3,259,786 9 mitt,-d through the sections 16 and apertures 24 to waveguide 10. The four separate wave portions entering member 23 from apertures 24 have the electric fields thereof directed in t-he same circular direction and, thereforei @merge in raember 23 to -provide the required TEol wave 5 mode for travel along waveguide 10. In a manner similar to that described above the amplified wave portions received by @apertures 30 and waveguide sections 19 are converted in mode and transferred to an output rectangular wave,-uide 55. 10 T-hus, the magnetic structure described in connection with FIG. 1 provides a magnetic field componen-t directed perpendi cularly to the axis of the waveguide and having extremel y -short axial periods, wherein the difficulties of flux leakage between adjacent niagnets of -the structure 15 and the problem of providing individual solenoids f<)r -the various magnets is eliminated. In another embodiment of the instant -invention shown in FIG. 5, @the magnetic -field to be shaped by the various magneti c elements is providedby a plurality of magnetic 20 pole,piec es 160 spaced apart along the length of the wave-guide, rather than by a solenoid. Pole pieces 60 alternate in magnetic polarity along the length of the waveguide ilO. Each pole piece provides a radially directed ma.meti c field. A north pole piece is disposed between 25 a pair of south pole pieces and la -south pole piece @is disposed between a pair of north magnetic pole pieoes. Pole pieces 60 may be circularly apertured and radially magnetized magnetic discs, and they may be either permanent magnets or the core members of electromagnets. 30 A plurality of hollow leylindrical magnetic members 61 is disposed along thelength of waveguide .10, at least one of @such members 61 being disposed between, bult spaced from, each pair of adjacent pole pieces 60. Magnetic members 61 are composed preferably of soft mag- 35 netic material of the type -referred to previously. The soft magnetic mem@bers 61 provide effective magnetic short,eirc uits for the magnetic flux prdvided by -the sources of magnetomotive force 60, except in the gaps between ea,ch pole piece 60 and the adjacent magnetic members 40 61. In such gaps a substantial fringing component of magnetic @field penetrates into the interior of waveguide 10 and provides the requisite radial con-lponent of magnetic field for forcing circular undul-atory motickn of an _ electron stream traveling along waveguide 10. 4a The g-aps between the magnetic pole pieces 60 and the soft magnetic members 61 may be filled with a plurality of nonmagnetic members 64 to provide -the desired vacuum-tight waveguide 10. The -structure of this embodiment also may be provided with longitudinal slots, indi- 50 cated as longitudinal -slots 62 and corresponding lands 63 with the advantage of increased interaction efficiency as discussed herein-before in connection with the embodi-ment of FIG. 1. The fringing @field shown in FIG. 5 provides an ex- 55 tremely strong third harmonic radial component of magnetic field, the variation of this radial component along the length of the waveguide being shown by the curve designate d Br in the figure. The -length of the period of t-his m-agnetic field haranonic is -one-third the period de- Go fined by the -axial distance between like pole pi--ces 60. Therefore, an extremely short axial magnetic period for undulating the electrons is provided although employing relatively @widely spaced pole pieces, thereby permitting operation with high;frequency electromagnetic waves and 65 avoiding the difficulties described heret-ofore associated with closely spaced pole pieces. Accordin-.IY, there has been described hexein an improved interaction structure for providing an extremely short magnetic period in a device effecting interchange 70 of energy between an undulating electron stream and an electromagnetic wave ;of very -high frequency traveling in synchronism therewith. The short magnetic penod is provided not only -without reduction in the strength of the magnetic field component required to -und-ulate the elee- 75 tron stream but, infact, with an inereased magnet-ic field component, thereby retaining a strong aniplitude of -undulation of the electrons and an effective exchan,@e of energy between waves and electron stream. The provision of angularly spaced longitudinal slots in the interaction waveguide formed by the magnetic structure results in increased interaction efficiency -by provid-in- a maximum electric field!of the electromagnetic wave close to @the waveguide wall in the region of maximum undulating @magnetic field. While the prinriples of -the invention have been made clear in the ill@ustrative embodiments, t-here -will be obvious to those skilled in the art, many modifications in structure, arrangement, pr<)portions, the elements, materials @and components, used in the practice of the inventio@, and otherwise, which are adapted for rpecific environments and operation requirements, without departirig from @these principles. The appended claims are -therefore intended to cover and embr-ace. any such raodifications within the limits only of the true spirit,andscope of the invention. Wb:at is cl,,aimed is: 1. An enera interchange device comprising: means .Y for projecting ;a istream of -char@ged particles alon-g an axis; a source of magnetomotive force disposed to provide a ma,-netiefield proximate to said axis and directed substantially parallel to said axis; means forming a waveguide @coax@ially ialigned w@ith said axis, said means providing @a s pa@tially,alternating -component of said magnetic field in;a direct-ion perpendicular to said axis for causing said streiam to undulate in a direction transver,,,e to said axis, said means fornung said waveguide beinformed with ;a plurality -of intemal slots p:arallel to said axis; and launching means foT launchin.