization by the discharge electrons, in a lasing environment, a discharge in accordance with the invention can be adjusted to the correct electron temperature for most efficient laser operation. Moreover, a laser in accordance with the invention is volumetric in the sense that the proper gas temperature and lower laser state concentrations are maintained by means other than diffusion through the gas to cooled side walls. Further, apparatus in accordance with the invention may be operated in the flowing gas as well as the static pulse mode.
Light amplification by stimulated emission of radiation (laser) has extended the range of controlled electromagnetic radiation to the infrared and visible light spectrum. A laser produces a beam of coherent electromagnetic radiation having a particular well-defined frequency in that region of the spectrum broadly described as optical. This range includes the near ultraviolet, the visible and the infrared. The coherence of the beam is particularly important because it is that property which distinguishes laser radiation from ordinary optical beams. On account of its coherence, a
laser beam has remarkable properties which set it apart from ordinary light which is incoherent. While the maser (microwave amplification by stimulated emission of radiation) and the laser are based on the same principles of statistical and quantum mechanics, the problems and the physical embodiments for achieving laser action are completely different from those for masers.
Coherence, the essential property of lasers is of two kinds: spatial and temporal. A wave is spatially coherent over a time interval if there exists a surface over which the phase of the wave is the same (or is correlated) at all points. A wave is time-coherent at an infinitesimal area on a receiving surface if there exists a periodic relationship between its amplitude at any one instant and its amplitude at later instants of time. Perfect time coherence is an ideal since it implies perfect monochromaticity, something which is forbidden by the uncertainty principle.
Laser beams have a number of remarkable properties. Because of their spatial coherence, they generally have an extremely small divergence and are therefore highly directional. For example, a ruby laser beam one inch in diameter at the source will be about four feet across on a surface ten miles away. The very best that could be accomplished over the same distance with an incoherent source, such as an arc lamp at the focus of a six-foot parabolic mirror, would be a beam spread over an area more than one-third of a mile across. Another important feature of lasers is the enormous power that can be generated in a very narrow wave length range. Under certain operating conditions, nearly monochromatic bursts of millions of watts can be produced. To get comparable radiation intensity from a black body, it would have to be raised to a temperature of hundreds of millions of degrees-a condition not practically achievable. A laser beam, because it possesses space coherence can be focused to form a spot whose diameter is of the order of one wave length of the laser light itself. Enormous power densities are thus attainable. For example, the focused output of a 50-kilowatt infrared burst from a laser can have a radiant power density of the order of 10 watts/cm; this is about million times the power density at the surface of the sun. Extraordinarily high temperatures, orders of magnitude greater than that at the sun, can be generated at the small area which absorbs this concentrated radiation. Furthermore, since the electric field strength of an electromagnetic wave is proportional to the square root of its intensity, the field at the focus of the laser beam can be millions of volts per centimeter. A promising potential of lasers comes from time coherence. It is this property which permitted prior art exploitation of radio and microwaves for communications. However, laser frequencies are millions of times higher than radio frequencies, and hence are capable of carrying up to millions of times more information. In fact, one single laser beam has in principle more information-carrying capacity than all the combined radio and microwave frequencies in use at the present time.
Accordingly, systems applications of lasers are useful for communication in space, on earth, and under sea. Military surveillance and weapons systems, mapping, medical, mining, manufacturing, and computer technology may also include lasers.
Two conditions must be fullfilled in order to bring about laser action: (1) population inversion must be achieved and (2) an avalanche process of photon amplification must be established in a suitable cavity such as, for example, an optical cavity. Population inversion can, for example, be accomplished if (I) the atomic system has at least three levels (one ground and at least two excited levels) which can be involved in the excitation and emission processes and (2) the lifetime of one of the most energetic of the excited states is much longer than that of the other or others.
When a system is in a condition where light (photon) amplification is possible, laser action can be achieved by providing l means for stimulating photon emission from the long-lived state, and (2) means for causing photon amplification to build up to extremely high values. In the usual embodiment, this is accomplished by fashioning the medium containing the active atoms into a cylinder with perfectly (as far as possible) parallel ends polished so highly that the surface roughness is measured in terms of the wave length of the laser. The ends may be simply polished metal or they may be silvered or dielectric coated so that they behave as mirrors which reflect photons coming toward them from the interior of the cylinder. Such a structure, whether the mirrors are within or outside the container, is called an optical cavity. If now pumping means, such as for example, an electric discharge acts on the medium and brings about population inversion of the long-lived state with respect to another lower energy excited state even though the long-lived state is only relatively longlived, in a small fraction of a second there will be spontaneous emission of photons. Most of these photons will be lost to the medium but some of them will travel perpendicular to the ends and be reflected back and forth many times by the mirrors. As these photons traverse the active medium, they stimulate emission of photons from all atoms in the long-lived state which they encounter. In this way the degree of light amplification in the medium increases extraordinarily and because the photons produced by stimulated emission have the same direction and phase as those which stimulate them and assuming the optical quality of the laser media is suitable, the electromagnetic radiation field inside the cylinder or cavity is coherent. In order to extract a useful beam of this coherent light from the cavity, one (or both) of the mirrors is made slightly transmissive. A portion of the highly intense beam leaks through the mirror, and emerges with regularly spaced wave fronts. This is the laser beam.
Parallelism of the mirrors is a rigorous geometrical requirement in low gain lasers. Thus, in low gain lasers, if the mirrors are not precisely parallel, the light rays that build up in the cavity will tend to digress further and further toward the edges of the mirrors as they are reflected back and forth between the mirrors, and finally they will be directed out of the cavity altogether. It is essential that any deviation from parallelism be so small that the coherent photon streams will reflect back and forth a very large number of times to build up the required intensity for laser action.
In all diffraction limited optical configurations such as those discussed above, coherent wave fronts appear to originate from a common center and so they can, by use of a lens, be made plane-parallel and hence; except for diffraction effects, non-divergent. In high gain lasers other optical configurations such as oscillator amplifier configurations and unstable resonators are used. A characteristic of these devices is that the photons only make a small number of passes through the laser medium. In present operational lasers, the photon is reflected about only two or three times.
By way of example, a continuously operating gas laser is disclosed in an article, Population Inversion and Continuous Optical Maser Oscillation in a Gas Discharge Containing I-Ie-Ne Mixture, Physical Review Letter, 6, page 106, 1961. In the usual embodiment of static gas, prior art gas lasers, the gas is statically contained in a tube about centimeters long. The mirrors which form the ends of the optical cavity are disposed either inside the tube or external to it. Pumping is accomplished in this system by electrical excitation (either radio frequency or direct current).
In addition to the helium-neon gas laser system, other gas laser systems have been achieved with helium, neon, argon, krypton, xenon, oxygen, and cesium (the last optically pumped in the gaseous state) as emitting atoms.
