US2975317A - Beam control device - Google Patents

Description

March 14, 1961 C, SUSSKIND BEAM CONTROL DEVICE Filed April 7, 1959 BEAM CGNTROL DEVICE Charles Sussitind, Berkeley, Calif., assignor to The Regents of The University of California, Berkeley, Calif., a corporation of California Filed Apr. 7, 1959, Ser. No. 804,620

Claims. (Cl. 313-87) The present invention relates to an improved electrode structure for controlling and modulating electron beams of high current density, particularly as employed in highpower mircowave tubes. Although high-current electron beams may be employed in a wide variety of applications, `one of the primary applications thereof is found in the field of high-power microwave tubes, such as klystrons or traveling-wave tubes, and thus the following description of the present invention is related thereto.

Many applications of klystrons or the like require amplitude modulation of the beam thereof with square-wave pulses or shaped pulses and these applications include the field of radar, communication systems, navigational aid systems, and linear accelerators. Substantial diniculty has been encountered in providing satisfactory modulation of such tubes because of the high voltages and high current densities employed in the tubes. Conventional vacuum-tube techniques have proven unsuited in this field and thus various other approaches have been attempted.

Referring again to high-power tubes of the klystron type for example, one immediately apparent method of controlling the electron beam thereof is-to control the beam voltage applied to the cathode. This is accomplished by applying to the cathode a negative voltage pulse obtained from a pulse transformer, storage network, or soft-tube pulse modulator. However, several serious disadvantages are inherent in this approach. The modulator required is both large and expensive and must supply both the full beam current and the cathode capacitance current so that special high-voltage, high-current switching tubes are required. Various other diiiiculties are also inherent in this system as, for example, the attendant phase modulation and frequency translation; however, the bulk and expense of associated control equipment will be seen to materially limit the versatility of this type of control. A further and also rather apparent approach to the problem of beam control is that of applying a modulating voltage to the anode in the type of tube structure wherein an isolated anode is provided. While certain material advantages attend this approach a major difficulty is encountered in the modulator itself for a full beam-voltage swing between anode and cathode is needed for cut-off of the beam current. Although this type of beam control is employed in certain commercially available tubes, it is apparent that a substantial cost attaches to the required modulator system so that, therefore, a material disadvantage is attendant thereto.

In analogy to conventional vacuum-tube techniques it is possible to employ a mesh-grid structure between the cathode and anode of a beam tube for controlling the beam current. As would be expected, the modulating voltage applied to this grid structure need only be a small fraction of the beam voltage to attain cut-off of the tube and, consequently, a relatively high value of tube amplification factor is possible. Serious difficulties are encountered, however, in the utilization of a mesh-grid structure because of the high-power characteristics of the application involved and consequently this type of struc- Patented Mar. 14, 1961 ture is limited to use in beam-power tubes of only moderate power output. Not only are mesh-grid structures difficult to construct, in that tolerances must be held very close, but also there is a serious danger of warpage due to the mesh grid being heated by the intercepted current from the cathode. Furthermore, such a grid structure is quite vulnerable to burn-out due to gas bursts or arcs between same and the cathode. An even greater difficulty is encountered in tubes employing oxide-coated cathodes for such oxide coating tends to evaporate from the cathode and to coat the grid structure. As the grid is heated by current intercepted from the beam it becomes a primary emitter upon reaching emission temperature of the coating deposited thereon from the cathode, and in such instances it is not possible to cut off the tube during pulsed operation thereof. It will thus be apparent that the utilization of mesh-grid structures for the control of beam current in a beam-power tube is limited in applicability to tubes of relatively low power and is almost wholly unsuited to tubes operating in middle and upper power ranges.

