Traveling Wave Tubes: Modern Devices and Contemporary Applications
From its conception in 1943 by Dr. Rudolf Kompfner in England,1 and later its development by Kompfner and John R. Pierce at the Bell Laboratories in the United States,2 the travelling wave tube (TWT) has become the microwave amplifier of choice for many commercial and military systems. Originally developed for communication, these devices have become fundamental to many military applications, including radar, electronic counter measures (ECM) and electronic warfare (EW) systems.
In simple terms all types of TWTs consist of an electron gun, a slow wave structure, magnetic focussing system, RF input and output couplers, and a collector. With operating voltages applied, the electron gun (containing an emitter) produces an electron beam, which is injected into the slow wave structure (SWS).
The magnetic focussing system constrains the electron beam, allowing it to travel longitudinally down the centre of the slow wave structure.
RF power of the appropriate frequency is injected through the input coupler onto the slow wave structure. The electron beam and the RF signal travel down the structure at similar speeds and an interaction between the two results in an energy transfer from the electron beam into the electromagnetic wave, thus achieving an amplification in the RF signal. The collector at the opposite end of the device to the electron gun is designed to collect the spent electron beam and dissipate the remaining energy efficiently.
Developments in material and manufacturing technologies over the past 50 years have aided the advancement of TWT capabilities. Improvements in high purity vacuum-compatible materials such as nickel/copper alloys and pure oxygen-free, high-conductivity (OFHC) copper have been a major contributor to improvements in both life and reliability.
Advances in thermionic cathode technology, resulting in increased operating life, and the development of high-energy product magnetic materials such as Samarium Cobalt, have enabled the reduction in size of magnetic circuits. The use of computer controlled processing systems and component-manufacturing machines have seen achievable tolerances reduce by an order of magnitude, along with a considerable reduction in unit cost.
Numerous structure designs have been conceived since its original conception, offering various advantages to different applications. Ervin Nalos’s paper, first published in the December 1959 issue of Microwave Journal (reprinted this month), focused primarily on high power travelling wave tubes.3 Other circuit types were discussed, including the simple helix, ring-bar and bifilar, demonstrating the considerable understanding and capability of different slow wave structures 50 years ago.
The major constraints to higher performance were materials technology, processing techniques and manufacturing capabilities. The 1959 paper discusses the simple helix, having the capability of continuous wave (CW) powers as high as 10 W at X-band. Today CW helix TWTs have achieved output power levels of several kilowatts at X-band, a considerable achievement, largely due to current material technologies and automated manufacturing processes.
Figure 1 Simple helix slow wave structure (top) and photograph of a tungsten helix structure (bottom).
The ‘simple’ helix continues to be the most commonly used SWS in TWTs since its inception by Kompfner. In its simplest form, a wire or tape wound in the form of a helix, it exhibits the greatest potential of all SWSs, in terms of dispersion control and thus greatest operating bandwidth. Performance characteristics can be controlled through the design of simple and complex pitch tapering, to enhance both narrow and broadband operation, optimising gain, power and efficiency. Figure 1 shows both a sketch of a simple helix structure and a photograph of a tungsten tape helix.
Dispersion characteristics can be controlled through design of helix supports, in terms of material choice and cross-sectional shape and electrically conductive dispersive vanes. Vanes offer the greatest opportunity for dispersion control and are commonly utilized within broadband TWTs of greater than an octave bandwidth.
Bifilar Contra-wound and Ring-bar TWTS
Figure 2 SBifilar contra-wound (top) and ring-bar (bottom) slow wave structures.
Variants on the simple helix include the bifilar helix (made up of two contra-wound helices of equal but reversed pitch), the ring-bar and the ring-loop structure. Sketches of both bifilar and ring-bar structures can be seen in Figure 2. These types of structures enable higher power handling through both thermal capability and higher voltage operation without giving rise to backward wave oscillation (BWO), a major constraint in simple helix structures. The downsides to these types of structures (in relation to the simple helix) are the limitation of bandwidth due to the high dispersion characteristics of the SWS and the increased complexity in manufacture, which directly impacts the cost.
