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On February 9th 2000, two large crates, each containing a Seismic Attenuation System (SAS) tower designed to the requirements of the TAMA interferometers, left the Caltech Synchrotron Laboratory en route for Japan. The shipment arrived there safely five days later, ready for installation. Soon after, Akiteru Takamori and Riccardo DeSalvo arrived also to install the two towers in the vacuum chambers of the three-meter Hongo campus interferometer.
This was a delivery whose process began in Autumn 1999, when TAMA scientists realized their seismic attenuation might be improved in the future with stacks of the SAS type. This would facilitate reaching better sensitivity, and the very low frequency isolation promised that the interferometer lock could be maintained for longer stretches of time, even during noisy periods of the day.
In January 2000, Seiji Kawamura of TAMA suggested that the LIGO SAS Group collaborate with TAMA in this research. While a quick study, using commercial active systems, was already planned, the SAS appeared to be a more robust solution to TAMA's longer-range R&D goals. SAS group expertise was seen as an advantageous way to move rapidly forward.
Below: SAS Group members enclose the tower for transportation (1), (2). Next, (3) Riccardo DeSalvo and Akiteru Takamori discuss the procedure.
The SAS group, led by Riccardo DeSalvo, quickly agreed to adapt an existing design for advanced LIGO to the TAMA requirements. From the LIGO point of view, the project was viewed as a mature test-bed for the LIGO SAS technique. The project was to become Akiteru Takamori's PhD thesis.
Pre-prototype tests and design began immediately. Production was to start in Summer 2000, and completion of two seismic attenuation chains were scheduled by the fall. In July and August 2000, while starting the adaptation of the seismic towers intended for the mirror suspension payload, Akiteru realized that an unacceptable mismatch was present between the existing TAMA suspensions and the TAMA-SAS already in production. He quickly produced a fresh proposal to redesign new suspensions and have them built together with the SAS, with only a small cost in schedule delay. Akiteru's plan was approved by the TAMA collaboration, and by September 2000 the new mechanical suspension redesign was completed and ready for production.
At right: A schematic overview of the TAMA SAS towers.
The SAS towers are based on the LIGO-developed Monolithic Geometric Anti-Spring Filters (MGASF), each capable of 60 dB attenuation. The TAMA-SAS towers also incorporated the Virgo inertial damping of the attenuation chain resonant modes, as well as the advanced accelerometers designed as part of the PhD thesis of Alessandro Bertolini and provided, in part, by the Univesita' di Pisa. The new SAS/SUS system is expected to deliver a few tens of nanometers of r.m.s. mirror motion below the observation frequency range. In addition, it provides passively attenuated residual seismic noise crossing the thermal noise level at or below 10 Hz. The control electronics are adapted from the Virgo DSP system.
The new suspensions, fully compatible with the SAS isolation towers, were based on a double pendulum with recoil mass mirror controls first developed by Virgo, along with the multiple pendulum passive mode damping developed and tested by TAMA scientists, in addition to the Caltech developed MGAS springs for low frequency vertical resonances.
Shown below: (1) The TAMA SAS assembly lab. (2) A top view of one of the towers. (3) An inner view.
The suspension production contract was awarded in October to Italy's Galli and Morelli, who were already finishing the SAS towers. In December 2000, Riccardo, Alessandro and Akiteru took delivery in Lucca, Italy from Galli and Morelli of the completed SAS/SUS mechanics. The SAS part was fully assembled and tuned at Caltech in January 2001, and in the first week of February the equipment was packaged in the two large, protective crates for shipment to Japan.
Equal to the technical challenges were the complexity of the international purchasing, integration of the subsystem, as well as the transport and proper entry into Japan. It was thanks to the ingenuity of Irena Petrac of Caltech, and Seiji Kawamura (a TAMA scientist who transformed himself into an administrator for the occasion) that all these challenges were met.
In the days between January 19th and 23rd, Riccardo and Akiteru, helped by Professor Kimio Tsubono, Kazuhiro Yamamoto, and others, unpacked the crates, installed, aligned and then tuned the two towers in the freshly arrived three-meter interferometer vacuum tanks. By March 3rd one of the Suspensions was already assembled and suspended from a SAS tower. In an already famous, perfectly summarizing comment, Akiteru stepped back, surveyed the arrangement and said conclusively, "Well, it looks just like the drawings!"
Above: (1) Beginning installation of a tower at TAMA. (2) Both towers now visible. (3) Akiteru smiles in satisfaction during assembly of the double pendulum. (4) Double pendulum assembly, but where's Riccardo?
In the next five months, Virginio Sannibale, Giovanni Losurdo, Szabi Marka and Alessandro Bertolini will assist in finishing the two systems with all their wiring, mirrors, instrumentation and tuning, so as to complete a Fabry Perot interferometer test. This test in the three-meter interferometer is expected to produce the final validation of the TAMA-SAS/SUS system. TAMA will then be able to consider this system for use in the pioneering TAMA 300 interferometer itself.
This effort was a true international scientific collaboration between almost all of the major gravitational-wave interferometer groups, Virgo, LIGO and TAMA. It is a precursor of even wider collaborations between all gravitational-wave experiments.
To view a complete viewgraph presentation of the the TAMA SAS story in .pdf format, click here. To read the TAMA SAS technical document in .pdf, containing detailed scientific results, click here.