Effects of electrode design on null point motion of spheres suspended in the electrodynamic balance

Abstract

Three similar but geometrically distinct electrode designs for an electrodynamic balance or quadrupole particle levitation cell have been compared in terms of the minimum periodic motion, or ‘‘null point’’ motion of single, suspended glass spheres of either 30 or 11 μm in diameter. The three electrode surface designs included the ideal hyperbolic shape, similar but spherical surfaces, and spherical surfaces in which the center ring electrode was split. The motion of the glass spheres was determined by forming an interferometer from scattered laser light and a reference beam using a single frequency He–Ne laser. A heterodyne signal was generated by shifting the frequency of the reference beam an arbitrary but known amount. No significant distinctions in terms of small amplitude motion were observed among the different cell designs. All three cells were able to suspend and stabilize a 30-μm sphere to an rms amplitude of motion of less than 0.14 μm. For each of these designs, dependence of the particle motion on the dc balancing voltage was measured, and the effect of other operating parameters determined. Also, previously unreported sensitivity with regard to the vertical tilt of the cell axis was observed.