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Test Mass Suspensions for AIGO

AIGO. Test Mass Suspensions for AIGO. Ben Lee. The University of Western Australia. 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. 10 -20. 10 -22.

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Test Mass Suspensions for AIGO

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

  2. 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. Ben Lee

  3. 10-20 10-22 10-24 10-26 100 101 102 103 104 Thermal Noise 1st Generation Sensitivities Thermal Noise Advanced Sensitivity H(f) / (Hz)1/2 • The thermal noise must be reduced below the quantum noise limit. Frequency (Hz) Ben Lee

  4. Suspension Thermal Noise Low thermal nose achieved with very high Q modes. H(f) / (Hz)1/2 Suspension modes may also affect control systems. Frequency (Hz) Ben Lee

  5. There is much more to consider….Thermoelastic loss Low Loss Materials From the dissipation dilution theorm: (300K) 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] References: • A.M. Gretarsson, G.M. Harry. Rev. Sci. Instrum. 70 (1999) 4081 • J. Ferreirinho in: D.G.Blair (Ed), The Detection of Gravitational Waves, Cambridge Universtiy Press, Cambridge, 1991. • S. Rowan, et. al. Phys. Lett. A. 5 (2000) 265 Ben Lee

  6. Fused Silica Sapphire Silicon Niobium (All 200µm thick) ,for fibres ,for ribbons Thermoelastic Loss Thermoelastic loss presents a significant frequency dependance to the loss value, Φ. 10-4 where Thinner ribbon 10-5 Loss angle, Φ and 10-6 10-7 100 10000 1 10 1000 Frequency (Hz) Ben Lee

  7. Fb Fb FT FT 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. The effective loss factor This value can be lower for ribbons compared to fibres with similar strength. Ben Lee

  8. AIGO suspension Removable Modular suspensions. Peg in Hole interface. Ribbon Suspension Control mass 30kg Test mass 4.2kg Sensitivity: 2nm/Hz1/2 (100Hz bandwidth) Force: 20µN (100Hz bandwidth) Ben Lee

  9. Result: Stress at contact points approaches yield strength of Nb High pressure contact points Ribbon Peg Test mass Peg – Hole Interface FEM model of pegs Ben Lee

  10. Thermal noise observed by a gaussian beam Include non-homogeneous losses Φh=10-2 Structural losses only Holes in the test mass FEM model of test mass. Homogeneous structural loss Φs=10-8 Question… • Will localised losses at the hole surface significantly affect the thermal noise? Model and results courtesy of S. Gras (ACIGA) Define the extra lossy elements. Apply Φs=10-2 to 10-8 Ben Lee

  11. Sh+s(spectral density of inhomogeneous model) Ψ= Ss(spectral density of homogeneous model) For AIGO interferometer with r0 = 20mm, - Φh cannot exceed 4×10-3 10% increase line Holes in the test mass Contour values, Ψ: Model and results courtesy of S. Gras (ACIGA) Ben Lee

  12. 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. Teflon coatings Fused silica fibre Reference: 1. Class. Quantum Grav 21 (2004) S923-S933 Ben Lee

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

  14. x direction modes y direction modes Direction of laser field Test Mass Reducing Violin Modes • The violin modes for the orthogonal ribbon can be calculated by solving the beam equation: Ben Lee

  15. Expected Suspension Thermal Noise Comparison, Normal Ribbon and Orthogonal Ribbon -8 10 Normal Ribbon Orthogonal Ribbon -10 10 -12 10 -14 10 m/Hz1/2 -16 10 -18 10 -20 10 -22 10 0 1 2 3 10 10 10 10 Frequency (Hz) 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. Ben Lee

  16. 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. Ben Lee

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