Millimetre wave double corrugated waveguide TWT

Lead Research Organisation: Lancaster University
Department Name: Engineering

Abstract

Traveling wave tubes (TWTs) are among the oldest electronic amplifiers, invented in 1943, but they remain the only devices able to provide high power in a wide frequency band at microwave & millimetre wave frequencies. Without TWTs, no satellite communications, high-speed wireless communications, radars and many other fundamental applications would be possible. A TWT consists of a filament wound in helical shape where an electron beam is generated. The helix is supported by longitudinal dielectric rods that are placed concentric to a metal vacuum envelope. The electromagnetic wave is slowed at about the same velocity as the electrons, causing bunching of the electrons. In turn, the electromagnetic field retards the bunches and their kinetic energy is transferred to the oscillating field, thereby amplifying it.
Current fabrication technology permits the realisation of helices with diameters below 1mm that support frequencies up to around 60 GHz in principle. To assemble a helix with these dimensions requires a highly skilled operator and takes a long time. Further, the uncertainty in the fabrication of the small parts causes a low yield. The cost of such a millimetre wave tube is very high, >£10k, limiting its use to some specific applications. On the contrary the demand for high power millimetre waves amplifiers is growing. One application, wireless gigabit data communications is a global business that requires wide band, high power, amplification at frequency above 50 GHz to support multigigabit free space transmission. An improved TWT addresses this market need directly.
This project aims to overcome the frequency limitation of the helix TWT by introducing a novel double corrugated waveguide (DCW), in place of the helix. The DCW was conceived by the PI to permit the design and fabrication of the first 1 THz, 1000 GHz, TWT amplifier. The behaviour of the DCW at lower frequency as a slow wave structure with similar performance to a helix, but much easier to manufacture, will be investigated. Precision CNC milling, with micrometer accuracy, will be used to define the DCW.

The great advantage of the DCW over the helix is the ease of assembly: the DCW is a metal structure made from two parts that can be aligned by features and then simply clamped together. One part is a hollow rectangular metal waveguide with two parallel rows of metal pillars along the guide: the other part is a top plate to close and complete the waveguide. No precision alignment is required and the assembly time is minimal: the skill of the assembler having been replaced by the precision of the machining. The introduction of the DCW in TWTs will substantially reduce the fabrication cost and overcome the frequency limitation of the helices. The innovation that the double corrugated waveguide will bring is of great importance and will foster the development of a new family of low cost, high performance vacuum electron devices, that will give the UK outstanding market perspective and employment opportunities. Lancaster University has a strong international reputation in the field and will design the novel TWT, also investigating the range of frequencies that can be amplified by devices based on the new DWP structure. e2v, the main UK vacuum electronics company will support the design process with its fabrication experience and facilities. e2v will also characterise electromagnetically the completed devices. The designs of double corrugated waveguide will be realised by state-of-the-art microfabrication facilities in the Millimetre Wave Technology Group, part of the RAL Space department at the STFC Rutherford Appleton Laboratory.

At the end of the project an optimised design of the first double corrugated waveguide TWT for millimetre wave frequency range will be realised for experimental verification at e2v. This successful realisation will be the prototype demonstrator for the following production engineering for eventual commercial supply.

Publications

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Paoloni C (2015) Double Corrugated Waveguide for Ka-Band Traveling Wave Tube in IEEE Transactions on Electron Devices

 
Description The mini-IPS project has been a collaboration between Lancaster University and the RAL Space department of the STFC Rutherford Appleton Laboratory with the support of e2v . The research has opened the route to a new generation of traveling wave tubes (TWTs) at millimetre wave frequencies. These tubes are fundamental for enabling applications in the field of wireless networks and satellite communications. The aim of the project was to demonstrate that the double corrugated waveguide (DCW) is a promising replacement for the conventionally used helix in TWTs. The DCW is a slow wave structure made of two rows of metal pillars in a rectangular waveguide. The pillars define an electron beam channel for the interaction with the radio frequency field. The main DCW advantages are the possibility of fabrication by one of several microfabrication processes and an ease of assembly, already tested up to 1 THz in the EU FP7 OPTHER project. The use of the DCW overcomes the difficulty, and high cost, of fabrication and assembly of the helices, which typically have diameters in the range of hundreds of micrometres for millimetre wave TWTs.

The study demonstrated that the DCW provides excellent performance when used as slow wave structures in TWTs. The TWT design specifications were determined by e2v on the basis of a possible industrial exploitation. Two sets of specifications were defined. One was a future target for the TWT: 30 dB gain with 100 W output power. The second set of specifications were for a single section TWT for this proof of concept: a frequency range 32- 37 GHz with 20 dB gain. Achieving the latter specifications were considered fundamental to demonstrate the validity of the approach.

The project started with an extensive study to fully characterise and optimise the electrical properties of the DCW. Different topologies were designed and simulated by commercial three dimensional electromagnetic simulators. The general structure of the DCW is based on a central section with uniform pillars where the interaction with the electron beam occurs. Entrance and exit regions with tapered pillars and channel cross sections are used to transform the TE01 mode needed at the rectangular waveguide flanges into the hybrid mode propagating in the DCW. A relevant simulation effort based on three dimensional particle in cell simulations were performed to study the large signal behaviour of the structure. Different DCW length, measured in terms of periods, were simulated for a full characterisation of the structure. The results fully satisfied the design specifications: predicted gain was of the order of 25 dB.with return losses better that -20dB and output power of 20 W.