The foregoing problems have long been recognized by workers in the field and various alterna-tive approaches have been employed in an effort to accomplish suitable beam control with a minimum of expense and bulk associated with the control circuitry. One such approach is `to employ the beam-forming electrode normally included in beam-power tubes as the control electrode for the tube. While certain disadvantages of `the above-noted methods of beam control are overcome by this approach, yet there remains an even more fundamental difficulty in that only relatively poor control is possible. While the beamforming electrode is not subject to bombardment by the beam and thus does not suffer the disadvantages of the above-discussed mesh structure, yet this very dislocation of the electrode from the beam path which provides the advantage `also results in the serious disadvantage of establishing unequal control fields in the beam path area. Various tests with this type of control has shown that it is not possible to equally affect the electron beam entirely across the cross-section thereof, but instead there is provided only a fringing effect which operates in the main upon the beam edges and to a much lesser extent upon the core of the beam. In order to provide beam cut-olf from the beam-forming electrodes it is necessary to employ a very large control voltage with the abovenoted attendant disadvantages thereof in addition to the material limitation upon the possible amplification factor of the tube so controlled. There have been additionally proposed various possible beam-control means suitable mainly for use with hollow-beam tubes wherein it is possible to provide electrodes out of the path of the beam and yet in sucient proximity to each other that certain of the disadvantages of the control by beam-forming electrodes noted above are reduced. Such approaches are, however, not generally applicable, for conventional beampower tubes do not employ hollow beams, and even in those instances where hollow-beam tubes are utilized the approaches suggested to date are limited to particular tube designs.

The present invention is particularly directed to overcoming the above-noted difficulties in the beam control and modulation of beam-power tubes. This is herein accomplished without a material reduction in the cathode emission area of the tubes by the provision of nonintercepting electrode structure placed in close proximity to the cathode and in such proximity between the individual elements thereof and such relation therebetween as to provide a substantially uniform control-field configuration across the surface of the cathode whereby beam modulation is readily accomplished. By the provision of such non-intercepting electrode structure the difficulties of over-heating the control electrode to thereby cause emission therefrom is herein precluded. Furthermore, thereis provided by this invention a relatively low-voltage beam cut-off wherein the amplification of the tube employing this invention is thereby maximized.

It is an object of the present invention to provide improved electrode means for controlling and modulating electron beams of high-current density.

It is another object of the present invention to provide non-intercepting electrode structure for the control and modulation o'f high-current-density beams of microwave tubes.

lt is a further object of the present invention to provide for a beam-power tube control-electrode'structure having a long life and providing a high amplification factor for the tube.

It is yet another object of the present invention to provide for high-power electron tubes an improved control means having advantageous cut-off characteristics.

1t is a still further object of the present invention to provide electrode structure for the Vcontrol and modulation of an electron beam wherein substantially uniform electric fields are generated to affect the beam and yet the electrode structure is positioned out of the beam path for minimum heating thereof.

Numerous other possible objects and advantages of the present invention will become apparent to those skilled in the art from the following description which is presented in terms of a single preferred embodiment of the invention, and thus is not intended as limiting, for a preoise delineation of the true scope of the invention is set forth in the appended claims.

The invention is illustrated in the accompanying drawings, wherein:

Fig. l is a center longitudinal section through a beampower tube and showing only schematically the elements thereof in one preferred embodiment of the present invention;

Fig. 2 is a transverse sectional view taken in the plane 2 2 of Fig. l;

Fig. 3 is a plan view of an alternative cathode and control-electrode structure in accordance with the present invention;

Fig. 4 is a sectional view or" the embodiment of Fig. 3 taken in the plane 4 4 thereof; and

Fig. 5 is a graph illustrating the manner in which tube amplification varies with certain identified control-electr-ode parameters.

Considering now the present invention in some detail and referring iirst to Figs. l and 2 of the drawings, there will be seen to be schematically illustrated therein as a part of a beam-power tube 11 a disc-shaped cathode 12 disposed adjacent to one end of the tube envelope 13, which may be conventionally formed of glass or ceramic, for example. This cathode may be provided with an oxide-coated outer surface 14 whereby same is adapted upon heating to emit electrons in copious quantities therefrom. Electron emission yfrom the cathode 12 is accomplished by heating of same and to this end there is provided a heater iilameut 16 wound upon the rear surface of the cathode disc and preferably insulated therefrom with suitable heat-shielding means 17 confining heat from the heater iilament from dissipating in other directions besides that of the cathode. Suitable heater leads or connectors 1S are provided in extension through the tube envelope 13 for energization of the heater to pass electrical current therethrough and to thereby raise the temperature of the heater in a conventional manner.

