travelling wave tube wj 10m28

ilar speeds and an interaction between the tworesults in an energy transfer from the electronbeam into the electromagnetic wave, thusachieving an amplification in the RF signal.The collector at the opposite end of the deviceto the electron gun is designed to collect thespent electron beam and dissipate the remain-ing energy efficiently.

TWT TECHNOLOGYDevelopments in material and manufactur-

ing technologies over the past 50 years haveaided the advancement of TWT capabilities.Improvements in high purity vacuum-compat-ible materials such as nickel/copper alloys andpure oxygen-free, high-conductivity (OFHC)copper have been a major contributor to im-provements in both life and reliability.

Advances in thermionic cathode technolo-gy, resulting in increased operating life, andthe development of high-energy product mag-

32 MICROWAVE JOURNAL n OCTOBER 2008

BRIAN COAKERe2v, Lincoln, UKTONY CHALLISe2v, Chelmsford, UK

From its conception in 1943 by Dr.Rudolf Kompfner in England,1 and lat-er its development by Kompfner andJohn R. Pierce at the Bell Laboratories in theUnited States,2 the travelling wave tube(TWT) has become the microwave amplifierof choice for many commercial and militarysystems. Originally developed for communica-tion, these devices have become fundamentalto many military applications, including radar,electronic counter measures (ECM) and elec-tronic warfare (EW) systems.

In simple terms all types of TWTs consistof an electron gun, a slow wave structure,magnetic focussing system, RF input and out-put couplers, and a collector. With operatingvoltages applied, the electron gun (containingan emitter) produces an electron beam, whichis injected into the slow wave structure(SWS). The magnetic focussing system con-strains the electron beam, allowing it to travellongitudinally down the centre of the slowwave structure.

RF power of the appropriate frequency isinjected through the input coupler onto theslow wave structure. The electron beam andthe RF signal travel down the structure at sim-

TRAVELLING WAVE TUBES:MODERN DEVICES ANDCONTEMPORARYAPPLICATIONS

COVER FEATUREINVITED PAPER

netic materials such as SamariumCobalt, have enabled the reduction insize of magnetic circuits. The use ofcomputer controlled processing sys-tems and component-manufacturingmachines have seen achievable toler-ances reduce by an order of magni-tude, along with a considerable re-duction in unit cost.

Numerous structure designs havebeen conceived since its original con-ception, offering various advantagesto different applications. Ervin Na-loss paper, first published in the De-cember 1959 issue of MicrowaveJournal (reprinted this month), fo-cused primarily on high power travel-ling wave tubes.3 Other circuit types

were discussed, including the simplehelix, ring-bar and bifilar, demon-strating the considerable understand-ing and capability of different slowwave structures 50 years ago.

The major constraints to higherperformance were materials technol-ogy, processing techniques and man-ufacturing capabilities. The 1959 pa-per discusses the simple helix, havingthe capability of continuous wave(CW) powers as high as 10 W at X-band. Today CW helix TWTs haveachieved output power levels of sev-eral kilowatts at X-band, a consider-able achievement, largely due to cur-rent material technologies and auto-mated manufacturing processes.

THE HELIXThe simple helix continues to be

the most commonly used SWS inTWTs since its inception by Kompfn-er. In its simplest form, a wire or tapewound in the form of a helix, it ex-hibits the greatest potential of allSWSs, in terms of dispersion controland thus greatest operating band-width. Performance characteristics canbe controlled through the design ofsimple and complex pitch tapering, toenhance both narrow and broadbandoperation, optimising gain, power andefficiency. Figure 1 shows both asketch of a simple helix structure anda photograph of a tungsten tape helix.

Dispersion characteristics can becontrolled through design of helix sup-ports, in terms of material choice andcross-sectional shape and electricallyconductive dispersive vanes. Vanes of-fer the greatest opportunity for disper-sion control and are commonly uti-lized within broadband TWTs ofgreater than an octave bandwidth.