- an electromagnetic, wave to travel -along @said,axis wherein an electric field component of said wave -is directed perpendicularly to said axis. 2. An enetgy interohan.-e device comprising: means forprojecting a st@reaxn of electrons along an axis; a source of -magnetomotive force disposed to provide a magnetic field proximiate @to said axis iand directed substantially p-arallel to said iaxis; -a plurality of altema:te magnetic -and non-magnotic elements immersed in said magnetic field along said axis, said elements forming a waveguide having a plurality of internal slots parallel to said iaxis; -and launching @means for launching an electromagnetic wave in the @transverse electric mode to travel along said waveguide. 3. An energy interchiange device @comprising: means for projecting a stream of electrons -along an axis, a source of magnetomotive force for providing a magnetic field proximate to said -axis and directed substantially parallel to said @axis, a plurality of magnetic, elements immersed in said magnetiefield and @spaoed apart along said axis, wherei-n said elements shape said magnetic field to have components perpendicular to said axis at points along said -axis between said magnetic elements: means -forming a wa @vegaidecoaxially aligned w-ith said axis in the -region of said perpendicular components of said magnetic field, said waveguide having a plurality of internal slots parallel to said axis; and launching means f@or launching an ele@tromagnetic wave in the transverse electric mode to travel ialong said waveguide wherein the electric field component of saicl wave is directed perpendicularly ito said axis and said magnetic field components. 4. In an energy interaction device, the combination of: a magnetic unit provided with -an elongated aperture therethrough, said unit,comprising a plurality of magnetic members havin.- circular apertures therein and means fGr supporting said magnetic membets in spaced apa@rt relationship with the circular apertures the-reof coaxially aligned to define @said elongated @aperture, a source Gf magnetomotive force for providing a @magnetic field, said

magnetic unit being immersed in @said magnetic field whereby the direction @of said ma.-netic field is substa-@it@ially para@llcl to said elon.-ated aperture, an electron gun disposed opposite one end of said mag@nctic unit for projecting a st-ream of electrons alon.- t-he len,-th of said elon-ated @aperture, and launchinmeans for launcliin.@ an clectroma,-netic wave in the transverse electric mode to travel alon.- the length of said elongated aperture, said aperttire bein.- tformed with a plurality of angularly spaced longitudinal grooves. 5. The combination of claim 4 wherein t-he @lectric field components of said electromagnet:tc wave are directed in @concentric circular paths, said patlis bein.- coaxial with the axis of said elongated aperture. @6. The combiriation of claim 4, -wherein said ma,-netic members comprise soft ma,-- netic material. 7. @In an energy intetaction device, ,the combination of: a ma.-netic unit provided Nvith,an clon.-ated aperture therethrough, said unit comprising a plurality of axially-ali-ned, sandwiclled, apertared members, alternate ones of said members bein-.,magnetic, and the ren-@ainin.- ones of sa;d meml-ars,being nonmagnetic, the apertures of said ,MP -mbers defirung said elongated aperture, a source of magnetomotive force for providin@ a ma-netic field, said ma.-netic unit bein.- imniersed in said ma,-netic field ,whereby the direction of said magnetiefield is substantial@ly parallel to the axis of said elongated aperture, an electron gun disposed opposite one end of said magnetic unit for projecting a stream of electrons alona the length o,f said clon.-ated apertnre, and launchin-.,m-@ans disposed near one end of said mag-netic unit for launchin.- an clectroma-netic wave in the itransverse electric mode to travel along the length of said elongated aperture, said aperture being formed witli a plurality of lon@itudi-nal slots. 8. In an energy interactlon device, the combination -of: a magnetic unit provided with an elongated aderture therethrou.-h, said unit comprising a source of m@a@netomotive force including a plurality of spaced apart - apertured elements, wherein ia magnetic field is provided between each pair ;of adjacent ones of said elements, a plurality of ma-,netic menibers havino. apertures therein, ,at least one of said Magnetic meniber-s being disposed bebween each (>f said pairs of elements, the apv .rtures 0 f said elements and mia@.netic members bein.- coaxially aligned to define said elongated aperture, an electron gun disposed opposite o-ne end of said magnetic unit for projecting a stream of elect-rons along the length of said elongated @aperture, and launoliin- means disposed near one end of said magnetic unit for launching an clectrotnagnetic wave in the transverse electric mode to ,travel alon- the length of said elongated aperture, said aperture bein,a fornied with a plurality of slots along the len,@th of said aperture. 9. The device of cla@m 8, wherein said magnetic field is oppositely directed betiveen adjacent pairs of said elements. 3,259,786 tl2 -10. The device of claim 8, wherein said magnetic elements are unifdrmly spaced apart, the distance betweeii adjacent ones of said mag-netic elements definin.a a mag-netic period, :and wherein said stream of electrons is projected with a velocity such that the electrons thercof progress through a distance @substantially equal to one ma@anctic period dii-ring a time equal to t@hat required for the electroma,-netic wa-ve to progress through a distance equal to one ;magnetic period plus one wavelength 10 of the electroma.