US2867745A - Periodic magnetic focusing system - Google Patents

Description

Jan. 6, 1959 J. R. PIERCE 2,867,745

PERIODIC MAGNETIC FOCUSING SYSTEM Filed Oct. 7. 1953 F/G. v 210 220 .2/ sa J/ a.: j

u /a g5 BVWI A TTOR/VF V United States Patent PERIODIC MAGNETIC FOCUSING SYSTEM John R. Pierce, Berkeley Heights, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 7, 1953, Serial No. 384,717

Claims. (Cl. S15-3.5)

This invention relates to periodic focusing arrangements which utilize a succession of regions of spatially alternating time-constant axially symmetric longitudinal magnetic field to focus a beam of charged particles.

The principles of periodic focusingare set forth in my U. S. Patent 2,847,607, issued August 12, 1958. In a typical focusing arrangement of this kind, annular magnetic elements suitable for serving as pole pieces are spaced apart alcng land surrounding a longitudinal path yof flow of an electron beam. Permanent magnets are then utilized to pole successive pole pieces o-ppositely whereby there is established along the path of flow a 'periodic succession of regions of longitudinal magnetic field, characterized in that the direction of the field is .reversed in successive regions. As is there pointed out, it is found that as the magnetic period is changed or as vthe magnetic strength is increased the focusing exhibits :successive pass and stop bands, i. e., successive regions lof good and had focusing.

An important advantage of periodic focusing arrange- `ments o-f this kind is the much improved magnet efiiciency provi-ded over uniform field permanent magnet focusing. It is found that the improved efficiency makes possible good focusing in this way with magnets a small| fraction of the size and weight needed for uniform field focusing.

Hitherto it had been thought most advantageous to operate at the pass band corresponding to the shortest 'magnetic period both since this resulted in the smallest ripple in the electron beam diameter andso provided the highcst beam transmission efiiciencies, and since it permitted the maximum saving in magnet weightand size.

However, when periodic vfocusing principles are employed to focus the electron beam in a traveling wave tube (a device which utilizes the interaction between a traveling electromagnetic wave and an electron beam over a relatively long path'to secure gain to the wave) it is often found desirable to have magnetic periods which are relatively long, i. e., long relative to the magnetic period corresponding to operation in the pass band requiring the shortest magnetic period but still short relative to the length of the path of lectron ow. It is thought that a magnetic period of from one-half to onetenth the length of the electron path will usually be desirable. Among other considerations, in ltraveling wave tubes long magnetic periods permit ease of fabrication and assembly, facilitating the introduction of wave guide coupling connections for applying and abstracting wave energy to the tube and minimizing the number of magnets and pole pieces necessary. Additionally, long magnetic periods make it possible to utilize conveniently the longitudinal magnetic field in successive regions to bias magnetically ferrite elements which may be inserted along the path of fiow to provide unidirectional attenuation to minimize the tendency towards self-oscillation which is characteristic of such tubes. This will be described in greater detail below.

A long magnetic period can be used consistently with the principles of periodic focusing by operation in 011e 2,867,745 Patented Jan. 6, 1959 2 of the higher pass bands. However, in operation with long magnetic periods, it is particularly important to minimize ripples in the electron beam diameter. The

cautions appear necessary lest the beam transmission efficiency be seriously impaired.

A primary object of the present invention is to minimize the deleterious effect of transition regions in periodic magnetic focusing arrangements. l

A related object is the better to adapt periodic magnetic focusing techniques for use in traveling wave tubes in which relatively long magnetic periods are desirable.

To this end, the invention provides a periodic magnetic focusing system which establishes along the path of electron flow a succession of regions of spatially alternating time-constant longitudinal magnetic field characterized in that along each of these regions thestrength of the magnetic field is higher near the two ends than therebetween.` By peaking the magneticfield strength before each reversal in direction, the mean square of the field strength across each of the transition areas (the peaked field strength portions being considered as part of the transition area) can be made substantially equal to the square of the field strength along the intermediate portions of relatively uniform strength of each successive region. Various specific arrangements will' be described hereinafter which will peak the magnetic field intensity before each reversal in field direction as is desired. Of those to be described in detail a common characteristic is the positioning adjacent to the path of flow of permeable means which selectively shunt magnetic flux `from the path of flow, whereby along the path of ow the strength of the magnetic field is greatest immediately before and after reversals in the direction of the field. Alternatively, it is possible to vary the magnetic field strength along the path of flow in a similar pattern by properly shaping the successive pole pieces between which is established the longitudinal magnetic field. However,

this latter 'technique ordinarily will prove less convenientA along a portion of the path o'f flow of the tube shown in l Fig. l;and i Figs. 3 and 4 show fragmentary portions of alternative forms of periodic focusing systems in accordancewith the invention for utilization with the tubev shown in Fig. 1.

