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Internal Meeting on V+MS noise budget, 2 Sept 2009, EGO. Diffused Light from External Benches. I.Fiori and E.Tournefier. Noise model. General formula, for phase noise into ITF from a back-scattered field:. Phase noise carried by back-scattered field.
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Internal Meeting on V+MS noise budget, 2 Sept 2009, EGO Diffused Light from External Benches I.Fiori and E.Tournefier
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.
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.
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.
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%
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
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)
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)
