thermal compensation review david ottaway ligo laboratory mit

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


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


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


5

Effect on Advanced LIGO Interferometers


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)


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


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


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


10

Heater Ring Thermal Compensation


11

Thermal Compensation of Point Absorbers in Sapphire


12

Sub Scale Scanning Laser Test


13

Scanning Laser Test Result

Uncorrected Optic (6712 ppm scatter from TEM00) Corrected Optic (789 ppm scattered from TEM00)


14

Predicted Effected of Thermal Compensation on Advanced LIGO


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)


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

thermal compensation review david ottaway ligo laboratory mit

Documents

sapphire ligo

advanced ligo sensitivity

thermal modeling

wmk slide

year slide

ppm slide

small scale compensation

compensation plates