Other systems include carbon dioxide, helium, and nitrogen. For a more complete discussion of the highpower flowing system including carbon dioxide, helium and nitrogen reference is made to patent application of C. K. N. Patel, Ser. No. 495,844, filed Oct. 14, 1965 abandoned in favor of continuation-impart application, Ser. No. 814,510 filed Mar. 28, 1969, now US. Pat. No. 3,596,202, and assigned to Bell Telephone Laboratories, Inc. Such a high-power laser typically includes two reflectors forming a suitable resonator or cavity, a tube forming the side walls of the laser, suitable pumping apparatus including a cathode, anode and directcurrent sources connected in appropriate polarity between the anode and the cathode; inlet apparatus; a source of carbon dioxide, helium, and nitrogen connected to the inlet apparatus; and equipment for exhausting the spent gases from the laser or for cooling and separating them for reuse.
As 'indicated hereinabove, a laser output may be generated in various media (i.e., crystals, semiconductors and gases) by pumping or introducing energy to create an inversion where a large number of the atoms are in high energy levels to support photon emission. In prior art gas lasers, whether flowing or static, the lasers were pumped or excited by using a diffusion controlled electrical discharge in a small tube maintained at a low pressure. Typically, in such gas discharge tubes (typically of the order of l centimeter in diameter) operating at low pressures (about l-IO torr) there is a loss of electron-ion pairs from the center of the plasma to the side walls of the tube by radial diflusion (so-called ambipolar diffusion of ion-electron pairs). For a steady state operation of the discharge, this loss must be made up by a net ionization rate in the plasma which exactly balances the diffusion loss rate. This required ionization rate dictates what temperature the electrons must have to sustain the discharge, and hence what applied E/N is needed to give the electrons that temperature. For long tubes E/N is defined by the applied voltage divided by the tube length and gas density.
In such situations, the discharge can be said to be ballasted by the tube walls, i.e., since radial diffusion of the electron-ion pairs is fast, any small local increase in electron density is reduced by diffusion. This fact makes such discharge radially and axially uniform as well as quite reliable and simple to produce.
The plasma (neutral gas plus electron-ion pairs) contained inside the electric discharge tube tends to remain radially smooth as long as the time required for the electron-ion pairs to diffuse to the surrounding walls is equal to the ionization time such as, for example, the time required to double the electron density.
Since the ambipolar diffusion time is generally proportional to the product of the gas pressure and the tube diameter squared for large diameters, this ambipolar diffusion time can, under some circumstances, become long compared to the ionization time in the tube, especially for high ionization rates, large diameter tubes and high pressures. In this latter situation, the discharge is no longer ballasted by the presence of the tube walls, i.e., local increases in the electron density are not immediately diffused to the walls where they are reduced by wall recombination, etc. Accordingly, local nonuniformities can be produced by these higher electron densities and the fast-growing non-uniformities can become worse. Often the result is that the previously uniform glow discharge turns into arcs, streamers or current spokes. This latter condition often is a plasma that is very inefficient, and often useless for certain purposes.
From the above, it will be seen that in high-pressure, large diameter discharge tubes the tendency is for any local increase in electron density not to be damped by diffusion to the confining walls. Upon occurrence of such disturbances, one can reduce their tendency to grow by reducing the ionization rate which means a lower electron temperature since the local ionization rate is a function of the local electron temperature. A lower electron temperature, however, requires that a lower electric field must be applied. The proper balance is a critical one: too high an electric field can allow the high pressure large diameter discharge to spoke, but if too low an electric field is applied, the discharge cannot be started in the first place. Further, at high pressures, it is generally found that an applied voltage or electric field large enough to start a discharge is also large enough to cause the discharge to be radially non-uniform and, for example, spoke. For the preceding reasons, it will be seen that if a discharge tube or cavity has a sufficiently large volume, maintenance of a controlled discharge therein by diffusion to the walls is not possible. As used herein the term controlled discharge means in a gaseous medium a discharge having predetermined properties which although such properties may vary in space and time, they remain at-least within desired limits for the time that the discharge exists. Such properties include but are not limited to the electronic and molecular states of the gaseous medium as well as the optical, electrical and chemical qualities of the medium, and its heating, ionization, dissociation, and recombination rates. A controlled discharge provided in accordance with the invention has a characteristic time which is substantially the duration of the time that sustainer current flows in the gaseous medium as a result of the motion of secondaryelectrons generated in the gaseous medium under the influence of an electric field termed herein a sustainer electric field more fully described hereinafter. For the case of a flowing medium wherein flow time through the cavity or working region is less than the duration of sustainer current in the gaseous medium, the characteristic time is the flow time through the cavity.
This invention is an improvement over that disclosed in pending patent application Ser. No. 859,424 filed Sept. 19, 1969 by James P. Reilly, abandoned in favor of continuation-in-part application Ser. No. 50,933, filed June 29, 1970, to which reference is made, and assigned to the same Assignee as this patent application.
It is an object of the invention to provide apparatus for and a method of producing controlled discharges in a gaseous medium.
It is another object of the invention to provide a controlled discharge in a gaseous medium in a controlled 7 manner with predetermined effect on background temperature, density and pressure of the medium.
A still further object of the invention is to provide apparatus for and a method of producing controlled, large, volumetric discharges without the inherent ionization instability that occurs when the discharge current itself produces the ionization.
A further object of the invention is to provide apparatus for and a method of producing spatially uniform discharges in a gaseous medium that can be used, for example, to provide a lasing medium chemical reaction processes, mediums for Ml-lD devices and the like and other applications where a conducting gaseous medium is necessary or useful to achieve a desired result.
Another object of the present invention is to provide apparatus for and a method of producing a population inversion suitable for use in a gas laser oscillator or amplifier.
It is another object of the present invention to provide apparatus for and a method of producing laser action in a flowing gas by electrical excitation.
A still further object of the invention is to provide a method of and apparatus for controlling the gas temperature in a gaseous laser by proper choice of gas flow velocity and input power to increase the efficiency of the lasing of the gaseous laser.
A still further object of the present invention is to provide a method of and apparatus for producing laser action in a flowing gas by generating free electrons, and an electrical discharge to maintain the optimum electron environment to produce the lasing action.
A still further object of the present invention is to provide a method of and apparatus for producing laser action in a flowing gas by electrical excitation comprising an electron beam to create electrons and a DC voltage to produce a discharge which maintains the optimum electron environment to produce lasing action.
Due to the ability to control the distribution of a discharge in a lasing medium, a still further object of the invention is to permit in an electrically excited gas laser an arrangement of electrical excitation means resulting in optimum optical qualities.
The novel features that are considered characteristic of the invention are set forth in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description when read in conjunction with the accompanying drawings in which:
FIG. 1 is a perspective view with parts broken away of the apparatus in accordance with the invention;
FIG. 2 is a sectional view taken on lines 2-2 of the apparatus shown in FIG. 1;
FIG. 3 is a sectional end view taken on lines 33 of the apparatus shown in FIG. 2;
FIG. 4 is a perspective view with parts broken away showing details of the electron source;
FIG. 5 is a perspective diagrammatic view illustrating method of operation and coordinates associated with electron generation, gas flow, and lasing activity; and
FIG. 6 is a block diagram of the circuitry associated with the electron gun and sustainer electrodes.
