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Thermal Compensation Review
David Ottaway
LIGO Laboratory
MIT
Advanced LIGO Technical Review
2
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
3
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
4
Thermal Comparison of Advanced LIGO to LIGO 1
Parameter LIGO ILIGO II
SapphireLIGO IISilica
Units
Input Power 6 125 80 W
PRC
Power0.4 2.1 1.3 kW
Arm Cavity Power
26 850 530 kW
Substrate Absorption
5 10-40 (30) 0.5-1 (0.5) ppm/cm
Coating
Absorption0.5
0.1-0.5(0.5)
0.1-0.5 (0.5)
ppm
Advanced LIGO Technical Review
5
Effect on Advanced LIGO Interferometers
Advanced LIGO Technical Review
6
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 shielding2. 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
7
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
8
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
9
Thermophysical Parameters Measurement (295-320 K)
Sapphire (C and A axes)
Parameter Value Error Units
dn/dT 7.2 0.5 ppm/K
a 5.1 0.2 ppm/K
c 5.6 0.2 ppm/K
ka 36.0 0.5 W/m/K
kc 39.0 0.5 W/m/K
Fused Silica (Corning 7940)
Parameter Value Error Units
dn/dT 8.7 0.3 ppm/K
0.55 0.02 ppm/K
kth 1.44 0.02 W/m/K
Advanced LIGO Technical Review
10
Heater Ring Thermal Compensation
Advanced LIGO Technical Review
11
Thermal Compensation of Point Absorbers in Sapphire
Advanced LIGO Technical Review
12
Sub Scale Scanning Laser Test
Advanced LIGO Technical Review
13
Scanning Laser Test Result
Uncorrected Optic (6712 ppm scatter from TEM00) Corrected Optic (789 ppm scattered from TEM00)
Advanced LIGO Technical Review
14
Predicted Effected of Thermal Compensation on Advanced LIGO
Advanced LIGO Technical Review
15
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
16
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