LIGO Document T020059-v3

Are Reaction Chains Needed for aLIGO HAM Optics?

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T - Technical notes
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Willems (2002) had concluded the noise induced on the second and third stages of the HAM triple suspensions by controlling them with iLIGO-style OSEMs mounted directly to the cage is ~4 orders of magnitude below the required displacement noise, and therefore reaction chains are not needed for the HAM triple suspensions. In this document we update the calculations made originally by Willems to be sure these conclusions still hold. These updates include final design parameters for the HAM triple suspensions, final design requirements for their displacement in the aLIGO recycling cavities, and updated performance results and requirements for the HAM isolation platform. We conclude that this noise will be at worst is a factor of ~5 away from the final length requirements (around HEPI support structure resonances) but typically around a factor of 60 and that no immediate action need be taken.
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Notes and Changes:
- Converted everything to displacement noise
- Corrected an error in interpreting Eq 2 of T020059-v1 as force noise instead of displacement noise
- Added "normal" transmission of platform motion through triple SUS for comparison
- Updated control model to use driven [m/N] TFs from T080311 and T080310, instead of simple "1/f^2" models used in T020059.
- Used "Flat above 30 Hz" displacement noise requirements for the triples (from T080192), instead of continuing at f^(5/2) (unspecified in T010007).

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