Attention is now directed to FIGS. 1-6 which show various details of laser apparatus incorporating the invention. Apparatus is shown in these figures wherein a gaseous medium capable of producing lasing action such as, for example, a mixture comprising 16% CO 34%N and 50% He is supplied from a suitable conventional source (which may comprise a plenum chamber and diffuser not shown) to suitable means defining a cavity or working region 10 of laser apparatus 12 via gas inlet means 11. The cavity or working region 10 of the laser apparatus (generally designated by the numeral 12) is shown by way of example as being generally rectangular in configuration. The term cavity as used herein means not only one that is defined by walls, but also one that is not defined by walls or the like since in certain cases such are not essential to carrying and/or using the invention. As best shown in FIGS. 2 and 3, the rectangular working region 10 comprises oppositely disposed top and bottom walls 14 and 15 adapted to receive and support respectively mirror holder and adjustment assemblies 21 and 22 more fully described hereinafter. Carried on the inner surfaces, the top and bottom walls 14 and 15 are oppositely disposed arcuate flow members 16 and 17 which are arranged and adapted to function to provide a smooth laminar flow through the working region 10. The mirror assemblies 21 and 22 are each disposed in members 16 and 17 and recessed to provide minimum disruption of flow and minimize spurious arcing. .Oppositely disposed side walls 18 and 19 are sealably attached to the top and bottom walls 14 and 15, side wall 18 being provided with a circular opening to sealably receive electron gun apparatus 25 more fully shown and described hereinafter. oppositely disposed to the aforementioned circular opening in side wall 18 is a recess in side wall 19 for receiving a flush mounted electrically conductive electrode plate 52 more fully described hereinafter. The aforementioned components other than electrode 52 defining the working region 10 are preferably comprised of an electrical nonconductive material, such as, for example, Lucite, Melamine, Fiberglass Epoxy, and the like. 3
As shown in FIGS. 1 and 4, the electron gun generally designated by the numeral 25 includes a rectangular electron source comprising an electrically conductive enclosure 26 constructed of stainless steel or the like and open at one end. Within enclosure 26, electrons are generated in conventional manner by thermionic emission from a plurality of spaced filaments 27 which are supported within and near the rear portion of enclosure 26 by a plate 28 comprised of electrically nonconductive material. Filaments 27 are supported by electrically conductive stand-offs 29 which are coupled to a source of filament current 30. The filaments are heated in conventional manner by source 30 to produce the thermionic emission. The enclosure 26 is mechanically supported within and insulated from the outer cylindrical wall 36 by supports 33 and 34 which also provide electrical connection to the pulse circuit '40. Supports 33 and 34 permit application to enclosure 26 the potential necessary to control the amount of electrons generated therein. One form of control may be provided as shown via a reticulated screen grid 35 electrically and mechanically connected to the enclosure 26 and covering the open end thereof.
A conventional pulse circuit 40 (see FIG. 6) coupled to grid 35 via supports 33 and 34 and enclosure 26 provides the necessary potential to control the amount of high energy electrons released by the electron gun. The pulse circuit 40 is triggered or actuated by a timing circuit 41. Actuation and control of the electron gun is more fully described hereinafter. Broadly, the electron emitter or gun provides an abundance of high energy electrons which are defocused and directed toward the working region 10 through the screen grid 35 (see FIG. 5).
The volume surrounding the electron gun within wall 36 is evacuated by a vacuum pump (not shown) in conventional manner and the electron gun maintained at a low pressure to provide an optimal environment for the free electrons generated therein to pass unhampered through screen grid 35 and be attracted and accelerated toward a reticulated electrical conducting plate 45. Plate 45, made of stainless steel or the like, is maintained at a potential high compared to that of screen grid 35. Electrons generated at the filaments 27 are strongly accelerated toward plate 45 and a portion passthrough the plurality ofholes 46 provided in plate 45. A thin sheet of material or diaphragm 47 is disposed between the working region 10 and the electron gun to permit the existence of separate pressure regimes. Diaphragm 47, which may be at least in part supported by plate 45, must possess adequate structural stability to withstand any required pressure differential (the vacuum in the electron gun 25 and the pressurized gas flow in the working region 10) and composed of a material arranged and adapted to transmit the maximum number of electrons without absorbing an excessive portion of their energy which can reduce efficiency and/or result in failure of the diaphragm. While preferably composed of metal, the diaphragm 47 may be composed of either nonconductive or conductive material.
After electrons from screen grid 35 pass first through the holes 46 in the plate 45 and then the diaphragm 47, they enter the working region 10 by passing through a reticulated cathode 50 which may be constructed of a wire mesh and insulated, if desired, from .the electron gun 25 by a ring of non-conducting material 51. In the working region 10, electron energy is maintained by a sustainer electric field between oppositely disposed anode plate 52 and previously mentioned cathode 50 which are coupled to the sustainer circuit 53. Cathode 50 which may be comprised of a wire mesh grid as previously noted prevents, for the electron beam and sustainer electric field arrangements as shown, damage to the diaphragm 47 from spurious arcs which may be otherwise inadvertently struck between the anode 52 and/or cathode 50 and diaphragm 47. A high voltage direct current potential is typically maintained between anode 52 and cathode 50 by a conventional sustainer circuit 53 which may comprise capactive discharge means charged by power supply 54 and triggered by timing circuit 41 for pulsed operation. The example hereinbefore given is for a shower head type electron beamwhich covers a broad area, however, the same result may be accomplished by the provision of a rapidly swept beam of electrons over a broad area.
The production of a volumetric controlled discharge, which for the embodiment illustrated in FIGS. 1-6 comprises the excitation and inversion of a gaseous medium in the working region between the sustainer anode 52 and cathode 50 is provided in accordance with the invention in two steps. A discharge as used herein is, in an ionized medium, the flow of current under the influence of a sustainer electric field or fields. While primarily described herein is the use of DC voltages with inter-cavity electrodes, to provide a sustainer field the invention described herein includes the use of radio frequency electromagnetic fields, inductive electrode structures, capactive electrode structures, movement of an electrically conductive medium in the presence of an applied magnetic field, and the introduction of laser energy into the working cavity to provide the sustaining electric fields. For a more complete discussion of the basic process here involved, reference is made to the aforementioned patent application Ser. No. 859,424 filed Sept. 19, 1969. The present invention comprises an improvement over the aforementioned patent application in the provision of ionizing radiation, such as, for example, the provision of highly efficient ionizing radiation through the use of electron gun means rather than high voltage discharge means or the like disclosed in the aforementioned application. The ionizing radiation provided in accordance with the invention provides a source of secondary electrons at very low temperatures and increased efficiencies heretofore unobtainable since theory indicates that the only way comparable conditions in high power, high pressure devices can be duplicated is to operate the pulser circuit of the aforementioned patent application at levels of the order of one million volts and/or high repetition pulse rates a result not easy if not impossible, of practical attainment.
