Traveling-Wave Tubes (Second Installment)

A SIDE FROM HELICES, the circuits most commonly used in travelingwave tubes are iterated or filter-type circuits, composed of linear arrays of coupled resonant slots or cavities. Sometimes the geometry of such structures is simple enough so that an approximate field solution can he obtained. In other cases, the behavior of the circuits can be inferred by considering the behavior of lumped-circuit analogues, and the behavior of the circuits with frequency can be expressed with varying degrees of approximation in terms of parameters which can be computed or experimentally evaluated. In this chapter the field approach will be illustrated for some very simple circuits, and examples of lumped-circuit analogues of other circuits will be given. The intent is to present methods of analyzing circuits rather than particular numerical results, for there are so many possible configurations that a comprehensive treatment would constitute a book in itself. Readers interested in a wider and more exact treatment of field solutions are referred to the literature. 12 The circuit of Fig. 4.1 is one which can be treated by field methods. This "corrugated waveguide" type of circuit was first brought to the writer's attention by C. C. Cutler. It is composed of a series of parallel equally spaced thin fins of height h projecting normal to a conducting plane. The case treated is that of propagation of a transverse magnetic wave, the magnetic field being parallel to the length of the fins. It is assumed that the spacing ( is small compared with a wavelength.