Travelling Wave Tubes: timeless precision that goes a long way
Do you know what a travelling wave tube is, what it does and how it is made? A new video highlights the innovation, craft and precision required in this technological marvel, refined by world-leader Thales over many decades.
In an age propped up by quintillions of solid-state devices - the semiconductors and microprocessor chips in all our consumer electronics - should you even care about vacuum tubes? Yes is the short answer, because without these intricate objects amplifying the signals coming from satellites and radars, many of the tools we rely on today to travel, to work and for our leisure would not be possible.
Put simply, the tube is an integral part of these transmitters, where the input signal is often very weak and the output needs to be high power. Solid-state devices – which need large amounts of energy to function and to keep them cool - cannot perform this role, especially given the immense distances the signals need to travel, the length of time the transmitters need to function and the amount of power available.
The story of these tubes – known as travelling wave tubes (TWTs) - goes back nearly 80 years, to research performed by Rudolf Kompfner in a British Admiralty radar laboratory during World War II. His invention centred on a vacuum electron tube that could amplify wide-band microwave signals to hugely increase the range of wireless communications.
Shortly after the war ended, Thales – then known as CSF – had set up its own specialised TWT R&D and production centres, producing radars and communication systems that would help launch the space industry in the 1970s. Today, Thales is the world's leading supplier of traveling wave tubes for space, defence and satcom markets.
Most data sent by satellites today uses a Thales amplifier. Many thousands of Thales TWTs have been launched into orbit since 1974 providing over 900 million hours of operation. They are even helping to transmit data from the Exo Mars mission, and to understand worlds at the very edge of our solar system through the New Horizons mission. You can see details on all the missions our TWTs are involved with here.
Over 60 individual skills to build one tube
For over 70 years, the women and men of Thales have been pushing the boundaries of physics to connect people through exceptional products that are designed to last in even the most severe environments. At our sites in Vélizy, Thonon and Ulm, engineers, technicians and operators share unrivalled expertise split over more than 60 individual skills. Each component is built and tested with an extreme precision because the success of each space mission – and the communications we all rely on – depend on our tubes.
See how all the elements are put together, and how the tube functions in the video below.
What is a Travelling Wave Tube?
A Travelling Wave Tube amplifies a modulated electromagnetic wave in order to transmit data. Inside the vacuum envelope, the electromagnetic wave interacts with an electron beam. Because both travel at almost the same speed, electrons transmit their kinetic energy to the wave, an effect known as the Cherenkov effect. The simplest analogy we can provide is an aircraft traveling slightly above the sound barrier and radiating its kinetic energy in a form of a sound wave.
Amplifying electromagnetic waves is what makes a large number of applications possible, from microwave ovens to radars and satellites.
Thales is widely recognized as a pioneer and innovator in TWTs. We have consistently developed this technology to reach decisive improvements in electrical efficiency and thermal performance. In microwave frequencies, TWT provides electrical efficiency that will be unreachable by competitive solid-state technologies for the foreseeable future. That makes them very attractive for space borne applications where cooling electrical devices could be challenging.
How does it work?
An electron beam is extracted from a heated cathode and accelerated by a static electric field inside the TWT gun. The electron beam then interacts with an injected electromagnetic wave in a slow-wave structure, usually a helix, where the beam releases approximately 30% of its energy. At the end of the line, electrons finish their journey in a depressed collector where a large part of their remaining kinetic energy is retrieved and reinjected into the system. The fact that the electron travels in a vacuum without facing ohmic resistance losses explains why the total efficiency is so high, above 70% even at over 10 GHz.