-netic wave:as measured alon@- the len@-th of said elongated ap--rture. 111. The devir-c of claim 8, wherein higher harmonic magnetic field components are se-t up in said elongated aperture, and whereiii said stream of electrons is pro15 jected with a velocity such that the electrons thereof progress throu@-h a distance substantially equal to one period of a predetermined harmonic of said magnetic field components durin@, a ftime equal to that required for the electromagnetic wave to progress throu.-h a 20 distance equ.al ito one such period plus one wavelength of the electroma,-netic wave as meastired @alon.@ the length of said elongated aperture. 12. An ener.-y interchange device comprising: means forming an elongated waveguide, said waveguide bein@ 25 formed with a plurality Of internal longitudinal slots; means for launchi-iig jan electroma,@netic wave to travel along said waveguide whereirl an electric field component of said wave is directed transversely to sa;d waveguide; mcans for projecting a stream of char,@ed particles 30 throu,-h said wave,@uide; and a magnet structure adjacent said waveguide, said magnet structure providing a magnetic field ha@ving afirst component directed longitudinally alon.- said wave.ouide and a second component which sp@atially altemates in a direction tra-nsverse to said wave3@' guide for causin@ said stream to follow an undulatin-. path through said - waveguide. 13. The device defined by claim,12 wherein said magnet structure comprises means providing al(>ngitudinally directed niagnetic field and a plurality of spaced mag40 neticalily permeable members imm-,rsed in said magnetic field. 14. The device defined by claim @12 wherein said magnet structure -comprises a plurality of spaced magnetic 45 poles -of alternlating maonetic polarity -and a plurality of soft magnetic members each positioned between and spaced from a pair of said poles. 15. The device defined by claiin 12 wherein said magnet structure includes a plurality of -abuttiiig alternate 50 manetic and nonmagnet members to fo@rm said wave,uide, said members ibeing formed with intemal angularly spaced notches, said @notches being in longitudinal alignriient to form said slots of said waveguide. 55 No references cited. HERMAN KARL SAALBACH, Pi-imai-y E.Yamiiier. S. CHATMON, JR., ASSiStant Exainiizei-.

Un'i'ted States Patent Office 3,259,786 3'259,786 UNDULATING BEAM ENEP.GY INTERCHANGE DEVICE Robert @4. Phillips, Redwood City, Calif., assignor to General Electric Company, a corporation of New York Filed Oct. 18, 1965, Ser. No. 497,154 15 Claims. (Cl. 315-3) Th-is application is a continu-ationin@paxt of a copending application of Robe@Tt M. P-hillips, Se-rial No. 194,935, filed May 15, 1962, now @aband-oned. This invention Telatesto -apparatus for provid-ing ititerc,bange of energy between a stream of char.-ed particles -and an electromagnetic wave and more par@tictilarly to improvements in ;apparatus whetein such interchange of enetgy,is effectedby forcin,- the Charged particle st-ream to travel w-ith an u-ndulatory motion in the presence of the e@lect-r-onia.-notic wave. Iln a travelin.- wavetube, @an electron.streani is exposed to itbe electric and ma.-netic flelds of a trave)lina el,-ctromagnetic wave over :an extended re,-ion along the axis of propagation of the tr@aveling wave. T-his exposure of the @stre-am is effected by projeoting the electron stream alon.- @such,axis f4DT a distance equal to several operat@ing wavelengths of the wave. Ener@gy is exchanged between the wave and the electron stream by appropriate adjustment.of -therelative veloo@ties of wave and, stream. In a copending -application SeTial No. 816,540, filed May 28, 1959, no@v Patent No. 3,129,356, by Rob@Drt M. Philhps .and assigned to @the as&i,-nee of the instant invention, a .t@ravelin.- wave tube is disclosed @vheTein the electron stre,am is forced to undulate a:@bout the axis of wave propagation intheregion of its exposure to th.- travelin.wave. The undulating motion is induced on the stream by ia static magnetic fi6ld,oriented perpendicularly rto -the axis ;of wave propagation rand alternatin.- as a funct-lon of distance,alongthe axis. Theundulatin,- stream h-as an - al,tematin.- componen-t of velocity -transverse to the axis, so thatthe cleetrons in the stream w@itll b@, accoleir-ated of ,retarded iby,a component:of the electric fleld of ithe wave parallel to this iransverse direction of veloci@ty. By @ad@ justing the axial velocity of the stream to have a par.t,icu-1a,r sy@,ch.,ronoas Telationship Nvith the axial phase veloc,i@ty of ithe eleet-roma:gnotic wave an energy-exchanging intetaction between electron st-ream and itraveling electromagnetic wave :occuirs. Thus, the wave may be amplffled by ex-tract-lon by energy from the stream, or ,t.he energy of the stmam may be inereased by extraction of energy -f-rom the wave; a -travelin.@ wave tube employing -the former type of inter-action is known as a traveling wave amplificr,and one employing the latter type jof interaction is known as an electron accelexator. Amon.- the advant-ages @of t@he a:bove-desopi@bed invention employin,@ !an undulating cleotron stream is t@hat an energyexchan.-in,g interaction is possible witho,ut e,mployin.@ ia slowwave structure, such @as a helix or loaded wavegaide, to slow the axial velocity of propa.-ation of ,thewave. Eliniin-ationofthe;slow-wavestractureelimina,tes the -accompanyin.- @disadvantages, such disadvantages inoluding costeness of ithe slow-wave structu@re @and its low-power handfing capabilities. Accordingly, the aforementioned pa-tent apiplhcation disoloses non44Daded rec,t,angul-ax, ciroular @or coaxial wa-veguides for guiding the ,tr,avol,in,g electromagnetic wave along an iaxis fOT in@tetr.ac,tion wi-th an und-ulating electron stream. The electromagne@tic wav,-s may have phase velodties gre,,ater than the velocity of light in such non-,Ioaded waveguides. In the above-iden@tifled patent application the axiod-ly periodic magnetic Reild for inducin.