An Atomic Gravity Gradiometer for Earth Gravity Mapping and Monitoring Measurements
IIP-10-0009
PI: Nan Yu, JPL
October 29, 2014
Task Co-Is: Jim Kohel, Robert Thompson, Xiaoping Wu, JPL Current team members: Sheng-wey Chiow, Jason Williams, and Dah-Ning Yuan, JPL Many others have been on the task and made significant contributions.
Gravity – Part of Whole Earth Science Measurements
Geodesy Earth and Planetary Interiors
– Lithospheric thickness, composition – Lateral mantle density heterogeneity – Deep interior studies – Translational oscillation between core/mantle
Earth and Planetary Climate Effects – Oceanic circulation – Tectonic and glacial movements – Tidal variations – Surface and ground water storage – Polar ice sheets – Earthquake monitoring
GOCE
N. Yu, J. M. Kohel, L. Romans, and L. Maleki, “Quantum Gravity Gradiometer Sensor for Earth Science Applications,” NASA Earth Science and Technology Conference 2002. Paper B3P5. Pasadena, California (June 2002).
Why Atomic Sensors?
σ−
σ+
σ+σ−
σ−
σ+
B field
coils
Laser-cooled Cs atom cloud at µK
Atomic beam
fringes
π/2 pulse π pulse
π/2 pulse
Atoms are stable clocks (laser-based atom optics) atom-wave interferometer
Freefall test mass Displacement Detection Atomic system stability
NIST atomic clock
+ +
• Use totally freefall atomic particles as ideal test masses identical atomic particles are collected, cooled, and set in free fall in vacuum with no external
perturbation other than gravity/inertial forces; laser-cooling and trapping are used to produce the atomic test masses at µK and even pK; no cryogenics and little/no mechanical moving parts.
• Matter-wave interference for displacement measurements displacement measurements trough interaction of lasers and atoms, pm/Hz1/2 when in space;
laser control and manipulation of atoms with opto-atomic optics. • Intrinsic high stability of atomic system
use the very same atoms and measurement schemes as those for the most precise atomic clocks, allowing high measurement stabilities.
• Enable orders of magnitude sensitivity gain when in space microgravity environment in space offers long interrogation times with atoms, resulting orders of
magnitude higher sensitivity compared terrestrial operations.
Inertial Phase Shifts in Atom Interferometers
Atomic fountain on ground
!ΔΦ = 2k a T2
With over 106 atoms, the shot-noise limited SNR ~ 1000. Per shot sensitivity = 2 × 10–10⁄ T2 m/s2.
Light-pulse atom interferometer accelerometer
π pulse
π/2" pulses
• Independent of atom initial velocity.!• The laser wavenumber k is the only
reference parameter.!• Sensitivity increases with T2.!
Raman Beam
Counter-propagating Raman Beam
z
t
π/2
π
π/2
0 T 2T
Master laser II
Master laser I
Slave laser 2
Slave laser 1
Cs lock
Raman lasers
Amplifier 2D MOT Laser 1
2D MOT Laser 2
3D MOT Laser 5
3D MOT Laser 6
3D MOT Laser 1
3D MOT Laser 2
3D MOT Laser 3
3D MOT Laser 4
3D MOT
2D MOT
Gravity Gradiometer = Atom Interferometer (x2)
Upper Detector
3D MOT
2D MOT
Raman
Lower Detector
Stabilized Retro-Mirror
Single Chamber
Laser and Optical Subsystem
Raman
• A ground transportable gravity gradiometer with the system design comparable with microgravity operation in space
LabVIEW Control
Data Acquisition/ Processing
Gradiometer CPU 2D MOT Coil Drivers
Timing Control
Control System
Frequency Control
Bias Field Coil Drivers
3D MOT Coil Drivers
IIP Gravity Gradiometer: Instrument Overview
2D MOT, 3D MOT, Repumps, Clearing beams
Fountain launch and detection.
The configuration for a space instrument will look like this without long fountain tubes.
Single Vacuum Chamber
Subsystem: Atomic Physics Package
Rack-mounted enclosures housing laser and optical system modules.
