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LIGO Livingston Observatory NewsBeam Tube Bakeout a Great Success
Installation Activities Forge Ahead
Student Swarms Spotted at LLO!
A number of significant commissioning milestones have been achieved here at the LIGO Livingston Observatory (LLO) since our last newsletter report. One major accomplishment was the completion of the beam tube bakeout. Begun in early 1999, the aim of this activity was to condition the 8-km of evacuated beam tube in the interferometer by heating the tube to remove residual water and other volatiles from the inner surface of the vacuum envelope. To achieve this, the beam tube had to be heated under vacuum to a temperature of around 170° C for about three weeks. The planning for this task (under the local direction of Allen Sibley, and with the invaluable technical assistance of Kerry Stiff) began in 1998, while a similar effort was already underway at the LIGO Hanford Observatory (LHO). LLO staff visited LHO for extended periods to become fully acquainted with the equipment and methods used. Following the successful completion of the LHO bakeout, the equipment and software were shipped to LLO. In parallel, the entire 8-km of beam tube was instrumented with thermocouples and then thermally insulated (as revealed in the trio of photos below).
The beam tube was heated in 2-km isolated sections by closing the mid-point gate valves along each interferometer arm. Then, an electrical current of about 2000 amps was injected into the beam tube to resistively heat it to the desired chamber. Each 2-km section was heated for about three to four weeks while it was continuously pumped by extended range turbo-pumps. Following cool-down, the vacuum was observed to improve in each module by a factor of about 100, to about 1 x 10-9 torr, with hydrogen due to outgassing of the 304 L stainless steel of the beam tube now the dominant source of residual gas. The water vapor pressure decreased by about six orders of magnitude. Each 2-km module required about a month to set up the instrumentation, another month of actual bakeout, and then another month of demobilization following the bakeout. The successful completion of this task represents the fruition of about two years of planning and more than one year of intense on-site activity. Table 1 below lists the summary results achieved for each module. Congratulations to all associated with this important program!
An important focus of activity here at the Livingston Observatory has been the installation and commissioning of the interferometer itself. The pre-stabilized laser (PSL), which has been operating since mid-1999, was studied in detail as the various servo-systems designed to control its performance were characterized. The frequency and intensity stabilization servos have been installed and well characterized. The pre-mode cleaner was observed to have the same barometric sensitivity as the one at LHO. It has now been replaced by a new sealed unit.
The mode cleaner optics and associated electronics have been installed and its operation has been integrated with the PSL. "Diagonalization" of the suspension servos (meaning the process whereby the mirror motions are decomposed into motions that are pure "pitch," "roll," "yaw," and displacements) remains to be completed. The glue joint of one mirror magnet was found to be defective and the position adjusting magnets (used on the small optics only at LLO) were observed to directly couple seismic motion into the mirrors, making the system unexpectedly noisy. This summer all of the small optics suspensions were removed, inspected, and rehung. The position adjustment magnets, which were added to the small optics to increase the adjustment range of the electromagnetic actuators, were removed. (They remain in place on the large optics at both LLO and LHO where the much larger mass of the large optics reduces the seismically induced displacement noise.) The diagonalization work will be revisited this fall.
Another important activity associated with the mode cleaner commissioning was a measurement of the frequency noise of the laser system using the mode cleaner cavity as a frequency reference. It was discovered that acoustic noise was readily coupling through the mirror mounts on the PSL optical table into the light itself. Joe Kovalik made systematic measurements of the acoustic sensitivity of each of the mirror mounts. The mounts were then replaced with much more robust custom mounts. These mounts (shown in the photo at right) do not use a slender post to support the magnet but instead integrate the mirror into a squat aluminum brick which has very high resonant frequencies relative to the gravity wave band. During the next pumpdown cycle, expected this fall, the acoustic sensitivity of the new robust system will be investigated.
This summer, installation of all of the large optics was completed (as seen in the first photo below). Alignment of the optics is now in progress and should conclude this month. Also, installation of all of the core optics support components (baffles, beam pick-offs, telescopes, etc.) is nearly complete (mid and right photos) and will also conclude this month.
We are working hard to finish all these installation activities so we can soon close up the vertex vacuum space, evacuate it, and return to commissioning studies. In addition to revisiting the laser frequency noise and diagonalizing the mode cleaner actuators, the broader plan for this fall is to operate the recycling mirror, beam splitter, and input test masses as a recycled Michelson interferometer as the next integration step towards having a fully working interferometer. We are also closely monitoring the residual water vapor levels in the end test mass vacuum chambers. Once the water vapor level has fallen sufficiently so as not to contaminate the beam tube vacuum, we will open the gate valves to begin 4-km long single arm studies similar to those conducted recently at LHO. The fall promises to be an exciting and challenging time at LLO.
The Livingston Observatory was buzzing with a veritable hive of LLO summer SURF students!
Student participants in Caltech's SURF (Summer Undergraduate Research Fellowship) program--in addition to students from LSU, the University of Texas at Brownesville, and several high school students from the Baton Rouge and New Orleans areas--all worked at LLO this summer on various LIGO related activities. Work topics included laser characterization, checkout of electronics, and studies of the seismic environment, among others. We conducted a "LIGO academy" with approximately twice weekly lectures on various aspects of the physics of LIGO as well as an introduction to General Relativity. Special thanks to Joe Kovalik, Anthony Rizzi, and Peter Saulson for their efforts. These lectures were supplemented with a few "enrichment" presentations on various special topics. Thanks to Professors Barry Barish (Caltech), Carl Brans (Loyola University), and Steve McGuire (Southern University) for their help.
We held special seminar days on July 28, August 11, and August 18 so that students could present the results of their studies to the LIGO staff. The participation of so many fine and enthusiastic students in this year's LLO summer SURF program made for a very enjoyable and productive season at LLO. Next summer, we hope to further expand this program and urge our readers to highlight these opportunities to those students who would like to participate.