-It will be appreciated that electrons are removed from the cathode area by an electric iield gradient established between lthe cathode and other elements of the tube, later defined, and in the present invention control of electron traverse from the cathode is accomplished by controlling this eld gradient. To this end there is herein provided a non-intercepting electrode structure 21 including a plurality of electrically conducting posts 22 extending in insulated relationship through the cathode disc 12. These electrode posts 22 may, as illustrated, extend through apertures 23 formed in the cathode disc so as to be thereby physically spaced from the cathode material and thereby insulated therefrom or, alternatively, insulating material may Ybe provided about the posts 22 to electrically insulate same from the cathode. The arrangement of electrode posts is made in argeometrical manner over the surface of the cathode, as in a spiral, or as in Fig. 2, in an hexagonal manner. Substantially equal spacing between the electrode posts 22 is necessary, and furthermore substantially equal distribution of the posts about the planar surface of the cathode is also of importance. Mounting of the electrode posts 22 may be accomplished by an electrically conducting plate 24 disposed behind the cathode and suitably mounted in the tube envelope inV insulated relationship to other tube elements, particularly asregards the cathode 12. Electron control is provided by applying a modulating or control voltage to the non-intercepting grid structure and this is herein accomplished by the provision of a prong 26 formed of electrically conducting material and extending from the post mounting disc 24 in insulated relationship through the tube envelope 13. Although the portion of each electrode post 22 extending through the cathode 12 may conceivably have a variety of physical configurations, yet it has been found particularly desirable to provide upon the inner ends of the electrode posts 22 electrically conducting spheres 27 having only a slightly greater diameter than that'of the attached post 2.2. Ellipsoidal or cylindrical post ends may also be employed.

Further to the configuration of the non-intercepting electrode structure as regards the cathode through which it extends, it is herein contemplated that the posts and aflixe'd spheres shall not be materially bombarded by electrons emitted from the cathode and this condition is herein furthered by making the diameter of each sphere 27 the same as the diameter of the cathode aperture 23 immediately behind same.

insofar as the foregoing description of the cathode structure and electrode structure are concerned it will, of course, be appreciated that no attempt has been made to illustrate and describe actual physical tube structure insofar as mounting means or insulating means are concerned for it will be readily apparent that appropriate insulating or physical spacing must be provided between such as the heater wire 16 and the cathode 12 as well as between the electrode structure 21 Vand other metallic elements of Ythe tube. As the physical tube structure, aside from the relationship between cathode and control electrode is concerned, does not bear upon the present invention no detailed description or illustration thereof is herein included. Such is further the case as regards the remainder of the beam power tube illustrated, and thus in Fig. l there is only schematically shown a beamforming electrode 31 provided in conventional form and disposition as an annulus about the tube envelope interiorly thereof and spaced from the cathode in front of same. Y In conventional manner there is further provided as elements of a tube 11, in succession away from the cathode 12 and the beam-forming electrode 31, a focusingY anode 32, a microwave amplifier or oscillator circuitry 33, and a beam collector 34. All of these elements are aligned longitudinally of the tube with the abovedescribed cathode and suitable electrical connectors are providedV for applying appropriate potentials to the individual elements whereby the normal functions of the beam power tube are capable of being accomplished.

