Seismic Isolation R&D Activities

Overview

The seismic isolation team focused on the pre-isolator development for initial LIGO described above. This advance implementation of the pre-isolator is, in addition to an important near-term aid for the Livingston interferometer, also a significant step forward for the Adv LIGO seismic isolation system. A photograph of the hydraulic-actuator variant is shown in Figure 1.

Figure 1 Hydraulic pre-isolator (HEPI) vertical isolator. On the left, the vertical actuator is shown; differential pressure in the bellows exerts force on the septum in the middle, which is carried to the load via the pyramidal flex joint at the top. On the right, the actuator is shown as installed at the MIT LASTI testbed.

A second-generation active isolation system prototype was designed by the LSC team and fabricated by the LIGO Laboratory. It is currently being commissioned at the Stanford Engineering Test facility. A photograph of the prototype is shown in Figure 2. This technology demonstrator will be used to (a) inform the development of the full-scale LASTI seismic systems for the HAM and BSC chambers, which will be developed this coming fiscal year and (b) serve as a controls test bed for the active isolation systems. Initial testing of the demonstrator show that a key measure of intrinsic mechanical alignment, the coupling from a requested horizontal actuation to an accidental tilt of the platform, is very low, which will ease the low-frequency controls design. Other measurements indicate that the first internal mechanical resonance, which will limit the maximum control loop bandwidth, is roughly 200 Hz, compatible with the design goal of 50 Hz for the loop bandwidth.

Figure 2 Photograph of the prototype at the Engineering Test Facility (ETF), at Stanford, of the in-vacuum seismic isolation system. The trapezoidal springs which support the outer and inner stages can be seen; the cavity at the lower left is one of six (3 outer, 3 inner) cavities that receive a plug-in unit containing sensors and actuators.

Testing and control law development will continue on this system during 2003. A request for bid for the next generation prototype is in preparation and will be issued in early 2003, enabling the delivery to LASTI in late 2003.