Normal wake and reversed resonant radiation of a charge moving along a slow-wave system

Coherent radiation forwarded ahead of a charged bunch passing through a slow-wave guide is considered. This kind of radiation appears as a particular case of existing theory of resonant wakefield when the group velocity exceeds the charge velocity which is synchronous with the phase velocity. Some of possible occurrences of the condition are discussed for anomalous media and new materials, moving media and plasma, and some types of periodic slow-wave systems. Potential applications include advanced methods of particle acceleration, beam diagnostics and coherently amplified generation of short high power electromagnetic pulses induced by low-energy beam. Important aspects of the group velocity concept and “superluminal” propagation, “unipolar” pulse and Cherenkov radiation are discussed as well.