A model is constructed to evaluate absolute instability which may lead to bandedge oscillations in a traveling wave tube. Under the assumptions (a) that all modes have forward group velocities, and (b) that the slow wave structure has a parabolic dispersion relation in the plane, the threshold coupling constant (Pierce’s parameter ) is calculated for the onset of absolute instability. The effect of distributed resistive loss in the circuit is included. The axial wave number and the characteristic frequency of the oscillation at the onset are given.
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1998
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