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. Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

Thermal Noise 10-22 10-20 10-24 10-26 100 101 102 103 104 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) Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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) Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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] There is much more to consider….Thermoelastic loss 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 Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

Thermoelastic Loss Thermoelastic loss presents a significant frequency dependance to the loss value, Φ. Sapphire Niobium Fused Silica Silicon (All 200µm thick) 10-4 where Thinner ribbon 10-5 Loss angle, Φ and ,for fibres ,for ribbons 10-6 10-7 1 10 100 1000 10000 Frequency (Hz) Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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 FT Fb This value can be lower for ribbons compared to fibres with similar strength. FT Fb Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

AIGO suspension Removable Modular Ribbon Suspension suspensions. Peg in Hole interface. Control mass 30kg Test mass 4.2kg Sensitivity: 2nm/Hz1/2 (100Hz bandwidth) Force: 20µN (100Hz bandwidth) Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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

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

Holes in the test mass Contour values, Ψ: 10% increase line Sh+s (spectral density of inhomogeneous model) Ss (spectral density of homogeneous model) Ψ= For AIGO interferometer with r0 = 20mm, - Φh cannot exceed 4×10-3 Model and results courtesy of S. Gras (ACIGA) Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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 Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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 Pendulum mode freq. and dilution factors Normal Ribbon Orthogonal Ribbon x y fpend (Hz) dilu 0.91 1.12 2.8×10-3 0.27 0.92 3.1×10-3 3.0×10-3 Little difference between normal and orthogonal ribbon. Orthogonal Section Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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

Reducing Violin Modes m/Hz1/2 Frequency (Hz) Expected Suspension Thermal Noise Comparison, Normal Ribbon and Orthogonal Ribbon 10 1 2 3 -22 -20 -18 -16 Frequency (Hz) -14 -12 -10 -8 Normal Ribbon Orthogonal Ribbon 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. Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee

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. Australia-Italy Workshop 3rd-6th Oct 2005 Ben Lee