Abstract
A superconducting energy-recovery linac (ERL) is under construction at Brookhaven National Laboratory (BNL) to serve as a test bed for an application to upgrades of the Relativistic Heavy Ion Collider (RHIC). The damping of higher-order modes in the superconducting five-cell cavity is of paramount importance and represents the topic of this paper. Achieving the damping by the exclusive use of two ferrite absorbers and the adoption of a space-saving step instead of the conventional taper are part of the exploratory study. Absorber properties which are portable to simulation programs for the ERL cavity have been obtained by measuring the absorber as a ferrite-loaded pill-box cavity. Measured and simulated results for the lowest dipole modes in the prototype copper cavity with one absorber are discussed. First room-temperature measurements of the fully assembled niobium cavity string are presented which confirm the effective damping of higher-order modes by the ferrite absorbers, and which give credibility to the simulated $R$ over $Q$'s in the ERL.
Highlights
A superconducting energy-recovery linac (ERL) is under construction at Brookhaven National Laboratory (BNL) to investigate concepts for electron cooling of ions and coherent electron cooling of high-energy protons in the Relativistic Heavy Ion Collider (RHIC), or for an envisioned electron-ion collider [1,2]
The ERL test facility is based on a niobium superconducting (SC) 703.75 MHz five-cell cavity capable of accelerating electrons to 20 MeV and a SC 2.5 MV electron gun
The design followed the techniques developed at Cornell [6,7] and KEKB [8] for ferrite absorbers, which lead to a cavity design with comparatively large iris apertures allowing propagation of higher-order modes (HOMs) to the external absorber
Summary
A superconducting energy-recovery linac (ERL) is under construction at Brookhaven National Laboratory (BNL) to investigate concepts for electron cooling of ions and coherent electron cooling of high-energy protons in the Relativistic Heavy Ion Collider (RHIC), or for an envisioned electron-ion collider [1,2]. A necessary intermediate step consisted in determining ferrite absorber properties in a form that is portable into the MWS program for simulation of the ERL and other relevant cavities The ERL string differs from the prototype cavity primarily by the presence of two ferrite HOM absorbers with step transitions to the beam tube, and by their asymmetric and significantly increased distances from the cavity ends This geometry lowers the signal level at accessible ports but several dipole modes have been identified via the comparison of the measured data with MWS simulations using the portable ferrite parameters. Of the results collected confirms the effective damping of HOM’s in the ERL by the ferrite absorber and allows making credible projections of R=Q values and estimates of dipole shunt impedance values for use in beam dynamic studies
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