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3. Thermal effects: a brief introduction  In TM, optical power predominantly absorbed by the HR coating and converted into heat  temperature gradient inside the substrate  Two different effects are generated: Non-uniform optical path length distortions (thermal lensing) mainly due to the temperature dependency of the index of refraction  wavefront distortions of the fields in the SRC and PRC cavities Change of the profile of the high reflective face due to thermal expansion (thermo-elastic deformation), in both ITMs and ETMs, affecting the FP cavity. This effect is negligible in current detectors, but becomes relevant in advanced IFOs 17/05/2010A. Rocchi - GWADW 2010 - Kyoto3

4. Virgo+ scheme Mirror B Mirror A Single AXICON used to convert a Gaussian beam into an annular beam. Size of the annulus hole can be set by moving L3 Half wave plate and fixed polarizer are used for DC power control. This system does not deviate the beam impinging on the AXICON To monitor the CO 2 beam quality, an infrared camera has been installed on each bench. 17/05/20104

5. TCS noise: already an issue  TCS can inject displacement noise into the detector (see LIGO-P060043-00-Z)  Coupling mechanisms: Thermo-elastic (TE)- fluctuations in locally deposited heat cause fluctuations in local thermal expansion Thermo-refractive (TR)- fluctuations in locally deposited heat cause fluctuations in local refractive index Flexure (F)- fluctuations in locally deposited heat cause fluctuations in global shape of optic Radiation pressure A. Rocchi - GWADW 2010 - Kyoto17/05/2010 TETRF Present detectors already require intensity stabilization of the CO 2 laser IR detector noise limited 5

6. A. Rocchi - GWADW 2010 - Kyoto 2.4W total TCS power 4W6W 6.5W7W No TCS, 12W IFO With 14.5W of IFO input power, Virgo+ TCS has been tested looking at the phase camera images to see the effects of compensation on the shape and position of the sidebands. The optical gain of the ITF increases by about 50%. Virgo+ TCS performances Coating absorptions play an important role: with new ITMs, you get the same result with only 3W of TCS 17/05/20106 8.5W total TCS power 14.5W IFO 17W IFO

7. TCS in Advanced detectors/1  One more effect to take care of: displacement of the HR face of all TMs Change of the ROC, decrease of the spot size on TMs, increase of thermal noise of about 15% (see LIGO-T060083-01-D) One more actuator: ring heaters to control ROCs of all TMs  Present level of intensity stabilization (10 -7 /√Hz) not enough to heat with CO 2 directly the TM (10 -9 /√Hz needed)  compensation plates required 17/05/2010A. Rocchi - GWADW 2010 - Kyoto7

8. TCS in Advanced detectors/2 17/05/2010A. Rocchi - GWADW 2010 - Kyoto  The heating profile must be much more precise than in present detectors Simple system like an axicon is not enough (see VIR-0182A-10) Too high HOMs content in RF sidebands for MSRC  Necessity to move to active optical elements (MEMs or scanning systems) to generate CP heating pattern axicon OHP Optimized Heating Pattern 8

9.  In a cryogenic IFO with silicon TMs, thermal lensing is likely to be negligible (to be verified with optical simulations) Thermal expansion coefficient tends to zero Thermal conductivity increases, higher than 10W/(cm K) between 10 and 100K dn/dT is small at low temperatures 17/05/2010A. Rocchi - GWADW 2010 - Kyoto What about 3 rd generation detectors? From S. Steinlechner et al, “Absorptions measurements on silicon”, 2 nd ET general meeting, Erice 9

10.  S. Hild et al presented a possible 2-tone configuration for ET at the Erice Meeting High frequency detector: ○ High optical power ○ Room temperature Low frequency detector: ○ Low optical power ○ Cryogenic ○ Silicon test masses 17/05/2010A. Rocchi - GWADW 2010 - Kyoto But if ET is a 2-tone Xylophone? (CQG 27, 2010) Does ET-HF need TCS? 10

11.  ET-HF uses Helical LG 33 modes Fused silica test masses Considering 3MW in the FP cavity and coating absorptions of 0.5ppm, absorbed power is 1.5W (3 times higher than in AdVirgo) 17/05/2010A. Rocchi - GWADW 2010 - Kyoto Thermal effects in ET-HF 11

12.  Compensation plates (same diameter as TMs) properly heated by CO 2 laser  Ring heaters to correct mirrors’ radii of curvature 17/05/2010A. Rocchi - GWADW 2010 - Kyoto AdVirgo-like TCS… 12

13.  If ET-HF and ET-LF are co-located: there would be a lot of cryogenics  Directional radiative cooling working principle (LIGO-G080414- 00-R) The cold source is imaged onto the centre of the test mass The central area of the test mass is in radiative contact with the cold source Heat radiated “towards” the cold source is not returned to the test mass The energy balance is negative, the test mass is cooled 17/05/2010A. Rocchi - GWADW 2010 - Kyoto … or radiative cooling S. Hild et al Experiment carried at Caltech with a parabolic mirror (Nucl.Instrum.Meth.A60 7:530-537,2009.) 13

14. 17/05/2010A. Rocchi - GWADW 2010 - Kyoto Cold Target Parabolic Collector ParabolicReflector System under test (see VIR-0302A-10)  From radiative cooling to Parabolic Radiative Cooling  The use of parabolic collectors allows to decrease the dimensions of the cold targets How PRC would look like in an IFO 14

15. 17/05/2010A. Rocchi - GWADW 2010 - Kyoto Experimental set-up  Scaled down system designed, simulated (optically and thermally) and assembled  First tests performed using LiN2 to cool the cold spots 15 1m

16. 17/05/2010A. Rocchi - GWADW 2010 - Kyoto First data  Some technical issues during first tests  Experimental data show some discrepancies with simulations  Work in progress to identify and mitigate stray effects 16 A 10% change in the width of the cooling profile causes a worsening of the thermal lensing compensation quality of a factor of 10 Optical path length. Strength of the residual lens: 3.4·10 -4 dioptres 10% larger 3.5·10 -5 dioptres ideal case -4.8·10 -4 dioptres 10% smaller 75 300 286 296 08hrs

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