Non-periodic perturbations in periodic RF structures

This paper presents analysis of the effects of non-periodic perturbations in periodic RF structures with the use of 3D finite element computer simulations. Small departures from periodicity change the phase shift experienced by a wave as it traverses an imperfect cell. Cell imperfections may be introduced intentionally or due to fabrication errors or material properties variations. The accumulated effects produce distortions of the fields. If the errors are large enough the structure modes become localized. We believe that this is the first demonstration of localization in a periodic RF structure in which the transverse and period dimensions are on the order of a wavelength. The phenomenon is similar to Anderson localization but has its own unique mechanisms. These results are demonstrated in a periodic structure being designed for a 670 GHz extended interaction klystron. In an ancillary result we document a limitation of finite element codes when applied to long periodic structures.