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Instituut voor Kern-en Stralingsfysica, K.U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium(Received 6 October 2010; published 17 February 2011)The HIE-ISOLDE project at CERN proposes a superconducting upgrade to increase the energyrange and quality of the radioactive ion beams produced at ISOLDE, which are currently postaccel-erated by the normal conducting radioactive ion beam experiment linac. The specication and designchoices for the HIE-ISOLDE linac are outlined along with a comprehensive beam dynamics studyundertaken to understand and mitigate the sources of beam emittance dilution. The dominant cause oftransverse emittance growth was attributed to the coupling between the transverse and longitudinalmotions through the phase dependence of the rf defocusing force in the accelerating cavities. Aparametric resonance induced by the coupling was observed and its excitation surveyed as a functionof transverse phase advance using numerical simulations and analytic models to understand and avoidthe regions of transverse beam instability. Other sources of emittance growth were studied and wherenecessary ameliorated, including the beam steering force in the quarter-wave resonator and theasymmetry of the rf defocusing forces in the solenoid focusing channel. A racetrack shaped beamport aperture was shown to improve the symmetry of the elds in the high- quarter-wave resonatorand reduce the loss of acceptance under the offset used to compensate the steering force. Themethods used to compensate the beam steering are described and an optimization routine written tominimize the steering effect when all cavities of a given family are offset by the same amount, takinginto account the different velocity proles across the range of mass-to-charge states accepted. Theassumptions made in the routine were shown to be adequate and the results well correlated with thebeam quality simulated in multiparticle beam dynamics simulations. The specication of the designtolerances is outlined based on studies of the sensitivity of the beam to misalignment and errors, withparticular emphasis on the phase and amplitude stability required for the independently phasedquarter-wave resonators.

Beam dynamics design studies of a superconducting radioactive ion beam postaccelerator