Inductively-loaded Extended Gaps For Relativistic Klystrons

Abstract

As the power and the frequency of a relativistic klystron increase, design of both modulation and extraction cavities becomes increasingly difficult. If the gap is narrow, the rf electric field begins to exceed breakdown limits. On the other hand, if the gap width exceeds a small fraction of a wavelength, typical extraction modes do not couple well, and space charge potential energy reduces efficiency and limits the current that can cross the gap. The authors present theoretical investigations, numerical simulations, and initial experimental results on a novel gap design which incorporates an inductively-loaded return-current structure. The structure consists of a series of washers (e.g., 25 in the device which has been built), mounted on a small number (e.g., four) of longitudinal conducting posts which bridge the gap. The unmodulated component of the beam current induces a return current which flows freely in the posts; as a result, the beam`s unmodulated space charge is nearly completely neutralized. The washers serve to increase the capacitive coupling between the beam and the structure, and to provide azimuthal symmetry for the charge on the structure. However, the inductance of the posts is sufficient to inhibit rf return current, and thus the structure sustains themore » longitudinal rf field in the cavity. With appropriate choice of resonant modes, it appears that gap widths exceeding a half wavelength can be used for modulation of high-current beams, as well as extraction of rf energy with high efficiency.« less