Spurious reflection of space charge fields in TWTAs

The reflection of waves at the termination of a beam loaded traveling wave structure is considered. The presence of a bunched beam in the structure induces additional currents and voltages on the structure, which give rise to an additional reflection at the termination. This effect is first illustrated in a simple transmission line model, and then a formal field-based derivation for the reflection in a periodic structure is presented. Simple estimates for the magnitude of the reflection can be made based on the amplifier´s Pierce parameter. Unlike the reflection due to a mismatched load, the spurious reflection due to the beam depends on signal power level. The spurious reflection can be simulated in large signal parametric codes, and examples of this are presented.