As taught in the aforementioned patent application, a principal feature in providing a volumetrically scalable discharge is the control of gas temperature and discharge uniformity wherein an electrical discharge or the like produces free electrons and ionization of the working medium in a sustainer electric field. Electron temperature, which is a function of E/N in any gas mixture, is controlled by adjustment of the sustainer electric field E and control of the gas density N. In flow applications, proper design determines the allowable temperature rises (A1) in the gas and the corresponding density (A N) in the gas. In pulsed applications, the heat capacity of the gas, the pulse width and the effect of pressure waves must be considered in the proper control of A N. If the electron temperature is kept sufficiently low so that the ionization due to the sustainer field is small compared to ionization due to the aforementioned free and preferably high energy electrons, the volumetric discharge can be maintained in a controlled manner to high pressures. For example, controlled discharges in accordance with the invention of up to one atmosphere have been established.
The aforementioned patent application disclosed in detail the provision of a short, high voltage pulse substantially inductively spread throughout the volume of the working medium. This is accomplished by the provision of a plurality of electrodes and a short pulse. The discharge is uniformly provided or spread throughout the cavity containing the working medium because the volumetric discharge through all of the electrodes offers the least inductive impedance and thereby makes the current in the short pulse flow reasonably uniform throughout the volume of the cavity containing the working medium. An important criterion for this arrangement to be practically effective is that the inductive impedance of the short pulse discharge circuit be comparable to the resistive impedance of the discharge. This has been accomplished in accordance with the aforementioned patent application and produced uniform ionizations over large volumes in times less than about 10' seconds with minimum disturbances of the working medium. In one case, the working medium was a mixture of N CO and Re which was used to produce a lasing medium.
An important feature of volumetric ionization in accordance with the aforementioned patent application as well as the present invention is to stabilize the discharge by suppressing the arcing. Much of the arcing which occurs in systems in accordance with the aforementioned patent application occurs due to the electrode configuration and various electrode configurations have been tested in conjunction with that invention and in all cases ionization in accordance with the present invention creates a stabilizing effect which allows the operation in areas that heretofore would have created arcing and breakdown.
In accordance with the present invention, an electron beam is provided to produce free electrons and ionize the working medium. The electron beam which replaces the short high-voltage pulse of the aforementioned patent application, is, among other things, more efficient in producing ionization of the working medium. For example, a 50 kv electron passing through air produces the order of 1000 secondary electrons along its path before losing its energy. The effective ionization potential of a gas mixture of N C0 and He is approximately 30 volts, with half the primary electron energy loss going into ionization.
When a laser application, for example, requires very high power, there is an advantage in working with relatively high gas pressure (such as, for example, up to one atmosphere or more) and large transverse dimensions (up to 30 centimeters or more). Such conditions would require voltage levels in excess of 1,000,000 volts in the pulser circuit of the aforementioned patent application. The present invention eliminates this high voltage requirement. An electron beam ionizer in accordance with the invention need be provided, for example, with only a voltage of the order of kv to achieve useful ionization for such distances and pressures. Further the provision of electron beam ionization in accordance with this invention permits continuous ionization through such large volumes thereby eliminating the necessity for repetitive pulse ionization in, for example, a laser application. In addition to the preceding, electron beam ionization can be simply and conveniently controlled by controlling the potential on a grid disposed in front of the electron emitting means. Thus, ionization level and, for example, laser output for laser applications may be simply and economically controlled by controlling the grid voltage which may comprise part of a low powered, easily controlled circuit. This feature of control along with the ability of an electron beam to ionize in a truly continuous fashion makes apparatus in accordance with the invention highly attractive for ionizing a working medium in any application where it is desired or convenient to separate ionization from maintenance of a discharge.
In apparatus in accordance with the invention, at least one wall of means defining a working region must transmit or provide high energy electrons which deliver their kinetic energy to the working medium in the form of ionization with a high efficiency. The electron beam voltage, i.e., the energy of the electrons in the beam providing the aforementioned high energy electrons must be sufficiently high that the electrons will enter the working region by, for example, penetration of a diaphragm or foil disposed in a wall of the container before passing through and ionizing the working medium. The intensity of the electron beam current is broadly determined by the ionization level requirements such that the volumetric recombination (or attachment) rate equals the production rate of ionization in the electron beam for a particular application. Increasing the intensity of the electron beam leads to increasing the level of ionization with a corresponding higher volumetric recombination rate. The diaphragm or diaphragms through which the high energy electrons enter the working region need be only such that they transmit the necessary number of electrons and are adequately supported and cooled during transmission of the high energy electrons. The support requirements are such that the diaphragm must withstand the pressure differences between the working gas and the vacuum region where provided on the other side of the diaphragm where the high energy electrons are created and accelerated toward the diaphragm. Typically, a suitable geometry is one where there is a high vacuum region exterior of one or more of the walls of the cavity defining the working region. Electrons are generated in the vacuum region by any suitable method such as, for example, plasma emission, thermionic emission, photo emission, electron bombardment and the like. Upon generation of the electrons, they are in conventional manner accelerated through a suitable electrostatic or electromagnetic structure and caused to pass through the diaphragm into the working region.
Irrespective of the method of generating electrons, they may be typically coupled to the working. region through the diaphragm. The diaphragm may be disposed over a reticulated member and in certain pulsed applications the foil temperature rise may be limited simply by its intrinsic heat capacity and may be cooled in any suitable manner such as by gas flow or conduction and may be comprised of Al, Be, T,, C, and
the like. Since the function of the diaphragm is to separate the working medium in the working region from the vacuum in the electron gun, it typically should be capable of withstanding a pressure difference of one atmosphere. Since the diaphragm is heated by absorbing energy from transmitted electrons in C. W. or numerous rapid pulse applications, it must be cooled. However, any suitable cooling means may be used.
While a shower-hea type electron beam arrangement is shown and described herein for irradiating a large area by a relatively low energy electron beam of the order of 50-150 kv, it is to be understood that the invention is not so limited and that other arrangements such as, for example, one or a plurality of small electron guns of the type used in electron beam welders and the like may be used where appropriate to the application. Further, if the use of a diaphragm is undesirable, a series of small holes in a plurality of plates defining a plurality of serially disposed chambers which are differentially pumped may be employed to provide separation of the electron gun from the working region without requiring the electrons to pass through a solid member. In this case, the electrons pass directly through one or more of a series of aligned holes in the plates and the gas in the working region will not diffuse rapidly enough through the hole adjacent the electron gun to substantially affect the generation of electrons. Suitable voltages may be applied to the space between plates to obtain maximum focusing of the electrons and the pressure between plates successively decreased in the direction of the electron gun.
