A gridless, variable perveance Pierce electron gun.
This thesis covers the design and development of a modulated Pierce electron gun used in the construction of experimental travelling wave tube (TWT) amplifiers. The gun incorporated an open aperture switching electrode, positioned mid-way between anode and cathode, to pulse the beam. This method of modulation did not have the same adverse effects on electron trajectories as in the case of a conventional mesh grid, but rather the electrode could be used to alter the focus conditions within the gun and subsequently improve certain beam characteristics. Ion focusing effects could also be eliminated with the electrode, allowing dual mode operation of the guns without the complications normally associated with such a practice. The switching electrode was simulated to ascertain its effect on electron trajectories within the gun, using finite element analysis as well as an electron optics design program. A test gun was constructed in a glass envelope in order to investigate the performance of the new design. The glass gun allowed a beam analysis to be performed, as well as thermal measurements to be made. Results from this gun compared favourably with earlier simulations. The results of two metal/ceramic construction TWTs are presented, showing the beneficial effects of the switching electrode on the performance of the tubes as a whole, and the electrode's potential to compensate for constructional anomalies. The joining of metals to ceramic using active brazing techniques is also an important aspect tackled by the thesis, with several innovative ideas being implemented in the construction of the devices. A simple yet reliable electrical feed-through was developed for those guns having a ceramic envelope. Extensive work was also performed on the manufacture of impregnated tungsten cathodes for use in the electron guns. Several test diodes, including a water-cooled demountable test vehicle, were constructed to test the performance of the cathodes. An analysis was performed on the patchy behaviour of some of the initial cathodes to improve the preparation methods used in the laboratory. The emission results obtained from the cathodes are documented, as is the successful incorporation of several of them into the new modulated gun design.