Subsystem: Laser and Optics
Repumper Laser module, complete
High Frequency AOM
module, x2 complete
Booster Laser Module, complete
Slave module stack
Laser module drawers
The optics and electronics rack contains all laser systems and electronics. Electronics has a combination of COTS, semi-custom parts, and in-house fabrications.
Subsystem: Electronics and Control
Added optical amplifier for a larger sized optical beam.
Instrument Analysis and Optimization
The plots show the expected and measured contrast of Raman fringes that impact the overall instrument performance.
Example: Atom interferometer contrast loss limitation and optimization. The fringe contrast is limited by the atom cloud residual thermal expansion and size of the laser beam.
Amplitude after Raman upgrade
0.2 0.3 0.4 0.5 0.6 0.70.2
0.3
0.4
0.5
0.6
0.7
Low
er N
EF
(a.u
.)
Upper NEF (a.u.)
Closed Loop Real-time Measurement
! Applicable for space-based missions
! Addressed SNR-dominant sensitivity discrepancy
! First-order insensitive to fluctuations in the atom interferometer contrast and offsets.
0.2 0.3 0.4 0.5 0.6 0.70.2
0.3
0.4
0.5
0.6
0.7
Low
er N
EF
(a.u
.)
Upper NEF (a.u.) NEF: Normalized Excitation Fraction of atoms
Instrument Analysis and Optimization
1.0
0.8
0.6
0.4
0.2
0.0
Phas
e100806040200
Run #
Upper Lower
Run # S
igna
l (N
EF)
Transportable gradiometer Instrument Sensor Rack
Instrument Sensitivity Evaluation
• 40E @ 1s (E: Eotvos, 10-9/s) • At the state-of-the-art reported in research lab experiments
0.2 0.3 0.4 0.5 0.6 0.7 0.80.2
0.3
0.4
0.5
0.6
0.7
0.8
Upper NEF (a.u.)
Low
er N
EF
(a.u
.)
1 5 10 50 100 500100012
51020
50100
Time HsL
GradientHEL
(Ellipse Fit)
Designed performance
(Closed loop)
Sensitivity Validation Measurements
• Five lead bricks (total 33kg) were placed near the apparatus • The instrument is sensitive to minute structural distortion due to additional mass
Vibration isolation platform
Mass supporting beam, independent of the instrument and platform
• Disturbance to the instrument was minimized by supporting the mass from outside of the vibration isolation box
Stack of 5 lead bricks
Sensitivity Demonstration
• Observed clear modulated closed loop signal of 36.4 (1.0) E • Agrees with the estimate of test mass gradient signal of 34.4 (4.0) E (Error due to 1
cm positioning precision.)
3300
3280
3260
3240
3220
3200
3180
Grad
ient (
Eotv
os)
300020001000Time (s)
No extra mass With lead mass
Modulation of 5 lead bricks (~30 kg)Differential gravity gradient: 36.4 +/- 1.0 Eotvos
4
5
6
789
0.001
2
3
4
Alla
n De
viat
ion
(1/S
NR)
12 3 4 5 6 7 8 9
102 3 4 5 6 7 8 9
1002 3 4
Averaging Time (s)
Apex Upper Return Lower Return
Apex measurement: Atoms launched from the lower chamber, detected in the upper chamber when stationary at apex. No degradation in SNR with stationary clouds.
Detecting stationary clouds
400 420 440 460 480 5000.20.40.60.81.01.2
Time HmsL
SignalHVL
Releasing vs launching: Atoms released from the upper chamber rather than launched, detected in the lower chamber. No degradation in atom number by releasing.
Launched Released
Generating stationary clouds
Microgravity Operation Evaluations
Current Status
• The JPL instrument is designed as a ground transportable gradiometer with an operation configuration compatible to space operation mode under microgravity condition.
• The instrument is capable of operating continuously with a sensitivity within a factor of four of the designed performance. Implementation to achieve the remaining factor is underway.
• The instrument sensitivity is current at the state of the art of atom-interferometer gravity gradiometers demonstrated in research labs.
• We are actively investigating and developing a technology infusion path to space missions for Earth Science gravity measurements.
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