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 sh ¼ 617 MΩ=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.

Investigations Into X-Band Dielectric Assist Accelerating Structures for Future Linear Accelerators

“…A special type of dielectric could be added to reduce the size of these copper structures for the same operation frequency. The DAA structures have the same geometry as the dielectric assist accelerating structures proposed by Satoh et al [48][49]. They consist of dielectric cylinders with irises, periodically arranged in a metallic enclosure, and operate in a TM02 mode with a π phase advance (standing wave).…”

“…The ceramic materials for the dielectric-based accelerating structures have to withstand high accelerating fields, prevent potential charging by particle beams, have good thermal conductivity, and generate low power loss. Magnesia (MgO) ceramic, with a relative permittivity = 9.64 and an ultralow loss tangent tan = 6 × 10 −6 near 10 GHz, which has been demonstrated by [49], is chosen as the dielectric material for the simulation of our DAA structures. Here the ceramic properties at 12 GHz are assumed to be the same as that near 10 GHz.…”

“…In the last two decades, different kinds of DLA structures with improved performance have been reported, such as a dual-layered dielectric structure [43], a hybrid dielectric and iris-loaded accelerating structure [44], a multilayered dielectric structure [45], a disk-and-ring tapered accelerating structure [46], and a dielectric disk accelerating (DDA) structure [47]. Among these DLA structures, a dielectric assist accelerating (DAA) structure proposed by Satoh et al [48][49] is of particular interest because it realized an extremely high quality factor and a very high shunt impedance at a C-band frequency. Building on these developments, a DAA structure operating at a high frequency (X-band) appears to be very promising for future linear accelerators considering a high A. Grudiev is with RF Group of Accelerator Systems Department, European Organization for Nuclear Research (CERN), Espl.…”

Dielectric Rf Resonators With Open Boundaries for Studying Radiation Effect on Electrodynamic Characteristics of Solid Dielectrics

“…One of the directions of these studies is based on the use of dielectric materials [1]. This applies both to acceleration methods in wake fields, excited by electron bunches [2,3], and to new dielectric-assist accelerating structures with the traditional RF power input [4]. Dielectrics for accelerating structures will be operated under conditions of strong ionizing radiation.…”

Multipactor suppression in dielectric-assist accelerating structures via diamond-like carbon coatings

“…The superconducting cavity has a high Q-value of greater than order O(10 10 ) at low temperature T < 4.2 [K], while the normal conducting cavity has a Q-value of O(10 4 ) working at room temperature, as determined by the ohmic loss of the metal. Recently, it was proven that a dielectricassist accelerating (DAA) structure could attain a high Q-value of O(10 5 ) at room temperature [9,10] through a reduction in the surface magnetic field using the reflection of a dielectric layer exceeding the limits of the Qvalue determined by the ohmic loss. Figure 1 (a) shows a five-cell DAA cavity with a Q ∼ 1.2 × 10 5 , which is one order of magnitude larger than the typical Q-value of a normal conducting cavity.…”