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

Noise model 2  δx opt (t) = displacement of scattering surface (optics and bench in this case) along ITF optical axis Phase noise carried by back-scattered field General formula, for phase noise into ITF from a back-scattered field: In note VIR-007A-8, Edwige derives K as function of ITF parameters and expected  f sc on 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.  K = “Coupling factor” which depends on ITF port.  f sc = Fraction of light power carried by back-scattered beam

NEB 3 Present: G=1.8E-21 (measured) (G includes  f sc 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,  f sc same  Gnew = G/sqrt(3)/2 = 0.5E-21 Mitigations: 1) Reduce 18Hz bump >> Mech-damper, and HVAC flux reduction 2) 45Hz bump is the turbo pump fan >> fan seismic isolation. Case of low microseism Factor 10 safe margin Confident in this formula because predicted K agrees with measured (VIR-007A-08)

WEB 4 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: 1)turbo pump fan seismic isolation. Case of low microseism

Microseism Statistics over 1 year 5 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 Low microseism (case of prev. plots): RMS<0.5micron = 60% High microseism (like July 7, or worst): RMS > 3 microns = 3%

Microseism High microseism July 7, VSR2 start, (RMS 0.2-1Hz WE Lvdt = 3microns) 6 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  f sc ? Lentone maybe. Measure its scattering

EDB 7 Case of low microseism 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)

EIB 8 Case of low microseism 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. 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 Conservative Projection assuming NO RESCALING of G (measured G<1 E-21)