In normal operation of a beam power tube, such as that above-described, the cathode 12 thereof is heated by the heater 16 with appropriate heater current being provided to the latter via the prongs 18 from a suitable filament supply, illustrated as a battery 36. Temperature of the cathode l2 is thereby raised to the point of thermal emission wherein electrons are readily emitted from the outer coated surface 14 thereof. In the absence of control means, such as is provided by the present invention, a highly positive potential applied to the focusing anode with respect to the cathode will thereby cause electrons emitted from the cathode to travel rapidly toward the focusing anode 32. In passing through the beam-forming electrode 3l suitable electrical potential applied thereto will operate upon the beam in a conventional manner to control the cross-sectional size and configuration thereof as desired for the tube in which it is employed. In the present invention control over the electron beam leaving the cathode is provided by the non-intercepting electrode structure 2l. In practice a suitable control or modulating voltage is applied between the electrode structure 2l and the cathode 12 and such is illustrated in Fig. l as a variable battery 37 which, of course, will be appreciated to be only a schematic illustration inasmuch as beam modulating voltages of varying frequencies may be employed as, for example, audio frequency and radio frequency. The voltage applied to the electrode structure 21 relative to the cathode produces a field about the posts 22 and spheres 27 thereon such that this field acts upon the cathode area to control the voltage gradient immediately in front of the cathode. As previously noted, the control of the electron beam is actually a problem in electrical fields inasmuch as the field gradient is determinative of the acceleration received by electrons emitted from the cathode. lt is possible with the multiplepost electrode structures illustrated in Figs. 1 and 2 to mathematically analyze the electrical fields immediately adjacent the surface of the cathode. This may be accomplished by employing the method of images with the simplifying assumption that the grid posts are considered as a system of point charges above the cathode surface. In accordance therewith there may then be determined an expression for the field gradient at the cathode surface and by setting this gradient equal to zero, which is the condition for beam cut-off, the cut-off amplification factor may be obtained. It is found from such determination that the cut-off amplification factor is directly proportional to the anode-cathode distance and to the radius of the spherical grid post ends. Additionally, such cutoff amplification factor increases with a decreasing separation between the posts. The amplification factor noted above may be further determined in terms of the ratio of post length above the cathode surface (a) to the distance between the posts and a point equidistant therebetween (L), holding constant the post diameter. A plot of this relation appears in Fig, 5 of the drawings as a solid line therein and will be seen to have a maximum value at some intermediate value of the ratio of post heightV to post separation. The solid line plot of Fig. 5 was calculated for a system of three posts which is the Y most easily solved arrangement insofar as relative fields are concerned. However, by the appropriate consideration of the contribution of additional groups of posts it is possible also to derive relationships covering more complicated electrode configurations as, for example, the seven-post arrangement of Fig. 2. Such calculations show that the addition of further posts in regular separation operate only to modify the curve of Fig. 5 as in the manner of the dashed line therein which has been calculated as applicable to a seven-post array. As may be seen from Fig. 5 of the drawing, maximum amplification is achieved at an increased value of the ratio of electrode height to separation for a larger number of electrodes. Calculations upon which the plots of Fig. 5 are based, as well as experimental observation of the present invention in operation, indicate that for the solid curve of Fig. .5 wherein only three electrodes are considered, a maximum amplification is reached for a ratio of a/L of about .75 and further that with increasing numbers of electrodes maximum amplification occurs at ratio values approaching 1.0. Calculations for an eighteen-post array, plotted as the dashed curve in Fig. 5, shows maximum cut-off amplification at a value of a/L of Iapproximately 0.9. Limitations are imposed upon the optimum post separation in that an overly large number of size of posts will materially reduce the cathode emission area. The electrode permeance must thus be maximized consonant with complete electron-beam control by desired maximum electrode potential and for any particular application of the invention an optimum electrode size and spacing may be readily determined.

Aside from the mathematical considerations of the present invention, it will be seen from la consideration of the structure illustrated in Fig. l that electrons emitted from the cathode 12 tend ton be attracted by the anode 2 in a direction perpendicular to the cathode surface. inasmuch as the electrode posts 22 and spheres 27 thereon are not disposed between an electron-emissive surface and the anode 32 along the line of travel of electrons moving perpendicularly to the cathode toward the anode, it then follows that substantially no electron bombardment of the electrode structure results. Inasmuch as the electrode posts, and the term herein is employed as including the spherical ends thereof, receive substantially no electron bombardment, it then follows that the electron beam does not operate to substantially raise the temperature of the control electrode structure. As was previously noted, one of the great difficulties in the control of electron beam tubes lies in the high temperatures to which control structures are raised by electron bombardment, wherein electron emission from such control structure materially interferes with the operation of the tube. In the present invention this difficulty is entirely overcome. Not only are the posts of the control electrode structure disposed so as to receive substantially no electron bombardment, and are thus herein termed nonintercepting electrodes, but also the physical structure of the control electrode is such as to rapidly and readily conduct away such heat as may be imparted to the posts 22. Contrasted to conventional mesh grid structures, the present invention is composed of relatively large members having good conducting properties. Furthermore, the posts of the present invention do not depend primarily upon radiation to maintain a constant temperature thereof, but instead, are quite capable of conducting heat from the outer ends thereof through the cathode structure to the post-mounting disc 24, which is herein made suiciently massive as to thereby provide a thermal sink. The above-noted heat shields above the heater wire 16 serve to prevent the radiation of heat from the heater 16 to either the posts 22 or mounting plate 24. Consequently, the problem of overheating of control electrode structure for a beam-power tube is herein entirely overcome. There is provided by the multiple-post arrangement of the present invention disposed in piercing relationship to the tube cathode, a beam control or modulation highly superior to that previously known. Full and complete control over the electron beam is provided by the multiple-post arrangement hereinbefore described.

