Radiation from a space charge dominated linear electron beam

It is commonly known that radiation loss in linear beam transport is largely unimportant. For a space charge dominated linear beam, however, radiation power loss can be an appreciable fraction of the injected beam power [Biswas, Kumar, and Puri, Phys. Plasmas 14, 094702 (2007)]. Exploring this further, the electromagnetic nature of radiation due to the passage of a space charge dominated electron beam in a "closed" drift tube is explicitly demonstrated by identifying the cavity modes where none existed prior to beam injection. It is further shown that even in an "open" drift tube from which radiation may leak, the modes that escape contribute to the time variation of the electric and magnetic fields in the transient phase. As the window opening increases, the oscillatory transient phase disappears altogether. However, the "bouncing ball" modes survive and can be observed between the injection and collection plates.