Physical Limitations in Electron Ballistics

T HE subject of this talk is "Physical Limitations in Electron Ballistics". It is pleasant to have a chance to talk about such physical limitations, because there is so little we can do about them. And, although these limitations are apt to be discouraging, a knowledge of them is very valuable, for it keeps us from spending time trying, like the inventors of perpetual motion machines, to do the impossible. As electron ballistics is particularly subject to physical limitations, there are so many that it is impossible to discuss all of them thoroughly at this time. Also, many of the limitations are of a rather complicated nature, and to deduce them from basic principles in a quantitative way requires much thought and patience. I think the best I can do is to try to mention most of the chief limitations, as a warning to the uninitiated that rocks lie ahead in certain directions, but to concentrate attention on only a few of them. I have chosen this evening to devote particular attention to limitations that bear on the production and use of electron beams in which considerable current is required, such as those used in cathode ray tubes and high-frequency oscillators, and to mention only briefly as a sort of introduction problems pertaining more closely to low-current devices such as electron microscopes. T H E WAVE NATURE OF THE ELECTRON