Referring now more particularly to the drawings, Fig. '1 illustrates schematically a traveling wave tube 10 of .the kind well known in the art which utilizes a periodic focusing system for keeping its electron beam aligned with the tube axis.

The various tube elements are enclosedin an evacuated envelope 11. The envelope is, for example, of glass or a suitable non-magnetic metal such'as copper, oreven a magnetic material such as Kovar so long as it is made sufficiently thin so as to become magnetically saturated so readily that it does not seriously reduce the magnetic focusing field along the path offlow. At opposite ends chosen.

fliet-ma;

of the` envelope are positioned an electron gun 12.which serves as the source of an electron-beam and a target or collector electrode 25. The electron gun and collector are aligned4 to provide a longitudinal pathy ofl flow along the tube axis. The electron gun includes; an electronemissive cathode surface 13, an electrodetsystem 1,4; for controlling the intensity of electron emission-arid` formingv the electrons into a well-defined cylindrical beam and-an accelerating anode 15 forV giving the electrons a, longitudinal velocity suited for interaction-with the signal wave' inthe manner characteristic of travelingl wave tube open ation. Disposed along the path ofV owy is a helically coiled conductor 16, ai plurality of operating wave lengths the. two sidewalls 21A, 2.1.3 and 2 2A 22B Of the. Wel/ e. guides 21 and 22 which are apertured for passage therethrough of the tube envelope also are of a magnetically soft conductor so that these walls may simultaneously serve as pole pieces. It may be desirable in some cases, to taper the wave guides so. thatV the spacing between side walls at the point of insertion of the tube envelope correspongls`V to that desired between Ysuccessivepolc pieces.

long, which servesy as. interaction circuit for propagating i a slow electromagnetic4 wave, in; coupled relation with the electron beam.

The helix 16 is joined at opposite ends to an input coupling strip 17 by an impedance matching section 13 and to an output coupling strip 19 .by an impedance t matching section 20. vThese matchmg sections and 20 are extensions of the conductor 16` in which theypitch of the' helixv isl gradually increased. An input wave is applied to. the, upstream end of the interaction circuit by way of'ifnput waveguide coupling connection 21 and the output wave. is abstracted at the downstream end by way of output wave guide coupling connection 22. Each of they wave guide coupling connectionsAZl and 22 i's a section of rectangular Wave guide which has a pair of opposite sidewalls, 21A, 21B and`22A, 22B, apertured for passage therethrough of the tube envelope, and which has a closed end 21C', 22C, and an open end 21D, 22D, by which it can be connected into a wave guide transmission system. Each of the input and output coupling strips 17 and 19 is4 supported in its corresponding wave guide connection. Input Waves are applied to the input Wave guide connection 21v to have a mode of propagation having an electric field vector parallel to the coupling strip 17. In this Way, an electromagnetic wave is introduced into the interaction circuit for travel therealong in a coupling relationship with the electron beam. The electron gun forms a cylindrical electron beam for projection coaxially through the helix. For accelerating the electron bearn longitudinally, the helical conductor 164 is maintained by suitable lead-in connections (not shown) at a potential which is positive with respect to that of the cathode 13 and which may be approximately the same as that of the collector 25 or may be substantially lower. For ecient operation, it is important that the electron ow be substantially parallel to the axis of the helix 16 whereby a minimum number of electrons are lost in striking the conductor. For this purpose it is usual to provide magnetic focusing of the electron beam to counteract the radial space charge forces'in the beam which tend to make it diverge.

The description hitherto has been of a conventional helix-type traveling wave tube. The present invention is directed in its principal application to an improved permanentmagnet periodic focusing system vfor use with such atube. f.

