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StatusOngoing
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Status date2024-12-21
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Activity Code5C.321
The achievement of high output power requires a higher energetic electron beam. Technological bricks are more and more challenging, developments for components and materials are getting marginal.
In order to achieve a high electronic flow, with reduced geometrical dimensions, an advanced new electron emission device (MMC) is needed, for a constant high beam current density. This MMC device shall be capable for 5 A/cm² electronic flow through the TWT interaction range. Obviously, the electron ejection has to function constantly for more than the 15 years of the cosmic mission life.
The MMC emitter is developed from actual MM-type technology, at THALES as a new high current density electronic source. It allows an increase of the current density by lowering the extraction energy for electronic emission.
The new MMC emitter-design is already in use in telecommunication tubes on ground, for high power TWT’s. The non-terrestrial qualification includes a complete batch of MMC-devices, further specific test vehicles and qualification tubes in order to verify the performance of the MMC devices either by accelerated or standard life testing.
Based on a successful qualification, the MMC emitters will serve THALES’ actual and new development projects, where the high emission current density is mandatory.
The development of the MMC emitter for 5 A/cm² with 15 years life time can be considered as an evolution out of the well approved and reliable MM-type technology. By lowering the extraction energy for electronic emission, the MM-technology could be systematically improved and verified. Consequently, the MMC device for non-terrestrial application represents the follow-on for the MM-type emitter, including a general process update, while maintaining the reliability aspects of the actual MM–type emitter. In detail, prior to the qualification of the MMC-emitters a further optimisation step for increasing performance and reliability is done by adjusting the manufacturing processes.
As the MMC emitters are the successor of the MM-type device, they use the same technology, i.e. production tooling and means are most identical. As expected, the MMC is flexible and after successful qualification will be used for the portfolio of THALES vacuum tubes, up to the highest frequencies. Either a strong electron beam with twice the current density of the current MM-type device may serve the high end power tubes, or - for the standard low power and medium class tubes a high density, but reduced beam diameter will fulfil the requirements.
THALES demonstrated by performing accelerated life testing on several MMC devices and on specific test vehicles that the new MMC emitter design is capable for this extensive qualification.
According to the principle of vacuum amplifiers (TWTs) the electronic emitter generates a high-density beam that interacts with the radio-frequency signal on the helix and thus transfers the electronic power to the microwave signal.
Currently developed (“state-of-the-art”) tubes require an elevated current, which cannot be achieved just by increasing the electron beam size, since the compression for matching the interaction beam tunnel would also increase the interference with it. Therefore, the MMC device with a current density of up to 5 A/cm² is not only one of the main components for this high-power family of Travelling Wave Tubes (TWT), but will be also applied for other high frequency TWTs, with a smaller electron beam diameter. The main advantage will be a reduced beam compression for focusing the beam inside the helix and therefore a reduced focussing and overall production effort which can also minimize the product costs.
The electron beam is generated by the MMC emitting device. By increasing the current density the electron beam becomes more powerful and a higher output power of the TWT can be reached.
High power TWTs are designed such, that they can handle maximum microwave energy, which is automatically correlated to the tube efficiency. So, - under the consideration of a sufficient margin – the following main parameters are adjusted to their physical / material optima:
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Temperature
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Thermo-mechanical stress
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Thermal gradients
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Thermal conductivity
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Electrical field strength
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Long-term stability
The MMC electron source has to fulfil all of the above criteria, which sometimes need to choose a compromise on materials and processes. Therefore, the selected materials and processes play an outstanding role in this assembly and need to be chosen very carefully.
The project is divided into three blocks, each finalized by a dedicated milestone.
The first is focused on the process engineering updated and closed by the Preliminary Design Review (PDR/TRR).
Followed by qualification tests on subassembly level and specific test vehicles, which were finalized by the Critical Design Review (CDR). Finally, the qualification test on tube level for a 170W Ku-band TWT THL12170C+ was successfully performed and evaluated for the final review.
The qualification campaign of the MMC cathode was successfully performed with real improvements for THALES and their customers. With a significant increase of the permissible emission current density up to 5A/cm² future gun designs can be improved with reduced beam compression and therefore improved beam focusing. The customer can expect improved live time behaviour, due to the fact that a chromium-doped MM cathode will ensure improved barium coverage over a long life time. The cathode size can be potentially reduced due to increased current density or kept constant to replace MM cathodes with improved life time behaviour.
