  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.
