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First Advanced LIGO Suspension Prototype arrives at MIT from GEO!

First Advanced LIGO Suspension Prototype arrives at MIT from GEO!

- Contributed by David Shoemaker

The Advanced LIGO concept calls for a test mass suspension system based on a design developed by the University of Glasgow as part of the UK-German GEO collaboration. The particular features are the use of fused silica suspension fibers, multiple pendulums in series, and the damping of all of the suspension modes via damping at the topmost mass. This design has a considerable history in test interferometers, and a similar design is installed in the GEO-600 interferometer near Hannover, Germany.

The development of the suspension for Advanced LIGO also involves the LIGO Lab Advanced Suspensions group at Caltech, as well as testing in the LASTI interferometer at MIT. As a first round of prototyping, an all-metal test system was designed at Glasgow (where Norna Robertson and Calum Torrie played the main design roles, with the rest of the Glasgow group and Caltech visitor Phil Willems helping). Once the parts kit was complete, it was shipped to MIT for the first assembly and characterization. Norna and Calum came along for a month visit to sometimes lovely Cambridge, Massachusetts. Many people from the MIT group chipped in to help assemble the system, as shown in Photos 1 and 2 above. Myron MacInnis was central to getting the pieces to fit together, and making missing parts. Other visitors and helpers during this phase included Janeen Romie and Virginio Sannibale from Caltech.

Working on the Advanced LIGO suspension (1). Working on the Advanced LIGO suspension (2).

Above: Working on the Advanced LIGO suspension.

The design is a quadruple suspension, as seen in Figure 1 at left below. Four masses are suspended in series, the upper two being more or less rectangular pieces of metal, and in the final design the penultimate mass is of a heavy glass, and the final mass of sapphire. There are two "chains" of pendulums for some of the suspensions, allowing forces to be exerted on the test and other masses from a similarly quiet reference platform. Vertical compliance is supplied by trapezoidal leaf springs as developed for the Virgo isolation system. The whole system sits in an octagonal frame, as shown in Figure 2 at right below.

The suspension's quadruple design. The system's octagonal frame.

Above: At left, an illustration of the suspension's quadruple design; at right, the system's octagonal frame.

This all-metal system is intended to check mechanical clearances, allow the modal frequencies to be measured, and the damping of modes observed and tuned up. Then the group at MIT (Rich Mittleman and Peter Fritschel are particularly active on this phase), along with visitors from Caltech and Glasgow, will look at detailed questions of actuator hierarchies. To hold an interferometer in lock, we will want to adjust the mirror axial position without introducing excess noise. The solution is to push high up on the suspension chain for large, slow excursions, where the noise of the actuator will be filtered by the multiple pendulum transfer function; and to push only with minuscule forces (but with a wide bandwidth) on the final test mass. This final actuation will ultimately be performed with nothing more than light pressure. The present phase of research will tell us how to optimize those forces and bandwidths.

Calum Torrie.

Finally, no description of this baby would be complete without a photo of its principal progenitor, Calum Torrie, shown above. And we have a very happy ending: Calum will join the LIGO Lab at Caltech in the fall to continue suspension work; and Norna Robertson will be visiting the west coast and spending some time at Caltech with the same goal.