Principles Of Klystrons, Traveling Wave Tubes, ... Review

All these devices use the "bunching" of electrons to provide signal amplification, though they differ in the exact process that causes velocity modulation.

The text you're referring to is the title of a standard textbook in the field of microwave engineering: by A. S. Gilmour, published by Artech House . Key Features of the Textbook Principles of Klystrons, Traveling Wave Tubes, ...

: A defining feature is its emphasis on clear, qualitative explanations of physical phenomena, deliberately minimizing the use of advanced mathematics to make the content accessible to a broader audience. All these devices use the "bunching" of electrons