In accordance with an illustrative o ,bodiment of the invention, each of a succession of annular disk-members 31 of material which is magneticallyl soft, i. e., highly permeable, such as soft iron, Permalloy, or one of the ferrites, is disposed around the tube envelope spaced apart along the tube axis for forming a succession of gaps 32 therebetween. The separation of these members determines a distance which is one `half the magnetic period so this spacing should be appropriately Generally, it will befdesirable to choose a magnetic period of from one half to one tenth the length of the path of the velectron flow. These members 3l serve as a series of pole pieces along the path yof flow between which will 'be setup regions of longitudinal magnetic field. To minimize any discontinuities introduced by the input and output wave guide connections,

The unapertured side walls of the wave guides, aswell asthe end closures 21C, 22C, are of a non-magnetic conductor such as copper tov avoid shunting the ilux to. besetv between the apertured side walls.

In accordance with a characteristic of periodic magnetic focusing principles, successive pole pieces must be oppositely poled whereby there results along the tube axis a spatially alternating magnetic field. Gne convenient arrangement for achieving the desired polarities is illustrated. It comprises bridging successive pole pieces 31 by annular cylindrical permanent magnets 33 magnetized longitudinally, successive permanentA magnets being reversed, in sense as shown. The annular magnets 33 advantageously have an inner diameter which is largey relative to the inner diameter of the annular pole pieces so that there are formed annular gaps 32 between successive pole pieces in the region between the tube envelope 11 and the cylindrical magnets 33.y The outer diameter of the magnets advantageously is equal to that of the pole pieces whereby a smooth outer surface is formed along the major portion of the tube length. Moreover, consistent with the purpose ofminimizing the discontinuities introduced by the wave guide input and output connections,y at each of the wave guide connections 21 and 22 a permanent horeshoe-shaped `magnet 24 is bridged between the two apertured side walls MA, ZB and ZZA, 22B thereof which serve as pole pieces andy its size is chosen to provide a magnetic field strength along that portion of the tube axis therebetween, corresponding to that desired along those portions of the tube axis between successive pole pieces. i

As has been stated above, one of the primary purposes of the invention is to make possible good focusing, consistent with periodic focusing principles, with relatively long magnetic periods. In the analysis set forth in my above-mentioned patent Where it is assumed that the eld intensity variation is sinusoidal, it is shown that one of the conditions for stable flow can be represented by a Mathieu function stability plot which is characterized by a succession of stable pass and unstable stop regions. It is found that when the field intensity variation is in the form of a square wave, the conditions for stable flow are similarly characterized by pass and stop regions. To utilize long magnetic periods stably, it is desirable -to operate in one of the higher .pass regions. An effective upper limit exists, however, on the length of the mag-V netic perio-d since the ripple on the beam tends tov increase with longer magnetic periods and it is generally desirable to limit the ripple on the electron beam so that the maximum beam. radius is less than twice the initial beam radius. These various conditions .are analyzed in considerable detail in my aforementioned patent, and for thevarious design considerations applicable to the design of a periodic magnetic focusing system reference can be ma-de thereto.

The principal improvement of the present invention is the increasing of the strength of the longitudinal com* ponent of magnetic held along the tube axis in 4the regions near the pole pieces relative to the intermediate regions. This eiectively minimizes the increase in ripple on the beam which is occasioned by an increase in the magnetic period of the spatially alternating field, and operation in the higher pass bands is made more attractive. In the plot of Fig. 2 there is illustrated as the solid line the desired variation in the magnetic eld strength B along the path of flow. The magnetic intensity B is plotted as the ordinate with the distance along the tube a:iis z aS the abscissa. For the sake of comparison the field variation which would be provided by the focusing structure described in my above-mentioned patent is shown as the broken line. It can be seen that it is desired to peak the magnetic field intensity preliminary to and subsequent to each reversal in direction. The end sought is to have the mean square field in the transition region, designated in the plot as T, substantially equal to the square of the field along the region, designated in the plot as U, of substantially uniform field strength. The points of zero iield along the tube axis correspond to points along the tube axis opposite the centers of the successive pole pieces, and one half the magnetic period corresponds to the distance between corresponding points on successive pole pieces. It is generally desirable to minimize the length of the transition region T, and more particularly to minimize the distance along which the magnetic field strength is very low. To this end, it is advantageous to make the pole pieces as thin as possible without having them become magnetically saturated.

