Concentration-modulation electron-beam devices

Electron-beam devices are considered in which a form of concentration modulation, as defined by Gabor, is utilized. In the devices described here a scalloped electron beam is produced by a combination of crossed electric fields and a magnetic field, and strikes a collector after passing through a fixed aperture (cf. Fig. 1). The points of minimum width of the scalloped beam are "images" of the cathode which are reproduced along the longitudinal axis of the system by virtue of the cyclotron motion of the electrons. The application of an additional alternating electric field, parallel to the fixed electric field, causes a periodic variation in the longitudinal position of the cathode images, giving rise to an alternating component in the current which reaches the collector. Concentration modulation is compared with other modulation methods. Transit-time limitations are evaluated and possible applications are considered, including frequency multiplication at microwave frequencies. The use of the concentration-modulation technique in modulating a high-density electron beam at audio frequencies in an experimental system is described.