- undalatory motion i,n the eleetron stre:am is provided iby a series of magnets r,paced @ap,art alongthe axis of propa-gat-ion of the wave. With,m the series of magnots the magnetic poles the-reof Patented July 5, 1966 2 ,altetnate in 'sense; i.e., -t-he pole of every second magnet is a north pule and the poles of t-he interj@acent magnets are sou@th poles. In the presence of such an arr-angement of ma,,-netic poles, the electron stream travelina alon.- -the axis of the ituaveling wave -tube encounters @a continuously Tever-qing, or spatiahy periodic, static component of manetc field that is directed parpendicularly to the ams. This st-a@tic, but spatially periodic, it-ransverse magnetic field component forces the eleetron stream to und-ulate 10 period-leally abdutt-he axis -as it travels ;along the length iof the tu-be, the periodicity @of the -undulatory path correspondingwith -the periodicity of ithe magnet@le field. It has be--n foundthat enorgy can be -interchanged@ be@ween wave and electron.stream over,abroad frequency 15 T-ange if a synchronous relationship @between wave and stream is established wherein the eleot-ron stream progresses through a distance equall tD one per-iod of ithe axially poriodic &tatic magnetic field (distance -along t-he axis bet@ween two like pole pieces) duping a -ti-me substan20 tial-ly equal to t-hat required forthe electroma.-netic wave to progress through a distance equal to such magnetic pediod p@lus one wavelen.-th of ithe electromagnetic wave in -the wave-,-ui-de cmiployed. This opitimum synchronous Telationship is the primary factdr determining the changes 25 tobe made intube @struct-ure and paxameter under different conditions of operation of the tube. Where it is desired to substantiailly inerease the frequeney of the eleotromagnet-ic wave empIDyed in a lube.of this type, t-heoreti-cally the optimum synchtonousxel-ationship can be main30 @tained if either the veloci-ty,of ithe electron st-ream is substantially increased or the length of the magnetic period ds subst-antiahy reduced, OT if an effective odmbinat-lon of thesetwo changes is made. However, the velocity of the elect-ron stream cannot be increased beyond t-he veloci@ty 35 ;ofkght. Moreover,,inthe.above-described trave wave tube the volodty of the ele-otron stream is norm-ally a substantial propomtion -of the velocity of h@',ght, so -that to increase ithe stream velocity substantially beyond this value Tequires -the ex-pendit@ure of uneconomica.1 amounts 40 of ener.-y !and the employment of unduly h-igh elect-ron accelerating v(yltages. These high v(ylta.-es are ac@companied @by diffieult electric insu(lation problems. Thexefore, -increasing the electron velocity is an impxactical sol@ution to the maintenanee of optimum synchronism 45 where-ahi.-herfreq@uency@waveistobeemployed. Conseq@uently, :opti-.rnum -synchroni-sm!at hivhor wave -frequencies must be effected by reducing the length of the magne,tic period. '50 5,. Theoretically, the magnetic period length may be reduced by spacin.- closer together the magnets of the structure described. However, although a direct decrease in magnet spacing will preserve the synchronous relationship for increasing wave frequencies, the spacing decrease has attendant disadvantages. Among these disadvantages is the increased tendency for magnetic flux to "leak" or cross the decreased gap between adjacent inagnetic poles without penetrating into the path of the electron stream. Accordingly, the transverse magnetic field component required for beam undulation will be reduced 60 for a particular series of magnetic poles as their mutual spacing is decreased. Additionally, if the magnetic poles constitute the core members of electromagnets, wherein a solenoid is wound around each core member, the reduced spacing between poles may be difficult to obtain 65 without reducing the size of the solenoid. Mailitaining the same magnetic field in the core member of a solenoid @of reduced size requires a correspondingly increased magnitude of current in the winding of the solenoid. If the current is not increased as the -size of the solenoid is 70 decreased the magnetic field strength provided by the poles is correspondingly reduced. Consequently, decreasing the spacing between the magnets of the magnetic

structure described above tends to decrease the transverse ma.-netic field component available to undulate the electron stream unless difficult, costly, and generally undesirable corrective measures are taken. An additional consideration in reducing the ma.-netic pedod is that if the magnetic field strength is maintained substantially constant as the Icngth of the magnetic period is reduced the necessary amplitude of ele@etron stream undulation is not (Yotained. It has been found that an adequate transverse electron velocity can be acquired by the undulating stream only if the magletig field intensity is increqsed proportionately to the increase in frequency. Thus, not only is it necessary to find a solution to overcome the tendency of the field strength to decrease as the ma.-nets of the above-described structure are brought closer to.-ether, but for efficient operation at increasin.- frequencies of the wave, means must be provided to actually increase the magnetic field strength. Accordin.-ly, it is the principal object of this inventiOR to provide improv,-d apparatus of the type wlierein an interchange of energy is effected between an electron stream travelin.