BIFILAR CONTRA-WOUND ANDRING-BAR TWTS

Variants on the simple helix includethe bifilar helix (made up of two con-tra-wound helices of equal but re-versed pitch), the ring-bar and thering-loop structure. Sketches of bothbifilar and ring-bar structures can beseen in Figure 2. These types ofstructures enable higher power han-dling through both thermal capabilityand higher voltage operation withoutgiving rise to backward wave oscilla-tion (BWO), a major constraint in sim-ple helix structures. The downsides tothese types of structures (in relation to

the simple helix) are the limitation ofbandwidth due to the high dispersioncharacteristics of the SWS and the in-creased complexity in manufacture,which directly impacts the cost.

COUPLED CAVITY TWTSThe feature that distinguishes the

coupled cavity TWT from other typesis the SWS, which consists of a seriesof cavities, is coupled by slots. Thebenefits of this are that the cavitiescan be designed to operate with highvoltage electron beams enabling peakoutput powers of 10s to 100s of kilo-watts, with high average powers.

The space harmonic coupled cav-ity circuit, favoured by most usersbecause of the high (up to 20 per-cent) instantaneous bandwidthachievable, is particularly suited tointegration of periodic permanentmagnetic (PPM) focussing. The re-sult is a very compact device that isused in mobile radar systems. Veryhigh average power and CW cou-pled cavity TWTs are available butthese utilise solenoid focussing,which requires significant electricalpower and weighs more than PPMfocussed devices. Figure 3 showstwo of the more commonly usedcoupled cavity type structures: slot-ted and clover-leaf.

CATHODE TECHNOLOGYDevelopments in the field of emit-

ters, the electron source of travellingwave tubes, have enabled the develop-ment of devices capable of 10s andeven 100s of thousands of hours of life.Fifty years ago the electron sourcesused in vacuum devices, including theearly TWTs, would have been of theoxide-coated type emitter, restricted topulsed or low current density CW ap-plications, ideally suited to high-powerpulsed devices, like the coupled cavityTWT, used for radar-type applications.

Today, with advances in cathodetechnology, materials and processing,a range of impregnated tungsten ma-trix cathodes are the cathode ofchoice. Capable of considerably high-er mean currents, operating CW athigh current densities (> 20 A/cm2),the coated tungsten matrix (M-type)cathode is the most commonly used.

Other advantages over the oxidecathode include higher resistance topoisoning, increased life and im-proved manufacturing tolerances be-


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s Fig. 1 Simple helix slow wave structure(top) and photograph of a tungsten helixstructure (bottom).

s Fig. 2 Bifilar contra-wound (top) andring-bar (bottom) slow wave structures.

s Fig. 3 Slotted (top) and clover-leaf(bottom) coupled cavity slow wavestructures.

cause of the machined emitting sur-face. In addition to this, coupled witha potted heater assembly, cathodeshave been manufactured to surviveand function under the most severevibration and shock levels.

Work continues towards makingadvances in cathode design and man-ufacture. Developments includemixed-matrix and reservoir cathodes,and more recently the field emittingcold cathode.4 Although in its infancy,

recent research has produced sam-ples nearing the capabilities requiredfor a TWT electron source.

TWT DESIGN AND VALIDATIONThe introduction of computer

modelling and its advances over thepast three decades have had amarked impact on the vacuum elec-tronics industry, taking design fromlong-hand calculations (sometimesonly comprehendible to the most ad-

vanced mathematicians) to user-friendly computer simulation of allaspects (electronic, mechanical, ther-mal) of the device design.

3-D electron beam simulation pro-grammes enable accurate simulationof beam entry, focussing systems andcollection. Figure 4 shows a plotfrom an electron gun model, usingOPERA 2D. Together with the con-stant advances in computing power,designs can be realised in hours oreven minutes, and once validated, thelatest software is capable of previous-ly unprecedented levels of accuracy.

Advances in Particle in Cell (PIC)and parametric codes, combined withcomplex optimisers, enables accuratesimulation of the interaction betweenelectron beam and the slow wave struc-ture. Increases in computing powerhave enabled the simulation of complexslow wave structures and complete RFcircuits. Figure 5 is a cross-section of ahelix SWS, showing dielectric helixsupport rods and dispersion vanes, typi-cally used in broadband helix TWTs.

