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1 Test Mass Suspensions for AIGO Ben Lee The University of Western Australia

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee2 Introduction Thermal noise in interferometers. Reducing the thermal noise: what we know so far. Suspensions for AIGO. Removable modular suspensions. Reducing violin mode Q factors.

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee3 Thermal Noise 10 -22 10 -20 10 -24 10 -26 10 0 10 1 10 2 10 3 10 4 Thermal Noise 1 st Generation Sensitivities Advanced Sensitivity The thermal noise must be reduced below the quantum noise limit. Frequency (Hz) H(f) / (Hz) 1/2

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee4 Suspension Thermal Noise H(f) / (Hz) 1/2 Frequency (Hz) Low thermal nose achieved with very high Q modes. Suspension modes may also affect control systems.

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee5 Low Loss Materials From the dissipation dilution theorm: Thus, materials with low Φ are better.Fused Silica: Φ~3×10 -8 [1] Silicon: Φ~2.8×10 -8 [2] Sapphire: Φ~3.7×10 -9 [3] There is much more to consider….Thermoelastic loss (300K) 1.A.M. Gretarsson, G.M. Harry. Rev. Sci. Instrum. 70 (1999) 4081 2.J. Ferreirinho in: D.G.Blair (Ed), The Detection of Gravitational Waves, Cambridge Universtiy Press, Cambridge, 1991. 3.S. Rowan, et. al. Phys. Lett. A. 5 (2000) 265 References:

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee6 Thermoelastic Loss Thermoelastic loss presents a significant frequency dependance to the loss value, Φ.,for fibres,for ribbons where and 110 100 1000 10000 Frequency (Hz) 10 -4 10 -5 10 -6 10 -7 Loss angle, Φ Sapphire Niobium Fused Silica Silicon (All 200µm thick) Thinner ribbon

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee7 Fibers vs Ribbons. Dissipation dilution factor: The ratio of restoring force supplied by bending elasticity to the restoring force supplied by tension. This phenomena has a significant effect on pendulum mode and violin mode Q factors. FTFT FbFb FTFT FbFb The effective loss factor This value can be lower for ribbons compared to fibres with similar strength.

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee8 AIGO suspension Sensitivity: 2nm/Hz 1/2 (100Hz bandwidth) Force: 20µN (100Hz bandwidth) Control mass 30kg Test mass 4.2kg Removable Modular suspensions. Peg in Hole interface. Ribbon Suspension

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee9 Peg – Hole Interface Test mass Ribbon Peg High pressure contact points Result: Stress at contact points approaches yield strength of Nb FEM model of pegs

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee10 Will localised losses at the hole surface significantly affect the thermal noise? Holes in the test mass Question… Model and results courtesy of S. Gras (ACIGA) FEM model of test mass. Homogeneous structural loss Φ s =10 -8 Define the extra lossy elements. Apply Φ s =10 -2 to 10 -8 Thermal noise observed by a gaussian beam Structural losses only Include non-homogeneous losses Φ h =10 -2

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee11 Holes in the test mass Model and results courtesy of S. Gras (ACIGA) Contour values, Ψ: S h+s (spectral density of inhomogeneous model) S s (spectral density of homogeneous model) Ψ=Ψ= 10% increase line For AIGO interferometer with r 0 = 20mm, - Φ h cannot exceed 4×10 -3

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee12 Reducing Violin Mode Qs It has been reported by Goßler et. al. [1] the need to reduce the Q factor of the fundamental and first harmonic violin mode. The purpose is to prevent interference with interferometer length control servo. This is achieved by adding lossy coatings. Reference: 1. Class. Quantum Grav 21 (2004) S923-S933 Fused silica fibre Teflon coatings

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee13 Reducing Violin Modes The Orthogonal Ribbon can reduce violin modes and Q factors. High Contact Peg End Flexure Orthogonal Section End flexures provide similar pendulum mode Q factors. Orthogonal ribbon section exhibits fewer, lower Q violin modes in the critical direction. Pendulum mode freq. and dilution factors Normal Ribbon Orthogonal Ribbon x y f pend (Hz)dilu x y 0.91 1.12 2.8×10 -3 0.27 0.91 0.92 3.1×10 -3 3.0×10 -3 Little difference between normal and orthogonal ribbon.

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee14 Reducing Violin Modes The violin modes for the orthogonal ribbon can be calculated by solving the beam equation: Direction of laser field x direction modesy direction modes Test Mass

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee15 Reducing Violin Modes Similar low frequency thermal noise Lower number, and lower Q factor x direction violin modes. Small increase in thermal noise, due to lower violin mode Q.

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Australia-Italy Workshop 3 rd -6 th Oct 2005 Ben Lee16 Conclusion Removable modular suspension can be achieved with only a slight increase in test mass thermal noise. Lowering all the violin mode Q factors can be achieved with an orthogonal ribbon. The orthogonal ribbon has little effect on pendulum mode thermal noise. AIGO facility can be used to test the practicality of the suspensions presented.

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