Electron beam current and ionization level required in a given working medium are determined by the application. Thus, many N CO laser applications require only a relatively low level of ionization and low volumetric beam current. Further, in this particular application, the cooling requirements of the diaphragm are modest and can be satisfactorily met by heat conduction to cooled support members. However, for MHD generator and accelerator applications, for example, higher ionization levels and higher volumetric beam currents are necessary for practical devices. Accordingly, a greater cooling of the diaphragm will be required for this type of application than with, for example, a laser.
The quality of the electron beam, i.e., the spread, energy and uniformity of the electron beam throughout the working medium are determined by the application. Thus, for many of the laser applications, the intensity of the electron beam must be substantially uniform (with variations not exceeding about a few percent) in order to produce a working medium with the substantially uniform ionization necessary to provide uniform gain and optical properties in the lasing medium.
While the provision of an electron beam is preferred for the embodiment disclosed by reason of the electron beam being a highly efficient method of producing volumetric ionization, it is to be understood that other applications may require ionizing radiation in the form of photons, alpha particles, x-rays, and the like and such are included within the scope of this invention.
As may be seen from the preceding discussion, the level of ionization that can be obtained using high energy electrons is determined by balancing the production rate of secondary electrons with the loss rate due to either recombination attachment or flow. Accordingly, it is important to understand the limits of high energy electron current density and energy to understand the relevant loss process discussed herein below.
In an embodiment actually reduced to practice, to produce laser action, an electron gun produced a stream of electrons which were directed at a thin metallic foil diaphragm supported by a perforated plate with 470 1/1 6 inch holes in an area 2 X 4 inches. The limiting condition on the electron beam current was that the thin metallic foil used not be heated to a temperature at which its structural strength was significantly reduced, since its function is to withstand the pressure difference between the working region and the gun and still transmit electrons. This temperature was arbitrarily set as 200 K, the foil being aluminum having a thickness of cm. Other materials of other thicknesses may be used, and the foil may be cooled by a variety of means, including conduction to cooled supports, or, for example, forced convection with gas blown across its surface in a pulse mode of operation.
In the pulse operation, we may assume that all the energy is deposited in the foil, a lower limit on the incident current density in terms of the incident beam energy E (volts) and the pulse length r (sec) is approximately E I t 0.5 joule/cm 1 If E=50 kv, t=20 p. sec. I 0.5 amp/cm, approximately 20 percent of the incident electrons will be transmitted with a mean energy reduction of about 10 kv and it is these transmitted electrons which are available for pulsed ionization of the gas. As will be seen later, the above limit represents a current density far in excess of that required for ionization of the gas system selected for lasing operation.
Consider now the processes of ionization and recombination in the gas used which is essential to proper operation of the laser:
The production rate, p, of ions in a gas per cm is dn ldt an -lp Where a is the effective recombination coefiicient and p is the production rate, it follows that in equilibrium, that is, for dn ldt 0, we have:
DE n. a]
0: T ME,
where P gas pressure, dynes cm T gas temperature, K, a effective recombination coefficient, cm lsec", M molecular weight of gas, m, proton mass, gm, k Boltzmans constant, erg/K.
The approximate maximum values of n for a typical electron beam and current density 1 mA/cm, in a mixture of Helium, Nitrogen and CO in the proportions 3:2:1 are given immediately below, with characteristic decay time T l/om and range R at E 50 kv, using E, 50 volts.
n, R r rov-r cm Cm [Lsec To better understand the invention, the process involved in the creation of lasing power by the use of an electron beam and a sustainer discharge in accordance with the invention will now be discussed.
Thermionic electrons from a tungsten filament were modulated by a grid whose potential could be varied with respect to the filament and the electrons were accelerated through a potential V The value of V was chosen by optimizing the ionization produced in the gas. For higher energies Aluminum foil is more transparent and more electrons are transmitted, but the ionization density produced is lower. Accordingly, in Eq. (2) it may be shown that 8 E/8 m z C 1n E/E, where C is a constant,
and 8 E/8 m decreases as E increases.
The optimum value of V used was approximately 50 kv and the electron gun was maintained at a vacuum (p 0.1 micron) and separated from the laser chamber by a thin foil of aluminum of thickness 10 cm. Aluminum was chosen simply because of its ready availability. The laser chamber may be at any pressure from below 1p. up to about one atmosphere or more.
After passing through the foil, the electron beam entered the working region through a relatively wide mesh grid of stainless steel. This grid constituted a cathode and a gold plated disk constituted an anode, between which a sustainer voltage V l0kv) was applied. The grid was provided to prevent damage to the foil, and the gold plate on the anode served to reflect a proportion of the incident primary electrons, thus increasing the ionization of the gas. The filaments in the electron gun were maintained at 50 kv with respect to the foil (which was at or near ground potential) by a 5 micro farad capacitor which supplied the pulsed electron beam current. The filaments were pulsed negative 500 V with respect to the grid. Many other schemes for projecting a beam of electrons into a gas are possible such as photoelectric, field emission, electron bombardment and ion bombardment.
The sustainer current was supplied by a 250 p. F capacitor at voltages up to about 10 kv. Either the anode or the cathode can be grounded. The velocity of gas comprising the working medium which flowed through the laser chamber normal to the electron beam can be varied up to about Mach 1. In preliminary tests velocities of about one meter per second were used in order to ensure that the gas was uncontaminated through leaks.
The existence of uniformity of the electron beam in the working region and the low intensity variations of the electron beam was corroborated by replacing the anode wall with a lucite wall coated with sodium salicylate, a substance that is flourescent when excited by high energy electrons.
Two mirrors in the laser chamber, whose axis was normal to both the gas flow and the electron beam, were positioned vertically in the apparatus. One mirror was copper and concave and the otherone was IRtran 98 percent reflecting at a wavelength of l0.6p.. The mirrors were spaced 18 cm apart and were supported in a tube whose orientation could be adjusted by means of screws. The mirrors were aligned using standard techniques and the output from laser action between the mirrors passed through a 10.6 1. filter into a germanium crystal infrared detector, the output of which was fed into an oscilloscope triggered by the electron beam current. The sustainer current was measured as well as the infrared detector signal and the infrared detector was calibrated with a thermocouple calorimeter. It was found that laser gain became sufficient to begin lasing action only some time after the electron beam pulse reaches its maximum. This time lag represents the time required to achieve a population inversion by pumping the CO molecules into their upper state and is sensitive to the temperature dependence of the electron pumping rate. Increasing the sustainer voltage, and, therefore, the electron temperature in the lasing medium decreased this time lag.
A useful level of ionization was achieved with a pulse of p. sec. duration. After the pulse, the gases ionized by the pulse recombine and it is in this recombining debris stage of the cycle that laser action occurs. In another experiment laser action was accomplished with an essentially C. W. (i.e., ionization compared to flow and cooling times) E-beam. Thus, the electron beam pulse length may be varied from infinite to continuous to very slow thereby creating either a truly cw laser, an effectively cw laser, or a pulsed laser. For high power operation, the working medium may be provided in the form of pulses and the E-beam and sustainer circuits actuated substantially between pulses. Such operation permits substantial heat removal while still providing a substantially homogeneous medium in the working region.