Various embodiments of the present invention are possible within the scope thereof, and there is, for examplc, illustrated in Figs, 3 and 4 of the drawings an embodiment of the present invention particularly adapted to an electron source such as that disclosed in U.S. Patents Nos. 2,640,949 and 2,640,950, issued to Leslie l. Cook. The electron source illustrated in Figs. 3 and 4 of the drawings includes an electrically conducting enclosure 51 formed, for example, of mating open-sided boxes S2 and 53 and filled with an activating material 54 as taught in the above-noted patents. A heating element or filament 56 is disposed within the enclosure surrounded by the activating material 54 and is electrically connected through posts 57 extending through the enclosure in insulated relationship thereto for external electrical energization Y the apertures 58 in the face.

7 to impart heat to the activating material for raising the temperature thereof. A suitable heat shield may be disposed about the rear portion 52 of the source. The forwardor front portion 53 of the enclosure S1 is provided witlra number of minute apertures 58 through which vaporized activating material is adapted to ow upon heating thereof to form with the material of the front face 53 a good electron-emitting surface. As regards a more complete description of this electron-emission surface phenomenon, reference is again made to the abovenoted patents to Leslie I. Cook. This described electro-n source is readily ladapted for application in such as a tube or an electron gun and a suitable connector 59 may be aliixed to the source enclosure 51 for applying appropriate electrical potential to the electron source relative to other elements of the device in which it operates.

As regards the control of electrons emitted from the face 53 of the above-described electron source, there is herein provided in accordance with the present invention, a strip or meander 61 upon this face 53 on the outer surface thereof, disposed in encircling relationship about This electrode strip 61 is composed, as may be seen in Fig. 4 of the drawing, of a central, electrical conductor or electrode wire or member 62 encased in insulation 63. It is contemplated by the present invention that there shall be applied between the electrode member or wire 62 and the electron source or cathode upon which is mounted an electrical potential establishing control or modulating iields operating upon the electron-emissive surface of the electron source, to thereby provide desired control over an electron beam generated thereat. It will thus be appreciated that the actual coniiguration of the meander 61 may be altered to suit the particular circumstances. Although this strip or meander 6I is herein shown as weaving in-and-out about the apertures S and the surface 53 of the electron source, yet it will be apparent that inasmuch as no current flow through this electrode conductor 62 is required, it is suitable to provide the strip in the form of complete circles or rectangles or the like about each of the apertures 5S. Alternatively, the electrode 61 may comprise a transmission line adapted to carry such as radio frequency modulating signals so as to apply high frequency modulating fields in the cathode vicinity. In the electron source illustrated in Figs. 3 and 4 of the drawings, electron emission occurs only in a minute area immediately surrounding each of the tiny apertures 58 in the front surface of face 53 of the source. Utilization of the source necessarily requires electron-attracting means, such as, for example, a focusing anode 32 of the tube Il illustrated in Fig. l of the drawing. The electron-attracting iield operates here again, as in the above-described embodiment, to initially accelerate electrons perpendicularly to the emissive surface, and by .the provision of the non-intercepting electrode conguration of the strip 61 there is provided means for establishing controlling electric iields upon this surface to thereby modulate or control the accelerating electron field. The application of a potential between the electrode conductor 62 and the front face 53 of the electron source establishes a potential gradient therebetween which, in turn, produces as desired, a particular electrostatic iield at the electron-emission surface of the electron source. In this particular embodiment of the present invention the control electrode 62 is surrounded by an insulating material inasmuch as it is herein' desired to place the electrode in as close a proximity to the electron-emission surface as possible. This is herein accomplished without modification of a prior-known and highly desirable electron source, by the provision of the insulated strip or meander 61, whereby the electrode is in fact very closely spaced with the electron-emission surface, and yet is not in position either to electrically contact same or to be bombarded by electrons emitted from same. It will be appreciated from a consideration of the embodiment of the invention illustrated in Figs. 3 and 4 that all of the advantages set 8 forth above regarding the embodiment of Figs. 1 and 2 of the drawings are equally applicable in this instance.

In addition to the above-noted advantages of the invention, the non-intercepting electrode structure hereof, and particularly the embodiment of Figs. 3 and 4, has an inherently lower capacitance than alternate structures. This is highly desirable in that the lower the capacitance the more rapid the possible rise time of impressed pulses of modulating voltage, and consequently the invention is well adapted for beam modulation with sharp voltage pulses. It is to be particularly noted that the present invention is in no way limited to planar cathode surfaces, but instead is well adapted to various cathode configurations. The present invention has in fact been employed on a spherical cathode structure wherein a l9-post electrode structure wasemployed and highly satisfactory results have been obtained therefrom.

