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LSC Suspensions/Isolation Design Summit Held at MIT

LSC Suspensions/Isolation Design Summit Held at MIT

- Contributed by David Shoemaker

With LIGO I installation well underway, plans are afoot to design the next improvements to the LIGO detector. The LIGO Science Collaboration (LSC) working groups addressing Suspensions/Isolation, Lasers/Optics, and Configurations have been hard at work to determine what science goals can be reached with those technologies that will ripen over the next five years or so. The first LIGO I science run is slated from 2002 to 2004. Our goal is to provide a series of upgrades, starting in 2004 and carrying us through much of the first decade of the new millenium.

In this context, a four-day intensive Design Summit was held May 13-16 by the working group which deals with the mechanical aspect of interferometer design. The suspension design is critical to the thermal noise (and other "internally generated" noise source) limits to interferometer performance. It also provides the actuation for many of the control systems, and the seismic isolation must eliminate forces on our mirrors which emit from the environment. Roughly 25 LSC scientists from nine institutions gathered to discuss in gory detail the state of the art. Topics included the projections for possible evolution in our knowledge and of the availability of materials, science goals that could be targeted, and strategies consistent with the constraints of the LIGO infrastructure, possible NSF support, as well as the science goals of LIGO I and beyond.

Lunchtime at the summit. The first day was spent sharing the outlook for NSF support, learning about sources and their uncertainties, and in discussing the philosophy and realities of setting performance requirements. We took away from this the sense that we should plan for a series of upgrades and observing runs which will carry us from 2004 to roughly 2008, allowing an impressive range of improvements. As always, it was difficult to find strong drivers for interferometer design from specific sources, but some generalities about the sensitivity as a function of low-frequency response could be made. With the sensitivity foreseeable given the state of technology, it was exciting to discover that the Newtonian background and the quantum limit are important contributors to possible interferometer designs.

At left: Lunchtime at the summit; Sam Richman (soon to join MIT from the BIPM),Warren Johnson (Louisiana State University), Rai Weiss (MIT), and Peter Saulson (Syracuse) tell tall tales.

The second day addressed thermal noise in detail. The pendulum thermal noise, related to the losses in the suspension fibers, can be "engineered" to the point of disparition by changing the geometry of the fibers from their present round cross section to a rectangular form. Basically, the wire can become much less stiff in the direction along the laser beam but still maintain the strength needed to support the mass. In addition, this change in "aspect ratio" causes the principal source of loss, thermoelastic damping, to become smaller. Roughly a factor of 10 decrease in the thermal noise is thus expected--a huge improvement! The other source of thermal noise in the experiment is that due to the internal modes of the test masses: drumhead and cylinder modes. New crystalline materials are at an early stage of development and will require a long and probably expensive industrial development before their use is practical. Still, it is important to start early on this.

On the third day, we discussed seismic attenuation and the associated control challenges. Both passive and active (servocontrolled) systems were presented, and the elements of the designs extracted for recombination into solutions "tuned" to the needs and timing of reasonable upgrades. Through modeling in preparation for the Summit and consequent discussion, some elements of the design of isolation system are simply necessary (several stages of passive pendulum isolation). Others can be solved in several ways, and we established several baseline designs which will now be quickly evaluated in an objective and time-proven way (to use a bit of clumsy but effective NASA-ese lingo) in a follow-up teleconference meeting in mid-June.

The last morning concentrated on organization of design teams, assignment of action items, and establishment of some principles of design.

While mostly a serious affair, befitting its noble goals, there were some moments of levity and lessons even larger than LIGO. Jon How proved himself to be a very persuasive discussion leader, carrying a rather large stick and not always speaking softly. After a particularly decisive action on his part, a motion arose from the audience that Jon might like a cup of herbal tea. Warren Johnson proposed a passive isolation stage of minimal height, well "tuned" to fit in our LIGO I vacuum system, with the motto "Save the Whales" (his one-syllable nickname for the LIGO Vacuum Equipment). Rai Weiss reminded us that it is easier to change a few locomotives than rebuild all the tracks. We also had a catastrophe. A fused-silica suspension, made by GEO and brought to MIT for characterization, was broken during a lab tour, so we will have to learn about repairing the suspensions a bit earlier than planned. But all in all the group felt it had made a great deal of progress toward both a unified design as well as a unified team, and we had a little fun along the way.

Now comes the hard part: taking the enthusiasm of that collective atmosphere back with us to our various corners of the world and plugging away--on a very fast track--at the tough problems that lay ahead.