  In modern rare isotope facilities, ion cooling and bunching lies at the heart
of the ion transfer along a low-energy beam line that consists of several
differential pumping stages. We present a conceptual design of an ion guide as
an alternative to the conventional linear Radio-Frequency Quadrupole (RFQ) for
cooling and bunching rare isotopes. The ion guide is composed of stacked ring
electrodes of varying apertures, to which a confining RF potential following a
rectangular waveform is applied. The thicknesses of the rings and the gaps in
between are varied accordingly to maximize the confining volume and to reduce
ion losses. Ion transport within the ion guide is facilitated by a
lower-frequency wave traveling on top of the higher-frequency confining field.
The former is induced by locally adjusting the duty cycle of the rectangular
waveform of the confining potential. Design parameters are first calculated by
analytical studies and then optimized by ion trajectory simulations with
SIMION. The results show that the ion guide enables high ion transmission and
produces well focused ion bunches. It will be used in the NEXT project, an
experimental study of atomic masses of Neutron-rich EXotic nuclei produced in
multi-nucleon Transfer reactions.