As discussed in the aforementioned patent application, various gases and gas mixtures may be employed to support laser action although a 3:2:1 ratio of He:N CO is discussed herein, any gas or combination of gases such as CO, E 0, S0 HCN, NO, H Ar, N0 N 0, HF and the like may be handled in the manner discussed hereinabove and other gases may be added if required or desired.
The present invention is applicable to substantially any useful laser gas mixture, the principal advantage of the invention being that it is applicable to suitable gas mixtures at high pressures, producing a controllable volumetrically scalable gas discharge over a wide range 16 of operating conditions and electrode configurations. The present invention permits the production of a stable and controlled discharge when the gas mixture constituents and electron temperature T, are selected so that the rate of one or more of the variety of available recombination process (atom recombination, molecular recombination, attachment, etc.) exceeds the rate of ionization. When this is established, the discharge will not be self-sustaining, i.e., it will not run without the ionizing means being actuated and it is this feature that permits the ionizing means to control the discharge characteristics. If (T.,),,,,, is defined as the condition for a specific gas mixture wherein ionization equals recombination, viable laser apparatus will be provided if an inversion is produced by electronic excitation (and/or appropriate gas kinetic de-activation of states related to the laser transition) at some electron temperature or temperatures T such that T (T A specific example is the N -CO laser mixture. lonizations become significant when T is of the order of 1.5 ev or higher. However, a net preferential excitation (producing an inversion) can be made to occur for electron temperatures of less than 1.5 ev in both N and C0 The prior art teaches a large number of atoms and molecules that can be excited electronically by a discharge. Any lasing species which may be inverted by direct electronic excitation or by excitation via an auxiliary species (as in the N CO system) at T (T may be expected to be susceptible to the ionizersustainer concept and especially the electron beam ionizer-sustainer concept of the present invention.
Further the present ionizer-sustainer concept may be expected to be applicable to use of a gas mixture containing a gas which has a high net attachment rate (producing an effective recombination) at high electron temperatures which occur, for example, in 0 for values of T up to about 3 ev. Use of such a gas mixture may be expected to permit operation at higher than usual electron temperatures wherein significant ionization of one of the lasing mixture constituents occurs. This may make lasing transitions acceptable which are not otherwise stably available (C.W.N
TABLE I Output wavelength 10.611. Output coupling 1% Peak pulsed output power 3 watts EB pulse width ==lO0 usec Sustainer pulse width s 800 u. see Laser pulse width s 600 u. see Gas 16% C0,, 34% N 50% He Gas pressure 30 Torr input velocity 1 mlsec. quasistatic Laser cavity size Diameter: 2.54 cm Broadly, as may now be seen, the electron beam creates a desired electron density uniformly using only a small amount of energy while the sustainer discharge provides a voltage to give these electrons a desired temperature sufficiently high for laser action for example, but not high enough to generate any appreciable increase in electron density. The sustainer discharge deposits the dominant amount of energy in the gas directly where it is desired. In the case of an N --C laser, the energy is put into the upper laser state of CO and into Nitrogen vibration, the optimum electron temperature assuring optimum laser efficiency. Upon creation by the electron beam of a uniform electron-ion cloud, the cloud stays uniform during the time of the electric field provided by the sustainer voltage as long as the sustainer voltage does not result in a rapid creation of electrons. If the level of the sustainer voltage or field is raised to the point where it too produces a rapid ionization, then discharge non-uniformities may be created. However, provision of a sustainer field selected to create negligible electrons results in maintenance of stable, uniform and controlled discharge for several flow times through the working region.
As will now be apparent, the present invention permits the provision in a flowing gas laser of a spatially uniform discharge at the optimum electron temperature required for efficient laser operation at arbitrary pressure levels and physical sizes. While the invention is not so limited, this may be accomplished by utilization of the aforementioned two-step process comprising preferably, first an electron beam which creates in the gas a non-spoking predetermined spatial distribution (preferably uniform) electron density or ionization which would ordinarily, if left on its own, disappear by volumetric processes and/or flowing out of the channel and be incapable of producing efficient high power laser action. However, the second step or sustainer discharge is provided which gives the electrons produced by the first step the necessary electron temperature for preferably optimum laser (or other) excitation, with no significant increase in electron density.
It is to be understood that the invention is not limited to the apparatus shown and described and that, for example, other methods of an apparatus for creating the initial electron density can be used such as ultraviolet radiation, electrical discharge, protons and the like provided by electron beam means for introducing one or more electron beams to produce ionization of the gaseous medium as and for the purposes set forth hereinabove. Irrespective of whether the electrons are generated in the above described manner or any other suitable manner, they must be heated to the correct electron temperature by the E/N applied by the sustainer discharge.
Reference has previously been made to the fact that the present invention may be used to produce or facilitate carrying out chemical processes such as, for example, the generation of ozone.
Heretofore, for industrial applications ozone has been principally produced by the use of the well-known Townsend or silent discharge. Recently a second process based on the use of a corona or high pressure glow discharge has begun to be used in commercial applications.
The Townsend discharge process is characterized by two significant operating characteristics that are at least somewhat interdependent the requirement of a low current density in the discharge and a low overall energy efficiency. The production of ozone requires high levels of activation energy even with low conversion rates of the order of one mole percent of the working medium; hence cooling is essential if an undesirable change in chemical kinetics due to temperature rise is to be prevented. The aforementioned low current density and low overall energy efficiency characteristic of the Townsend process has not only resulted in high production costs but has severely limited the application of processes incorporating the Townsend discharge.
Details of the formation of ozone by a Townsend discharge are wellknown. Thus, while high positive column energy efficiencies have been consistently measured, a low overall efficiency results because of the severe potential drop at the electrodes. While the dielectric layer used to stabilize the discharge is principally responsible for the aforementioned electrode drop, without stabilization provided by this dielectric layer, the Townsend discharge does not function satisfactorily as an industrial process.
The glow discharge process is not subject to the two basic deficiencies of the Townsend discharge of low current density and high cathode drop. In the glow discharge process the current density is about 2-3 orders of magnitude higher than that of the Townsend discharge and the cathode drop is generally less than about 1,000 volts. However, compared to the Townsend discharge, the positive column energy efficiency of the glow discharge process is significantly lower.
Glow discharge processes are generally conducted under low pressure and with walls cooled to liquid air temperature. On the other hand, the high pressure discharge, or corona discharge, is subject to a more limited stability range than the low pressure glow discharge. Further, with the exception of a high frequency electrodeless discharge, electrode geometry is generally a critical factor in achieving stabilization of a corona discharge.
Due to the necessity of stabilization by a dielectric layer or specific electrode geometry, the abovedescribed processes are essentially surface processes. In such processes the kinetics in the active volume of the working medium is not homogeneous and optimum or near optimum conditions cannot be uniformly established.
