The traveling-wave phototube part I: Theoretical analysis

The traveling-wave phototube (TWP) is a broad-band photodetector which is useful for demodulation of light that has been amplitude-modulated at a microwave frequency, and for optical heterodyning of coherent light signals with microwave difference frequencies. In either application the light beam(s) results in the emission of a current-modulated electron beam from a photocathode; interaction between this beam and a slow-wave circuit such as a helix is employed for broad-band detection of the microwave modulation. Part I of this paper presents a detailed analysis of the TWP beam-circuit interaction process, including the effects of loss, space charge and asynchronism. Computer results are given for both current modulation and velocity modulation excitations, since the latter can be important under certain operating conditions. Conclusions regarding the effects of beam current, circuit length and circuit loss are drawn which are important in the design, understanding and use of TWP´s. The results of this analysis are also generally applicable to other cases where a slow-wave circuit is excited by an electron beam. In Part II of this paper, a detailed experimental analysis of the TWP will show good agreement with the predictions of this theoretical treatment.