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  Zero-g frees test masses completely from the housing (f 0 < 10  3 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 10  18 at  10 4 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 10  15 at  10 4 km by using capacitive accelerometers.

Paik-6 SMART  Same scientific goal as STEP.  Outer test masses are spherical.  Suspension and alignment by a current along a single tube  CMRR  10 8  Drag-free system may not be needed  SMART uses wire-wound coils.

Paik-7 LISA  To detect GW at 10   1 Hz.  Laser interferometry between three spacecrafts separated by 5  10 6 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