ABSTRACT
High power microwaves (HPMs) are generated by transferring the kinetic energy of moving electrons to the electromagnetic energy of the microwave fields. This process typically occurs in a waveguide or cavity, the role of which is to tailor the frequency and spatial structure of the fields in a way that optimizes the energy extraction from certain natural modes of oscillation of the electrons. In analyzing this process, we deal with the interactions between two conceptual entities: the normal electromagnetic modes of the waveguides and cavities and the natural modes of oscillation of electron beams and layers. The two exist almost independently of one another except for certain values of the frequency and wavelength, for which they exchange energy resonantly. We will therefore begin the chapter by reviewing the basic concepts of electromagnetics and considering the fields within waveguides in the absence of electrons. Our emphasis will be on two key properties of the electromagnetic fields within the waveguide: the spatial configuration of the fields and the relationship between the oscillation frequency and wavelength measured along the system axis. Our treatment will include both smooth-walled waveguides and periodic slow-wave structures (SWSs), the treatment of the latter requiring a discussion of Floquet’s theorem and Rayleigh’s hypothesis. We will also touch on two features that play a role in determining the power-handling capability of high-power devices: the relationship between the power in a waveguide or cavity and the peak perpendicular field at the wall, a key factor in breakdown, and resistive wall heating in high-average-power devices, either continuous or rapidly pulsed. From waveguides, we will graduate to cavities, which have normal modes of their own that can be treated largely by extension from the treatment of waveguides. The important cavity parameter Q, the so-called quality factor, will be a focus of the discussion.