Coupled Cavity TWTs
Figure 3 Slotted (top) and clover-leaf (bottom) coupled cavity slow wave structures.
The feature that distinguishes the coupled cavity TWT from other types is the SWS, which consists of a series of cavities, is coupled by slots. The benefits of this are that the cavities can be designed to operate with high voltage electron beams enabling peak output powers of 10s to 100s of kilowatts, with high average powers. The space harmonic coupled cavity circuit, favoured by most users because of the high (up to 20 percent) instantaneous bandwidth achievable, is particularly suited to integration of periodic permanent magnetic (PPM) focussing. The result is a very compact device that is used in mobile radar systems. Very high average power and CW coupled cavity TWTs are available but these utilise solenoid focussing, which requires significant electrical power and weighs more than PPM focussed devices. Figure 3 shows two of the more commonly used coupled cavity type structures: slotted and clover-leaf.
Developments in the field of emitters, the electron source of travelling wave tubes, have enabled the development of devices capable of 10s and even 100s of thousands of hours of life. Fifty years ago the electron sources used in vacuum devices, including the early TWTs, would have been of the oxide-coated type emitter, restricted to pulsed or low current density CW applications, ideally suited to high-power pulsed devices, like the coupled cavity TWT, used for radar-type applications.
Today, with advances in cathode technology, materials and processing, a range of impregnated tungsten matrix cathodes are the cathode of choice. Capable of considerably higher mean currents, operating CW at high current densities (> 20 A/cm2), the coated tungsten matrix (M-type) cathode is the most commonly used. Other advantages over the oxide cathode include higher resistance to poisoning, increased life and improved manufacturing tolerances because of the machined emitting surface.
In addition to this, coupled with a potted heater assembly, cathodes have been manufactured to survive and function under the most severe vibration and shock levels.
Work continues towards making advances in cathode design and manufacture. Developments include mixed-matrix and reservoir cathodes, and more recently the field emitting cold cathode.4 Although in its infancy, recent research has produced samples nearing the capabilities required for a TWT electron source.
TWT Design and Validation
The introduction of computer modelling and its advances over the past three decades have had a marked impact on the vacuum electronics industry, taking design from long-hand calculations (sometimes only comprehendible to the most advanced mathematicians) to user-friendly computer simulation of all aspects (electronic, mechanical, thermal) of the device design.
Figure 4 OPERA 2D finite element software-electron gun model.
3-D electron beam simulation programmes enable accurate simulation of beam entry, focussing systems and collection. Figure 4 shows a plot from an electron gun model, using OPERA 2D. Together with the constant advances in computing power, designs can be realised in hours or even minutes, and once validated, the latest software is capable of previously unprecedented levels of accuracy.
Figure 5 Microwave Studio parametric model of a helix SWS.
Advances in Particle in Cell (PIC) and parametric codes, combined with complex optimisers, enables accurate simulation of the interaction between electron beam and the slow wave structure. Increases in computing power have enabled the simulation of complex slow wave structures and complete RF circuits. Figure 5 is a cross-section of a helix SWS, showing dielectric helix support rods and dispersion vanes, typically used in broadband helix TWTs.
Figure 6 3D thermal simulation model of TWT collector assembly.
In addition to the advances in electrical design enabled by new codes and improved processing speeds, commercially available codes can be utilised for thermal and mechanical stress analysis. Thermal analysis of TWT collectors enables improved thermal management of new designs. Figure 6 shows a simple thermal model of a single stage collector. The modern-day designer now has a complete package of modelling and simulation codes that, when fully validated with real device data, enable a right-first-time design approach significantly reducing development times and costs.
Present State of the TWT Art
TWT production is limited to a handful of manufacturers throughout the world; major suppliers include CPI, L-3 and Teledyne in the US, e2v, Thales and TMD in Europe, NEC in Japan, and several developing manufacturers in both India and China.
