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LIGO Hanford Observatory News
That's All Folks! Last LIGO-I Mirror Installed at Hanford
As the month of June drew to a close, the last
mirror of the initial LIGO detector (a.k.a. LIGO-I)
was installed at the Hanford Observatory. This was
the mirror known as WA4K-ETMy, the end mirror (or
"test mass") for the 4-km interferometer. The photo
at left shows the mirror illuminated by a flashlight
as it hangs in its chamber. (The camera angle shows
the view from inside the vacuum chamber looking
towards equipment on the outside.) This installation
follows on the successful earthquake repairs of the
Washington 2-km (WA2K) interferometer, damaged in
the February 28, 2001 Nisqually Earthquake, as well
as remedial work on suspensions, sensors and actuators
in the LA4K interferometer in Livingston. The
installation of the WA4K optics in the corner station
had been done concurrently with earthquake repairs on
the WA2K system and the WA2K-ETMx had been installed
previously.
Although this was the last planned installation activity
for LIGO-I, it would be a mistake to surmise that this
equipment is "more of the same old stuff." Since we have
run commissioning of components in parallel with completion
of further installations, the components have been steadily
improved as we learned from studying the performance of
previously installed hardware. The WA4K uses the latest
design of shadow sensors, perfected after it was discovered
that the earlier design suffered from susceptibility to
scattered light. The WA4K interferometer is also the first
LIGO interferometer to use a newly-developed, digital
suspension controller. The new controllers feature digital
tuning throughout the modules, replacing component-level
analog filters. This allows software tuning of the controller's
features rather than a "cut and solder" approach to reshaping
the electrical characteristics of the controller.
We did not quite have the installation finished by the end of June. There were still some tasks associated with alignment of the mirror, and an extensive list of checkout procedures remained before we could put the doors on the end chamber and begin pumpdown. The photo at right shows scientist Rick Savage using a glove-encapsulated radio to guide team members in threading an alignment beam through the vacuum chambers. These remaining tasks were completed and the pumpdown was initiated shortly after July 4. Typically, we have a wait of several weeks between initiation of pumping the chambers in a building and opening the large gate valves that connect the buildings to the beam tube. This hiatus allows time for water vapor, introduced into the chamber with the new equipment, to dissipate enough so that the cold traps guarding the beam tubes can reduce moisture leakage into the tubes to the required level. Nonetheless, there were many commissioning tasks running on WA4K equipment in the corner station while we were completing work in the end station.