- witli undulatory motion and an electromagnetic wave. Another object of this invention is to provide apparatus of the type wherein interchange of energy is effected between an undulatin,@ electron stream and an electromagnetic wave traveling in synchronism therewith for wave freqtiencies - reater than heretofore obtainable. Anotber object of this invention is to provide a ma.netic structure of small magnetic period for inducidg undulation of an electron stream in a traveling wave tube. Another object of this invention is to provide a ma.-netic structure of small magnetic period for a device effecting interchan---e of ener.-y between an undulating electron stream and an electromagnetic wave of very high frequency traveling in synchronism herewith without conseqtient reduction in the ma.-netic field component. Another object of this invention is to provide an improved magnetic structure for a device etecting interchange of energy between an undulatin.@ electron stream and an clectroma.anetic.wave of very high frequency trav,-Iing in synchronism herenvith by providing an in,creased magnetic field component. The fore.-oing objects are achieved by providing, in a traveling wave tube of the type above described, a static periodic magnetic field having a high harmonic content of ma.onetic field distribution. The improved magnetic structure provided comprises a source of magnetomotive force disposed to provide a magnetic field directed parallel to the axis of wave transmission and a plurality of ma.-netic elements imrnersed in the magnetic field and spaced apart along the axis. This improved magnetic structure provides hi,-her spatial harmonic components and, consequently, shorter harmonic periods of the ma-netic field in which the magnetic elements are immersed. In one embodiment of the instant invention, the source of magnetomotive force consists of a pair of opposed spaced ap,-rtured magnetic end members, the magnetic -field directed parallel to the axis being provided between the end members. In this embodiment the magnetic elements are also apertured and are disposed in linear spaced array along the axis between the end members. The resultant magnetic field is shaped by the magnetic elements to have a substantial cornponent perpendicular to the axis of the traveling wave tube at points along the length oj' the array bet,@Neen adjacent magnetic elements. It is a further feature of the invention to provide a plurality of angularly spaced Ion-itudinal slots in the wall of the circular interaction wave.-uide formed by the magnetic elements for the purpose of brin.-ina, the maximum electric field of the electroma.-netic wave close to the waveguide wall which is the re.-ion of maximum undulatin.- magnetic field. 3,259,786 4 The invention will be described @vith reference to the accompanying drawings, whcrein: FIGURE I is an elevational view, partly in cross-section, of one embodiment of the invention; 5 FIGURE 2 is a -cross-sectional view of theembodiment of FIG. 1, illiistrating details of the mode transformer employed; FIGURE 3 is a cross-sectional view of the embodiirent of FIG. 1, illustrating the electric field configuration 10 of the wave transmission mode employed; FIGURE 4 is a schematic cross-sectional view of a portio-@i of the ma,-netic circuit of FIG. 1, illustrating the mode of operation of the embodiment; and FIGURE 5 is a scliematic cross-sectional view of a 1.5 porlion of the ma.-netic circuit of another erwoodiment of this inventio-@i. The travelin.- wave tlibe of FIGS. 1-4 provides an ener,-y-exchangini_z interaction between an electron stream and a traveling clectroma-netic - wave in an evacliated 20 wavegu,d-,- 10. Wave,,,-,,iide 10, of eirelilar cross-section, is adap;ecl to p-opagate ali clectroma.-netic wave alop.the direct-@on of axis 11 thereof in a maprer well known in the art. An cle--tron gun 13 is disposed to p.-ojoet an electron stream 14, sqown pictorially, along axis 11 for 25 interaction with tqe wave travelin.- therealon.-. A plurality of reclan.-ular wavegiiide sections 16 are coupled to waveguide I 0 near one end t,@ereof for lalinching an electroma,-netic wave therealon.-I wave,-Ilide sections 16 rectiviii.- input electroma.-r@etic energy throu.-h respec'Live 30 gas ti.-ht dielectric windows 17. A plura',Iity of rectan.-ular wave-uide sectiois 19 are coupied to waveguide 10 rcar the 'other end thereof for receivin-, amplified cle,-tromagnetic waves from wave.-uide 10, waveguide sections 19 transmittit-ig the electronia-net,'@c epergy of these waves 35 to utilization apparatus tbrou@-h respective gas tigbt dielectric windows 20. Wi:lldow,s 17 and 20 are composed of one of several well-'@-nown niater@'als that are transparent to electromagnet;c waves, but whici.1 do not allow the passa,@e of gaseous moiecules, thereby pormitt,@'n.- the 40 maintenance of a vactium in waie,-uide 10 and waveguide sections 16 and 19. An e'@ectron collector 21 receives the electrons leavi-ii- wave,@uide 10 and dissipates the kinetic ener-y of such electrons. Collector 21 may be provid--d wit'h passa,@es for a suitable coo.ant to flow therethrotigh to prevent excessive temperature of the @to- collector. One end of each of waveguide sections 16 curves toward the central portion of ivavc,-Uide 10 and is affixed to the outer surfice of a circular inember 23, which is an extension of wave,-ti;de 10. Each of waveguide sec50 tions 16 transfers electroma-n.-tic energy to m-@mb,-r 23 and, conseqliently, ivaveguide@ 10 through a respective one of rectan.-Ular apertures 24. The size of each a-perture 24 is equal to the internal cross-sectional area of wave-uide sectio@-is 16. By so curving wave,-uide s,ctions 16, 55 a directional transfer of electroma-netic energy to r@iemb-@r 23 is eff-ected, whereby most of'the eiier.