Weak Equivalence Principle Test on a Sounding Rocket

1. Weak Equivalence Principle Test on a Sounding Rocket James D. Phillips, Biju R. Patla, Eugeniu E. Popescu, Emanuele Rocco, Rajesh Thapa, Robert D. Reasenberg Smithsonian Astrophysical Observatory Harvard-Smithsonian Center for Astrophysics and Enrico C. Lorenzini Faculty of Engineering, University of Padova, Italy CPT10, Indiana University, 29 July, 2010 Phillips, et al, SR-POEM

2. Mission Concept • WEP test in sounding rocket payload. • Galilean (dropping) test. • Experiment duration about 600 s. • Long free fall yields sensitivity. • Inversion between drops controls error. • Payload ≈ 200 kg. • Payload: non-recoverable (like orbiting payload). • Low cost (not like orbiting payload). • For a single pair of substances, σ(η) ≤ 10-16. • 1000 fold advance over present best result. Phillips, et al, SR-POEM

3. Top view Experiment Concept • 2 test mass assemblies (TMA) in free fall for 40 s per drop. • TMA are about 0.9 kg. Each is a dumbbell comprising two cubes. • Experiment includes 8 drops. • Payload inversion between drops. • Reduces systematic error (4 drops in each orientation). • Drops placed symmetrically around apogee. Phillips, et al, SR-POEM

4. Optics Chamber Dropping Chamber TFG plate CG housing TMA TMA Plate Hexapod Payload Servo Instrument Concept • Derived from POEM (derived from JILA test). • 2 test mass assemblies (TMA) observed by 4 tracking frequency laser gauges (TFG).

5. Experiment Time Line • Below 800 km (~300 s): uncage, electrostatic capture, discharge. • Calibration with payload on side. • Locate CM: 1 nm in Z; 1 μm in X & Y. • Above 800 km (~530 s), series of 8 drops. • Payload orientated vertically. • 40 second drop. • Inversion of payload. • 40 second drop. • Below 800 km (~300 s), recalibration with payload on side. } } 8 drops symmetric around apogee: apogee > 1100 km 7 times Phillips, et al, SR-POEM

6. Mitigation of Systematic Error • Differential distance by TFG (laser gauge) from comoving instrument to TMA (test mass assembly) • Second difference: TMAAvs TMAB: coincident CM’s. • Third difference: payload inversion, which cancels: • Gravity from local masses • Earth’s gradient (not higher order term, but it’s small & known). • Electrostatic force. • Outgassing. • Radiometer effect. • Thermal radiation. • ONLY SOMEmagnetic terms. • 1,3: TMA-payload distance constant: payload servo • 4-6: S(Temperature difference) < 0.5 mK Hz-1/2 at 0.007 Hz. Phillips, et al, SR-POEM

7. Magnetic Force • Upon inversion, some components of magnetic force are unchanged, like the WEP signal (inertial coordinates). • Symmetry about xy plane => ∂Bi/∂z vanishes. • Rotate about y: x and z reverse. • Residual gradient depends on asymmetries. • Test magnetic moment (U. Wash). • Purer material (Al & material B), degauss. • Test shield & reduce gradient. Phillips, et al, SR-POEM

8. Tracking Frequency Laser Gauge (TFG) • First developed around 1990 for POINTS. • See Phillips & Reasenberg, RSI, 76, 064501, 2005. • Now being developed under NASA-APRA. • Using DFB (semiconductor) lasers at 1.55 micron. • Goal: 0.1 pm/√Hz in a cavity. Presently 2 pm/√Hz, non-resonant. • New alignment system planned. • TMA will rotate wrt instrument. • Cavity-based alignment to within 10-8 rad/√Hz. • Employs 2f detection, allowing use of reflected beam. Phillips, et al, SR-POEM

9. Laser Δν1 Laser Δν2 TFG Advantages • Free of the cyclic bias characteristic of heterodyne laser gauges. • Uses one beam, not two. • Distance measured as radio frequency, notRF phase: more accurate transport & measurement. • Can operate in a resonant cavity: improves sensitivity, suppresses error, & supports alignment. • Able to suppress reflection-phase errors. • Absolute distance at little added cost or complexity. Phillips, et al, SR-POEM