The basic problem in the above-described processes is believed to exist because of close coupling between the emission of electrons from the electrodes and the electric field in the active volume. Where such coupling exists, it is very difficult to maintain a steady and uniform discharge without arcing.
Ozone may be more easily and efficiently produced than heretofore by utilizing, in accordance with the present invention, an independent source of electrodes in the form of an electron beam or, alternately, repeated short electron beam pulses superimposed on a sustaining electric field as and for the purposes hereinbefore described. As in a laser application, in this case the electric field in the active volume is also decoupled from the requirement of the electron emmission, whereby optimum conditions for ozone formation may be provided without for example, severe requirements on ballasting as required for a Townsend discharge or severe requirements on electrode geometry as in the case of a corona discharge. The production of ozone in accordance with the present invention is a truly volumetric process; hence for large scale applications, not only are scaling problems simpler than with prior art processes, but overall equipment size can be drastically reduced. Furthermore, the uniform conditions provided in the active column in accordance with the present invention provides an improvement in the overall energy efficiency and minimizes heat dissipation involved in the process as compared to that of the prior art. Accordingly, ozone may be produced in accordance with the present invention in higher concentrations than that heretofore available without the necessity of cooling.
Ozone may be produced with apparatus substantially as shown and described hereinabove with the exception that the mirror means defining the optical cavity are not required. The working medium for the production of ozone may be air or preferably pure oxygen. Electrons are generated by the electron gun in the manner previously described, enter the working region, and collide with oxygen molecules to form secondary electrons and ions. The electron temperature in the working region must be maintained at a level which is favorable to ozone production.
in the working region, electrons are generated through ionization by primary and secondary electrons and are lost by attachment to the molecules of oxygen. Both the ionization rate of secondary electrons and the electron attachment rate to oxygen molecules are strongly influenced by the electron temperature. Since, in accordance with the invention, there is an excess of electrons due to ionization by the primary electrons, a spatially uniform current can be maintained with an ionization rate of secondary electrons less than the attachment rate. Accordingly, the stability of the discharge process in the working region is substantially greater than that in conventional discharge processes where net ionization by secondary electrons is essential to sustain the discharge process.
If a secondary electron temperature in the range of approximately 2-3 electron volts is provided, a large percentage of the energy lost in elastic collision goes into dissociation of oxygen which is essential to the production of ozone in high concentrations. The electron temperature range suitable for the production of ozone is higher by about a factor of 2 than the range necessary to produce laser action. Further, since a low ambient temperature may be easily provided in the working region, this permits the production of higher concentration of ozone with much greater efficiency than heretofore possible.
Another application of the present invention is to magnetohydrodynamic power generation '(MHD). Electric power can be extracted from an electrically conductive stream of plasma by passing the plasma through a magnetic field transverse to the direction of flow. The magnetic field creates an electric field perpendicular to the magnetic field and to the direction of flow, and suitably constructed electrodes arranged parallel to the electric field permit the kinetic energy of the plasma to be coupled out as electrical energy. For a more complete discussion of Ml-lD devices, reference is made to U. S. Pat. No. 3,264,501.
In this type of application, an electron beam in accordance with the invention is injected into the plasma to maintain the required level of ionization independent of the electron temperature. In this manner, a stable plasma discharge may be usefully produced in the plasma wherein the ionization is volumetric and stabilized not by ambipolar diffusion of ion pairs to the walls as in a conventional discharge, but by equilibrium between ion recombination and ion production by the electron beam. It is to be emphasized that the parameters set forth below and their numerical values are given only by way of illustration.
Requirements on Recombination Coefficient MHD Generator Gas: Helium Flow Velocity l .5 X l0 m/sec Flow time: -10 sec Energy Extracted from plasma: ===O.2 eV/particle lonization Level: n, l0 electrons/cm Gas Density: 3 X 10" cm" Effective energy per ionization 50 eV Energy required per particle to ionize: Assuming I00 ionizations per flow time, energy required to maintain ionization Ratio:
This must be H 100 of the flow time, giving a requirement for the recombination coefficient: a2 l0ln, 10 cmlsec Recent experimental results (Berlande et al, Phys Rev A1, 887, 1970) indicate that in the preferred working region, n,=10, electron temperature T, z 3 X 10 l(, gas temperature Tg==l300l(, the upper limit on the effective recombination coefficient is a 10 em /sec.
The electron beam current density required to maintain an equilibrium ionization level n, 10 cm is ob tained by equating production rate and recombination rate. Thus: om, IE/eE R(E), approximately where a effective recombination coefficient I EB current density, amp cm E electron energy, volts R(E) range of electrons of energy E, cm
E, 50 eV per ion pair e 5/3 X 10' coulombs For E 100 keV, and density N,, 2.6 X 10 cm', then R(E) z cm in helium 7.5 X 10" amps/cm when a l0" cm lsec n An electron beam as set forth above is easily produced as and for the purposes previously described and may include for example using a jet of helium to cool the foil or diaphragm, the beam of electrons being injected into the MHD channel at a suitably chosen angle relative to the direction of the applied magnetic field.
manna The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims:
1. In the method of producing a spatially uniform controlled discharge substantially throughout a gaseous working medium in a working region, the steps comprising:
a. providing a gaseous working medium at a pressure in a working region disposed in a cavity having imperforate walls for confining the gaseous working medium that upon the production of secondary electrons in said medium said medium has ambipolar and thermal diffusion rates incapable of damping local increases in secondary electron density in said medium;
b. generating ionizing radiation externally of said cavity; introducing said ionizing radiation into said cavity through one of said walls to produce substantially throughout said working region a substantially spatially uniform predetermined density of secondary electrons in said medium by ionizing said medium, said one wall being impervious to gases and pervious to said ionizing radiation; and
d. providing a sustainer field for providing substantially uniformly throughout said working region a predetermined electron temperature effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization, said electron temperature producing said controlled discharge substantially uniformly throughout said working region at a predetermined level.
2. The method as defined in claim 1 wherein said electron temperature is controlled at least in part by flowing said medium through said cavity.
3. The method as defined in claim 2 wherein said density and temperature are maintained at said values less than that which will produce uncontrolled arcing for times less than the characteristic time of said discharge.
4. The method as defined in claim 1 wherein the density of said secondary electrons is controlled at least in part by flowing said medium through said cavity.
5. The method as defined in claim 1 wherein said working medium is passed through said cavity.
6. In the method of producing a spatially uniform controlled discharge substantially throughout a gaseous working medium in a working region, the steps comprising:
a. passing a gaseous working medium at a pressure through a working region disposed in a cavity having imperforate walls for confining the gaseous working medium that upon the production of secondary electrons in said medium said medium has ambipolar and thermal diffusion rates incapable of damping local increases in secondary electron density in said medium;
b. generating ionizing radiation externally of said cavity;
c. introducing said ionizing radiation into said cavity through one of said walls to produce substantially throughout said working region a substantially spatially uniform predetermined density of secondary electrons in said medium by ionizing said medium, said one wall being impervious to gases and pervious to said ionizing radiation;
d. providing a sustainer field for providing substantially uniformly throughout said working region a predetermined electron temperature effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization; and
. providing a pressure and velocity of said medium in said working region to produce said controlled discharge substantially uniformly throughout said working region at a predetermined level.