Table 1 Global Selection of Current Helix and Coupled Cavity TWTs
Determining the current state of the art is difficult; many government-funded programmes restrict the publication of data and commercial confidentiality is high due to the competitive markets. Table 1 shows a cross-section of products from various manufacturers, giving a broad view of current capabilities.
Satellite Communications (Ground-based)
Low cost, high reliability and high linearity are key in this commercially competitive market. Offerings are available from all the major manufacturers, whether it is earth stations, Satellite News Gathering (SNG) mobile systems, network hubs or small lightweight flyaway pack systems. Demands for bandwidth are forcing the move towards higher frequencies (Ka-band) and the onset of digital broadcasting requires higher powers.
Notable performance advances have been achieved by NEC and L-3 in the development of Ka-band helix TWTs for this market, with CW power levels as high as 500 W. Another growth area is in small lightweight amplifiers used in flyaway and hand portable systems. Reductions in luggage weight, by most airlines around the world, has forced demand for these systems to become smaller and lighter. In a market where solid state amplifiers and travelling wave tube amplifiers (TWTA) compete head to head, e2v has launched a range of TWTAs (StellarMini™) that are the smallest and lightest currently available.
Advances in multi-octave TWTs developed originally for military applications has lead to opportunities in multi-band TWTAs for both commercial and military communications. Dual- and tri-band devices have been developed by Teledyne, CPI and e2v.
Satellite Communications (Space)
Key attributes of the space TWT include long life (mission life greater than 20 years), high reliability, low power consumption (high efficiency) and low mass. The majority of all TWTs in space have been manufactured by Thales (France) or L-3 Electron Technologies Inc. (US; formerly Boeing/Hughes) with developments progressing at CEERI (India).
Future demand is moving up in frequency as advances in solid state technology capture the low frequency end of the operating spectrum (up to Ku-band) and the overcrowding of traditional bands forces the need for greater bandwidth utilization. The number of satellites being launched at Ka-band is growing fast and is set to continue.
Traditionally the realm of high peak power helix and coupled cavity TWTs, the development of active phased-array radar has seen a significant shift away from vacuum devices towards solid state technology, more suited to compact packaging required in an array system.
Although as requirements become more demanding, requiring higher efficiency, lower thermal dissipation and greater reliability, customers of microwave amplifiers are turning back to TWTs as the preferred option. Over the past three decades TWT reliability has increased considerably; space TWTs have achieved MTBFs of six million hours with efficiencies reaching 50 percent, which makes the TWT a viable alternative to solid-state amplifiers (SSA). Advances in mini TWT technology, driven by airborne towed decoys and MPMs, has lead to compact high efficiency devices ideally suited to phased-array and Synthetic Aperture Radar (SAR) applications.
ECM and EW
The largest market for the helix TWT is in ECM and EW applications, which has seen tens of thousands of devices built into expendable decoy systems and ECM pods around the world. The demands on devices tend to be a combination of those for all other applications, with the added complexity of operation over multi-octave bandwidths. Current demands are for greater bandwidth and higher efficiency in smaller lighter weight packages that are able to operate over extreme temperature ranges and high altitudes. With the growth of unmanned air vehicle (UAV) applications, the military business for TWTs continues to grow.
Over the past decade, the likes of L-3, CPI, Thales and e2v have developed ranges of mini TWTs, predominantly for airborne applications with bandwidths of greater than 2 octaves covering 4.5 to 18 GHz and power levels now exceeding 100 W CW across the full band. Devices have been proven to survive and operate at temperatures ranging from –55° to > 150°C, altitude > 70 kft and shock levels in excess of 500 G.
Figure 7 Deployed fibre optic towed decoy.
Utilization of multi-stage depressed collectors has seen mid-band efficiencies top 50 percent, resulting in reduced thermal footprints and prime power requirements. CPI12 has, over the past two decades, delivered many thousands of mini TWTs into the Raytheon12 Goleta ALE-50 towed decoy programme, which is a notable achievement. Figure 7 shows a typical fibre optic towed decoy (FOTD) TWT platform being deployed.