-,y transferred from wave-uide sections 16 travels along the len-,th of waveguide '10 and very little of such energy travels toward electron gun 13. The other end of each of @wave60 giiide sectl'ons 16 is coupled to receive electroma.anetic energy from a respective one of channels 26 of a wave divider 27 (FIG. 2), to be described hereinaft.-r. One end of each of waveguide sections 19 curves toward the central portion of waveguide 10 and is affixed 65 to the outer surface of a circular member 29, which is an extension of waveguide 10. Each of wave,@uid-- sect;ons 19 receives clectroma,@netic energy from member 29 and, consequently, wavegu;de 10 througb a respective one of rectan-ular apertures 3-9. The size of each aper0 ture 30 is eq,,i,,il to the internal cross-sectional area of wave,@uide sections 19. By so curvin.@ wave.-uide sections 19, a directioiial transfer of clectroi-na@netic ener@y from member 29, whereby most of the ene'r,@y trave,in'g 75 along wave.@uide 10 tra@nsfers to waveguide sections 19

3,259,786 5 and very little of such energy continues along member 29 beyond waveguide sections 19. The other end of each of waveguide sections 19 is coupled to transfer electromagnetic energy to a res-nective one of chailnels 26 and a wave divider 27. 5 Electron gun 13, one type of e'ectro-n gun su@table for use with the instant inv,-ntion, comprises an indirectly heated cathode 35 and a cathode heater, not shown, mou-@ited immediately behind cathode 35. The cathode heater is connected to a suitable energizing source for 30 heatirig the calhode 35 to a temperature to emit electrons. A centrally apertured focusing electrode 37 and a correspon-din@ly apertured accelerating anode 33 are provided for projecting the electrons emitted by cathode 35 through circular meniber 23 and along axis 11 of wave15 guide ]LO. Electrode 37 and anode 38 also function to focus the electrons into the concentrated stream 14. An electric potential, not shown, is provided between cathode 35 and anode 38 to project the electron stream alonthe waveguide 10 with the proper velocity for an energy20 exchanging interaction with the electromagnetic wave therein. A cup-shaped member 39, supported from a base member 40, the latter being affixed to one end of circular niember 23, supports the cathode and anode in prop-.r position to form and project the electron stream25 in the operation of the invention, electron stream 14 interchanges energy with an electromagnetic wave traveling to the right in waveguide 10 of FIGS. 1 and 4. As describo,d in the aforementioned patent application, an energy-exchangin.- interaction between electron stream 30 and wave is effected in waveguide 10 by forcing the electron stream to undulate about the axis of the waveguide. The undulatory motion is forced on the electron stream by a static magnetic field component oriented perpendicularly to the axis of waveguide 10 and alternating as 25 a function of distance along the length of the wave,-uide. In the instant invention this static magnetic field component is provided by the novel magnetic structure shown in FIG. 1. This novel magnetic structure includes a magnetic unit which comprises a plurality of axial',Y 40 @aligned, sqndwiched, apertured members. Alternate ones of the apertured members are magnetic and the remaining members are non-magnetic, the aligned apertures of the sandwiched members defining waveguide 10 through the length of the magnetic unit. 45 Specifically, in FIG. 1, the magnetic unit coirprises . a plurality of circularly apertured magnetic discs 42 uniformly spaced apart along the lenath of the tube and a plurality of circularly apertured non-magnetic discs 43 sandwiched alternately between discs 42. Discs 42 and 50 43 have the adjacent flat surfaces thereof brazed together in vacuum-type relationship, whereby the aligned cylindrical inner surfaces of the array of discs 42 and 43 comprise the boundary of circular waveguide 10. The apettures of the discs 42 and 43 are forn-led with 55 angula,rly spaced radial -nbtehes, these notches being longitudinafly allgned to form a iplurality of angularly spaced longitudinal grooves or slots 52 and corresponding lands 53 best shown in FIG. 5. The longitudinal slots increase the circumferential path leng-th of the wall GLirrents in 60 waveguide 10 and provide a maximum of the electric field of the electromagnetic wave close to the wall Cf the waveguide. Since this is allso the region of maximum unduaating magnetic field, the result is a significant increase in interaction efficiency. (For a waveguide 10 of an 65 in-ternal diameterof about one inch, the slots 52 may be labout 1/8 inch in depth and abiout 10 degrees wide with the lands 53 being about twenty degrees wide.) Discs 42 are preiferably selected from the class of material@s catagorized as "soft magnetic materials," this class 70 being,generally characterized as those magnetic mate)rials having relatively small coercive force. Such a class @of magnetic materialr, is described by A. E. De Barr in Soft Magnetic Materials Used in Industry, The Institute of Physics, London, 1953 and by P. R. Barde@ll in Magnetic 7 5 Materials in the Electrical Industry, Philosophical LibTary, New York, 1955. The soft magnetic material employed in the instant embodiment is known under t'ne trade name "Permendur," which is an alloy :of iron cobalt characterized by a relativelly low coercive force but an extremely latge value of saturation magnetic induction. The nonrnagnetig discs 43 are composed preferably of copper. The magnetic unit described is immersed in the magnetiv, field provided by a source of magnetomotive force, such magnetic field being directed proximate to the axis of waveguide 10 and generally parallel thereto. The source of ma@gnetomotive -force comprises an elongatedhollow cylindrical