A 15 Minute WEP Test (SR-POEM)

1. A 15 Minute WEP Test (SR-POEM) R.D. Reasenberg, E. Hirose, B. Patla, J.D. Phillips, E.M. Popescu Smithsonian Astrophysical Observatory Harvard-Smithsonian Center for Astrophysics Cambridge, Massachusetts, USA and E.C. Lorenzini Faculty of Engineering University of Padova, Italy Q2C4 September 2009 Bremen Reasenberg et al., SAO

2. Mission Concept • For a single pair of substances, σ(η) ≤ 10-16. • 1000 fold advance over present best result. • WEP test in sounding rocket payload. • Experiment duration 400 to 800 s. • Payload ≈ 200 kg. • Non-recoverable payload (like orbiting payload). • Low cost (not like orbiting payload). Reasenberg et al., SAO

3. Instrument Concept • Derived from POEM (which was derived from JILA test). • 2 test mass assemblies (TMA) observed by 4 tracking frequency laser gauges (TFG). • Double difference observable: • Observations made from co-moving reference plate. • Difference of observations yields quantity of prime interest. Reasenberg et al., SAO

4. Experiment Concept • TMA (about 1.0 kg) in free fall for 40 s per drop. • Experiment includes 8 drops. • Payload inversion between drops. Inversion cancels most remaining systematic errors: In payload frame, gravity from local mass is fixed and WEP signal reversed by inversion. Earth’s gravity gradient is symmetric and thus the same after inversion (except for higher order term which is too small to matter.) Reasenberg et al., SAO

5. Experiment Housing Precision instrument inside vacuum chamber inside 14 inch payload tube. Not shown here are the two vacuum ports at the upper end of the chamber, the capaci-tance gauge electrode sets, and the TFG optics. Reasenberg et al., SAO

6. Tracking Frequency Laser Gauge (TFG) • Developed around 1990 for POINTS. • Recent further development. • Now being developed under NASA-APRA. • See Phillips & Reasenberg, RSI, 76, 064501, 2005. • Based on Pound-Drever-Hall locking • Converts distance change to frequency change • Easily and reliably measured. • Hops to new fringe when it runs out of range. • Has advantages over traditional heterodyne laser gauge. (5 applicable here) Reasenberg et al., SAO

7. TFG: Six Advantages • Intrinsically free of the nm-scale cyclic bias characteristic of the heterodyne laser gauge (e.g., Hewlett-Packard, Zygo). • Uses one beam, not two: simplifies the beam launcher. • Distance changes converted to Δ(radio frequency): more stable and more easily measured than RF phase. • Able to operate in a resonant cavity: improves precision, suppresses misalignment error and supports servo-based alignment. • Suppresses polarization error from non-normal incidence in a cornercube (nm scale). • Measures absolute distance with a minimum of added cost or complexity. Reasenberg et al., SAO

8. Laser Gauge Alignment Reasenberg et al., SAO

9. Wavefront Sensing (Morrison, Meers, Robertson, Ward) Sampas-Anderson SB1 mirror1 θ E0 θ mirror1 CR pick-off SB2- E0 CR SB1 fold mirror mirror2 SB2+ QPD1 mirror2 QPD QPD2 • Carrier resonates with TEM00, and SB2+ resonates with TEM10. • Transmitted light is used. • Only Carrier resonates with TEM00. • Reflected light is used. • Used by LIGO. Reasenberg et al., SAO

10. Comparison of Methods Reasenberg et al., SAO

11. TMA Suspension System • Can observe and control 6 degrees of freedom. • Capacitance gauge sensing. • Electrostatic forcing. • All active during setup and inversion; off during WEP measurement. • Coriolis acceleration: measure E-W velocity. • Transverse position measurements made before and after WEP measurements. Reasenberg et al., SAO

12. Interelectrode gaps are 0.25 cm. The long bars are drive electrodes and the large rectangle is the sense electrode. Not shown, grounded shield around the sense electrodes. The capacitance gauge is the sensing portion of the TMA-SS Dimensions are in cm. Capacitance Gauge Plates These electrodes are deposited on glass plates (ULE or Pyrex) that are later joined to form a box. Any significant open areas are filled with grounded shields, and the small remaining gaps are covered with resistive material, which leaks off charge. A ground plane on the back of each plate covers the drive electrodes but not the sense electrode (because it would add capacitance and thus decrease sensitivity.) Plan of capacitance gauge electrodes. Reasenberg et al., SAO

13. TMA CG Electrode Alternatives • Metal plates, insulated from and attached to, a stable conductive housing. • Insulators are well-hidden behind metal electrodes. • Facilitates attachment of leads. Reasenberg et al., SAO

14. POEM Capacitance Gauges + + - - ~ Collaboration with Winfield Hill, Rowland Institute at Harvard Vacuum TMA Correlator s/w in PC f1, f2, …, f6 ADC 24 bit 100 kHz Estimates of 6 positions per TMA, at 1 kHz Cal. f1 Moving Static Reasenberg et al., SAO

15. f2 Vacuum Chamber Correlator f2 ADC - TMA + f1 Correlator f1 ADC - + Possible Capacitance Gauge for SR-POEMTwo of six position measurement circuits shown Reasenberg et al., SAO

16. Charge on TMA • TMA potential at separation  few × 100 mV. • Measure by applying DC field to capacitance gauge electrodes and neutralize, e.g., with UV LED. Sun, et al. LISA-LIGO Charging Workshop, 2007. • Before reaching altitude at which drag is low (800 km). • Make TMA voltage  rms variation over surface. • Effect of small constant charge on TMA cancels in payload inversions. Reasenberg et al., SAO

