On the basis of a rigorous field theory, two different physical models of attenuator and sever have been proposed. One is named High attenuation (HATT) model in which both attenuator and sever are considered as a unified attenuator, but the sever is regarded as an area of very high loss; the other is called Sever and attenuator (SATT) model in which the sever is modelled as a drift area in which the electric and magnetic fields both vanish. A complex function is derived and potential sinking effect is also considered. Thus, a set of more practical self-consistent equations of nonlinear beam-wave interaction is formulated. Simulations are carried out under the conditions of the two different physical models, and the simulation results are compared with the experimental data. The results show that in the case of single signal drive, the unknown second harmonic should be included for predicting the saturated output power. It is also evident that the SATT model and the HATT model predict the same physical nature, whereas the results predicted by the HATT model are much closer to the experimental data than those obtained from the SATT model. Therefore, these results provide a strong theoretical basis for designing broadband and high gain helix travelling wave tubes.