sole-noid 45 clisposed between a pair of pole pieces 46, the latter being preferably of steel. Pole pieces 46 are disc-shaped and are provided with circular apertures therein, the cylhidrical inner surfaces of the pole pieces engagin- the cylindrical outer surfaces of the respeotive magnetic discs 42 disposed opposite ends of the magnetic unit. Solenoid 45 is energized by a suitable source @of direct electric @current, not shown. In the absence of the magnetic unit, the magnetic field provided by solenoid 45 passes through one of pole pieces 46, emerges from the cylindrical inner surfaces of such pole pieces, extends through -the holilow central c(>re of solenoid 45 in a d;rection substantially parallel to the axis of the solenoid and enters the cylindrical inner surface of the other one of pole pieces 46. The spaced array of soft magnetic discs 42 shapes the niagnetic field provided by the magnetomotive force to have strong components perpendicular to the axis of the tube at Points along thelength of waveguide 10 between adjacent ma,@netic discs, as -shown in FIG. 4. These perpendicular components are greatest at the outer radius of waveguide 10 and progressively decrease in strength aS the Tadial distance from axis 11 derreases. This shaped maglietic field comprises higher order spatial harmonics, these spatial harmonics having relatively short ma@@netic p--riods for operatign with electroma@,Pnetic waves of very high frequencies. The magnetic field lines are shown in the regions between adjacent ones of the uppet halfrections of the magnetic discs 42. Magnetic discs 42, having a very high permeability, function, in effect, as magnetic short circuits for the magnetic flux lines. Therefore, very lit@le magnetic ffux is found within the aperture of each magnetic disc. However, in the regions between ma-,@nctic discs 42 the non@magnetic disc 43 are equivalent to air gaps, so that a sulbstantial fringing of the maanetic flux lines occurs. The veotor sets 48 and 49 illustrate the compo@nents of the magnetic field lines which are directedparallel and perp,-ndicular to the axis 11 at a radiusless than the inner radius @of the ma.@netic discs. The magneti,- field components directed parallel to 'axis 11 are denoted as axial or z-components, the z-direction being in@dirated at the rigl,@t end of FIG. 4. The magnetic field components diirected perpendicularly to axis 11 are denoted as radial, or i,-components, the r-direction being also indizated in the figures. Thus, vector sets 48 and 49 illustrate that the magnetic field between magnetic disrs 42 has a subst@ntial component of magnetic field directed radially; i.e., perpendiculaioly to waveguide axis 11. Vector set 48 illustrates that corresponding radial magnetic field component is directed toward axis 11 and vector sret 49 illustrates that the corresponding radial component is directed away from axis 11. Therefore,.a spatially alternating radial coinponent of n-iagnetic field is encountered by the electron stream as it travels along the length of waveguide 10. The curve designated B, represents the fundamental variation cif the radial component of the magnetic fie)ld as a function of position alon@g the waveguide. Tlius, an electron stream traveling down the tubio encounters a complete sinusoidal variation in the radial component of magnetic field as it progresses from one disc 42 to another.

7 It is shown in the aforementioned pate,,it application ,that if the electron stream, in movin.- along the wave,-uide, is forced to undulate so as to haire a periodic velocity component p@arallel to the direction of th-- electric field comiponent of the electroma,@netic wave, an energy-exchanging interaction between stream and wave may ocelir. FIGS. 3 and 4 illustrate the mode of 'Lindi-ilation of electron streai-n 14. TIie curve designated d, represe@its th-. patti of a re-.presentative electron cif the st-eam. T.Ile electro-q undtilates on tne stirface of an ima,-inary cylinder as it travels along the length of the waveguide, periodi.cally alternatipg in the 0-direct,.on about a st@raight line path on the surface of the imaginary cylind-.r and parallel to axis 11. (The circumferential, or 0-direction is illustrated by the arrow at the center of FIG. 3, and represents displacement along a ci@rcular path cone,-ntric ivith respect to axis 11.) This period:c, alternation of th-, electron is due to the elertron stream cro@qsin,@ the alternating rad;al components of static magnetic field as it moves Eelong waveguide 10 so that it is subjected alternat--Iy to positive and negative circumferential forc-@s. These iforces induce in the stream periodic 0-comi)on@-nt 'of ieloc;ty that is porpendicular to the velocity in tl@e axiil direction, a cornpilete cycle of the 0- component of velocity occurin.- in a distance equal to that between the center Of ,t@v,o adjacent discs 42. An electric field exerts a force on a charged particle such as an electron, this force taking place in the direction of the electric field. If the electron is moving, an electric field parallel to the direction of motion will either increase or decrease the velocity of the electron, according -to the relative direction of the electron and the electric field. If, therefore, each time that segments of electron stream 14 have a maximum component of velocity in the 0-direction the segments are immersed in a decelerating electric field of a traveling wave, the stream velocity will progressively decrease and the stream will give up kinetic energy tD the electric field. Accordingly, the wave will be amplified as wave and electron stream progress alon- ithe length of wave-u'de 10 and the device is termes a traveling wave a'm@'plifier. If, on the other hand, the electron stream segments are immersed in