Advances continue to be made at higher frequencies covering the 18 to 40 GHz band for countering and jamming new threats. With continually changing and advancing threats, plus the upgrades to existing systems, demands on the microwave amplifiers in this market are increasing, continuing to enhance efficiency and expand bandwidth will be necessary to keep ahead of the advancing SSA sector and meet the expectations of customers.
Status of Coupled Cavity TWTs
Many modern radar systems, including new developments, continue to use coupled cavity TWTs. This is because, contrary to popular opinion, coupled cavity TWTs are often more robust, long-lived, reliable and efficient than the solid state alternative. Coupled cavity TWTs currently manufactured cover the frequency ranges from D-band up to M-band. Instantaneous bandwidth of 10 percent is required for most applications, but various techniques have been employed to increase this to 20 percent (normally compromising efficiency or power considerations).
The conventional manufacturing technique for coupled cavity TWTs employs individual cavities and coupling plates brazed together. At Ka-band and above, this technique becomes very expensive, as the machining tolerances become extremely tight.
Alternative methods of production for high frequency TWTs have been investigated, with the ladder structure used by CPI being the most popular. Modern computer aided design techniques have been used to redesign existing coupled cavity TWT designs; the result of this has been much higher manufacturing reproducibility, and hence yields.13
New radar transmitter specifications continue to demand more from the TWT designer; the areas of particular interest are higher mean power, faster warm-up time and higher efficiency. The use of computer aided design tools to investigate these areas has been successfully employed. Notably, e2v has developed and built RF circuits for high mean power that overcome the natural limitation of heat being conducted through iron pole pieces.14 Other manufacturers have increased mean power by improving the electron beam confinement under RF conditions. There is no reason why both techniques cannot be combined to produce coupled cavity TWTs of higher mean power capability.
Figure 8 Frequency and power capabilities of present amplifier technologies.
With the recent development of compound semiconductors into the power amplification domain, a number of power applications have now migrated from tube-based to solid state amplification. This is especially true of sub-kilowatt, narrow-band requirements, with recent developments in Silicon Carbide (SiC) and Gallium Nitride (GaN) extending these devices into multiple-kilowatt capability, to frequencies around 10 GHz and above.15 Figure 8 shows the current solid-state and vacuum tube landscape, with respect to frequency and power.
As solid-state devices increase in capability, more applications will migrate from a tube to a transistor embodiment. However, for the present it is clear that travelling wave tubes continue to offer a compact and efficient amplifier solution, particularly under harsh operating environments. TWT amplifiers can also span a broad frequency bandwidth, approaching three octaves of coverage from a single tube.
Future Direction for TWTs
The future remains bright for TWTs, albeit in a tougher and more competitive market place. The continued progress of solid-state amplifiers will eat into the edges of the TWT domain, but there will remain to be requirements for the amplification of microwaves beyond the present capabilities of solid-state.
For systems with limitations on size, weight, power dissipation and consumption, there are, and will continue to be, numerous applications for vacuum electronic devices (VED). Higher power levels and higher frequencies are areas where tubes have no equal. The continued advances in VED technology will sustain growth.
In the commercial market, High Definition Television (HDTV) and the onset of the digital age are demanding higher powers and higher frequencies. These are major opportunity areas for the TWT.
The defence business worldwide continues to grow, upgrades to existing systems and new platforms, such as UAVs, require higher efficiencies, smaller lighter payload packages and improved reliability. Higher definition radar systems such as SARs and phased-array radar offer opportunities for small, lightweight, high-efficiency devices. Also, government and defence funding is being made available to the industry to continue developing products for the future. An area of considerable interest at present is in the terahertz and sub mm-wave frequency regimes. Research and development in this area include CAD design of MEMS type structures, manufacturability, detection techniques and prototype manufacture. Programmes are as yet undefined but potential uses include UAV SAR for tactical targeting and terrain avoidance and security imaging.16
Inputs on TWT product history and technology development have been provided by Alan Griggs (e2v principal TWT engineer) and Ian Milsom (e2v cathode development and test manager). The overview of current power amplifier technology was compiled by Dr. Cliff Weatherup (e2v strategic technology manager) and Dr. Trevor Cross (e2v chief technology officer). Product and application photographs were provided by Andy Bennett (e2v marketing).
