A new torsion pendulum for gravitational reference sensor technology development

Abstract

We report on the design and sensitivity of a new torsion pendulum for measuring the performance of ultra-precise inertial sensors and for the development of associated technologies for space-based gravitational wave observatories and geodesy missions. The apparatus comprises a 1 m-long, 50 μm-diameter tungsten fiber that supports an inertial member inside a vacuum system. The inertial member is an aluminum crossbar with four hollow cubic test masses at each end. This structure converts the rotation of the torsion pendulum into translation of the test masses. Two test masses are enclosed in capacitive sensors which provide readout and actuation. These test masses are electrically insulated from the rest of the crossbar and their electrical charge is controlled by photoemission using fiber-coupled ultraviolet light emitting diodes. The capacitive readout measures the test mass displacement with a broadband sensitivity of 30 nm∕Hz and is complemented by a laser interferometer with a sensitivity of about 0.5 nm∕Hz. The performance of the pendulum, as determined by the measured residual torque noise and expressed in terms of equivalent force acting on a single test mass, is roughly 200 fN∕Hz around 2 mHz, which is about a factor of 20 above the thermal noise limit of the fiber.

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“…The torsion pendulum facility developed at the University of Florida serves as a test bed for gravitational reference sensors and associated technology for LISA [33]. The torsion pendulum consists of a suspended cross-bar assembly with four hollow, gold-coated, aluminum cubic test masses with the same dimensions as the LISA TMs (Figure 8.…”

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“…where r is the pendulum arm, 0.22 m, V mod is the excitation voltage applied to the two x electrodes in the GRS, FIG. 9: Torque sensitivity curve of the UF torsion pendulum from [33] dCx dx is the derivative with respect to position of the capacitance between the electrode and test mass. N 1ω is determined from the pendulum angular readout.…”

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“…The best way to run the apparatus in these conditions is to close the feedback loop on the optical lever, without closing the control loop with the interferometer, and to recover the interferometer sensitivity by subtracting the optical lever noise with an off-line analysis. The actuation system is composed of electrostatic actuators as typically used in the control of torsion pendulums [42,43]. In our case, the actuators are four metallic plates located at the sides of the balance beam.…”