an accelerating electric field of a traveling wave each time - that the segments have a maximuni 0component of velocity, the stream velocity will progr6ssively increase and the electron stream will gain energy from the wave. Such a device is termed an electron accelerator. Thus, by adjusting the axial veiocity of the electron stream so that the maximum 0-component of velocity of segments of the undulating stream always encounters of decelerating electric field or always encounters an accelerating clectric field, a synchronous relationship is established and energy is exchanged between stream and wave. As has been described previously, an optimum synchronous relationship takes place when the electron stream progresses through a distance equal to one period of the axially periodic radial magnetic field component during a -time substantially equal to that required for the electroma.-netic wave to progress throu.-h a distance equal to such magnetic period nius one wavelength of the electromagnetic wave in waveguide 10, as measured along the length of the waveguide axis. When the tube is operating in this synchronous manner, as the electron stream originally encounters the wave near the entrance of the magnetic unit some stream segments have their 0-component of velocity increased and other segments have their 0-compc)nent of velocity decreased. However, the alternating radial magnetic component periodically converts the entire 0- component of stream velocity to a totally z-component as the stream moves down the waveguide. Therefore, the synchronous electron stream contains segments havin.- increased axial velocity and segments havin.- decreased axial velocity. Electron bunches thereby form in the stream in a manner well 3,259,786 known in prior art traveling wave tubes. With proper relative wave and stream velocities the undulating electron bunches remain in synchronism@ with decelerating electric @fields ol" the -@,vave and progressively lose energy to the wave to provide traveling wave amplification. In the apparatus described, it is required that the electron stream be forced to undulfte with a periodic component of velocity parallel to the direction of the electric field component of the electromagnetic wave in order to have an energyexchangin- interaction between stream 10 and wave. Inasmuch as the undulatory motion of the electron stream in FIGS. 3 and 4 occurs in the 0-dircetion it is desirable that the electromagnetic wave have a predominate portion of the electric fi-.Id component 15 thereof also parallel to the 0-direction. The electric field coriipoi-ient of such a wave is shown by the dashed circles in FIG. 3, this wave propa-ating along the waveguide in a mode known as the transverse electric mode. In the transverse electric mod-. of wave propagation all 2o electric iield components of the wave are directed perpendicularly to the direction bf wave transmission and therefore may be depicted as lying in transverse planes. Thus, the dashed lines of FIG. 3 illustrate that the electric field components of the wave propagating along 25 axis 11 lie in circular concentric paths about axis 11. The particular circular waveguide mode shown in FIG. 3 is known as the TEO, mode. The TEO, mode requires the sriiallest d-ameter waveguide for propagating a wave of particular @;requency wherein the electric field lines 30 lie only in circtilar paths. Thus, the 0-components of velo,city of the undtilatipg electron stream are parallel to the electric field components of the travelin- wave employed, so that an energy interchanging action between stream and wave will occur if the optimum conditions 35 for synchronism are satisfied by the relative velocities of stream and wave. FIG. 3, which is a transverse cross section view of one of the magnetic discs 42, also illustrates the prev,'@ously meritioned internal Ion F itudinal slots 52 and cor40 responding lands 53. As previoiisly mentioned, the slots 52 increase the path of the currents in the wall of the interaction waveguide 10 formed by the magnet structure whereby the electric field, represented by the dashed lines of FIG. 3, is increased close to the wall of the 45 waveguide. As shown in FIG. 4, the maximum undulating magnetic field also occurs close to the waveguide wall. Because the maximum electric field and maximum undulatin- magnetic field now occur in the same region, a substantial increase in interaction efficiency is pro50 vided as compared to the use of a smooth-walled waveguide. To provide the desired TEO, mode a first wave divider 27, shown in FIG. 2, is employed to convert,a wave traveling in the conventional doitunant mode in a rertangular 55 waveguide to the circularly sy@nmetric TEO, mode shown in FIG. 3. A similar wave divider 27 converts the ampefied circularly symmetrir- wave received by waveguide sections 19 to a rectangular waveguide mode for transmission to a utilization device. In operation, wave di6o vider 27 receives a wave in the dominant rectangular waveguide mode in an input channel 55. Input channel 55 comprises a conventional rectangular waveguide, the channel dimensions shown in cross-section in FIG. 2 being the narrow dimension of the var@ious rectangular 65 waveguides. The broad dimensions of the waveguides @are oriented perpendicularly to the plane of the figure. The energy traveling in channel 55 is divided equally into two equal leng-th waveguide channels 56. The energy in each of channels 5@6 is again divided equally in two 7o waveguide channel-s 26. All channels 26 are of equal length. Channels 26 are coupled to respective ones of wave.-uide sections 16. Thus, the energy entering channel 55 is divided into four equal portions, all portions entering the correspond75 ing waveguide sections 16 in like phase and being trans-