Compensation of Transverse Field Asymmetry in the High-beta Quarter-wave Resonator of the HIE-ISOLDE Linac at CERN

TL;DR

This paper discusses methods to compensate for transverse field asymmetry in high-beta quarter-wave resonators at CERN, aiming to improve beam quality in the upgraded REX-ISOLDE linac.

Contribution

It introduces cavity geometry modifications, including a racetrack-shaped beam port and inner conductor adjustments, to optimize transverse field symmetry in high-beta QWRs.

Findings

Optimized cavity geometry reduces transverse field asymmetry.

Modified design enhances beam quality and emittance control.

Effective sputtering of niobium film onto the cavity surface is enabled.

Abstract

The superconducting upgrade of the REX-ISOLDE radioactive ion beam (RIB) post-accelerator at CERN will utilise a compact lattice comprising quarter-wave resonators (QWRs) and solenoids, accelerating beams in the mass range 2.5 < A/q < 4.5 to over 10 MeV/u. The short and independently phased quarter-wave structures allow for the acceleration of RIBs over a variable velocity profile and provide an unrivalled longitudinal acceptance when coupled with solenoid focusing. The incorporation of the solenoids into the cryomodule shortens the linac, whilst maximising the acceptance, but the application of solenoid focusing in the presence of asymmetric QWR fields can have consequences for the beam quality. The rotation of an asymmetric beam produces an effective emittance growth in the laboratory reference system. We present modifications of the cavity geometry to optimise the symmetry of the transverse fields in the high-beta QWR. A racetrack shaped beam port is analysed and a modification made to the inner conductor with a geometry that will enable a niobium film to be effectively sputtered onto the cavity surface.