Silica Research in Glasgow

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Silica Research in Glasgow

• Gianpietro Cagnoli IGR - University of Glasgow
• GEO Collaboration
• Ginzton Lab, Stanford
• Caltech

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Summary

• Silica suspensions in GEO600
• Direct Measurement of Thermal Noise
• Coating Losses
• Non-Linear Thermoelastic Effect

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Silica suspensions in GEO600GEO600 Collaboration

• Two monolithic silica suspensions have been
  installed in GEO600 (middle of June)
• GEO600 is the first interferometer to use such
  suspension to reduce thermal noise
• Some numbers
• fibres length 285 mm
• vertical freq. 16 Hz
• mirror mass 5.6 kg

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Silica suspensions in GEO600

• Features
• 4 fibres welded on ears
• 2 silicate bonded ears each mass
• Selective damping of violin modes with teflon
  coating on fibres
• Masses are hold in a rigid frame during the
  welding and transportation inside the tank
• Once the stage is suspended the holding frame
  becomes a catcher

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Silica suspensions in GEO600
Coating
Cutting
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Silica suspensions in GEO600
Welding (test)
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Direct measurement of thermal noise GEO600
Collaboration

• During the last year the 10m interferometer in
  Glasgow have been upgraded
• A sensitivity limit lower than 10 18 m/Hz ½
  has been achieved above 500 Hz (blue curve at
  right)
• Shot noise 210 19 m/Hz ½
• Damping of the internal modes of one mass was
  increased by spreading 3 strips of grease on the
  barrel and noise was again measured (red curve)

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Direct measurement of thermal noise

• Q of several modes were measured
• Q 1800 for greased mass
• Q 1.210 5 for clean masses
• Thermal noise estimation
• - semi-infinite mass
• - homogeneous structural damping (Levin Yu.,
  Phys.Rev. D, 57 (2), 659-663)
• Results
• Fluctuation-Dissipation Theorem has been
  verified experimentally

3 masses Q 120000 1 mass Q 1800
4 masses Q 120000
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Coating losses Glasgow and Stanford Collaboration
Identify and measure the coating loss from Q
measurements on silica cylinders

• Sample
• - Corning 7980
• - 127 mm diameter, 100 mm length
• - IR coating on front faces and barrel

• Measurements
• - ring down method, 7 modes measured
• - 15 different sample suspensions tested

• Finite elements analysis
• - used for energy ratios (surface to bulk)
• - convergence with N. of points checked
• - agreement to a few with measured
• frequencies

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Coating losses

• Two parameters model
• Cff is the front face to bulk energy ratio

• Results comments
• - all modes but 1st asymmetric are lined up
• - low loss measurement is not accidental
• - 1st symmetric mode loss too high?
• - but then the linear fitting is not good

then another source of loss is significant
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Coating losses

• Results comments
• - apart for the 1st symmetric mode (high loss)
  the other points are aligned

• other sources of loss are currently under
• investigation to justify the high value
• measured for the 1st symmetric mode

Coating losses could be a serious limit to the
next generation interferometers
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Non-linear thermoelastic effect Glasgow and
Caltech Collaboration
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Non-linear thermoelastic effect
In non-linear thermoelastic effect thermal
fluctuations are transformed in strain
fluctuations through the thermoelastic
coefficient b 1/EdE/dT
This second order effect becomes relevant due to
the large longitudinal static stress so in the
suspension fibre
P
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Non-linear thermoelastic effect
An article will be submitted soon to Phys. Rev. D
by Cagnoli G. and Willems P.