The E7 Run: Taking the Interferometers Out
for a Test Drive
by Kent Blackburn, Mark Coles, Szabolcs Marka, Fred Raab, and John Zweizig
In January, for the first time, LIGO operated all three
laser interferometers at the Hanford and Livingston sites
simultaneously during a 16-day engineering run. This was
our seventh engineering run to date (dubbed with hip
brevity "E7") and was the most extensive test
conducted of the interferometers thus far. The run
provided an opportunity to test interferometer performance,
assess the reliability of the hardware and software during
sustained operation, and to test the data handling and data
analysis hardware, software, and methods. During the run,
the ALLEGRO resonant bar detector at
Louisiana State University
laser interferometer near Hannover, Germany, were also in operation.
LIGO engineering runs serve the same purpose for interferometers that test-drives serve in developing a new car. They confirm those aspects of the new equipment that work well, and those needing a bit of fine-tuning to optimize performance. Unlike previous runs, which focused mainly on characterizing each of the interferometers and improving their reliability, the E7 generated large amounts of data from sustained operation that will now be used to develop and exercise the LIGO data analysis pipeline. Of course, work is still underway to tune up the LIGO interferometers to their design sensitivities, and much remains to do. Still, the data recorded during E7 is vital in that it will provide LIGO Scientific Collaboration (LSC) scientists with real interferometer data needed for perfecting and tuning gravitational wave search algorithms.
The four kilometer long interferometer at the LIGO Livingston Observatory (LLO) was operated in a recombined, but not recycled, configuration. While we have demonstrated the operation of the LLO interferometer in a power recycled configuration, we are able to operate for longer time periods in the non-recycled mode. Seismic noise is significantly higher at Livingston than at Hanford, and use of the non-recycled mode reduces seismic sensitivity. In addition, a newly installed active compensation system was used to filter the large microseism present at LLO.
At Hanford, the two kilometer long interferometer was operated in a power-recycled configuration, while the site's second instrument, the four kilometer long interferometer, was operated in a recombined, but not recycled, configuration. A new system to compensate for the tidal distortion of the earth's crust by the sun and moon was used to allow for extended continuous locking of the instruments. Large levels of vibration were present at Hanford each weekday from approximately 7:00 am to 3:30 pm, due to a building construction approximately 700 feet from the corner station. Our machines were unavailable during these periods as expected. There were also some high-wind conditions and when wind speeds exceeded 25-30 miles per hour we were able to observe effects in our data. The four-kilometer interferometer used the new digital suspension controllers that will eventually be retrofitted into the other interferometers. Microseism compensation for the Hanford interferometers was not yet available.
The Fig. 3 below shows the calibrated strain sensitivity of the three LIGO interferometers at the time of the E7 run. Commissioning continues on all the interferometers and further improvement in noise performance is already observed.
Meanwhile, commissioning of all three interferometers continues around the clock. The interferometer sensitivities described in Figure 3 have already been significantly improved upon by the implementation of additional features of the interferometer control system, such as the common mode servo system and further tuning of the optical alignment and electronic gains in the control electronics. As our first real test-drive of all three interferometers together, the success of the E7 was not measured by any race to a finish line but by the number of laps driven, and the rich data and experience furnished during the course. Fueled by this knowledge, we now return to our labs and "garages" to tune up for the next big drive, a LIGO milestone--the first scientific data taking run, scheduled later this year.