C. Trenkel

A preliminary study of a torsion balance based on a spherical superconducting suspension

We present details of the design and construction of a new superconducting, magnetically suspended torsion balance in which the levitation coil and the lift surface of the float lie on the surfaces of concentric spheres. We compare results from calculations of the variation of the inductance with the levitated height and transverse motion of the float with experimental measurements and show that the levitation system is stable. Angular motion of the torsion balance is detected using superconducting pick-up coils whose inductance is modulated by float rotation. The subsequent change in current flowing in the persistent circuit containing the coils is measured using a flux-gate magnetometer. The pick-up coils exert a restoring torque on the float which can be modified by adjusting the persistent current stored. Periods down to 60 s should be obtainable for a current of 2.5 A. Preliminary results of ring-down experiments in He gas at a pressure of 53Pa show that periods of angular oscillations of 24 s with quality factor, Q, of about 200 can be obtained. The moment of inertia of the float is 2 × 10-5 kg m2. The observed period of 24 s indicates that there is an additional restoring torque in the system which may be due to trapped flux. The observed value of Q is consistent with gas damping.

Novel torsion balance based on a spherical superconducting suspension

We report on the progress that has been made on the development of a spherical superconducting torsion balance in which the traditional fiber is replaced with a Meissner effect suspension operating at 4.2 K. The instrument can levitate a mass of 18 g in persistent mode and currently operates in a helium exchange gas pressure of a few mTorr. A superconducting rotation detector incorporating a superconducting quantum interference device magnetometer has been developed to interrogate the angular position of the torsion balance and provide its natural period. The natural period can be programmed from 25 s to 150 s by varying the current stored in the detector. The detector also offers the possibility of applying external torques onto the torsion balance for the purpose of servocontrol, and a simple derivative control has been developed. The properties of the rotation detector can be predicted from measurements of the inductances of the circuit components. The total measured torque noise is 2×10−13 Nm/√Hz at a...

Photolithographic manufacture of a superconducting levitation coil on a spherical substrate

Abstract We have developed a photolithographic method to pattern a superconducting levitation coil made from Pb on a spherical MACOR substrate. The cross section of the windings is approximately 150 × 40 μm, and a typical coil consists of 70 windings with a minimum pitch of 350 μm. The substrate radius is about 4 cm, and the coil extends from 15 to 60° in polar angle. We present the manufacturing process and suggest future improvements.

Corrigendum: STE-QUEST—test of the universality of free fall using cold atom interferometry (2014 Class. Quantum Grav. 31 115010)

The theory of general relativity describes macroscopic phenomena driven by the influence of gravity while quantum mechanics brilliantly accounts for microscopic effects. Despite their tremendous individual success, a complete unification of fundamental interactions is missing and remains one of the most challenging and important quests in modern theoretical physics. The spacetime explorer and quantum equivalence principle space test satellite mission, proposed as a medium-size mission within the Cosmic Vision program of the European Space Agency (ESA), aims for testing general relativity with high precision in two experiments by performing a measurement of the gravitational redshift of the Sun and the Moon by comparing terrestrial clocks, and by performing a test of the universality of free fall of matter waves in the gravitational field of Earth comparing the trajectory of two Bose–Einstein condensates of 85Rb and 87Rb. The two ultracold atom clouds are monitored very precisely thanks to techniques of atom interferometry. This allows to reach down to an uncertainty in the Eötvös parameter of at least 2 × 10−15. In this paper, we report about the results of the phase A mission study of the atom interferometer instrument covering the description of the main payload elements, the atomic source concept, and the systematic error sources.

The torsion balance as a tool for geophysical prospecting

We discuss whether the torsion balance can again become a key tool for geophysical prospecting. We outline the acknowledged disadvantages of the Eotvos torsion balance and seek designs of a torsion balance beam that would enable the torsion balance to be used on moving platforms. A key result is that torsion balance beams designed to be insensitive to the curvature and horizontal gradients of the gravity field are insensitive to the angular motion of the platform about horizontal axes. We suggest that a double torsion balance using these balance beam designs could be used on moving platforms. We point out that second gradients of the gravitational field (third derivatives of the potential) can be determined with reasonable sensitivity with current technology. We describe double torsion balance schemes where the mass, range, and azimuth of localized mass anomalies could be estimated or where local anomalies could be rejected using information from the second gravity gradient.

Precision Charge Control for Isolated Free-Falling Test Masses: LISA Pathfinder Results

The LISA Pathfinder charge management device was responsible for neutralizing the cosmic-ray-induced electric charge that inevitably accumulated on the free-falling test masses at the heart of the experiment. We present measurements made on ground and in flight that quantify the performance of this contactless discharge system which was based on photoemission under UV illumination. In addition, a two-part simulation is described that was developed alongside the hardware. Modeling of the absorbed UV light within the Pathfinder sensor was carried out with the Geant4 software toolkit and a separate Matlab charge transfer model calculated the net photocurrent between the test masses and surrounding housing in the presence of AC and DC electric fields. We confront the results of these models with observations and draw conclusions for the design of discharge systems for future experiments like LISA that will also employ free-falling test masses.