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As manufacturer of Travelling Wave Tubes (TWT) THALES Electron Devices (TED) started the low-frequency TWT development for space applications (1GHz to 3GHz) in 1996 to re-enter this market. Within the commercial programs with their very tight production schedule only limited room remained for more peripheral improvement efforts.
With the project 'L- and S-Band Travelling Wave Tubes' TEDG realized in a project together with ESA (ESTEC) the improvement of the existing tube designs and the extension of the portfolio in the S-Band and also in the L-Band. Depending on the output power range, conduction cooled and radiation cooled versions are now available.
The present project is the last part (Step 3 and 4) of the development program dealing with L-band medium and high power class as well with S-band low to medium output power. Both steps were started at the end of 2003 by ESTEC and THALES Electron Devices (TED) and were finished successfully by the end of 2006.
The starting point for Step 3 of the project was the development of improved 150W L-band TWTs based on the first TED L-band design TL 2140 which is successfully in orbit for several years now. The challenge was to improve the efficiency by 7% over a frequency bandwidth of 200MHz and to reduce the mass by 400g. In addition the portfolio in the L-band was extended to a radiation cooled design and to the 250W output power class (conduction cooled only) with comparable electrical performance and mass.
In Step 4 the excellent results for electrical performance in the S-band high power class (Step 1 and Step 2) were transferred to the medium output power classes with up to 180 W in conduction and radiation cooled design.
These topics were a big success for TED to define an improved generation of Travelling Wave Tubes (TWT) with more than 61% in efficiency for L-band and 63% for S-band.
Each of the Steps was divided into three phases:
The goal for the electrical performance are:
THALES Electron Devices (TED) has outstanding experience in the development and production of Travelling Wave Tubes. The summarised in Orbit operational time of TED TWTs is meanwhile more than 271 million hours (in May 2007). After re-starting the low frequency development (L- and S-band) in 1996 TED delivered more than 435 tubes in this frequency ranges.
The development program strengthened the competitive position for TEDG against the Solid State Power Amplifier (SSPA) with respect to the satellite market. It consolidates TED's lead position in the low frequency range with very high reliable and flexible power amplifiers.
The application of a Travelling Wave Tube (TWT) is the power amplification of RF signals on ground and on satellites. The following figure shows a schematic description of a TWT:
A focused electron beam with a pre-defined beam current is needed. The beam is generated in a cathode which is designed for more than 15 years life time and is focused to an certain diameter by an electron optic. Both are part of the electron gun. In the interaction area the beam transfers the energy to the RF wave. The component of the field on axis must have almost the same velocity than the electron beam. Therefore the RF wave is delayed by a delay line i.e. a helix line. The electron beam is focused by a magnetic field in the interaction area and are recovered in a multistage collector to increase the efficiency of the tube. To avoid oscillation of the tube the amplification length is separated by an attenuator. In the input section the information of the RF wave is transferred to the electron beam. The RF wave itself is attenuated. In the last section the RF wave is finally generated and amplified including the information to be amplified. Therefore the helix line has a tapered pitch.
Depending on frequency, bandwidth and efficiency a lot of parameter have to be investigated or especially designed like beam current, magnetic field, diameters, attenuation, collector geometry and the RF window.
Starting points of the development were the existing tubes for both frequency classes and the results of Step 1 and 2 of the project. The first action was the theoretical examination of the existing materials, dimensions and design by the help of suitable development tools. Then the theoretical results were tested on subassembly level to verify the basic data and finally on Bread Boards to fulfil the requirements.
After the verification the theory was transferred to the new power classes - again with the computer simulations. Finally the housing for conduction and radiation cooled versions were designed.
After being tested electrically the tubes were subjected to qualification tests concerning electrical performance, mechanical and thermal design and environmental conditions. The individual temperature limits were investigated during TV testing. All designs passed the qualification tests successfully.
The project was successfully finished.