AND DE.VELOPMENT BRANCH DEFENCE

1 AND N4 -Mf::N 7 OF NATIONAL DEFENCEC A N A { ALi oD"FENCE RESEARCHTABI IgaSHMEO OTTAWAUREO TECHNICAL NOTE NO. TN 801A PfMP uE SIP E U u iir DrvIEW OFTRAVELL NG-WAVE TUBE TECHNOLOGYbyR. LuizL Ji7-JP; 'JECr NOi. NOVEMBER 1979 OTTAWA2 -58 75 CAUTION~ This~~tt~r~ iifurr Irshei tip cypteIv. understandidngI tai U roppietarv and patent rtghtS will be AND DEVELOPMENT BRANCHC? .DEPARTMENT OF NATIONAL DEFENCECANADADEFENCE RESEARCH 'ESTABLISHMENT OTTAWA/TECHNICALjcjA COMPREHENSIVE REVIEW OF TRAVELLING-WAVE TUBE TECHNOLOGYR adar ESM SectionDefence Electronics DivisionPROJECT BOOABSTRACTA review of TWT technology is presented comparing selected aspectsand design procedures relative to general theory of operation of various types of TWT designsis discussed together with a review of principles of their constructionand trade revue de la technologie des tubes A ondes progressives (TWT)est presentge, comparant certains aspects et procedures par rappart thgorie g~nirale d'opfration de plusieurs modales de tubes Aondes progressives 'est presentee, avec une revue des principes de fabricationainsi que des avantages et desavantages de chaque ftfIODi+ t r ib -LB c vp-- -iTABLE OF CONTENTSV: PAGEABSTRACT/RESUME.}

2 IiiTABLE OF CONTENTS .. # ..v:; il1 .0 INTRODUCTION .. DESCRIPTION OF THE TWT .. BASIC OPERATION .. THE CHARACTERISTICS OF TWT'S AND VARIOUS CONFIGURATIONS .. The Gain, Bandwidth and Efficiency .. The $ever ..,.. Power and Perveance .. Large Signal (Non-linear) Operation .. Noise .. Milli eter-wave Tube .. Backward-wave Amplifier .. Saturation .. Dual Mode .. Modulation and Control .. The Crossed-field TWT .. SLOW-WAVE STRUCTURES .. Amplification Process .. The Helix .. 111gh Power Limitations .. 16" Other .elical Structures ..,.. 17a) Twin Crosswound Helix .. 17b) Sheath Helix #. *. 17 TABLE OF CONTENTS (CONTINUED)PAGEc) Tape Helix .. 18d) ml Hei ..Ba .. 18e) Rig and Bar Contrawound Helix .. "Forbidden" Zones .. Dielectric Loading .. Derived Structures .. Folded Transmission Lines .. Couled-cavity Structures .. THE ELECTRON BEAM SYSTEM .. Electron -uns .. Lens Compensatod Guns .. Anode Projection -I11er's Principle.

3 Imode-lip Gun (Mathias and King Version of MllerGun .. Hollow Beam Gun .. T W1E BEA AND TRE FOCUSING SYSTEM .. BMgnetic Field Prallel to the Axis .. Be m F cu in .. 2f Uniforni Magnetic Field .. 27f i'}i Periodic Pertiuient Magnet (PPM) Focusing .. Periodic Electrostatic Focusina (P .. Collectors .. Depressed Potential Collectors .. Methods of Cooling .. 30viTABLE OF CONTENTS (CONTINUED) MATHEMATICAL FORMULATION OF TWT .. Travelling-wave Tube Gain .. The Effect of Space Charge .. The Helix as a Spiral Slot Transmission Line .. 38I !i ~~ Phase Velocity and Group Velocity .. Coupled-cavity Structures -A Lumped Circuit Model .. Hollow Beams .. CONCLUDING REMARKS .. -FRNE .. Ve4iyadGopVlct .." FIGURE 1 -PORTION OF THE TRAVELING-WAVE AMPLIFIER PERTAINING TOELECTRONIC INTERACTION WITH RADIO-FREQUENCY FIELDS AND)RADIO-FREQUENCY GAIN .. 51 FIGURE 2 -SCHEMATIC OF THE TRAVELING-WAVE AMPLIFIER .. 51 FIGURE 3 -TWO-SECTION SEVERED TRAVELLING-WAVE TUBE.)