In addition to the advances inelectrical design enabled by newcodes and improved processingspeeds, commercially available codescan be utilised for thermal and me-chanical stress analysis. Thermalanalysis of TWT collectors enablesimproved thermal management ofnew designs. Figure 6 shows a sim-ple thermal model of a single stagecollector. The modern-day designer

36 MICROWAVE JOURNAL n OCTOBER 2008Visit http://mwj.hotims.com/16346-YYY

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s Fig. 4 OPERA 2D finite element softwareelectron gun model.

s Fig. 5 Microwave Studio parametricmodel of a helix SWS.

now has a complete package of mod-elling and simulation codes that,when fully validated with real devicedata, enable a right-first-time designapproach significantly reducing de-velopment times and costs.

PRESENT STATE OF THE TWT ARTTWT production is limited to a

handful of manufacturers throughoutthe world; major suppliers includeCPI, L-3 and Teledyne in the US, e2v,

Thales and TMD in Europe, NEC inJapan, and several developing manu-facturers in both India and China.

Determining the current state ofthe art is difficult; many government-funded programmes restrict the pub-lication of data and commercial confi-dentiality is high due to the competi-tive markets. Table 1 shows across-section of products from vari-ous manufacturers, giving a broadview of current capabilities.

HELIX TWTSSatellite Communications (Ground-based)

Low cost, high reliability and highlinearity are key in this commerciallycompetitive market. Offerings areavailable from all the major manufac-turers, whether it is earth stations,Satellite News Gathering (SNG) mo-bile systems, network hubs or smalllightweight flyaway pack systems. De-mands for bandwidth are forcing themove towards higher frequencies(Ka-band) and the onset of digitalbroadcasting requires higher powers.

Notable performance advanceshave been achieved by NEC and L-3in the development of Ka-band helixTWTs for this market, with CW powerlevels as high as 500 W. Anothergrowth area is in small lightweight am-plifiers used in flyaway and handportable systems. Reductions in lug-gage weight, by most airlines aroundthe world, has forced demand for thesesystems to become smaller and lighter.In a market where solid state ampli-fiers and travelling wave tube ampli-fiers (TWTA) compete head to head,e2v has launched a range of TWTAs(StellarMini) that are the smallestand lightest currently available.

Advances in multi-octave TWTsdeveloped originally for military ap-plications has lead to opportunities inmulti-band TWTAs for both commer-cial and military communications.Dual- and tri-band devices have beendeveloped by Teledyne, CPI and e2v.

Satellite Communications (Space)Key attributes of the space TWT

include long life (mission life greaterthan 20 years), high reliability, lowpower consumption (high efficiency)and low mass. The majority of allTWTs in space have been manufac-tured by Thales (France) or L-3Electron Technologies Inc. (US; for-merly Boeing/Hughes) with develop-ments progressing at CEERI (India).

Future demand is moving up infrequency as advances in solid statetechnology capture the low frequencyend of the operating spectrum (up toKu-band) and the overcrowding oftraditional bands forces the need forgreater bandwidth utilization. Thenumber of satellites being launchedat Ka-band is growing fast and is setto continue.


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RadarTraditionally the realm of high

peak power helix and coupled cavityTWTs, the development of activephased-array radar has seen a signifi-cant shift away from vacuum devicestowards solid state technology, moresuited to compact packaging re-quired in an array system. Althoughas requirements become more de-manding, requiring higher efficien-cy, lower thermal dissipation andgreater reliability, customers of mi-crowave amplifiers are turning backto TWTs as the preferred option.Over the past three decades TWTreliability has increased consider-ably; space TWTs have achievedMTBFs of six million hours with ef-ficiencies reaching 50 percent,which makes the TWT a viable alter-native to solid-state amplifiers(SSA). Advances in mini TWT tech-nology, driven by airborne towed de-coys and MPMs, has lead to compacthigh efficiency devices ideally suitedto phased-array and Synthetic Aper-ture Radar (SAR) applications.

