5/19/201529 jun 2010phillips, et al, sr-poemslide 1 weak equivalence principle test on a sounding...

04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 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


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.


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.

Top view

Instrument Concept

• Derived from POEM (derived from JILA test).

• 2 test mass assemblies (TMA) observed by 4 tracking frequency laser gauges (TFG).

TFG plate

CG housing

Optics Chamber

Dropping Chamber

TMA Plate

Hexapod Payload Servo

TMA


• 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

Experiment Time Line

} 7 times }


Mitigation of Systematic Error• Differential distance by TFG (laser gauge) from comoving

instrument to TMA (test mass assembly)

• Second difference: TMAA vs TMAB: coincident CM’s.

• Third difference: payload inversion, which cancels:1. Gravity from local masses

2. Earth’s gradient (not higher order term, but it’s small & known).

3. Electrostatic force.

4. Outgassing.

5. Radiometer effect.

6. Thermal radiation.

7. ONLY SOME magnetic terms.

• 1,3: TMA-payload distance constant: payload servo• 4-6: S(Temperature difference) < 0.5 mK Hz-1/2 at 0.007 Hz.

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.



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.


TFG Advantages

• Free of the cyclic bias characteristic of heterodyne laser gauges.

• Uses one beam, not two.• Distance measured as radio frequency, not RF

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.

LaserΔν1

Δν2

Laser


Incremental distance

-2 0 21

10

100

1000

log10

(Averaging Time, s)

All

an D

evia

tion,

pm

Zygo 4100:

6/23/096/24/097/14/092/03/10


Absolute Distance

1 10 1001

10

Averaging time, s

Dev

iati

on,

m


4 4

4

V

H

V

H90°

4 4

x2

Tip-Tilt Controller

4

Δ

2 f2

Tunable Laser

EO

~

P-D-H Hopping Controller (TFG)

λ/2

Mode matching lens

λ/4

PBS

+f1

φ Mod

Δ~

~f2

m1

m2

lens

m3

φ Adj

A

f1

Optical spectrum at A:

Cavity modes: 00 01 00 01

f1

f2

Free Space Beam

Beam Inside FiberElectronicQuadrant Photodiode

Mixer

Amplifier

Misaligned light

BSP

Reference Laser

TFG Out


TMA Suspension System• Can observe and control 6 TMA degrees of freedom.

– All active during setup and inversion.

• Coriolis acceleration: measure difference of TMA transverse 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.

TMAMetal plates, insulated from and attached to, a stable conductive housing. Insulators are well-hidden behind metal electrodes.

Design facilitates attachment of leads.


Thermal Stability

• 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.

• 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


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


Caging (Uncaging)

• 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.

`TMA

Cage~1 kN

TMA

E

Uncage & capture


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).


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.


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!


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).

[email protected]

617-495-7108

[email protected]

617-495-7360

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.)

5/19/201529 jun 2010phillips, et al, sr-poemslide 1 weak equivalence principle test on a sounding...

Documents

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inversion of payload

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tmapayload distance

sounding rocket payload

test mass assemblies

tma test mass assembly

phillips reasenberg