Beam loading in traveling wave tubes

The paper describes an improved model of helix TWT´s with circuit and beam separated in the radial direction, corresponding better to the actual physical configuration of TWT´s. The effect of separation of beam and circuit is in the appearance of beam loading effects of the circuit and field loading effects of the beam, both of which are unaccounted for in the usual TWT model where circuit and beam are treated as being at the same radial position. In the TWT theory of the improved model both loading effects turn out to be purely reactive, introducing no loss but an apparent increase in the forward circuit wave and an apparent decrease of the dc beam velocity. This can be interpreted as a desynchronization effect which must be offset by increasing the beam voltage. For typical TWT parameters the required increase in beam voltage is between one and four percent.