Re-examination of absolute instability near band edges in a traveling wave tube

We re-examine the beam mode and its interaction with the circuit mode in the immediate vicinity of the lower and upper band edges in a traveling wave tube (TWT). Contrary to previous findings, we show here that an absolute instability may arise, according the the Briggs-Bers criterion, at the lower band edge if the beam current is sufficiently high, even if the beam mode intersects with the circuit mode with a positive group velocity. However, the upper band edge was found to be more susceptible to absolute instability than the lower band edge, according to the threshold conditions that we derived analytically. Near the band edge, the Green's function shows possible transient growth in a TWT that is free of absolute instability.