Numerical simulation of backward wave excitation in Helix-travelling wave tubes

Summary form only given. The maximum achievable gain in individual sections of helix type traveling wave amplifiers is limited by the requirement that the device be stable with respect to the excitation of backward waves, Generally, there is a critical length above which the backward wave is absolutely unstable. Whether this length is exceeded in a specific device depends on a number of details that must be computed numerically. We will present a model, based on the TWT simulation code CHRISTINE, in which BWO stability is determined including a number of important effects. These are: the placement of severs, their reflection and transmission coefficients, the profile of attenuation along the interaction length, the presence of a driven signal, and the coupling of forward and backward waves due to asymmetries in the helix support structure. Specifically our model calculates the gain for the coupled forward and backward waves in a specified frequency band. Asymmetries result in a stop band near the pi point formed by the coupling of the forward and backward waves. Here the beam interacts with a composite mode formed by the fundamental spatial harmonic of the forward wave and first spatial harmonic of the backward wave. These modes will have differing coupling coefficients with the beam which we calculate using an improved solution of the tape-helix dispersion equations.