Design and performance of microwave phototubes using transmission electron multipliers

This paper describes the design and performance of a new type of microwave phototube. This tube combines the advantages of secondary emission gain with the high output efficiency of a helix, and its sensitivity is orders of magnitude greater than that of conventional microwave phototubes. In this tube the light to be demodulated passes through an optical window onto a semitransparent photocathode (e.g. S-1, S-20, etc.). The photoelectrons emitted by the cathode, which are bunched at the modulation frequency of the light, are multiplied by transmission secondary emission dynodes. The amplified electron beam then passes through a helix of the type used in a TWT, exciting a traveling wave that is taken out by an output coupler. In addition to discussing the details of the construction and performance of L-band tubes, consideration will be given the ultimate limits on frequency response and sensitivity set, by the use of secondary emission multiplication. The velocity distribution of photoelectrons and secondary electrons is of primary importance in determining these limits. Some of the applications of this class of tubes for optical communication, optical ranging, etc. will be reviewed.