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Internal Meeting on V+MS Noise Budget and Sources of Diffused Light (September 2, 2009)

This document summarizes the key discussions and findings from the internal meeting held on September 2, 2009, focusing on V+MS noise budget and the impact of diffused light from external benches on the resulting noise model. General formulas for phase noise related to back-scattered fields are derived, and a detailed analysis of coupling factors and their dependence on various parameters is presented. Strategies for mitigating noise due to microseism, including specific actions, measurements, and projections, are outlined to ensure optimal system performance.

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Internal Meeting on V+MS Noise Budget and Sources of Diffused Light (September 2, 2009)

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  1. Internal Meeting on V+MS noise budget, 2 Sept 2009, EGO Diffused Light from External Benches I.Fiori and E.Tournefier

  2. Noise model • General formula, for phase noise into ITF from a back-scattered field: Phase noise carried by back-scattered field • δxopt(t) = displacement of scattering surface (optics and bench in this case) along ITF optical axis • fsc= Fraction of light power carried by back-scattered beam • K = “Coupling factor” which depends on ITF port. • In note VIR-007A-8, Edwige derives K as function of ITF parameters • and expected fscon benches from measured BRDF, • then compares to measured G. •  End Benches and EDB diffused light path seems understood, EIB is not yet. • Tentative Projections for V+MS: • >>> using parametrized K we can predict how G rescales for V+MS • >>> Check Microseism.

  3. Case of low microseism NEB Confident in this formula because predicted K agrees with measured (VIR-007A-08) • Present: • G=1.8E-21 (measured) • (G includes  fsc reduction due to new • tower window and mir AR coating, • which is about a factor 2) • T=10ppm, F=50 • V+MS: • T = 5ppm , F = 150,  fsc same •  Gnew = G/sqrt(3)/2 = 0.5E-21 Factor 10 safe margin • Mitigations: • 1) Reduce 18Hz bump • >> Mech-damper, and • HVAC flux reduction • 2) 45Hz bump is the turbo pump fan • >> fan seismic isolation.

  4. Case of low microseism WEB • Present: • G=20E-21 (measured) • T=40ppm, F=50 • V+MS: • assume Gnew is same as NEB • (assume same improvement • associated to tower window • and mir AR coating) • Mitigations: • turbo pump fan seismic • isolation.

  5. Microseism • x-axis= RMS 0.2-1Hz of WE Lvdt, in m • On y-axis is the percent of time • this RMS is above a given x value • Statistics over 1 year • Low microseism (case of prev. plots): RMS<0.5micron • = 60% • High microseism (like July 7, • or worst): RMS > 3 microns • = 3%

  6. Microseism • High microseism July 7, VSR2 start, (RMS 0.2-1Hz WE Lvdt = 3microns) • NEB ok, WEB is at design. • WEB is worst because microseism • is (always) twice stronger at WE • than at NE, • because of more proximity to sea • WE (15km), NE and CB (18km) • Do we accept this? • Possible actions: • Go to T=2ppm (gain factor 2) • Feasible? • Further reduce fsc ? • Lentone maybe. Measure its scattering

  7. Case of low microseism EDB • Suspect (tappings, check on measured K ) • major contribution is from B1s and B5 • (B1p has small power) • B5 coupling scales with 1/F • B1 coupling does not scale • Mitigations: • 1) send a small fraction (1%?) of B1s on EDB • ( reduction of B1s diffused light by a factor 100) • and dump the rest inside the tower. • For that we need a high power beam dump. • 2) move the Faraday isolator before the OMC • in order to filter B1s diffused light • (gain at least a factor 100) • 3) Reduce EDB motion. Mech damper? (...not efficient). Isolation with sorbothane, damped springs? Conservative Projection assuming NO RESCALING of G (measured G=1 E-20, in March)

  8. Case of low microseism EIB • Diffused light path not understood, • Back reflection from ITF do not explain measured G (old INJ) • With new injection G reduced by > 10 times, • suspect better isolation of FI • or reduced scattering on bench • Projected UPPER LIMIT: • Major contrubution is upconv. • of 18Hz • (bench mode, and HVAC line) • Mitigations: • > 18Hz with Mech damper • > anyhow seismic isolation of EIB seems required to mitigate beam jitter, • solutions under study. Conservative Projection assuming NO RESCALING of G (measured G<1 E-21)

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