17. Surface Potential Variation • Classical solution: gold, graphite (Aerodag). • Recent LISA work by Robertson et al. • Class. Quantum Grav. 23 (2006) 2665-2680 • New materials studied: Au over Nb, diamond-like carbon, TiC, indium tin oxide (ITO), Au over ITO. • Many achieve 1 to 2 mV rms wrt mean. • Measurements done with 3 mm Kelvin probe. • Needs to be smaller and more sensitive. • They, GSFC and PNNL are investigating improvements. Reasenberg et al., SAO

18. Surface Potential Variation for SR-POEM • Assume: 1 mV rms, 4 mm spacing (top and bottom) =>2×1.7×10-16 g. • Measured time variation of surface potential at 1/140 Hz, averaging over 4 cycles: δa=1.2×10-17 g. • Very conservative estimate. • Good enough, and further … • Temperature in SR-POEM 100- to 1000-fold more stable (below). => SR-POEM has stable vacuum environment. • Need additional testing under SR-POEM conditions. • [Does voltage at inversion cause change of potential?] Reasenberg et al., SAO 10-9 Torr

19. Caging (Uncaging!) • Synergy with LISA. • Clean metals tend to cold weld. • Welds can reach a large fraction of strength of the metal. • Candidate design concepts: • Non-stick materials with possible separate ground point. • Graphite gas bearing to push off. • R-S-H compounds (long chain thiol or mercaptan). • S → Se ? • Contact at bottom of hole to hide the surface potential of contact area. Reasenberg et al., SAO

20. Thermal Stability • Two concerns: • Direct effect on apparent double-difference acceleration measured with TFG (thus on η). • Indirect by moving payload mass. • Direct effect made small by: • Use of ULE glass for precision structure. • Layered passive thermal control. • Symmetry of thermal leaks. Thus far, unable to find a problem. Reasenberg et al., SAO

21. Thermal Time Constants • The precision instrument hardly sees the external temperature changes. • Vacuum chamber gold coated inside and out. • Emissivity, ε = 0.02. • Payload tube (ε = 0.1) to chamber, τ = 1.5 x 105 s. • Chamber to metering structure (ε = 1), τ = 1.4 x 105 s. • Chamber to TFG plate (ε = 1), τ = 5.5 x 105 s. Reasenberg et al., SAO

22. Thermal leaks Vacuum flange to TMA plate, radiative: 6 mW/K Support ring to TMA plate, conductive: 56 mW/K Vacuum flange to support ring, conductive: similar Vacuum flange to TMA plate, total: 34 mW/K (τ=105 s) Cables? Reasenberg et al., SAO

23. Earth’s Gravity Gradient, Iat 1000 km altitude • For a TMA 1 mm (z) from payload CM plane, it moves 1.6 μm in 40 s. (2 x 10-8 ms-2 = 2.7 x 10-9gh) (We are investigating how closely the CM can be placed to the expected plane of the TMA.) • For a TMA 0.1 mm radially (ρ) from payload cm, it moves (radially) 80 nm in 40 s. (1 x 10-10 ms-2) • These motions are very predictable and change slowly with payload trajectory. • Symmetry: rising vs falling part of trajectory. • Nearly perfect! Reasenberg et al., SAO

24. Earth’s Gravity Gradient, IIat 1000 km altitude • Objective: matched conditions in pairs of drops. • Each pair has instrument in both orientations. • Use data with same altitude range from each drop. • Symmetrically placed with respect to apogee. • A small term remains. • 3 x10-11 m (per micron of TMA relative centering error) • Gravity gradient = 2.7 x 10-7 g/m (Earth only, 1000 km alt.) • Acceleration error < 8x10-18 g (per micron …error) Reasenberg et al., SAO

25. Systematic Error Estimation(major contributions) Reasenberg et al., SAO

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28. g = is the acceleration due to gravity at 1000 km Δ →change after inversion Reasenberg et al., SAO

29. Local Gravity Stabilization, I Earth thermal radiation: ≈10-10 g => 0.6 μm in 40 s Pseudo-drag-free instrument. Useful because local masses produce a much more complex gravity field than the rest of the spacecraft. Alternative: Map the local gravity field by making measurements on the way up to WEP-measurement altitude. Or a real drag-free system. Needs considerable study. Reasenberg et al., SAO

30. Local Gravity Stabilization, II • Earth gravity daz/dz = 2.7 x 10-7 g/m (well known) • Local gravity daz/dz ≈ 10-8 g/m (poorly known) • Require δz < 3 nm. • Non-grav. force & ACS noise => ~1 μm. • Servo to keep payload fixed w.r.t. combined TMA. • Control (zCM1+zCM2), θa, θb. • TFG is sensor. • Hexapod is actuator. • Need initial ΔvCM<10 nm/s. • Use local gravity model to correct for actual positions. • In early stage of investigation. Reasenberg et al., SAO

31. Why Does SR-POEM Work? • TFG supports quick measurements. • 0.1 pm/√Hz – requires development. • Double difference observable. • Symmetry maintained. • Local gravity stabilization (if used). • Payload inversions. • Cancel systematic errors. • Thermally benign environment. Reasenberg et al., SAO

32. Concluding Comments • Goal: σ(η) ≤ 10-16 for single pair of substances. • Sounding rocket experiment is low in cost. • Additional flights could test other substances. If the sounding rocket had launched at the start of this talk, the experiment would be over now! Reasenberg et al., SAO

33. Two new post docs started this month: Rajesh Thapa & Emanuele Rocco http://www.cfa.harvard.edu/PAG Papers and sounding-rocket proposal available. reasenberg@cfa.harvard.edu 617-495-7108 jphillips@cfa.harvard.edu 617-495-7360 This work has been supported by NASA-UG through grant NNC04GB30G. It is now supported by NASA-ATFP through grant NNX08AO04G Reasenberg et al., SAO