To achieve the field strength variation illustrated, the arrangement shown in Fig. 1 utilizes a succession of thin permeable annular disks 34,'or washers, with several spaced apart along and surrounding the path of flow in each of the regions 32 between successive pole pieces, the spacing from washer-to-washer in each region being less than the spacing between washer and pole pieces. The presence of such permeable washers serves to shunt a portion of the magnetic field from the path of flow. This shunting effect is enhanced the closer the washer spacing. Accordingly, because of the distribution described the field strength will have the desired variation along the path of flow, being higher along the portions corresponding to the ends of each region 32 where there is a wide washer-to-pole piece spacing than along the portions corresponding to the intermediate portion of each region 32 where there is a close washer-to-washer spacing.

In Fig. 3 there is shown a portion of a modified periodic focusing system in accordance with the invention also for use with the traveling wave tube shown in Fig. 1. As above, there is positioned around the tube envelope 1l a succession of annular permeable elements 31 which servel as pole pieces spaced apart along the path of flow by the gaps 32 and associated therewith is the succession of permanent magnets 33 which polarize successive pole pieces oppositely for achieving along the path of fiow a succession of longitudinal field regions characterized by a reversal of field direction with'successive regions. In this instance to shape the field strength variation along the path of flow as is desired in accordance with the principles of the present invention, a succession of thin Walled cylinders 36 of permeable material are positioned closely surrounding the tube envelope in the successive gaps 32 between pole pieces for serving as flux guides. The wall thickness of each cylinder is sufiiciently small to become readily magnetically saturated so that the magnetic field intensity along the path of flow is not too considerably reduced thereby. Additionallv, to provide the desired selective shunting effect on the magnetic flux along the electron nath the wall thickness of each cylinder 36 is less at its two end portions 37, 38 than the intermediate portion 39 therebetween whereby more of the flux is diverted from the path of flow along portions thereof corresponding to this intermediate portion 39 than along portions thereof corresponding to the thinner end portions 37, 33. It should be evident that the greater the difference in wall thickness of the end portions and the intermediate portions the greater the differential shunting effect. For the largest differential effect, the end portions 37, 38 may be eliminated thereafter leaving only a short section of cylinder in each gap 32 spaced apart a predetermined distance from the two poles pieces `defining the ends of each gap.

It should be further evident that by making the tube envelope of a permeable material such as Kovar, the envelope itself may be made to serve the function of the succession of 'cylinders for achieving the, desired fieldl strength variation along the electron path.' "For use in this way,` the thickness of the permeable envelope should be made less at regions adjacent the pole pieces than along the intermediate regions betweenpole pieces.

The lpresent invention has another important advantage in that it facilitates the solution of another problem associated with the operation of traveling wave tubes. This is the problem of minimizing the undesirable effects arising from reflections resulting from mismatches at they by` the insertion distributed along the path of ow of ferrite material whichris appropriately biased magneticalf ly. In particular, it is pointed out that in a helix-type traveling wave tube selective attenuation of the kind desired can be had by surrounding the helix interaction curve with a cylindrical ferrite element which isrbiased in a circumferential stateof magnetization, and furtherthat a helical section of ferrite placed in a longitudinall magnetic field will become biased Vin a state of circumferential magnetization and so will be suited for use in achieving the desired selective attenuation.

Fig. 4 shows a portion of a periodic focusing system in accordance with the invention for use with the tube4 shown inA Fig. 1 which is modified for use with selective. attenuation techniques of the kind just described. A plurality of helical ferrite sections 40 are disposed around the tube envelope 11 along the path of electron flow. To provide the circumferential magnetic bias needed for'. operation as unidirectional attenuation, each ferrite section is positioned in a region 32 oflongitudinal fieldl between Successive pole pieces 31, the longitudinal field inducing a circumferential field in each helical ferrite section.` Because the directivity of the attenuation properties of these ferrite elements depends on the direction of the biasing circumferential magnetic field, and since the direction of the longitudinal magnetic field in the successive regions between pole pieces reverses as is characteristic of periodic magnetic focusing, reversing the direction 4of the circumferential field set up in the helical ferrite elements, special precautions must be taken to have the direction of high attenuation the same for each ferrite section. In the arrangement shown, successive helical ferrite sections in successive gaps 32 are made to have their pitches in opposite senses to compensate for the reversal in direction of the biasing longitudinal fields. Alternatively, successive helical ferrite sections might be of materials having different characteristics to the same end, or ferrite elements may be inserted only in alternate gaps. y

