  The first phase of the HIE-ISOLDE project at CERN consists of a
superconducting (SC) linac upgrade in order to increase the energy of
post-accelerated radioactive ion beams from 2.8 MeV/u to over 10 MeV/u (for A/q
= 4.5). In preparation for the upgrade, we present beam dynamics studies of the
booster section of the normal conducting (NC) REX-ISOLDE linac, focused on the
longitudinal development of the beam in the 101.28 MHz Interdigital H-mode
Structure (IHS), employing a Combined Zero Degree Structure (KONUS), pulsing at
a high gradient of over 3 MV/m. The evolution of the transverse emittance in
the SC linac depends critically on the injected longitudinal phase space
distribution of particles from the existing linac and, with a better
understanding of the beam dynamics upstream, the performance of the upgrade can
be optimised. Data taken during the commissioning phase of the REX-ISOLDE linac
is analysed to understand the properties of the beam in the booster and
combined with beam dynamics simulations which include the realistic fields of
the IHS, determined from both simulation and perturbation measurement.
