Hubert HalloinScientific Committee 2013 WP Interface I3(APC/IPGP) : Fundamental Physics and...

Hubert Halloin Scientific Committee 2013

WP Interface I3(APC/IPGP) : Fundamental Physics and Geophysics in Space

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



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 (c

ompactness, 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



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)

TM1

TM2

TM3

TM5

TM4TM6

CoM3-6

O12 (T)

x

yCoM12

CoM34CoM56

X : State vectorM : Mass matrixA : State matrixB : Input (torques) matrix



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 µm after 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.



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



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



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 10-15 m/√Hz at 10 mHz

Influence of the tilt misalignment (seismometer arm movement) on the coupling into the fiber

• Simulated … and found to be negligible …



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



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



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

Hubert HalloinScientific Committee 2013 WP Interface I3(APC/IPGP) : Fundamental Physics and...

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