What is claimed is:

l. A controlled electron source for producing highdensity electron beams comprising a cathode having an electron-emissive surface on the face thereof and a plurality of apertures therethrough spaced in hexagonal array, an anode in spaced relation to said cathode electronemissive face, a plurality of electrode posts extending perpendicularly through said cathode apertures to form an equidistant hexagonal array between said'cathode and anode, and a member behind said cathode face mounting said posts, said posts and cathode being adapted to have a potential impressed therebetween for controlling the electrons emitted from said cathode.

2. In a high-power electron beam device having a cathode and an anode adapted to attract electrons perpendicularly from the former in a beam, control means comprising a plurality of thermally and electrically conducting posts extending through said cathode normal thereto in equidistant hexagonal array and having spherical ends thereon in front of said cathode, said cathode having apertures therethrough for passage of said posts of a diameter at least equal to that of the spherical ends of said posts whereby said cathode and posts are relatively electrically insulated and electron-emissive surface of said cathode is limited to areas other than the projection of said post ends normally onto said cathode, said posts extending beyond the cathode face a distance substantially equal to 0.75 to 1.0 of the distance between any three adjacent posts and a mid-point therebetween, and means mounting said posts behind said cathode and including connections adapted to receive a potential relative to said cathode for controlling by the iield about said posts an electron Ibeam formed at said cathode.

3. A high power electron beam tube comprising a cathode having an electron-emissive face and pierced by a plurality of equally spaced apertures, an anode in spaced relation to said cathode, and a plurality of electrode posts extending perpendicularly through the apertures in said cathode and insulated therefrom so as to form an equally spaced array over said cathode face between said cathode and anode and adapted to establish a potential relative to said cathode for controlling the potential gradient at said cathode, thereby controlling and modulating the electron beam.

4. The device of claim 3 further defined by said posts being composed of a material of good thermal conductivity and additionally comprising a member of Vhigh heat capacity behind the electron-emissive face of said cathode and afxed to said posts.

5. The device of claim 3, wherein each said post has an enlarged ellipsoid on the end located between said cathode and anode and the surface of which is contained within a perpendicular projection of the perimeter of its associated aperture.

. 6. The device of claim 3 wherein each said post has an enlarged sphere on the end located between said cathode and anode, said sphere diameter being no greater than the diameter of its associated aperture.

7. The device of claim 3 wherein said posts are located and spaced so that the ratio of the distance which a post extends beyond the cathode-emissive surface to the distance from a point equidistant between any three adjacent posts and any of the three posts is within the range of 0.75 to 1.0.

8. The device as set forth in claim 6 with the ratio further deined as substantially equal to 0.90.

9. A high power electron beam tube comprising a cathode having a continuous, essentially planar surface with an electron-emissive face thereon and pierced by a plurality of equally-spaced apertures, a means focusing the electrons from said cathode-emissive face into a beam, a plurality of thermally and electrically conducting electrode posts perpendicularly extending through the apertures in said cathode and insulated therefrom so as to form an equally-spaced array over said cathode-emissive surface, maximum diameter of said posts being no greater than the diameter of its associated aperture, said posts extending beyond the cathode-emissive face for a distance substantially equal to 0.75 to 1.0 of the distance from a point equidistant to any three adjacent posts and any of the three posts, and means mounting said posts behind said cathode and including connections adapted to receive a potential relative to said cathode for controlling the electron beam generated at said cathode.

10. The device as set forth in claim 8, wherein said apertures are equally spaced in hexagonal array and said posts form an equally-spaced hexagonal array over said cathode face.

References Cited in the file of this patent UNITED STATES PATENTS 2,068,287 Gabor Ian. 19, 1937 2,130,280 Knoll Sept. 13, 1938 2,316,214 Atlee et al. Apr. 13, 1943 2,358,542 Thompson Sept. 19, 1944 2,581,243 Dodds Ian. 1, 1952 2,640,949 Cook June 2, 1953 2,727,177 Dailey et a1 Dec. 13, 1955 2,801,361 Pierce July 30, 1957 2,810,090 MacNair Oct. 15, 1957 2,864,965 Wang Dec. 16, 1958

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