Hudanski, Ludovic (2008) Modulation of an electrom beam from a carbon nanotube based cathode - Applications to microwave tubes. PhD thesis PICM, PICM, EP/X p.223.

Alternative Locations: http://www.imprimerie.polytechnique.fr/Theses/Files/Hudanski.pdf

Abstract

Since their removal from radio receivers and other devices around the end of the fifties and their replacement by s!
olid state devices people have been reluctant to study electron tubes as microwave amplifiers. Nevertheless they currently remain indispensable for some applications because they are well better than solid state devices in terms of frequency operation, power, efficiency and reliability. That is why telecommunication satellites for example are launched with a hundreds of such electron tubes (travelling wave tubes) which remain heavy and bulky. To reduce their weight and their size and to reach better efficiency several laboratories study cathodes able to produce a frequency modulated electron beam. TRT, Nanocarb (mixed laboratory Ecole Polytechnique / Thales) and TED, where this PhD has been done, study new field emission cathodes based on carbon nanotube arrays.

This work has dealt with the study and the development of methods to directly modulate the electron beam provided by these cathodes. The carbon nanotube growth is realised by plasma enhanced chemical vapour deposition. Previous studies have developed and optimised a growth process to obtain individual vertically oriented carbon nanotubes. They have also studied their dc emission characteristics.
The first part of this PhD has consisted in the demonstration of the emission modulation by using resonant cavities which allow to create high microwave electric fields. We have thus demonstrated at 1.5 GHz the modulation of a 30 mA peak current corresponding to a current density of 12 A.cm-2, and then at 32 GHz the modulation of a 3.43 mA peak current. These results are the first and only microwave modulation demonstration with carbon nanotube based cathodes.

Nevertheless the use of resonant cavities limits the future of these cathodes to narrow bandwidth applications. The second part of the PhD has thus been dedicated to the development of a new photocathode concept based on the association of p-i-n photodiodes and carbon nanotubes.
The electron emission of such devices is controlled by the incident optical power, and cavities are not required any more. The control of the modulation is so made compatible with large bandwidth operations. A first generation of photocathodes based on carbon nanotubes and silicon p-i-n photodiodes has allowed us to demonstrate the modulation of a 500 µA current with a modulation ratio of 97% for an incident optical power of 12 mW.

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