ECM and EWThe largest market for the helix

TWT is in ECM and EW applica-tions, which has seen tens of thou-sands of devices built into expendable

decoy systems and ECM pods aroundthe world. The demands on devicestend to be a combination of those forall other applications, with the addedcomplexity of operation over multi-octave bandwidths. Current demandsare for greater bandwidth and higherefficiency in smaller lighter weightpackages that are able to operate overextreme temperature ranges and highaltitudes. With the growth of un-manned air vehicle (UAV) applica-tions, the military business for TWTscontinues to grow.

Over the past decade, the likes ofL-3, CPI, Thales and e2v have devel-oped ranges of mini TWTs, predomi-nantly for airborne applications withbandwidths of greater than 2 octavescovering 4.5 to 18 GHz and powerlevels now exceeding 100 W CWacross the full band. Devices havebeen proven to survive and operate attemperatures ranging from 55 to > 150C, altitude > 70 kft and shocklevels in excess of 500 G.

Utilization of multi-stage depressedcollectors has seen mid-band efficien-cies top 50 percent, resulting in re-duced thermal footprints and primepower requirements. CPI12 has, overthe past two decades, delivered manythousands of mini TWTs into theRaytheon12 Goleta ALE-50 towed de-coy programme, which is a notable


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TABLE IGLOBAL SELECTION OF CURRENT HELIX AND COUPLED CAVITY TWTs

Market Product Manufacturer Frequency/ OutputBand (GHz) Power (W)

TH3977[5] THALES Ku 750

VTU-6397H1[6] CPI Ku 1200 (Peak)/600 (CW)

MTG5338X[7] TELEDYNE C/X/Ku 350/600/350MEC

LD7314[8] NEC Ka 350 (Peak)/250 (CW)

9100HR[9] L-3 K 50 to 130

TH4725B[5] THALES Ku 100 to150

TH4626[5] THALES Ka 30 to 60

VTS5754F[6] CPI S 130K, 8% Duty

N10570A[10] e2v X 20K, 8% Duty

TL35038[5] THALES Ka 1K, 12% Duty

L6049[11] L-3 4.5 to 18 110

N20160[10] e2v 4.5 to 18 50

TH3893[5] THALES 6 to 16 1500

SatelliteComms(Ground)

SatelliteComms(Space)

Radar

ECM/EW

s Fig. 7 Deployed fibre optic towed decoy.

CO

NTI

NU

OU

S P

OW

ER (

W)

CONVENTIONAL SOLIDSTATE TECHNOLOGY

DOMINATES

1000000

100000

10000

1000

100

10

1

0.10.1 1 10 100 1000

MARINE RADAR

REGION OFTECHNOLOGY OVERLAP

SATELLITECOMMUNICATION

AMPLIFIERS

BROADCASTTV CANCER

RADIOTHERAPY

ELECTRONIC TUBESREQUIRED

LIMIT OF SOLID STATE

TECHNOLOGY

FREQUENCY (GHz)

TOWED DECOYS

s Fig. 8 Frequency and power capabilities of present amplifier technologies.

s Fig. 6 3D thermal simulation model ofTWT collector assembly.

achievement. Figure 7 shows a typicalfibre optic towed decoy (FOTD) TWTplatform being deployed.

Advances continue to be made athigher frequencies covering the 18 to40 GHz band for countering and jam-ming new threats. With continuallychanging and advancing threats, plusthe upgrades to existing systems, de-mands on the microwave amplifiers inthis market are increasing, continuingto enhance efficiency and expand

bandwidth will be necessary to keepahead of the advancing SSA sector andmeet the expectations of customers.

STATUS OF COUPLED CAVITY TWTS

Many modern radar systems, in-cluding new developments, continueto use coupled cavity TWTs. This isbecause, contrary to popular opinion,coupled cavity TWTs are often morerobust, long-lived, reliable and effi-

cient than the solid state alternative.Coupled cavity TWTs currently man-ufactured cover the frequency rangesfrom D-band up to M-band. Instanta-neous bandwidth of 10 percent is re-quired for most applications, but vari-ous techniques have been employedto increase this to 20 percent (nor-mally compromising efficiency orpower considerations).

