Evolution and radiation of a relativistic electron beam in a vacuum cavity without a magnetic field

The dynamics of a relativistic electron beam injected through a conducting coil into a cavity is studied, when the current is close to the limiting vacuum current. A numerical simulation shows that under conditions such that beam self-focusing is observed the firehose instability develops close to the foil, accompanied by radiation with an efficiency of up to 15-20%. In the case of a ribbon beam the TM2n mode is radiated. In the three-dimensional case the beam acquires a spiral shape. The instability has a threshold and grows for beam currents exceeding a certain value. 2 refs., 8 figs