A Inertial Clock for Precision Measurements

A precision inertial clock to be used for gravitational redshift measurements requires a stability of 1 part in 10('15) for averaging times equal to or greater than 10('2) s. A room temperature forerunner of an ultra-stable low temperature inertial time keeping device is described. In this version, a protected proof rotor is encapsulated by and doubly suspended with a shroud rotor which, with the use of feedback, is forced to corotate with the proof rotor under vacuum and thereby reduce the residual gas drag on it. Physical improvements to the apparatus increased the stability of the double suspension making possible hundreds of experimental runs, some lasting longer than 48 h, under various suspension and control conditions. The behavior of the fedback corotating rotors is shown to agree with the solutions of a pair of coupled differential equations which model the rotors, and which include terms for the previously inadequately treated proof rotor bearing drag and interrotor torsional coupling. Experimental frequency stability studies investigate the effects of angular derivative feedback to a rotor and compare the results with theoretical calculations for an ideal rotor. The experimental results agree in character with the theoretical predictions, but have a noise amplitude four to five orders of magnitude greater than predicted. It is shown that feedback is necessary if the long-term frequency stability is to improve with longer averaging times. The coupled rotor modeling and the frequency stability studies result in a much firmer understanding of the system performance and optimization processes. It is recommended that work on a superconductive version of the inertial clock commence.