The conventional manufacturingtechnique for coupled cavity TWTsemploys individual cavities and cou-pling plates brazed together. At Ka-band and above, this technique be-comes very expensive, as the ma-chining tolerances becomeextremely tight.

Alternative methods of productionfor high frequency TWTs have beeninvestigated, with the ladder struc-ture used by CPI being the most pop-ular. Modern computer aided designtechniques have been used to re-design existing coupled cavity TWTdesigns; the result of this has beenmuch higher manufacturing repro-ducibility, and hence yields.13

New radar transmitter specifica-tions continue to demand more fromthe TWT designer; the areas of partic-ular interest are higher mean power,faster warm-up time and higher effi-ciency. The use of computer aided de-sign tools to investigate these areas hasbeen successfully employed. Notably,e2v has developed and built RF cir-cuits for high mean power that over-come the natural limitation of heat be-ing conducted through iron polepieces.14 Other manufacturers haveincreased mean power by improvingthe electron beam confinement underRF conditions. There is no reason whyboth techniques cannot be combinedto produce coupled cavity TWTs ofhigher mean power capability.

FUTURE DEVELOPMENTSWith the recent development of

compound semiconductors into thepower amplification domain, a numberof power applications have now mi-grated from tube-based to solid stateamplification. This is especially true ofsub-kilowatt, narrow-band require-ments, with recent developments inSilicon Carbide (SiC) and Gallium Ni-tride (GaN) extending these devicesinto multiple-kilowatt capability, to fre-quencies around 10 GHz and above.15Figure 8 shows the current solid-state


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and vacuum tube landscape, with re-spect to frequency and power.

As solid-state devices increase incapability, more applications willmigrate from a tube to a transistorembodiment. However, for the pre-sent it is clear that travelling wavetubes continue to offer a compactand efficient amplifier solution, par-ticularly under harsh operating envi-ronments. TWT amplifiers can alsospan a broad frequency bandwidth,

approaching three octaves of cover-age from a single tube.

FUTURE DIRECTION FOR TWTSThe future remains bright for TWTs,

albeit in a tougher and more competi-tive market place. The continuedprogress of solid-state amplifiers will eatinto the edges of the TWT domain, butthere will remain to be requirements forthe amplification of microwaves beyondthe present capabilities of solid-state.

For systems with limitations onsize, weight, power dissipation andconsumption, there are, and will con-tinue to be, numerous applicationsfor vacuum electronic devices(VED). Higher power levels andhigher frequencies are areas wheretubes have no equal. The continuedadvances in VED technology willsustain growth.

In the commercial market, HighDefinition Television (HDTV) andthe onset of the digital age are de-manding higher powers and higherfrequencies. These are major oppor-tunity areas for the TWT.

The defence business worldwidecontinues to grow, upgrades to existingsystems and new platforms, such asUAVs, require higher efficiencies, small-er lighter payload packages and im-proved reliability. Higher definitionradar systems such as SARs and phased-array radar offer opportunities for small,lightweight, high-efficiency devices.

Also, government and defencefunding is being made available to theindustry to continue developing prod-ucts for the future. An area of consid-erable interest at present is in the tera-hertz and sub mm-wave frequencyregimes. Research and developmentin this area include CAD design ofMEMS type structures, manufactura-bility, detection techniques and proto-type manufacture. Programmes are asyet undefined but potential uses in-clude UAV SAR for tactical targetingand terrain avoidance and securityimaging.16 n

ACKNOWLEDGMENTSInputs on TWT product history

and technology development havebeen provided by Alan Griggs (e2vprincipal TWT engineer) and IanMilsom (e2v cathode developmentand test manager). The overview ofcurrent power amplifier technologywas compiled by Dr. Cliff Weatherup(e2v strategic technology manager)and Dr. Trevor Cross (e2v chief tech-nology officer). Product and applica-tion photographs were provided byAndy Bennett (e2v marketing).