1. R. Kompfner, Travelling Wave Electronic Tube, US Patent no. 2630544, Filed 20th March 1948, Issued 3rd March 1953.
2. J.R. Pierce, “Travelling-wave Tubes,” Proc. IRE, Vol. 35, No. 2, February 1947, pp. 108-111 .
3. E.J. Nalos, “Present State of Art in High Power Travelling-wave Tubes: Part I,” Microwave Journal, Vol. 2, No. 12, December 1959, pp. 31-38.
4. D.R. Wahley, et al., “Operation of a Low Voltage High-transconductance Field Emitter Array TWT,” Proc. IEEE Vacuum Electronics Conference, April 22-24, 2008, pp. 78-79.
5. Product data from Thales web site: http://components-subsystems.thales-catalogue.com.
6. Product data from CPI, Microwave Power Products Division web site: http://www.cpii.com/product.cfm/1/19/65.
7. Product data from Teledyne MEC web site: http://www.teledyne-mec.com/products/productCatalog.aspx.
8. Product data from NEC Microwave Tube web site: http://www.nec-mwt.com/english/products/twt/seihin/index.html.
9. Product data from L-3 Electron Technologies 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/stories/2007/09/17/daily44.html?ana=from_rss.
13. C. Ar, A.V. Piring and P. Tibbs, “F-Programs TWT Design Upgrades,” Proc. IEEE Vacuum Electronics Conference, April 27-29, 2004, pp. 20-21.
14. A. Griggs, “A New Coupled Cavity Circuit for High Mean Power Travelling-wave-Tubes,” IEEE Transactions on Electron Devices, Vol. 38, No. 8, August 1991, pp. 1952-1957.
15. R. Trew, “Wide Bandgap Semiconductor Transistors for Microwave Power Amplifiers,” IEEE Microwave Magazine, Vol. 1, No. 1, March 2000.
16. M.J. Rosker and H.B. Wallace, “Vacuum Electronics and the World Above 100 GHz,” Proc. IEEE Vacuum Electronics Conference, April 22-24, 2008, pp. 5-7.
Brian Coaker joined the English Electric Valve Co. (a GEC subsidiary, later known as EEV, now e2v technologies), Lincoln, UK, as an Apprentice Technician Engineer. He then read BEng Physical Electronic Engineering at Lancaster University, before reading for a Total Technology PhD at the University of Aston in Birmingham. He is a Chartered Electrical Engineer (CEng) and Chartered Physicist (CPhys), member of the Institution of Engineering and Technology (MIET) and the Institute of Physics (MInstP), a Chartered Scientist (CSci) and is a Whitworth Scholar (WhSch). He is currently engaged as general manager of the microwave business of e2v technologies (UK) ltd., with particular interests in the military, commercial and maritime radar sectors. He has authored technical papers in the fields of microwave electronics and electrical breakdown phenomena in vacuum.
Tony Challis joined the English Electric Valve Co., Chelmsford, UK, as an Apprentice Technician Engineer in 1983. He received his HNC in electromechanical engineering from Anglia Polytechnic University (APU), Chelmsford, UK, in 1987. In 1988 he joined a team of development engineers within e2v, developing new products and re-engineering existing devices. With a strong background in mechanical engineering and experience gained in vacuum technology, he progressed to Technical Authority for Helix TWTs. Achievements in electron gun and PPM stack design led to his involvement in the successful development of a range of mini TWTs designed for airborne decoy applications. With this knowledge of TWT design and manufacture, allied with an appreciation for the vacuum electronics business, he is currently product manager for TWTs and microwave amplifier systems.