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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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

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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.

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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

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xa y

xa2L xb1L xb2L

L xa1

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¯ x̄a2Lx̄b1L x̄b2L

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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