  Small, controllable, highly accessible quantum systems can serve as probes at
the single quantum level to study multiple physical effects, for example in
quantum optics or for electric and magnetic field sensing. The applicability of
trapped atomic ions as probes is highly dependent on the measurement situation
at hand and thus calls for specialized traps. Previous approaches for ion traps
with enhanced optical access included traps consisting of a single ring
electrode or two opposing endcap electrodes. Other possibilities are planar
trap geometries, which have been investigated for Penning traps and rf-trap
arrays. By not having the electrodes lie in a common plane the optical access
in the latter cases can be substantially increased. Here, we discuss the
fabrication and experimental characterization of a novel radio-frequency (rf)
ion trap geometry. It has a relatively simple structure and provides largely
unrestricted optical and physical access to the ion, of up to 96% of the total
4pi solid angle in one of the three traps tested. We also discuss potential
applications in quantum optics and field sensing. As a force sensor, we
estimate sensitivity to forces smaller than 1 yN Hz^(-1/2).