4 52 FIGURE 4 -DIAGRAM SHOWING DIRECTIONS OF ELECTRON AND POWER FLOWFOR: (a) NORMAL TRAVELLING-WAVE TUBE (FORWARD WAVEAMPLIFIER); (b) BACKWARD) WAVY AMPLIFIER .. 52 FIGURE 5-CROSSED-VIEID TWT .. 53 FIGURE 6 SLOW WAVE lhULTRS~5 SFIURE6 sLW WVESTRUC-TURES .. 53 FIGURE 7 -TWIN CROSSWOUND HELIX .. 53 FIGURE 8 H , ELIX .. 54 FIGURE 9 TAPE HLLIX ..54 FIGURE 10 HELIX STRUCTURES .. 55 FIGURE 1-SLOW AVE STRUCTURES ..** ..*..*.. 5 FIGURE 12 -COUPLED CAVITY STRUCTURE (CRODOROW-NALOS STRUCTURE) .. 57iFIGURE 13 -L "HUGIIE" STRUCTURE .. 57viiTABLE OF CONTENTS (CONTINUED)PAGEFIGURE 14 -THE "CENTIPEDE" STRUCTURE (COUPLING PLATE) .. 57 FIGURE 15 .. 58 FIGURE 16 -PIERCE GUN .. 58 FIGURE 17 -TWO ANODE LENS-COMPENSATED GUN .. 59 FIGURE 18 -GUN DESIGNED BY MATHIAS ACCORDING TO MULLER'S PRINCIPLE 59 FIGURE 19 -MATHIAS AND KING'S HIGH VOLTAGE VERSION OF MUILLER-TYPEGUN ..* * ..tq 60 FIGURE 20 -PERSPECTIVE VIEW SHOWING PATHS OF OUTER ELECTRONS .. 60 FIGURE 21 -A PRACTICAL ARRANGEMENT FOR LAUNCHING BRILLOUIN FLOW.

5 61 FIGURE 22 -A PRACTICAL ARRANGEMENT FOR LAUNCHING SMOOTH FLOW ATFIELDS ABOVE BRILLOUIN .. 61* FIGURE 23 -ELECTRON GUN CONTROL ELECTRODES .. 62"FIGURE 24- BOWING OF FIELD LINES .. 63 FIGURE 25 -C-MAGNET .. 63 FIGURE 26 -SOLENOID WITH SEPARATE COILS AND IRON END PLATES .. 63 FIGURE 27 -CYLINDRiWAL POT PE0 MUNENT MAGNET .64 FIGURE 28 -PPM-FOCUSIN6 STRUCTURE EMPLOYING AXIALLY 4 AGNETIZEDRINGS .. 64--'AMXE 29 -PERIODIC FOCUSING BY CONVERGING MAGNETIC LENSES .. 64 FIGURE 30 -iEPRESSED COLLECTOR " .. 65 FIGURE 31 -A DEPRESSED COLLECTOR WHICH SUPPRESSES SECONDARY CURRFNT 65 FIGURE 32 -EQUIVALENT CIRCUITS .. 66 FIGURE 33 -SPACE CHARGE EFFECT .. 67 FIGURE, 34 .. * 67 FIGURE 35 -EQUIVALENT LUMPED CIRCUITS FOR TiLE COUPLED-CAVITY-STRUCTURE .. 68n viiiI ~ ~TABLE OF CONTENTS (CONTINUED) 36 HOLLOW BEAM GUNS .. 69 FIGURE 37 CROSS-SECTION OF HOLLOW BEAM .69iixHIil l .. ii: S] i fr. PAS4 ix// INTRODUCTIONThe travelling-wave tube (TWT) was invented by R. Kompfner in 1943,and the principle of TWT amplification was demonstrated by him at BirminghamUniversity in the same year.

