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Diffused Light from External Benches

Internal Meeting on V+MS noise budget, 2 Sept 2009, EGO

I.Fiori and E.Tournefier

Noise model2

)))((4sin()( 0 txxGth optnoise

scfKG

δxopt(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 fsc 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.

fsc = Fraction of light power carried by back-scattered beam

NEB3

FT

LK end

end

24

• 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

• 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 becausepredicted K agrees with measured(VIR-007A-08)

WEB4

• 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

FT

LK end

end

24

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 WEthan at NE, because of more proximity to seaWE (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

EDB7Case of low microseism

• Suspect (tappings, check on measured K )major contribution is from B1s and B5(B1p has small power)

28,

5ARBS

B

RFL

K

2

211,

21

8

PFCCFL

Kdf

• 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)

EIB8Case 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)