10. Incremental distance Phillips, et al, SR-POEM

11. Absolute Distance Phillips, et al, SR-POEM

12. PBS λ/4 BSP V 4 4 Tip-Tilt Controller 4 4 H V 4 4 H 90° Δ x2 2f2 Free Space Beam Optical spectrum at A: Beam Inside Fiber Electronic Quadrant Photodiode Mixer Cavity modes: Amplifier Misaligned light A TFG Out λ/2 Reference Laser EO lens Mode matching lens f1 ~ m1 + m3 Tunable Laser f2 φ Mod m2 ~ φ Adj Δ ~ f1 P-D-H Hopping Controller (TFG) Phillips, et al, SR-POEM

13. TMA TMA Suspension System • Can observe and control 6 TMA degrees of freedom. • All active during setup and inversion. • Coriolis acceleration: measure difference of TMAtransverse velocities. • Transverse position measured before and after WEP. • WEP measured with TFG. • During WEP measurement, CG drive signals reduced. • Payload inertially pointed with ACS off. • Metal plates, insulated from and attached to, a stable conductive housing. Insulators are well-hidden behind metal electrodes. • Design facilitates attachment of leads. Phillips, et al, SR-POEM

14. Thermal Stability • Two concerns: • Direct: TFG plate warps, changing apparent differential acceleration (and thus η). • Indirect: Payload mass moves, changing local gravity. • Direct effect made small by: • Use of ULE glass for precision structure. • Layered passive thermal control. • Symmetry of thermal leaks. Direct • Indirect term calculated for worst case • Transmitter, 0.04K/s => 6K/ks average rise in 1 m tube. • 36 kg off center by 5 cm => 1.4 10-18 g (before inversion). Thus far, we have not found a problem. Indirect Phillips, et al, SR-POEM

15. Thermal Time Constants • The precision instrument hardly sees the external temperature changes. • Vacuum chamber gold coated inside and out. • Emissivity, ε = 0.02. • A: Payload tube (ε = 0.1) to chamber, τ = 1.5 x 105 s. • B1: Chamber to metering structure (ε = 1), τ = 1.4 x 105 s. • B2: Chamber to TFG plate (ε = 1), τ = 5.5 x 105 s. A B2 B1 Phillips, et al, SR-POEM

16. E Cage ~1 kN Uncage & capture Caging (Uncaging) `TMA TMA • Problem: Clamp for launch, uncage, capture electrostatically. Clean metals tend to cold weld. • Synergy with LISA. • Candidate design concepts: • Non-stick materials with possible separate ground point. • R-S-H documented: S-Au bond. R-Se-H speculative. • Contact at bottom of hole to hide the surface potential of contact area. • Use high E-field to capture TMA after uncaging. • Higher than LISA; 2 orders less than MICROSCOPE. • Must remove “fingers” beyond local reference plates. Phillips, et al, SR-POEM

17. Lab Demonstrations • TFG: 2 ULE plates to model TFG plate + TMA. 2f alignment & 1×10-13 m Hz-1/2 at 0.007 Hz. • Reference laser using temp-compensated fiber. • Capacitance gauge calibration. • Uncaging: clamp hard, release gently. • Magnetic testing: TMA (U Wash), shield (SAO). • TMA surface potential • Uniformity characterization (PNNL). • Total force, torsion balance (SAO, help from U Wash). • Payload position servo (SAO or contractor). Phillips, et al, SR-POEM

18. Why Does SR-POEM Work? • Free fall >500 s. TFG supports quick measurements. • 0.1 pm/√Hz To be demonstrated; expected in cavity. • Payload inversions. • Cancel systematic errors. • Differential measurement from co-moving platform. • Symmetry maintained. • Thermally benign environment. • Test many systems on zero-g aircraft flights before rocket launch. Phillips, et al, SR-POEM

19. Concluding Comments • Goal: σ(η) ≤ 10-16 for single pair of substances. • Sounding rocket has clear benefits. • Additional flights could test other substance pairs. If SR-POEM had launched at the start of this talk, it would now be finished! Phillips, et al, SR-POEM

20. This work has been supported in part by NASA through grants NNX08AO04G (ATFP) and NNX07AI11G (APRA). Four post docs are now working with us on SR-POEM. We anticipate opening one or two more positions soon. (Physics Today, Aug.) http://www.cfa.harvard.edu/PAG Papers and sounding-rocket proposal available. “A weak equivalence principle test on a suborbital rocket.” 2010arXiv1001.4752R, CQG 27, 095005 (2010). jphillips@cfa.harvard.edu 617-495-7360 reasenberg@cfa.harvard.edu 617-495-7108 Phillips, et al, SR-POEM