  We demonstrate a general technique to achieve a precise radial displacement
of the nodal line of the radiofrequency (rf) field in a linear Paul trap. The
technique relies on selective adjustment of the load capacitance of the trap
electrodes, achieved through the addition of capacitors to the basic resonant
rf-circuit used to drive the trap. Displacements of up to 100 micrometer with
micrometer precision are measured using a combination of fluorescence images of
ion Coulomb crystals and coherent coupling of such crystals to a mode of an
optical cavity. The displacements are made without measurable distortion of the
shape or structure of the Coulomb crystals, as well as without introducing
excess heating commonly associated with the radial displacement of crystals by
adjustment through static potentials. We expect this technique to be of
importance for future developments of microtrap architectures and ion-based
cavity QED.
