Introduction and context

1. Introduction and context • Fundamental physics and geophysics in space share common constraints and technologies, e.g. : • Long term acquisitions for detecting low frequency, low amplitude phenomena (fractions of second to hours) • Precise understanding of the gravity (test of the theory of gravity, gravity field perturbations due to geologic effects) • Two ‘instrumental’ problems are addressed in this WP : • How to improve the sensitivity of the present planetary seismometers ? • What are the limiting noises of the next generation of drag-free satellites ? • Two on-going studies at the interface between fundamental physics and geophysics in space : • Development of an interferometric read-out system for planetary seismometers • System simulation and noise characterization of the LISA Pathfinder drag-free mission

2. Objectives • Optical readout • Use of interferometric techniques (Fabry-Perot cavity) to measure the displacement of the seismometer arm. • Noise frequency bandwidth : 10-2 to 10 Hz • Current capacitive techniques seem to have reached their limit at about 10 pm/√Hz displacement noise • Theoretically, optical techniques can improve this performance by 1 to 2 orders of magnitude • The design should be compatible with space-based operations (compactness, robustness, low power consumption, etc.) • Simulations and analyses of LISA Pathfinder data • Understanding of the limiting noises • In-flight characterization of µ-Newton (cold gas) thrusters • Improvement of the LPF simulator (State Space Model) • Pave the way for an eLISA simulator

3. Main results on the LISA Pathfinder analysis • Work performed by Henri Inchauspé (PhD) • A technical simulator for eLISA based on the LPF experience • State Space Model (as for LISA Pathfinder) developed under MATLAB • Simplified model with 1 test mass (TM) / spacecraft (SC) TM2 TM1 CoM12 y O12 (T) x CoM3-6 TM5 TM3 CoM34 CoM56 TM6 TM4 X : State vector M : Mass matrix A : State matrix B : Input (torques) matrix

4. Main results on the LISA Pathfinder analysis • Test case : computation of the S/C drift w.r.t the TM due to the gravity gradient within the S/C on pure Keplerian orbits • Expected drift of ~3m over 1 year • Comparison between analytical model and SSM : • error <10 µmafter 1 year • On-going work on a Drag Free and Attitude Control System (DFACS) for eLISA, objectives : • Keep the TM within its cage • Guarantees the laser pointing from one S/C to the others.

5. Main results on the LISA Pathfinder analysis • Study of cold gas thrusters • On-going implementation of (approximate) analytical state space equations into the SSM of LISA Pathfinder • Participation to LPF mock data challenges • Realistic simulations provided by ESA and analyzed by scientists to retrieve the system characteristics (use of the SSM simulator …) • In the same conditions as for the mission

6. Main results on the seismometer development • Work performed by John Nelson (postdoc) • Global design of the experiment • Provision of the laser source, fiber components and photodiodes • Development of a force balance • First cavity proto-type being mounted

8. Main results on the seismometer development • In-depth study of the thermal noise sources for the cavity • The dominant noises are from spacer and mirrors (incl. coating) brownian noise. • Thermal noise level is theoretically compatible with 10-15m/√Hz at 10 mHz • Influence of the tilt misalignment (seismometer arm movement) on the coupling into the fiber • Simulated … and foundto be negligible …

9. Main results on the seismometer development • Optimization of the cavity geometry and sensitivity to vibrations • Mirror focal length optimization • Optimization of support points • First (quick and dirty…) simulations of cavity deformations due to vibrations • ~10 µm max deflection under 1g but almost no bending (hence no tilt), nor cavity length changes

10. Project objectives for 2014 – 2015 • LISA Pathfinder analyses • Improvement of the LPF SSM simulator, knowledge transfer towards a SSM simulator for eLISA • Tests procedures for characterizing the µNewton cold gas thrusters (especially their noise levels) • Model updates based on the final AIVT of the satellite • LPF is planned for launch in July 2015 • Participation to the inflight commissioning during the cruise of LPF to its final position (2nd semester 2015) • Optical readout for seismometer • Manufacturing of the Invar prototype and first tests • Determination of the sensitivity to vibrations and thermal perturbations (with a fixed arm) • New cavity design (if needed) for mounting on the force balance • Comparison to existing capacitive readout systems • Duplication and characterization • Design constraints for implementation in a space-based VLB seismometer

11. Relevance to the LabEx themes • LISA Pathfinder analyses • Immediate need for the mission, contribution of the LabEx to a fundamental physics project (GW detection) • Prepare collaborations and expertise for future drag-free missions, e.g. in planetary sciences • Optical readout for seismometers • R&D for the next generation of seismometer, potential technological breakthrough • Technological interface between fundamental physics and planetary missions : new technical approach and development of tests beds with similar constraints (low freq. perturbations) • Leverage effects • Interdisciplinary University grant awarded in 2012 : 20 k€ • purchase of the main optical equipment for starting the experiment