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Matters of Gravity
LIGO Update for Fall 1994

by Stan Whitcomb


     A major step for LIGO took place this spring when construction of the first of the two 4-km LIGO facilities began at Hanford, Washington. The official ground-breaking ceremony was held on July 6, against a backdrop of earthmoving equipment already at work leveling the site and preparing the soil for the foundation. This inital earthwork is expected to be completed later this fall.

     At the other LIGO site in Livingston, Louisiana, the purchase of the land by Louisiana State University (LSU) and a lease transferring the site to NSF are in the final signature stage. With the cooperation of the original landowner and LSU, we were able to arrange for the clearing of the site, begin the on-site geotechnical investigations, and apply for the necessary wetlands permit from the Army Corps of Engineers, in advance of the final land transfer. Progress on these items should help speed the start of construction at the Louisiana site.

     The engineering design of the beam tubes which span the full 4km arms of the two facilities has been completed. A qualification test of the design using full diameter tube segments and the field assembly procedures is being carried out by our design contractor, Chicago Bridge and Iron, Inc. The successful completion of this test, expected near the end of this year, will clear the way to begin fabrication of the required 16 km of beam tube. The other major engineering design contracts are for the building and site design and for the remainder of the vacuum system. The Request for Proposals (RFP) for the building and site design has been released to qualified Architect-Engineering firms, and the RFP for the remainder vacuum system is in preparation.

     As exciting as the beginning of construction is, some of the best news has come from the R&D work. The reconstructed 40 m interferometer at Caltech, called "Mark II" to distinguish it from its predecessor, has come into full operation now. This new version of the interferometer is housed in a new vacuum system, one which gives us much more room for testing concepts and hardware for the full-scale LIGO interferometers. The most significant change to the interferometer itself, namely, the replacement of the seismic isolation stacks, has resulted in an improvement in the low frequency performance of the interferometer by as much as a factor of 100. The next major change to the 40 m interferometer (currently underway) is the replacement of the old test masses with new ones which have the required supermirror coating deposited on on a polished face of the test mass itself, eliminating the need to have a mechanical attachment of a separate mirror to the test mass. The preliminary indications are that this change will reduce the amount of thermal noise in the interferometer and give yet another improvement in performance.

     A major effort to demonstrate the optical phase sensitivity required for the LIGO interferometers is underway at MIT. A 5 m interferometer is being built with seismically-isolated, suspended mirrors. This interferometer is designed to operate with the same laser power incident on its beamsplitter as the full-scale LIGO interferometer. Thus, it should have the same shot noise (measured as an uncertainty in optical phase) as the LIGO interferometers. In this way, it will test for any other optical noise sources and verify that they can be controlled with adequate precision. Together, the results from the 5 m and 40 m interferometer should provide a firm foundation on which to base the LIGO interferometer design.

     Many of the other R&D activities have less readily explained results (for the nonexpert), though the work is not necessarily less important or less difficult! We have made steady progress in understanding the complexity of the alignment systems required, in establishing the capability of industry to fabricate the high precision LIGO optics, in developing the input optics which control and stabilize the laser beam, and in modeling the various aspects of the interferometer. All of these efforts are essential to a success project.

     The past six months have also seen a reorganization of the project to enhance the project management. Barry Barish has been appointed as Principal Investigator, and Gary Sanders has been recruited from Los Alamos to become Project Manager. The new management plans will be reviewed by NSF in September along with an updated cost estimate. Success at this review (which we expect) will give us the go-ahead for the increasing level of effort required to complete LIGO on schedule.


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Last modified August 26, 1994