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LIGO Hanford Observatory NewsVacuum Chambers Bolted In and Cooking!
Hanford Hosts Second LIGO Scientific Collaboration Conference
During March, Process Systems International (PSI) finished bolting together the vacuum chambers at the Hanford Observatory. Now that the chambers are installed, the process of plumbing the vacuum lines and electrical connections is underway. These plumbing and electrical connections have been completed in three of the five experimental halls that house the vacuum chambers and these chambers have been pumped down and baked to remove water vapor and trace contaminants.
A view of one of the chambers that hold the beam splitters and the main interferometer mirrors is shown in Figure 1 at left. This chamber, known as a BSC (for Beam Splitter Chamber), stands about 18 feet high with a diameter of 9 feet. Each of the two interferometers at Hanford uses five BSC chambers for a total of ten chambers. The laser beam path will be approximately six feet off the ground, at about the height of the large tubes entering the BSC. Figure 2 at right shows more of the vacuum system from a few steps back. To the left of the BSC are two smaller chambers referred to as HAMs, or Horizontal Access Modules. The big, seven-foot diameter, dish-shaped doors on the sides of the chambers are removable, allowing scientists easy access to install optical assemblies inside. These HAMs hold the optics used to filter the laser light to the exacting specifications required by the main interferometer. They also contain telescopes to expand the beam to the correct size, and the optics to inject the light into and extract it back from the main interferometer. There are six HAMs for each interferometer, making twelve altogether at Hanford.
There are many more parts to the vacuum equipment in the experimental halls, including numerous pumps, manifolds, bellows, gate valves and liquid-nitrogen-filled cryopumps. Once all the equipment was bolted together, pipefitters and electricians were able to make the plumbing and electrical connections that allow the equipment to work together. Then the vacuum chambers in each experimental hall can be pumped down. The pumps do an effective job of getting air out of the chambers in less than a day, but molecules of gas that are lightly stuck to the chamber walls are harder to remove. Like Stick-On Notes, they do eventually come loose but take many years to do so and would degrade the vacuum in the chambers for a long time. Our solution is to bake the chambers for several days at about 150 degrees Centigrade (or a cookie-baking 300 degrees Fahrenheit). The extra heat gives the gas molecules enough energy to break free of the walls and make it to our pumps--like ripping off the Stick-On Note and throwing it in the trash.
Bake outs have now been completed in the two mid-station buildings and in the end-station on the Y-arm. Figure 3 at left shows the Y-arm mid-station experimental hall being baked. The shiny covers on the BSC and other hardware are heating blankets with aluminum-foil covers used during the bake. The blue boxes are power supplies that feed current to the blankets. There are temperature monitors underneath the blankets and a control system in the boxes adjusts the blanket currents to get uniform temperatures everywhere.
Once the chambers cool, the pressure is approximately a trillionth of an atmosphere. A mass spectrometer is then used to identify the nature of any residual gas left behind. This will be analyzed by scientists to ensure the chambers are clean enough to start the installation of interferometer components this summer.
The second meeting of the LIGO Scientific Collaboration (LSC) was held this March at the Hanford, Washington site. Attended by about 100 people, the plenary sessions of the meeting were convened to discuss a draft "Publications and Presentations" policy. This is a very important policy as it describes how the Collaboration will determine who is an author on each scientific publication, a key to earning recognition for the work reported. In addition a forum was held on the criteria for the detection of gravitational waves.
Discussion of the Publications policy lead to several suggestions for improvements in the draft version that had been circulated. The committee responsible will revise the draft and bring it to a vote of the Collaboration Council at the August meeting in Boulder, Colorado. When ready, the revised draft will be posted on the LSC web site, hopefully by May 1998.
Also conducted was a forum on the criteria for detection of gravitational waves, intended as a first discussion of significant issues. One matter addressed was the coordination of all detectors operating in the world with sufficient sensitivity to influence the confidence of detection for all classes of gravitational wave sources: impulsive, periodic, quasi-periodic and the detection of a stochastic background which will require the correlation of detectors over several baselines.
The meeting also provided time for the LSC technical development groups to continue the formulation of priorities and to design an ordered research program. The three groups are:
1) a development group to reduce stochastic forces--isolation systems and suspensions; David Shoemaker (MIT), Chair.
2) a development group to reduce sensing noise--improved optics and higher power lasers; Eric Gustafson (Stanford), Chair.
3) a development group for new interferometer configurations; Ken Strain (University of Glasgow), Chair.
Another function of the meeting was to organize the LSC effort in data analysis. Presentations were made by the LIGO Laboratory scientists and engineers responsible for the LIGO Control and Data System (CDS) and the LIGO Data Analysis System (LDAS). Three LSC data analysis working groups were formed, each with a chair and a LIGO Laboratory liaison. The initial functions of these groups are to set priorities and to establish an LSC program. The groups are organized around the following areas:
1) Astrophysical source identification and signatures; Bruce Allen (University of Wisconsin)/Tom Prince (Caltech-LIGO)
2) Detector Characterization; William Hamilton (LSU)/Daniel Sigg (Hanford-LIGO)
3) Detection confidence and statistical analysis; Sam Finn (Penn. State)/Albert Lazzarini (Caltech-LIGO)
The next meeting of the LIGO Scientific Collaboration will be August 13-15, 1998 in Boulder, Colorado.