7. In apparatus for producing a controlled discharge for providing molecular excitation of a gaseous working medium, the combination comprising:
a. means defining a cavity having a working region disposed therein, said cavity having imperforate walls for confining a gaseous working medium and defining a predetermined cross section and volume;
. a working medium in said cavity and working region at a pressure that upon the production of free electrons in said medium at said pressure said medium has ambipolar and thermal diffusion rates incapable of damping local increases in electron density in said medium;
. first means for generating ionizing radiation externally of said cavity;
. second means for introducing said ionizing radiation into said cavity through one of said walls and producing substantially throughout said working region a substantially uniform predetermined density of secondary electrons in said medium by ionizing said medium, said one wall being impervious to gases and pervious to said ionizing radiation; and
. third means for providing a sustainer field for providing substantially throughout said working region a predetermined electron temperature of said secondary electrons effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization, said electron temperature producing said controlled discharge substantially uniformly throughout said working region at a predetermined level.
8. The combination as defined in claim 7 wherein said cavity includes gas inlet and gas outlet means, and additionally including fourth means coupled to said gas inlet for flowing said medium through said cavity.
9. The combination as defined in claim 8 wherein said fourth means includes further means for providing a predetermined pressure and velocity of said medium in said working region.
10. The combination as defined in claim 8 and addi-l;
tionally including diaphragm means separating said first means and said cavity, said ionizing radiation being introduced into said medium through said diaphragm intermediate said gas inlet and said gas outlet and normal to the direction of flow of said medium through said cavity.
11. The combination as defined in claim wherein said third means includes electrode means for providing a sustainer electric field in said cavity normal to the direction of flow of said medium, said electrode means comprising a first electrode adjacent the wall through which said radiation is introduced and through which said radiation passes, and a second electrode oppositely disposed to said first electrode adjacent the opposite wall of said cavity.
12. The combination as defined in claim 11 wherein said second means includes a perforate plate member and a thin diaphragm covering and carried by said plate member, said diaphragm being disposed between said plate member and said medium.
13. The combination as defined in claim 7 wherein:
a. said medium has an upper and lower laser state;
b. said first and second means provides a density of secondary electrons in said medium sufficient to support a population inversion; and
c. said third means increases the average energy of said secondary electrons to a level to produce a population inversion in said medium in said cavity.
14. The combination as defined in claim 13 and additionally including:
a. means for passing said medium through said cavity in the form of pulses; and
b. means for actuating said first, second and third means intermediate said pulses to produce said population inversion intermediate said pulses.
15. The combination as defined in claim 13 wherein said medium is continuously passed through said cavity.
16. In the method of light generation by stimulated emission of radiation substantially throughout a gaseous active medium in a working region, the steps comprising:
a. providing a gaseous active medium at a pressure in a working region disposed in a cavity having imperforate walls for confining the gaseous working medium that upon the production of secondary electrons in said medium said medium has ambipolar and thermal diffusion rates incapable of damping local increases in secondary electron density in said medium, said medium having an upper and lower laser state;
b. generating externally of said cavity a broad area electron beam having a cross sectional area conforming substantially to that of said working region;
c. introducing said electron beam into said cavity through one of said walls to produce substantially throughout said working region a substantially spa tially uniform predetermined density of secondary electrons in said medium having an average energy insufficient to produce a population inversion in said medium, said one wall being impervious to gases and pervious to said electron beam; and
d. providing a sustainer field for providing substantially uniformly throughout said working region a predetermined electron temperature effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization, said electron temperature producing an average energy level sufficient to support a population inversion in said medium.
17. The method as defined in claim 16 wherein said medium is at least sequentially passed through said cavity at a pressure and velocity to produce substantially uniformly throughout said working region a population inversion in said medium.
18. The method as defined in claim 17 wherein the electron temperature is controlled by providing a sustainer electric field in said medium and said pressure and velocity are provided to produce substantially maximum population inversion in said working region.
19. The method as defined in claim 16 wherein said population inversion is serially provided in the form of pulses and the energy added to the medium by the in- W troduction of said free electrons is less than the energy added to the medium by said sustainer field.
20. In high powered laser apparatus the combination comprising:
a. gas supply means for producing a flow of a gaseous medium having a predetermined velocity and pressure and an upper and lower laser state;
. means defining a cavity including a working region for receiving said medium from said gas supply means and through which said flow passes;
c. first means for generating externally of said cavity a broad area electron beam having a cross sectional area conforming substantially to that of said working region, said means defining said cavity including walls for confining said medium, one of said walls including a diaphragm impervious to said medium and pervious to said electron beam;
d. second means for introducing said electron beam into said cavity through said diaphragm forming a part of said one of said walls of said cavity and produce a substantially uniform spatial distribution of secondary electrons in said medium in said working region by ionizing said medium, said secondary electrons having an average energy insufficient to produce a population inversion in said medium; and
e. third means for providing a sustainer field for controlling the electron temperature of said secondary electrons in said medium to substantially uniformly throughout said working region increase their average energy without substantially increasing the density thereof by self-regenerative ionization at said velocity and pressure and produce a population inversion in said medium in said working region.
21. The combination as defined in claim 20 wherein said third means includes means for generating a sustainer electric field in said cavity.
22. The combination as defined in claim 21 wherein said means for generating said sustainer electric field includes first and second electrode means in said cavity.
23. The combination as defined in claim 22 wherein said second electrode means is comprised of a perforate member and disposed in spaced relationship over said diaphragm.
24. The combination as defined in claim 23 wherein said working region includes means for passing a light beam through said working region.
said first means adds energy to said medium in an amount that is less than that of said third means.
29. The combination as defined in claim 20 and additionally including:
a. A perforate plate member carried by said means defining said cavity and covered by said diaphragm, said diaphragm being disposed over said plate member and between it and said medium in said cavity.
UNETEE STATES PATENT @FFECEE I CERHFEQATE @F CURREQTEQN Patent No. 9 Dated November 14, 1972 Jack D. Daugherfiy, Dial" id H. Douglas-Hamilton, Inventor(s) Richard M. Patrick and E an R. Pugh It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below? E" u i -1 In Abstract, line 14, for "lasting read--lasing--; Column 13, line 51, for ken/anew read-- 8 E/a m Column 13, line 57, for "dn /dt (In p" read--dn /d1i= 1n p--; Column 14, line 39, for 5E/5m read-- 8 E/B rn Column 14, line 40, for 6E/i5rn read-- B E/B m andColurnn 15, line 58, for "HezN CO read--He:N
Signed and sealed this 29th day of Ma) 1973'.
EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesfting Officer Commissioner of Patents