References1. R. Kompfner, Travelling Wave Electronic

Tube, US Patent no. 2630544, Filed 20thMarch 1948, Issued 3rd March 1953.

2. J.R. Pierce, Travelling-wave Tubes, Proc.IRE, Vol. 35, No. 2, February 1947, pp.108-111 .


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3. E.J. Nalos, Present State of Art in HighPower Travelling-wave Tubes: Part I, Mi-crowave Journal, Vol. 2, No. 12, December1959, pp. 31-38.

4. D.R. Wahley, et al., Operation of a LowVoltage High-transconductance FieldEmitter Array TWT, Proc. IEEE VacuumElectronics Conference, April 22-24, 2008,pp. 78-79.

5. Product data from Thales web site:http://components-subsystems.thales-cata-logue.com.

6. Product data from CPI, Microwave PowerProducts Division web site:http://www.cpii.com/product.cfm/1/19/65.

7. Product data from Teledyne MEC website: http://www.teledyne-mec.com/prod-ucts/productCatalog.aspx.

8. Product data from NEC Microwave Tubeweb site: http://www.nec-mwt.com/eng-lish/products/twt/seihin/index.html.

9. Product data from L-3 Electron Technolo-gies Inc. web site: http://www.l-3com.com/eti/product_lines_space_twt.htm.

10. Product Data from e2v.11. Product Data from L-3, Electron

Devices web site: http://www.l-3com.com/edd/products_mini_tubes.htm.

12. ALE-50 Contract reference, Business Journal, September 19, 2007,

http://www.bizjournals.com/sanjose/sto-ries/2007/09/17/daily44.html?ana=from_rss.

13. C. Ar, A.V. Piring and P. Tibbs, F-Pro-grams TWT Design Upgrades, Proc.IEEE Vacuum Electronics Conference,April 27-29, 2004, pp. 20-21.

14. A. Griggs, A New Coupled Cavity Circuitfor High Mean Power Travelling-wave-Tubes, IEEE Transactions on ElectronDevices, Vol. 38, No. 8, August 1991, pp.1952-1957.

15. R. Trew, Wide Bandgap SemiconductorTransistors for Microwave Power Ampli-fiers, IEEE Microwave Magazine, Vol. 1,No. 1, March 2000.

16. M.J. Rosker and H.B. Wallace, VacuumElectronics and the World Above 100GHz, Proc. IEEE Vacuum ElectronicsConference, April 22-24, 2008, pp. 5-7.

Brian Coaker joinedthe English ElectricValve Co. (a GECsubsidiary, laterknown as EEV, nowe2v technologies),Lincoln, UK, as anApprentice TechnicianEngineer. He then readBEng PhysicalElectronic Engineeringat Lancaster

University, before reading for a TotalTechnology PhD at the University of Aston inBirmingham. He is a Chartered ElectricalEngineer (CEng) and Chartered Physicist(CPhys), member of the Institution ofEngineering and Technology (MIET) and theInstitute of Physics (MInstP), a CharteredScientist (CSci) and is a Whitworth Scholar(WhSch). He is currently engaged as generalmanager of the microwave business of e2vtechnologies (UK) ltd., with particular interestsin the military, commercial and maritime radarsectors. He has authored technical papers inthe fields of microwave electronics andelectrical breakdown phenomena in vacuum

Tony Challis joinedthe English ElectricValve Co., Chelmsford,UK, as an ApprenticeTechnician Engineer in1983. He received hisHNC inelectromechanicalengineering fromAnglia PolytechnicUniversity (APU),Chelmsford, UK, in

1987. In 1988 he joined a team of developmentengineers within e2v, developing new productsand re-engineering existing devices. With astrong background in mechanical engineeringand experience gained in vacuum technology,he progressed to Technical Authority for HelixTWTs. Achievements in electron gun and PPMstack design led to his involvement in thesuccessful development of a range of miniTWTs designed for airborne decoyapplications. With this knowledge of TWTdesign and manufacture, allied with anappreciation for the vacuum electronicsbusiness, he is currently product manager forTWTs and microwave amplifier systems.


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