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Hydraulic External Pre-Isolator Installation Underway

Hydraulic External Pre-Isolator Installation Underway

- Contributed by Mike Zucker

Our Livingston, Louisiana, observatory site has some fantastic assets, including the rich local culture, wonderful cuisine, and abundant wildlife. Alas, other items you might add to such a list aren't quite so appreciated, at least from our scientific standpoint. The nearby blue Gulf waters and our vast tracts of pine forest have actually turned out to be as challenging in their way as they are picturesque.

Winter storm waves crashing on the coast, and the "whumps" from felling pines for the paper mill, wreak seismic havoc with the sensitive LIGO interferometer. Frequently, these disturbances cam overload our electronics and prevent us from reaching our most sensitive operating mode. This takes a painful bite out of our observing time, raising the chances we could miss a rare astrophysical signal.

But LIGO is fighting back. Last month we began installing a unique system designed to actively smooth out the rolling waves and "whumps" in real time, shielding the delicate interferometer components from nature's seismic racket. Over two years in development, the Hydraulic External Pre-Isolator (HEPI ) was conceived by our LIGO collaborators at Stanford University and implemented full-scale for testing on the LIGO Advanced System Test Interferometer at MIT (see our February 2003 Newsletter for more). Successful trials showed that the system could remove the offending disturbances, effectively simulating the quiet of our calmest summer night 24 hours a day, 365 days a year. A combined team of Livingston, LSU, Stanford, MIT and Caltech scientists and engineers, aided by major subcontractors in five states, sprung into action to build enough production copies to equip the seven sensitive optics platforms in the Livingston interferometer.

Tested HEPI hydraulic actuators. Gary Traylor assembles the vertical piston.

Each optics platform, previously supported directly from the ground, will now be placed on a movable stage powered by eight novel hydraulic pistons. Like normal hydraulic rams, these pistons can move massive loads (each LIGO "payload" weighs close to four tons). Unlike conventional pistons, however, they are based on steel bellows with no sliding joints or seals, they are exquisitely sensitive, nearly noiseless, and capable of sub-nanometer precision.

Bi-helical springs.
Ken Mailand monitors pressure testing.

Seismic sensors monitor the ground and the interferometer's "payload." Digital signal processing electronics synthesize these readouts into a continuous set of commands for each piston. Every movement of the earth is met with an equal and opposite thrust, recalculated 2,048 times each second, to keep the delicate payload stationary.

Katrina Carter calibrates the valves.
Mike Fyffe prepares interface electronics.

We're well into the installation as of this writing and expect to have the systems in operation on all seven active LLO interferometer chambers this summer. At that point, we'll all be looking forward to finally having some peace and quiet!

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