Progress Report on the Thompson–Lampard Calculable Capacitor at LNE

Abstract

We report on a new calculable electrical capacitance standard, which will enable realizing the farad in the SI with a combined uncertainty (k=1) of the order of one part in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> . To achieve this result, we developed various original mechanical systems to position and control macroscopic elements at the nanometer scale. A measurement system allowing the in-situ control of this positioning was integrated into the standard. The electrodes were held in position in a hexapod structure, which was mechanically isolated from the standard’s carrier structure to ensure that no force is applied to change the electrodes’ positions. An additional electrode, so-called “movable guard”, was introduced into the cavity formed by the electrodes to vary the cross capacitance of the standard. The lateral position of this guard in the cavity must be controlled to within 100 nm along its entire length of travel, which was achieved by means of two piezoelectric actuators. An additional capacitive measurement device between the moving guard and the five electrodes ensures that it is correctly positioned in the cavity. We demonstrated that the system described here guarantees the correct positioning of the guard within 80 nm in the cavity, regardless of its vertical position.