Development of Dielectric-Based High Gradient Accelerating Structures

Abstract

High gradient accelerating structures using dielectric-lined circular waveguides have been developed for a number of years at Argonne National Laboratory. In this article, we first report the experimental results of high power rf testing on the quartz based Dielectric-Loaded Accelerating (DLA) structure carried out on Feb. 2006 at the Naval Research Laboratory. The motivation for this experiment is to test the multipactor effect on different materials under high power and high vacuum condition. Up to 12 MW pulsed rf went through the tube without breakdown. Multipactor appeared during the experiment but with different features compared to other materials like alumina. Photomultiplier Tube (PMT) measurements were introduced into the experiment for the first time to observe the light emission time and intensity. In the second part of this paper, ways to achieve higher gradient for DLA structures are proposed: 1) smaller ID and longitudinal gap free DLA structures to reduce multipactor and obtain higher gradient; 2) new coaxial type coupler to avoid dielectric gap and improve impedance matching; 3) double layered DLA structure to reduce rf loss and enhance shunt impedance as well.

High power testing of a fused quartz-based dielectricloaded accelerating structure

“…Two issues were addressed during the development of X-band traveling wave DLA structures: multipactor and dielectric joint breakdown. The dielectric joint breakdown due to the local field enhancement is an engineering issue that may be solved by an approach based on a newly designed gapless rf coupling technique [2]. The issue of multipactor, which leads to strong rf power absorption, is associated with some physical properties of the loaded dielectric materials.…”

“…This structure is designed to have a large E r /E z ratio. Based on the physical model, this may enable the entire multipactor region to be investigated with the available amount of rf power [3].…”