6 A small signal theory of this wasextensively developed by TWT has been constructed to operate at many wavelengths, rangingfrom one meter to one millimetre. It has found wide application primarilybecause at any given frequency or power level, it offers much more bandwidthSrthan any of its travelling-wave tube, though structurally simple, is a *Con-structed Complication*. Because of its structural simplicity, TWTs havebeen built to operate at frequencies as high as 50 GHz. Modern require-ments for TWr amplifiers often call for very wide bandwidths combined withhigh power any practical application an ideal WIT is seldom available as an#oef-the-shelfV item mainly because more than two dozen Independent parametersare required in order to completely describe the the introduction of the 4T two chief limitations of Magnetronsand Klysvrons, naely, their narrow: bandwidth and s1iw capabilities,were largely overcoue. The field of Electronic Counter Measures (ECM) hasimmensoly benefited from the uniqua capabilities of the T1' is onq of the thoroughly researched technologies.

7 Thezefore,appropriate references should be consulted if the reader i1 interested inji : further dutails on any of the topics described In this report. Severalreferences used in the preparation of this report are DESCRIPTION OF THV TWTThe W1JT is a device to transker energy from a stream of electron6 toan alternating, radio frequency, current, (Figure 1). Unlike Klystrons and,agnetrons which depend upon standinugwaves to produce the high frequencyfxolds with which the beam Interacts locally, the TWT makeb use of the princ-iple of a distributed Interaction mechanium between the electron beam and thetravelling electromanettc waves. Referring to Figure 2, the T1T consists ofa Gun assembly (hpater, cathode and ',ode) which produces a stream of electronscalled an electron beam, a beam focussing system which produces sufficientlongitudinal magnetic field to keep the electron beam from spreading, acollector to absorb the spent beam, a cooling system to help dissipateexcess heat and a helix through which the electron beam passes before beingabsorbed by the BASIC OPERATIONR eferring j Figure 2, at the input the wave is transferred from awave guide to the helix by means of a short antenna and, similarly, at theoutput the wave is transferred from the helix to a short antenna from whichit is radiated into the output wave guide.

8 The purpose of the helix is toslow down electromagnetic wave which is travelling, nominally, at the speedof li, ht, to some required velocity. The signal amplification of a TWT iscreated from the interaction between the electron beam and the electromagneticwave travelling through the helix. The amplification is most effective whenthe speed of the electrons in the beam and the speed of the travelling waveare about equal. For example, the speed of electrons produced by a beamvoltage of 1500V is about 1/13 of the speed of light. Therefore, in orderto slow down the electromagnetic wave travelling at the nominal speed oflight, a helix made of a wire of length 13 times the length of the helixitself is helix, in the absence of the electron stream, supports the pro-pagation of a wave with an axial electric field and such a wave may travel ineither direction. It is found that in the presence of the electron beam thewave travelling against the electron motion is little affected, but the wavetrawvllina in the d.

9 Rectioli of electron motion is broken up into threecomponents. For a lossless helix one of these forward waves is attenuated,one is unattenuated, and the third iucreases in amplitude as it travels (orexperiences uegative attenuation). On the average, the electronS travel a-* :little faster than the amplified the electrons move in synchronism with the at'plified wave, theybecome bunched. I _ese bunches, while moving rapidly in opposing field regions* of the helix, give up energy to. the field. For a given electromagnetic energytravelling doun the helix, the stronger the field that acts on the electrons,the more the bunching and the more energy the bunches give up to producegreater signal amplification. It is found that 4s the frequency is increasedthe EM wave tonds to cling close to the helix such that the electric fieldintensity decreases near the axia of electron flow. Eor this reason the gaintends to decrease as the frequency increases because the field is constrdinedcloser to the helix.

10 At very low frequencies the helix will be only a fewwavelengths long and therefore the gain is low at very low frequencies. Wej. thus obtain a broad gain taxim=m with respect to frequency.(i~ " THE CHARACTERISTICS OF 1VT'S AND VARIOUS CONFIGUMtTIONSTwo basic types of slow-wave structures (or circuits) have receivedconsiderable attention as the interaction structures for travelling-wave are the helix and the periodically-loaded wave guide, which is a microwavo. 3i The helix has many advantages when used as a travelling-wavepropagation structure. It is by nature a broad band device to which itis relatively easy to couple over a wide frequency range. Attenuationmay easily be added at various places along the helix to prevent the tubeS'from oscillating when used as an , high power requirements coupled with broad bandwidthpersuaded scientists to design many modified forms of helices, each onespecifically suited for a particular application. In spite of all these,pulse operated power tubes requiring outputs of several megawatts, withduty cycle ranging from a fraction of a percent to several percents, wererequired.)