5/19/201529 jun 2010phillips, et al, sr-poemslide 1 weak equivalence principle test on a sounding...
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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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 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.
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 3
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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 5
• 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 }
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 6
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.
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 7
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 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.
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 9
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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 10
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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 11
Absolute Distance
1 10 1001
10
Averaging time, s
Dev
iati
on,
m
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 12
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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 13
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.
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 14
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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 16
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
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 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).
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 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.
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 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!
04/18/2329 Jun 2010 Phillips, et al, SR-POEM Slide 20
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).
617-495-7108
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.)