Abstract
Exploratory measurements of the Brookhaven Energy Recovery Linac (ERL) cavity at superconducting temperature produced a long list of high order modes (HOMs). The niobium 5-cell cavity is terminated at each end with HOM ferrite dampers that successfully reduce the Q-factors to levels required to avoid beam break up (BBU) instabilities. However, a number of un-damped resonances with Q≥106 were found at 4K and their mode identification forms the focus of this paper. The approach taken here consists of bead pulling on a copper (Cu) replica of the ERL cavity with dampers involving various network analyzer measurements. Several different S21 transmission measurements are used, including those taken from the fundamental input coupler to the pick-up probe across the cavity, others between beam-position monitor probes in the beam tubes, and also between probes placed into the cells. The bead pull technique suitable for HOM identification with a metallic needle or dielectric bead is detailed. This paper presents the results for HOMs in the first two dipole bands, the un-damped modes in the first quadrupole band, and the identification of several high-Q modes beyond.
Highlights
Several past measurements of the Brookhaven Energy Recovery Linac (ERL) at superconducting temperature produced a long list of higher order modes (HOMs)
The Niobium 5-cell cavity is terminated with HOM ferrite dampers that successfully reduce the Q-factors to tolerable levels
Identification of several low frequency HOMs in the ERL cavity was made possible for dipole modes, beam tube modes and quadrupole modes by matching their frequencies to corresponding resonance frequencies in the ferrite damped copper model
Summary
H. Hahna, R. Calaga a,b, Puneet Jainc, E. C. Johnsonc, W. Xua aCollider-Accelerator Department, Brookhaven National Laboratory, Upton, NY 11973 bEuropean Organization for Nuclear Research, 1211 Geneva, Switzerland cPhysics & Astronomy Department, Stony Brook University, Stony Brook, NY 117943 Presented at the International Workshop of Higher Order Mode Diagnostics and Suppression in SC Cavities (HOMSC12) Daresbury, UK June 25-27, 2012 Collider-Accelerator Department
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