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A dual hybrid ion-neutral trap has been built to study collisions between a cloud of ultracold atoms and small numbers of co-trapped atomic or molecular ions, in a common volume. Ultracold Na atoms are trapped in a vapour-cell magnetooptical trap (MOT), via a single-mode frequency-stabilized ring-dye laser, with repumping. A linear rf quadrupole Paul ion trap, centred on the MOT, co-traps selected atomic or molecular ions, for example Ca þ . Control of the initial ion temperature involves laser cooling Ca þ ions via the resonant Fraunhofer K line at 397 nm. We showed the rf ion trap can operate simultaneously with the MOT without destroying the ultracold atom cloud. The goal is to investigate the physics of ion-neutral sympathetic cooling, the cooling by neutrals of translationally cold and vibrorotationally hot molecular ions (e.g. Na 2 þ (v*, J*)), and the observation of other ion-neutral processes near 0 K, of potential interest in astrophysics.

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