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Paik-1
Exploring Gravity
with Proof-Mass Technologies
Ho Jung Paik
University of Maryland
July 6-10, 2008, Warrenton, VA
Paik-2
Inertial Technology
Gravity experiments and experiments searching for gravity-like forces invariably employ test masses.
To overcome the vibrations of the platform, these experiments often make a differential measurement over two or more test masses.
The test mass response is monitored by using an electric field (Microscope, LISA), magnetic field (GP-B, STEP, SMART), or light (LISA).
Paik-3
Advantages of Space
,/
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220
j
mFx
Zero-g frees test masses completely from the housing (f0 < 103 Hz) and eliminates many g-induced errors. GP-B, STEP, SMART, LISA
Extremely quiet dynamic environment free from the seismic and gravity noise of the Earth. GP-B, STEP, SMART, LISA
Bigger gravity signal achieved by rotating the spacecraft with respect to the Earth. STEP, SMART
Much longer baseline achievable in space.
LISA
Paik-4
GP-B
To search for dragging of the local inertial frame by a rotating mass, 41 milliarcsec per year.
A spinning superconductor generates a magnetic moment, called “London moment.”
As the gyro precesses, the magnetic flux through the superconducting loop varies and generates a signal, which is detected by the SQUID.
The spacecraft is rolled to modulate the signal at 1.6 mHz.
Paik-5
STEP
To test EP to 1018 at 104 km.
To eliminate gravity gradient coupling to Earth, a nested cylinder geometry is used for test masses.
The differential acceleration is detected magnetically by using thin-film superconducting coils coupled to a SQUID.
Microscope To test EP to 1015 at 104 km by
using capacitive accelerometers.
Paik-6
SMART
AuxiliarySuspension Tube
MainSuspension
TubeSuspensionCurrent Loop 2
SuspensionCurrent Loop 1
DifferentialCurrent Loop
Outer Test MassSensing Coil
Inner Test MassSensing Coil
TantalumNiobiumCoil-form
Same scientific goal as STEP.
Outer test masses are spherical.
Suspension and alignment by a current along a single tube
CMRR 108 Drag-free system may not be needed
SMART uses wire-wound coils.
L xa1
x
Ixa
xa y
xa2L xb1L xb2L
L xa1
x
¯ x̄a2Lx̄b1L x̄b2L
¯
xbI
xaI¯ x̄bI
Paik-7
LISA
To detect GW at 104-101 Hz.
Laser interferometry between three spacecrafts separated by 5 106 km.
Test mass position with respect to the spacecraft is measured by an LC capacitor bridge.
Paik-8
Error Sources
Brownian motion of the test masses
Cryogenic, low loss
Amplifier noise
Soft suspension, SQUID, laser interferometer
Platform vibrations
Differential measurement, drag-free system
Gravity noise
Liquid helium control, no moving parts
Parasitic forces
Electrostatic (trapped charge, patch fields), magnetic
Metrology errors
Precision machining