LIGO Document G1601144-v1

Factors pertaining to the strength of four-fiber monolithic silica test mass suspensions

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The diameter of the silica fibers used in the 40 kg quasi-monolithic aLIGO test mass suspensions was chosen as d = 400Ám to keep the bounce frequency below 10 Hz, and the violin mode frequencies above 500 Hz [1]. For further improvement of detector performance at low-frequency reducing the vertical bounce mode frequency would be beneficial. To make the bounce frequency smaller, the fibres need to be made thinner. Article [2] suggested that the fiber diameter can be reduced to 288 Ám; this thickness is sufficient to give fiber strength three times larger than the static load in aLIGO suspension. The factor 3 is chosen as a reasonable safety margin [2]. In this poster we analyze the overall strength of welded 4-fiber. The additional factors such as strength of welded joints or stock misalignments may limit the suspension strength. None of these weakening factors is crucial: the strength of the suspension is sufficiently large and able to provide long-time operation of the gravitational interferometer. The fiber's thickness is one of the few competitive limiting factors. In such case the fiber's diameter can be made ~ 300 Ám and the full suspension strength should not be significantly affected.

1. Heptonstall et al, Invited Article: CO2 laser production of fused silica fibers for use in interferometric gravitational wave detector mirror suspensions. Rev. Sci. Instrum. 82 (2011) 011301.
2. Heptonstall et al, Enhanced characteristics of fused silica fibers using laser polishing. Class. Quant. Gravity, 31 (2014), 105006.

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