The tapered entrance and exit sections add length, but no gain, to the TWT. It is important to reduce the length of the slow wave structure to permit a shorter magnetic focussing system and to increase the interaction region. Novel coupler configurations were devised, bending the DCW and accommodating the tapers in the bent sections, achieving an overall length reduction.

Three simulated structures were fabricated both in aluminium and copper at RAL Space by micro CNC milling machine. Linear measurements of transmission and reflection of the TWTs as function of frequency were in excellent agreement with the simulations, confirming the accuracy of the 3D simulations for the design of the DCW.
Exploitation Route The research outcomes of the project bring a novel improvement to the design and fabrication of millimetre wave TWTs. The result have been published in IEEE Transaction on Electron Devices and presented to International Vacuum Electronics Conference 2016 (IVEC2016). IVEC is the most important conference in the vacuum electronics field. A new collaboration with Prof Yulu Hu, School of Physical Electronics , University of Electronic Science and Technology of China , is established to define a novel analytical models based on the solution of the electromagnetic field for the fast design of DCW TWT.

A PhD student will benefit from working in this research topic, writing a Lagrangian code for the fast simulation of DCW TWT.
The structure was considered for a PhD studentship partially funded by the European Space Agency.

Companies in the field, as e2v, and institutions as European Space Agency, RAL or Met office, can now define a long term development plan to incorporate the results of the research in new devices for their applications.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Healthcare,Culture, Heritage, Museums and Collections,Security and Diplomacy

 
Description The project had one year duration and was an the first step of a long term activity to demonstrate the feasibility of a new family of traveling wave tubes for millimetre wave applications. One of the main obstacle to the widespread of millimetre wave applications is the lack of affordable wideband, high power TWTs. In particular, the need of high capacity communication links to support the increasing data traffic can be satisfied by the very wide band available in the millimetre wave frequency range. The work has shown that the double corrugated waveguide.is a promising replacement for the currently used helix structure, with the potential to reduce substantially the manufacturing cost of millimetre wave TWTs. This mini-IPS has set the background for e2v, the main UK company in the field of vacuum electron tubes to compete in the emerging market of millimetre wave applications. The cost reduction expected by the use of the DCW in TWT will provide UK companies of with a competitive advantage in enabling new high frequency applications where high power in a wide frequency band is required. The results of the project have exceeded the scope of the grant, introducing a new topology based on a relevant modification of the original structure which improves the performance and reduce the dimensions of the DCW TWT. It is expected that the next step in the development will be the fabrication and measurement of a complete DCW TWT.
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Security and Diplomacy
Impact Types Economic

 
Description Chist-era
Amount £1,000,000 (GBP)
Funding ID EP/P015883/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2017 
End 03/2019
 
Description DLINK - D-band Wireless Link with Fibre Data Rate
Amount £880,000 (GBP)
Funding ID EP/S009620/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2019 
End 12/2022
 
Description Horizon 2020
Amount € 3,330,000 (EUR)
Funding ID 644678 
Organisation European Commission 
Department Horizon 2020
Sector Public
Country European Union (EU)
Start 01/2015 
End 12/2017
 
Description Horizon 2020 ULTRAWAVE Ultra capacity wireless layer beyond 100 GHz based on millimetre wave Traveling Wave Tubes
Amount € 2,971,368 (EUR)
Funding ID 762119 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 08/2017 
End 09/2020
 
Description DCW Ka band TWT 
Organisation e2v Technologies
Country United Kingdom 
Sector Private 
PI Contribution mini IPS
Collaborator Contribution Meeting to define the specifications of the DCW, support for the design of the main parts of the TWT, market analysis, feasibility analysis. The PDRA Mauro Mineo visited e2v many times to discuss with experts there the development of the project.
Impact One conference paper and one journal paper in IEEE transaction on Electron Devices
Start Year 2014
 
Title WAVEGUIDE 
Description The present invention is a rectangular waveguide (100) providing amplification of an electromagnetic wave via interaction with an electron beam (107) in a channel defined by two parallel rows of alternately provided substantially identical pillars ((109), (111)). The pillars are attached to the basen (113) of the waveguide, with a void between the top of each pillar and the roof (103) of the waveguide. 
IP Reference WO2015063519 
Protection Patent granted
Year Protection Granted 2015
Licensed No
Impact Trans on Electron Device on the structure.
 
Title WAVEGUIDE 
Description The present invention is a rectangular waveguide providing amplification of an electromagnetic wave via interaction with an electron beam in a linear interaction channel where the electron beam enters the waveguide at a first curved part of the waveguide, traverses the linear interaction channel and exits the waveguide at a second curved part of the waveguide. 
IP Reference WO2016059388 
Protection Patent granted
Year Protection Granted 2016
Licensed No
Impact Trans on Electron Device on the structure.