TM-FEL with a longitudinal wiggler and an annular beam

We investigate theoretically the interaction of an electron beam with a TM mode in the presence of a longitudinal wiggler. The electron motion is longitudinal and radial therefore adequate for interaction with the electric field components of the mode. There are two facts which make this device unique: firstly, the equations which describe its dynamics are practically identical to the equations of the traveling wave tube (TWT) rather than the free electron laser (FEL) thus its sensitivity to beam quality is significantly lower, and the efficiency in the absence of tapering is higher. Secondly the interaction is possible only with an annular rather than a pencil beam as in a regular FEL-therefore the amount of current it is possible to inject is larger. Optimal operation at 20.56 GHz is examined