In the usual selective attenuationferrite arrangements, the degree of discrimination between the two directions. is related to the length along the ywave path of the selective attenuation elements. For large discriminations, a relatively long length of ferrite material is desirable. It can be appreciated 'that it would be preferable from the viewpoint of ease of fabrication and assembly to minimize the number of separate helical ferrite sections necessary in the arrangement shown in Fig. 4 at the expense of increasing the length of each helical section. To this end,- it is desirable that the magnetic period provided by the periodic focusing structure be long. As has been discussed above, with long magnetic periods it is important to peak the magnetic field strength before each reversal in the direction of the magnetic field. In operation, it is found that the helical ferrite sections serve as flux guides..

each endl for the same-purposevinv themannerI analogous: to variations in theI-wall'thiekness of the -cylindrical 'uxl guides shown in Pig. 3;

i' It is to be understood that the-various embodimentswhich have been describedare-*merelyillustrative of the principlesof the invention. `Various other arrangements may be devised by oneV skilled irr4 the art without departing fromy the spirit andscope-of vthe invention. For` eitample, it-should be possible-tov approximate the desired field strength distribution along the path of iiow by properly shaping-the'pole pieces, Moreover, although the invention hasbeen described with particular-reference to incorporation in helix-,type traveling wave tubes, theI applications thereof are not so limited, The invention may be utilized inV any apparatus vwhere it is desired to focus a beam of/charged-particles 4overaarelatively longV path.

lWhat is,Y claimed is; n

l: In combination, .an envelope, anelectronsource and a target spacedA apart'withinsaidflenvelope-for-delining therebetween a longitudinal path-offelectron ow, a suc-l cesion of annular'permeableclements -uniformly spaced apart along and 4surroundingthe pat-h;V of electron -liow for serving asl asuccessionof pole-pieces, and a--perma nentmagnet` structure forpolarizing successive pole pieces oppositely for establishing a'continuous successionA ofspatially alternating-focusing-fields Valong the p ath'offlow, and magnetically permeable meansA closelyl surroundingthe envelope and' disposed 'along the path of flow between successive -pole pieces vfor vmaintaining along the path of flow the strength; of-the magnet-ic lield -at values higher at'portions correspondingltoregions adjacent the pole pieces thanfat portions corresponding-to regions intermediate the pole pieces.

2. inl combination, an envelope, an electrony source anda target spaced apart within said envelope for defining therebetween a longitudinal,path-offelectron flow, magnetic means closely surrounding the envelope and ldisposed uniformly along the ypath -of flow for-establishing therealong a continuous succession of regions -of longitudinal magnetic field,I the ldirection of the magnetic -iield reversing in successive regions, and means-for peaking the intensity of the longitudinal magnetic field lalong the path of tiow immediately-before and aftereach reversal in the direction of the magneticlield.-

t 3. in combination-an envelope, an electron sourceand atar-get electrodel spaced apart :within said envelope -for defining therebetween alongitudinal path of flow, uniformiy spaced means ineludingapermanent magnet structure for establishing-along the-path'of iiowra continuous succession of regionszrof longitudinal magnetic field, the `direction of the magnetic field reversing with each successive region, and magnetically permeable means closely surrounding the enveloperfor reducing the strength of the magnetic field along the path of liow in the intermediate portions of each region whereby the strength of the ymagnetic eld is higher near the ends lof each region than along Vsaid intermediate portions.

4. In combination,` an electron source and a'target electrode deningtherebetween a path of electron fiow and means for focusing the electron beam along this path of flow comprisinguniformly spaced means for establishing along said path of ilow'acontinuoussuccession of spatially alternating time-constant Amagnetic Alields- Vcharacter-ized in Alternatively, the cross-sectional that the;intensity-offthezmagnetic riield'is: at a peak' along short',` regions adjacenty the regions@ of. reversal in direc@` tion of the magneticfield;v

5; In combination,- anenvelope, an electron source' andv atarget electrode-spaced apart withinsaid envelope for, deningtherebetweenfaz path of .electron`ow, a succession of'A thini pole' pieces; positioned to` closely f surround the envelope.'and'tunifo-rmly spaced, apart along the path-of iiow,. permanentl magnet;means, for biasing successive pole pieces oppositely, fortestablishing a continuous suc# cession of time-constant--spatially:A alternating magneticV fields along the` path?l of ilowf,,and aplurality of magnetically permeable; elementssurrounding the envelope andsp,aced apart alongthe -path of liow between each pairof successive;pelenpieees, ,characterized'inz that element-to-elcment spacingzgis ,rsmallerr than the` element-to-y pole spacing. c

