The lasing mechanism in fast-wave devices

It has been found that ac space-charge effects play an important role in typical electron beam-wave interactions. In conventional traveling-wave tubes (TWT's), the beam mode splits into two branches, a 'fast' and a 'slow' plasma wave, when space-charge effects are taken into account. It might be expected that in fast-wave devices the beam modes would also split into two branches when ac space-charge effects are taken into account. Many attempts have been made to derive such a dispersion relation for gyrotron beams. However, none of the considered theories has been able to fully simulate the complex space-charge and current distributions which occur in typical gyrotrons. This note is concerned with some of the basic results obtained by Dohler (1982, 1984). The lasing mechanism occurring in fast-wave devices is discussed, giving particular attention to gyrotrons. The relative importance of coherent and incoherent lasing in gyrotron theories is examined.