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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 spurious modes. The excited modes may be of two types: backward waves, which are absolutely unstable, or forward waves, which are unstable in the presence of reflections and regenerative amplification. Whether a specific device is unstable depends on a number of details that must be computed numerically. We will present a model, used in the traveling-wave tube simulation code CHRISTINE, in which 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. Asymmetries result in a stopband near the "π" point formed by the coupling of the forward and backward waves. For this case, a model is developed to evaluate the maximum stable length.