Millimetre wave double corrugated waveguide TWT

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.