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Thermal Compensation Review David Ottaway LIGO Laboratory MIT Overview 1. Overview of Problem 2. Road map for design choices (Set by other systems) 3. Summary of current results from subscale tests and modeling 4. Current known unresolved issues 5. Plans and resources required for next year Advanced LIGO Technical Review 2 Thermal Distortion Absorption in coatings and substrates => Temperature Gradients Temperature Gradients => Optical path distortions 3 Types of distortions, relative strengths of which are shown below: Sapphire Fused Silica Thermo-optic 1 26 Thermal Expansion 0.8 1.6 Elasto-optic Effect 0.2 - 0.3 Advanced LIGO Technical Review 3 Thermal Comparison of Advanced LIGO to LIGO 1 Parameter LIGO I LIGO II Sapphire LIGO II Silica Units Input Power 6 125 80 W PRC Power 0.4 2.1 1.3 kW Arm Cavity Power 26 850 530 kW Substrate Absorption 5 Coating Absorption 0.5 10-40 (30) 0.5-1 (0.5) ppm/cm 0.10.5(0.5) 0.1-0.5 (0.5) Advanced LIGO Technical Review ppm 4 Effect on Advanced LIGO Interferometers Advanced LIGO Technical Review 5 Adaptive Thermal Compensation Compensate for distortions in the substrates Essential for Advanced LIGO sensitivity to be realized Two parts to thermal compensation: 1. Coarse compensation of thermal lensing using heating ring and shielding 2. Small scale compensation using scanning CO2 laser Accurate measurement of sapphire and fused silica thermal mechanical properties enable accurate models Good propagation models to set design requirements (Melody and FFT Code) Advanced LIGO Technical Review 6 Requirements that flow from other systems • • Core Optics (Down select) Sapphire -Significant possible inhomogeneous absorption -> Small spatial scale correction (scanning laser) -Large thermal conductivity -> Small amount of coarse compensation (ring heater) on compensation plates Fused Silica -Poor thermal conductivity and homogenous absorption (ring heater) DC or RF read out scheme (Down select) -Reduces dependence on sidebands, might affect design requirements Wavefront Sensing (LIGO 1 experience, not fully understood) -High spatial quality sidebands are probably necessary for accurate alignment control, may negate the effect of read out scheme Advanced LIGO Technical Review 7 Summary of Subscale Experiments and Modeling Accurate measurements of fused silica and sapphire material properties Experimental demonstration of shielded heater ring coarse spatial correction Experimental demonstration of scanning CO2 laser fine spatial scale correction Accurate models of Advanced LIGO Interferometers style interferometer using Melody and finite element analysis (Femlab), (Thermal modeling without SRM) Scaling from subscale to full scale understood Work done by Ryan Lawrence Advanced LIGO Technical Review 8 Thermophysical Parameters Measurement (295-320 K) Sapphire (C and A axes) Fused Silica (Corning 7940) Parameter Value Error Units dn/dT 7.2 0.5 ppm/K Parameter Value Error Units aa 5.1 0.2 ppm/K dn/dT 8.7 0.3 ppm/K ac 5.6 0.2 ppm/K a 0.55 0.02 ppm/K ka 36.0 0.5 W/m/K kth 1.44 0.02 W/m/K kc 39.0 0.5 W/m/K Advanced LIGO Technical Review 9 Heater Ring Thermal Compensation Advanced LIGO Technical Review 10 Thermal Compensation of Point Absorbers in Sapphire Advanced LIGO Technical Review 11 Sub Scale Scanning Laser Test Advanced LIGO Technical Review 12 Scanning Laser Test Result Uncorrected Optic (6712 ppm scatter from TEM00) Corrected Optic (789 ppm scattered from TEM00) Advanced LIGO Technical Review 13 Predicted Effected of Thermal Compensation on Advanced LIGO Advanced LIGO Technical Review 14 Current Known Unresolved Issues Gravitational wave sideband distortion and its effect on sensitivity. Generated within the cavity no distortion nulling due to prompt reflection. Greater understanding through incorporation in Melody (Ray Beausoleil) Fabry-Perot mode size change due to input test mass surface deformation => Spot size change (actuate on arm cavity faces) Accurate 2D absorption maps of Sapphire to aid in actuator selection (negative or positive dN/dT actuator plates) Advanced LIGO Technical Review 15 Plans and Resources for Next Year Plans: Work with the Gin Gin Facility to determine prototype Further modeling Design requirements (29th Oct 2002) Preliminary design (14th Apr 2003) Resources: Staffing: Mason (1/5 time), Ottaway (1/4 time) Ryan Lawrence graduating and leaving LIGO Resources: $50 K in MIT LIGO budget to build prototype Advanced LIGO Technical Review 16