Slow-wave structures for M-type devices

Abstract

M-type devices such as magnetron oscillators, magnetron amplifiers and backward-wave oscillators always contain a sole plate to provide the static electric field which, together with a transverse static magnetic field, provides electron focusing. The sole plate unavoidably becomes part of the RF structure. It is the purpose of this paper to show the effect of the sole plate on the RF characteristics of the more common slow-wave structures. The results of the study indicate that no slow-wave structure containing a sole plate can be truly a backward-wave structure. The fundamental space harmonic must contain a region wherein the phase and group velocities are of the same sign. These results are predicted by a field analysis of the uniform-vane, strapped magnetron anode system. Such an analysis is made possible by two simplifying assumptions. The first assumption is that the interaction space between anode and sole can be replaced by an equivalent linear structure. In this way, the derived expressions for admittance become infinite series of exponential functions rather than infinite series of untabulated higher order Bessel functions. The second assumption is that the fields peculiar to the straps can be described by TEM waves. This assumption allows one to formulate a strap admittance which is in parallel with the admittance of the interaction space and the side cavity. The resultant dispersion equation can be solved graphically for the frequency-phase shift characteristic of the composite system. This analysis predicts that the straps may be either capacitive or inductive depending upon mode number, frequency and geometry. It shows that the frequency-phase shift characteristic is not simply derivable from the unstrapped anode system by the addition of a constant phase shift. It shows further that the fundamental space harmonic must contain a region wherein the phase and group velocities are of the same sign, i.e., the anode system is not, and cannot be, a simple backward-wave structure. These results are verified in detail by a careful and complete experimental study on a standard strapped magnetron anode structure. They are further verified by a second experimental study of the very common interdigital line structure.