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Go Back Two Steps for Earthquake Repairs!

Go Back Two Steps for Earthquake Repairs!

- Contributed by Fred Raab

Helena Armandula stands beside one of the LIGO optics. Interferometers do not like to be shaken. The 6.8 Nisqually Earthquake on February 28, 2001, was the largest temblor to strike Western Washington in over 50 years. Even while engineers were assessing the damage done to the Capitol Dome in Olympia, LIGO staff on the opposite side of the Cascade Mountain range in Eastern Washington were investigating to determine what damage may have been caused to the two Hanford Observatory interferometers. The 2-km interferometer was scheduled to participate in a weekend-long engineering run, named E3, starting in 10 days. This was to be the first coincidence run between the two LIGO observatories. The full-up Hanford interferometer (WA2K) was scheduled to take data in coincidence with one of the 4-km arms of the LIGO Livingston detector. People were rushing to implement a feed-forward tidal compensation system for WA2k, hoping to be ready for the E3 run. (The feed-forward system would prevent the Earth Tides from stretching the interferometer out of range for its control system by anticipating the stretching of Earth's crust due to lunar and solar gravity, and gently pushing the optic within the buildings to compensate. This would cancel the large tidal drifts--typically measured in tenths of a millimeters--and extend locked intervals to several days at a time.) Meanwhile, another crew was actively installing mirrors into the chambers for the 4-km long Hanford interferometer (WA4K). And finally, a visiting National Science Foundation review panel, composed of scientists with extensive experience with major science facilities, was on site to review our operations and the proposed funding of operations for the next five years. Then the ground shook, startling some but escaping the notice of others who had been focused on their tasks. But very quickly it dawned on people that the interferometer had been shaken up. We knew this was not good.

Attachments to the mirrors. One of the first questions often asked of serious accident victims is, "Can you wiggle your toes?" Stan Whitcomb, the leader of the commissioning effort at Hanford, went to the control room shortly after the shaker passed and tried wiggling mirrors, one by one. It quickly became obvious that some of the mirrors had sustained damage and repairs would be needed. Immediately we began to mobilize efforts to enter the vacuum chambers and start inspections of the mirrors. Since much of the development of LIGO's suspended mirrors was done at Caltech--in the midst of California's earthquake country--we were not unfamiliar to the damage earthquakes can do to suspended mirrors. The LIGO prototype 40-meter interferometer at Caltech was typically jolted about once a year by shakers during the 90's, and we learned to incorporate earthquake safety into our designs. In the photo at left above, those shiny screws aimed at the periphery of the purplish optic are called "Earthquake Stops" and their job is to catch the mirror as gently as possible when a jolt comes. Earthquake stops are also incorporated into the optical tables and vibration isolation systems to limit their motions. We have enough confidence in these stops to know that the mirror substrates, surfaces and coatings would do just fine, even in a fairly large local earthquake. Still, the "Achilles heel" of the design is the set of attachments glued onto the mirrors. By design, the mirrors are supposed to float as freely in space as possible, so that they are free to follow the currents of space induced by gravitational waves. That means the attachments between the mirror and the rest of the world have been made as tenuous as possible. There are two sets of attachments: a suspension fiber and guide rod assembly; and the magnet/standoff assembly. These are shown in the photo on the right.

The 25-pound mirror is balanced on a single loop of thin (0.01 inch) steel wire to an accuracy of about 1/100th of a degree. This is done with the guide rod that has a small groove to locate the wire. Balancing requires getting the guide rod in the right place and the wire properly seated in the groove to an accuracy thousands of times smaller than the wire diameter. Not surprisingly, a good shaking changes the balance and this needs to be trimmed up by hand. This alignment trimming is not a difficult procedure, but it does require a person to enter the vacuum chamber to tweak it up.

Much more worrisome is the magnet/standoff assembly (visible below the guide rod in the photo at right). The standoff is used to isolate the magnetic material--with its high internal friction--from the pristine fused silica of the mirror. This keeps the thermal vibrations of the atoms in the magnet from driving large (meaning more than 1/1000th of a proton diameter) wave-like distortions of the mirror's surface. Unfortunately, this assembly has a long lever arm and a small footprint by design, features that make the structure fragile if knocked. Topping that off, clearances between the magnet and the surrounding coils and shadow sensors are restricted to about 1-2 millimeters. In our post-quake check up, we found that these attachments had come unglued on about a dozen mirrors. Repairing the damage requires removing the mirror assemblies from vacuum chambers, removing the attachments and recleaning the mirrors, and then gluing new attachments onto the mirrors, following all the requisite vacuum baking and balancing steps.

How do we do better with our earthquake safety procedures in future? A clue is that the damaged attachments were part of the earliest optics installed. Typically the gaps between the installed optics and the stops were set to approximately one millimeter. We will now set the gaps closer to one-half millimeter and tighten the tolerance on that spacing. Hopefully this will improve survivability in subsequent quakes.

We are taking the occasion of opening the chambers to make detector improvements as well as repairs, by installing new shadow sensors throughout WA2K. These will be less sensitive to disruption by scattered light and the replacement will make it possible to inject more laser light into the interferometer to increase our sensitivity.