6,.,In,combinationganrelectronrsource anda target electrode for -d eiiningtilerebetweena.' path of electronilow, a helical conductor positioned: alongaisaid:V pathof flow:1 forpropagating air-,ayelinggwayeffor interaction withV the Cletrbnwaaapluralitycof p ole.- pieces spaced apart along -thegpathoftflow andsurrounding the helical con-V ductor, asuccession of permeable helical ferrite elements positioned along;the,path-of ilowpsuccessive ferrite elements being interposed `between and spaced from successive pole-,pieces, theY pitch of Aadjacentjferrite elements of the,-succession,being` of oppositeesense, and permanent magnet means for polarizing successive pole pieces op-V positely and for; maintainingacircumferential,magnetic bias inthe ferriteizelementsk thereby providing lnonreciprocalattenuationvfor awave .passing along said slow wave structure.-

7. Incombination-1an; evacuated -envelope,gan electronx source-1 and faftargetA electrode spaced apart in said er1- velope-for-detining therebetween Aa path of electron flow, a,- helical conductor for propagating anl electromagnetic wave ,for'interactionwith=the eleetronliiow, af succession ofhelical ferriteelementsfhaving,a magnetic permeability greater than one positioned to'surround -the envelope, a succession of pole pieces positioned along they path of liow, each pole pieceofsaid succession interposed be` tweenl and spaced from successive ferrite elements, and magnetic meansfor-'polarizing successive pole pieces oppesitely and forfmaintaininga circumferential magnetic bias in saidfhelical ferrite element.

8. In combination, an evacuated envelope, two elec. trodes spaced apart in saidwenvelope for defining therebetween a path of electron iiow, means for propagating electromagnetic wave energy in coupling relation with thel electron; flow, and meansfor focusing said electron4 flow and for providing nonreciprocal attenuation to the propagating electromagnetic wave energy, said means comprising a plurality or" magnetically permeable ferrite' windings positioned to surround the path of electron liowv and arranged in spacedsuccession along the length of said electron path, ya succession of pole pieces interleaved withthe succession` of ferrite windings such that the pole piecesand windings are positioned alternately in spaced succession along U,the path of flow, and magnetic means for maintaininga circumferential magnetic bias inthe fers` rite windings and Yfor polarizing adjacent pole .pieces oppo-sitely, whereby Athe magnetic focusing field will be spatially alternating along the path of flow and have greater magnetic intensity inthe region adjacent to the pole pieces than in the region Vof saidferrite windings positioned between adjacent pole pieces.

9. A periodic focusing arrangement, comprising Vaplu.- ralityy of annular pole pieces uniformly; positioned in spaced succession along a common axis, magnetic means for polarizing adjacent pole pieces-of the succession op'- positely for establishing a continuous succession of time` constant spatial alternating fields and magnetically permeable means interposed in the region between adjacentpole pieces andspaced apart therefrom for modifying'the field along the vcommonnaxis such'that the average of themean square of the eld intensity along the axis in the region of the transition between successive half-cycles of the spatially alternating eld is approximately equal to the square of the field intensity along said axis between these transition regions.

10. In a traveling wave tube comprising an evacuated envelope, an electron source and a target in said envelope for dening therebetween a path of electron How, a slow wave transmission circuit for propagating wave energy in coupling proximity to said electron flow, a plurality of annular magnetically permeable elements arranged in a uniformly spaced succession along said envelope and surrounding said envelope, alternate elements serving as pole pieces and having an inside diameter substantially equal to the outside diameter of the envelope and magnetic means for maintaining pole pieces oppositely polarized for forming a continuous succession of spatially alternating magnetic elds along the electron path having 10 a greater eld intensity in the region adjacent the pole pieces than in the region intermediate successive pole pieces.

References Cited in the file of this patent UNITED STATES PATENTS 2,200,039 Nicoll May 7, 1940 2,259,531 Miller et al Oct. 21, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,305,884 Litton Dec. 22, 1942 2,369,796 Ramberg Feb. 20, 1945 2,741,718 Wang Apr. 10, 1956 2,807,743 Ciotii Sept. 24, 1957 OTHER REFERENCES Article by Frank G. Broekman, Electrical Engineering for Dec. 1949, pages 1077-1080.

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