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LIGO Livingston Observatory NewsBack in the Vacuum
IRET Program at LLO
This summer, after undertaking venting, installation, and pumpdown work since May, we finally opened the gate valves along the X and Y arms for brief periods to assess the overall alignment of the interferometer. Problems had been discovered earlier in the year with the positioning actuators that the interferometer uses to control the position and orientation of each of the suspended mirrors. The chief problem with the actuators was that they were sensitive to scattered laser light from the main beam of the interferometer. Each actuator contained an on-board light source and photodiode which were used to sense fiducial points on each of the mirrors as a way of providing a local static orientation reference. When we turned the laser up to a power higher than a few hundred milliwatts, scattered laser light from the optics fell onto the photodiodes within the actuators, and this created noise which caused the suspended mirrors to move uncontrollably. To achieve full design sensitivity with the LIGO interferometer, we need to operate with a laser power of at least six watts. So it was clear that we needed a re-design and installation of an alternative design actuator. (See the Hanford columns in the May and June newsletters for more details regarding the engineering fixes implemented for these positioners.)
Above: A view of one of the end test masses. The white cylindrical caps visible at four places around the mirror's circumference are the end faces of the actuators which were replaced this summer. A fifth actuator, visible on the left, acts on the side of the mirror to control its position transverse to the laser beam. The cylindrical steel tube at lower right is the output telescope. A small amount of light is transmitted through the mirror so it can propagate through the telescope and onto an external photodiode. This allows us to monitor the amount of light power resonating in the arms of each Fabry-Perot cavity.
Although we had "buttoned up" the interferometer in preparation for pumpdown around the end of July, we could not immediately open the gate valves that isolate the 4-km long arms from the suspended test masses. Whenever the interferometer is vented, there is an unavoidable uptake of water onto the surface of the steel and into the bodies of the viton spring seats which are part of the seismic isolation system. The beam tubes, however, were separately baked out at around 180 degrees C under vacuum in 1999 and 2000 and are extremely clean. Their residual pressure is dominated by hydrogen and the water content is nearly four orders of magnitude less than the hydrogen pressure. Contamination of these long, clean, beam tube arms would be a catastrophe. Therefore, we try to be extremely conservative with respect to activities that connect this clean environment to the rest of the apparatus. Consequently, as the chambers containing the suspended test masses are evacuated, we carefully monitor the residual gas within the vacuum space to be connected to the arms. The gate valves are only opened when the rate of water influx into the beam tube arms from the test mass chambers at the end and corner stations is acceptably small. This takes about one month of continuous pumping after initial "button up."
Below: A view through the end mirror into the transmitted light telescope (the large lens seen behind and below center). This is a view down the telescope tube shown in the lower right corner of the photo above. The front faces of the actuators are visible through the suspended mirror. Note the parallel white strips within the central aperture of each actuator. These are the light sources and photodiodes which sense the end position of small cylindrical magnets glued around the circumference of each mirror and at one place along the cylindrical side surface of the mirror. The strips closest to the mirror edge are the light sources, while those closest to the sensor are the photodiodes. The circular objects between the white strips are positioning magnets. The three inch long hex bolts located around the circumference are earthquake stops. Soft Teflon tips on the hex bolt ends limit the mirror motion should excessive ground motion occur.
So, after a lot of waiting to find out if the test masses at the vertex and the end stations had been properly oriented with respect to each other, we finally got our first opportunity near the end of August to open the gate valves and take a peek. We were very pleased to find that the light beams were "right on the money." It required only a few minutes to orient the laser spots so that they were well centered on each of the end test masses, and then to tweak the end test mass alignments slightly so that their reflections were well centered on the input test masses. By month's end we had succeeded in once again locking the interferometer arms individually as Fabry-Perot cavities at input laser powers of more than one watt!
This summer, for the first time, the LIGO Livingston Observatory (LLO) was host to two high school teachers participating in a pilot program aimed at providing research experiences for teachers. The program, dubbed "IRET," is modeled on a National Science Foundation (NSF) sponsored program that seeks to strengthen the relationship between NSF funded research and teachers involved in K-12 education. Our enthusiastic trailblazers during this summer were Mr. John Thacker, a physics teacher for the last two years at Springfield High School in Livingston Parish (and next year at Covington High School in St. Tammany Parish); and Mr. Wilson Doucette, a long time teacher at Istrouma High School in East Baton Rouge Parish.
John Thacker, shown in the photo at left below, holds a Master's Degree in Physics from the Naval Postgraduate School in Monterey, and is a 20+ year veteran of the Coast Guard. He recently obtained an alternative certification as a high school physics teacher and has been a frequent and enthusiastic visitor to LIGO, accompanying high school classes on visits to the Livingston Observatory. Wilson Doucette (seen at right) was recommended to LIGO with the highest praise by both the Louisiana Science Teachers Association and the Physics Department at Southern University, where he has been an active collaborator on outreach activities for many years.
Above: John Thacker (left) during his "graduation lecture" as part of the IRET program. At right, Wilson Doucette hard at work creating lesson plans based on his summer experiences at LLO.
The goals of this program were to provide an opportunity for the teacher participants to be part of a research environment and to use this experience to create teaching resources that can enrich their classrooms as well as be shared with other teachers. So that they would become well-informed about LIGO and contribute to the overall scientific mission of the site, Wilson worked closely with Doug Lormand on the characterization of the PEM sensors and John worked with Micke Fyffe and Rus Wooley on the installation and checkout of the data acquisition and control electronics.
We asked each of the teachers to create lesson plans related to LIGO that they could use in their classrooms next year. We also asked them to format these plans as web based resources so that they could be placed on the internet and shared with other teachers. See http://www.ligo-la.caltech.edu/teach.htm for details about the lesson plans created. Each plan is self-contained and uses materials easily available to a classroom teacher. Also, each one references Louisiana State Science Benchmarks (these are based on the National Academy of Sciences benchmarks and are probably generally applicable in other states as well), and includes references for supplementary information or other web resources. Both John and Wilson have provided their e-mail addresses in case interested teachers would like to contact them for additional information.
Our pilot program this summer was a great start! Not only did John and Wilson help with the commissioning effort at LIGO and create lesson plans, they also helped to build some hands-on gadgets for our fledgling outreach center in the multipurpose room. (The next time you visit LLO, check out our brachistochrone and our tautochrone pendulum). We hope to enlarge the program in future years thanks to the great start by John and Wilson, and we hope to see them with their classes back at LLO during the coming school year.
Above: At left, John Thacker and Wilson Doucette with the brachistochrone. Balls placed on each track simultaneously show that the shortest path length in space is not the shortest path length in time. Next, at right, John with a tautochrone. A tautochrone is a pendulum with elliptic corrections to the pendulum length that allow the period to be independent of angle, even for large angular displacements.