Generation of density modulated electron beam in an open resonator

The energy carried by density modulated electron beams with low spread in velocity can be converted into high power microwave radiation with high efficiency. The density modulated beam is generated by applying both RF and DC fields. This is possible in an open cavity that consists of a Bragg reflector region. The authors examine the conditions for beam generation taking into account regular field emission (Fowler-Nordheim) and space-charge effects. The concept is demonstrated when a density modulated beam is injected in a tapered traveling wave structure; simulations indicate that the highest efficiency is achieved when in the first 15% of the interaction region the phase velocity of the wave is higher than the average velocity of the electrons.