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First results from the `Violin-Mode' tests on an advanced LIGO suspension at MIT

Lockerbie, N. A. and Carbone, L. and Shapiro, B. and Tokmakov, K. V. and Bell, A. and Strain, K. A. (2011) First results from the `Violin-Mode' tests on an advanced LIGO suspension at MIT. Classical and Quantum Gravity, 28 (24). ISSN 0264-9381

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This paper describes the first results from `Violin-Mode' measurements made on the four suspension fibres of a fully suspended 40 kg test mass. These measurements were made at the LIGO lab, Gravitational Wave Observatory test facility, at MIT. Here, an aluminium-alloy (dummy) test mass, simulating an advanced LIGO (Laser Interferometer Gravitational Wave Observatory) test mass/mirror, had been suspended in air from a test suspension by four fused-silica suspension fibres, each measuring 400 mu m in diameter x 600 mm long. Violin-Mode measurements were made on these highly tensioned fibres by retrofitting a prototype system of four novel shadow sensors to the test suspension, one per fibre, these sensors having, collectively, a displacement sensitivity of (6.9 +/- 1.3) x 10(-11) m (rms) Hz(-1/2), at 500 Hz, over a measuring span of +/- 0.1 mm. Violin-Mode fundamental resonances were detected in all four fibres: with frequencies similar to 485 Hz when the test mass was supported lightly from below, and at similar to 500 Hz when it was fully suspended. In the latter case the Violin-Mode detection took place whilst the test mass, together with its suspension fibres, was undergoing relatively large-amplitude `pendulum-mode' motion, at similar to 0.6 Hz. This motion was measured to have a peak-peak amplitude at one of the suspension fibres of up to similar to 140 mu m (35 mu m, rms)-the shadow sensors each having subsidiary outputs for monitoring such low-frequency, large amplitude, motion. Under fully suspended conditions, a calibrated Violin-Mode `free-oscillation' amplitude of 430 +/- 20 picometres, rms, was measured at 500.875 Hz, in the same suspension fibre which was found to be undergoing, simultaneously, the similar to 140 mu m peak-peak motion. Over the bandwidth monitored (dc to 3.2 kHz), Violin-Mode harmonics up to the sixth were recorded in an evoked response. It was concluded that the prototype system had demonstrated amply its practical viability as a detector of Violin-Mode resonances in the test-mass suspension fibres of gravitational wave interferometers, such as those of advanced LIGO.