Fabrication and cold test of dielectric assist accelerating structure

Abstract

We present the detailed description of a successful design and cold testing of the dielectric assist accelerating (DAA) structure. The DAA structure consists of ultralow-loss dielectric cylinders and disks with irises which are periodically arranged in a metallic enclosure. The advantage of the DAA structure is that it has an extremely high quality factor and a very high shunt impedance at room temperature since the electromagnetic field distribution of accelerating mode can be controlled by dielectric parts so that the wall loss on the metallic surface is greatly reduced. A prototype of the five-cell DAA structure was designed and built at C-band (5.712 GHz), and cold tested. Three types of dielectric cell structure, ``regular,'' ``end,'' and ``hybrid'' dielectric cells, are fabricated by sintering high-purity magnesia. The prototype was assembled by stacking these cells in the hollow copper cylinder, whose two ends are closed by copper plates. The resonant frequency of the prototype was tuned to the desired frequency by machining only end copper plates. The unloaded quality factor of the accelerating mode was measured at 119,314 and the shunt impedance per unit length of the prototype was estimated from the experimental results of the bead pull measurement as ${Z}_{\mathrm{sh}}=617\text{ }\text{ }\mathrm{M}\mathrm{\ensuremath{\Omega}}/\mathrm{m}$, which were within 2 percent of the design values. The field distribution of accelerating mode was also measured by the bead pull method, and its results agreed well with simulation results.