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LIGO Livingston Observatory NewsLivingston Status Update
Construction Continues on the LLO Staging Building
Toward the close of summer, we resumed operation of the interferometer under vacuum after a long vent period when new actuators were installed as well as over a month of pump down time to remove residual water vapor absorbed in the end vacuum chambers. The water vapor is now nearly at acceptable levels, and we have begun brief periods of operation of the interferometer. We are picking up where we left off in May, tuning and optimizing the various control loops of the interferometer so that the entire apparatus can operate robustly in a power recycled configuration with the Fabry-Perot arms in resonance.
A significant technical obstacle which must be overcome is the unique seismic environment in Livingston--impulsive man-made noise in the 1 to 10 Hz band with velocity amplitudes of 1 to 10 microns/second. Impulses this large are sufficient to cause the interferometer to "lose lock." When this happens, the spacing between the mirrors forming the boundaries of an optically resonant cavity is no longer close to an integer number of half wavelengths of the laser light and consequently the light no longer resonates in the cavity. Most of the seismic noise comes from the south, towards the town of Livingston and in the direction of the major highway and freeway in the area. The plot below, obtained with a two dimensional array of seismometers located along the Y-arm of the interferometer, shows the direction that the impulsive seismic signals propagate (i.e. the direction opposite to the source, where northwards is at the top of the plot at 90 degrees and the radial direction indicates the velocity amplitude of the seismic motion). The data show the distribution of events where the ground motion exceeded one micron per second during a three hour interval on a typical weekday. It is apparent that the noise doesn't come from any single source, but rather arises almost entirely from sources that are located to the south of the observatory.
Above: Plot showing the distribution of ground motion events.
We are pursuing several technical approaches to surmount this obstacle. Foremost is a new controller for the actuators that has been designed to supply significantly more current to the actuators for brief periods. This allows the system to apply much more force (by more than a factor of five) to counteract impulsive seismic disturbances. This apparatus, designed by Ken Watts and Rai Weiss, is now undergoing thorough testing, earlier on the bench and then in the interferometer, and initial results are promising. We have been able to lock a single Fabry-Perot cavity even during the day mid-week, and this is a significant improvement in the robustness of the overall system. (A safety circuit, which is an integral part of this new controller design, automatically disables the controller whenever the rms current exceeds about 150 mA. This protects the actuators from heating up and outgassing into the interferometer vacuum while still allowing instantaneous currents which are a few times higher than this.)
On a second front, we are planning to implement a "feed forward" compensator for the seismic ground motion. This effort, conducted primarily by Joe Giaime and Ed Daw, will sense the ground motion along the direction of the Fabry-Perot arms using seismometers mounted on the floor near the suspended test masses. The seismometer outputs will be used to control external positioners ("fine actuators," essentially long stroke piezoelectric stacks) that will compensate for the ground motion. Initial testing will be done over the frequency band of the micro-seismic peak (about 0.14 Hz), where simulations of the procedure indicate that such a simple one-degree-of-freedom compensation technique appears likely to be quite successful. The micro-seismic motion is also about one order of magnitude larger in Livingston than it is at Hanford. We will also experiment with extending this technique into the 1 to 10 Hz band to further reduce ground noise. See the Livingston LIGO Electronic Log entry on this topic for additional explanation of the technical approach being pursued. (Enter "reader" for user and "readonly" for password at the security window.)
Finally, on a third front, we are examining the possibility of pursuing an early implementation of some of the active seismic isolation techniques being developed for advanced LIGO. For example, the hydraulic actuator prototypes now being readied for testing in the LASTI interferometer at MIT could possibly be replicated for integration into the external supports for the seismic stacks at LLO. We will closely watch the initial testing of these MIT prototypes to evaluate their effectiveness in isolating the presently installed test masses from ground motion typical of the Livingston site.
Slowly but surely, progress is being made in the construction and refurbishing of the Livingston Staging building. Enjoy our little mini slide-show here to find out about the latest developments.
Figure 1 (above). The roof over the new addition is nearing completion, keeping the wind and rain out of the interior. This will allow the work of installing the drywalls to begin.
Figure 2. The auditorium is also taking shape and looks like it's really going to turn out great. Notice the beginnings of the curved ceiling, designed to provide better acoustic performance.
Figure 3. The stairs leading to the upper floors have been erected which make inspection of ongoing work a whole lot easier. This view is from the upper floor entrance to the new addition of the building.
Figure 4. The walls of the old storage building are ready to be cut to allow for the installation of the windows. This will not only bring some light to the second floor area, but also provide a pleasant view overlooking the erosion control ponds.
Figure 5. To house the LDAS equipment, a portion of the second floor has been partitioned off. This set aside area will be air-conditioned separately to provide the required environment for the computer equipment. Windows will allow a view of the LDAS equipment from the adjacent office and lab area.