1. Thermal Compensation System TCS V. Fafone for the TCS Subsystem

2. AdV 1st Project Review, Cascina - 3.11.2008 2 Thermal Effects in Advanced Virgo Thermal compensation for Advanced Virgo, must take into account the much greater circulating laser power (~800kW) Optical power is predominantly absorbed by the HR coating and converted into heat, producing a gradient of temperature inside the substrate. Two different effects originate from the heating of the test mass: – nonuniform optical path length distortions (thermo-optic effect, or thermal lensing) mainly due to the temperature dependency of the index of refraction. – change of the profile of the high reflective surface, due to thermal expansion (thermo-elastic deformation).

3. AdV 1st Project Review, Cascina - 3.11.2008 3 3 Thermal Effects in Advanced Virgo In AdV, thermal effects will create distortions both in the recycling and in the Fabry- Perot cavities: – Wavefront distortion in the PRC in presence of thermal lensing, which changes the cavity mode, the input laser no longer matches the PRC cavity, the coupling coefficient between the laser TEM 00 and the cavity TEM 00 is less than one. – HR surface elastic deformation The cavity becomes less concentric, and the spot sizes at the mirrors will shrink (increase of thermal noise and reduction of arm cavity coupling to the input beam) Necessity to control the radii of curvature of all test masses, TCS will have to act on both ITM and ETM. From S.Chelkowski presentation at 1.7.08 beweekly meeting

4. AdV 1st Project Review, Cascina - 3.11.2008 4 4 Simulation of thermal effects in Advanced Virgo Test Masses Effect of the absorptions on ITM: 0.5ppm (coating) and 2ppm/cm (substrate). FP cavity power ~ 800 kW Absorbed power ~ 0.5 W Thermo-elastic deformation of the HR surface  Change in the ROC Optical path length increase in the substrate. The focal length of the equivalent lens is 4.5 km

5. AdV 1st Project Review, Cascina - 3.11.2008 5 TM heated on both faces, with the same profile, but different powers: 2.5 W on the HR face, 7.5 W on the AR face. Simulation of thermal effects in Advanced Virgo Test Masses Effect on the ROC Evaluated possibility to correct both thermal effects with one actuator: It is not possible if we shine only one face It is possible with CO 2 heating on both faces Front heating increases ROC (~3.1 m·W) Back heating decreases ROC (~2 m·W)

6. AdV 1st Project Review, Cascina - 3.11.2008 6 Advanced Virgo 1st Review - 23.10.20086LIGO Laboratory6 TCS Noise Coupling Mechanisms Thermoelastic (TE)- fluctuations in locally deposited heat cause fluctuations in local thermal expansion Thermorefractive (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

7. AdV 1st Project Review, Cascina - 3.11.2008 7 7 Ad Virgo TCS noise In case the CO 2 is applied on the HR face, to have TCS noise 10 times smaller than AdVirgo sensitivity @ 50 Hz RIN must be 5·10 -9 /  Hz Flat RIN over the entire frequency band No relevant change with TM 30 cm thick In case of heating on both sides of the TM, RIN requirements are released, due to cancellation mechanisms, to 10 -8 /  Hz.

8. AdV 1st Project Review, Cascina - 3.11.2008 8 Advanced LIGO and Advanced Virgo share the same thermal issues. A possible TC System has already been proposed in the LIGO Collaboration Green dots: shielded heating ring Blue rectangles: CP Compensation plates will correct thermal effects in the PRC Shielded heating rings will compensate HR surface deformations TCS scheme In the case of Advanced Virgo, the positioning of CPs in the PRC is a delicate issue due to its interaction with the suspension systems

9. AdV 1st Project Review, Cascina - 3.11.2008 9 Compensation plate Transmissive optic that lives entirely in the recycling cavity; TCS noise coupling is much lower: RIN requirement is 10 -6 /  Hz; CPs interact with the IFO beam: it must satisfy requirements like those of a core optics with respect to displacement noise, absorption and scattering, index homogeneity, antireflection coatings, and the like – In order to minimize etalon effect in the CP, a wedge or a tilt is foreseen. – If the CP is wedged, there will be sensitivity to transverse motion. A 10 arcmin wedge is considered. safety factor of 10 wrt the AdV sensitivity

10. AdV 1st Project Review, Cascina - 3.11.2008 10 Compensation plate – The CP is foreseen to be attached to the ITM reference mass, noise motion requirement is expected to be easily met. – The most feasible solution is to fix the CP on the back of the reference mass. – The clear aperture due to the coil-magnet actuators on the back of the RM is 280 mm, a maximum diameter of 280 mm for the CP is foreseen, while the thickness will be of about 7-10 cm. – Design is in progress

11. AdV 1st Project Review, Cascina - 3.11.2008 11 CP effect on Longitudinal Sensing and Control: simulation Simulation of CP displacement noise coupling to longitudinal error signals and gravitational channel A mirror with 100ppm reflection added behind IMY, inside PRC No wedge on CP, worst case Full 125 W input power Transfer function from CP motion to all longitudinal signal simulated with Optickle Compensation plate

12. AdV 1st Project Review, Cascina - 3.11.2008 12 LSC: estimate noise projection CP motion estimate projected linearly to h Motion of CP reconstructed in a very conservative way: – Assume in AdVirgo we actuate on input RM – Assume the AdVirgo longitudinal correction is the same as in Virgo – Assume RM and Mirror to have the same mass Therefore CP motion is assumed to be the same we would have in Virgo AdVirgo correction should be much lower Inserting wedges (or tilting) will largely reduce this coupling Another reason to have a wedged or tilted CP is that if a plate with faces perpendicular to the beam is moving longitudinally in a cavity, it induces power fringes; with 100 ppm AR coating, the relative power variations would be 4%, on a time scale of 1 s. These variations may fool the DC locking of DARM.

13. AdV 1st Project Review, Cascina - 3.11.2008 13 Simulation done with Finesse A mirror with 100ppm reflection added behind IMX Longitudinal and Angular Lock active Simulations: CP etalon effect on the Alignment optical gains Effects of the CP misalignment with respect the IMX mirror (1 deg wedge on CP - worst case) Introduction of thermal effects CP effect on Automatic Alignment: simulation Compensation plate

14. AdV 1st Project Review, Cascina - 3.11.2008 14 Etalon effects on the Alignment optical gains The CP etalon tuning produces an optical gain variation of the order of 1‰ on the AA error signals CP misalignment effects on the AA control loop The misalignment of the CP front face produces a big effect on the alignment error signals (1.5 nrad of correspective EMX misalignment for a CP misalignment of 10 urad) Thermal effects included in the CP: lens with ROC of about 2700m (IFO at full power). No relevant changes from the CP plane configuration Angular and longitudinal locks active

15. AdV 1st Project Review, Cascina - 3.11.2008 15 Conclusions Longitudinal system – Noise introduced in h by CP should be negligible – No significant effect on longitudinal control system Angular system – The CP etalon effect results to be negligible – The CP front face misalignment produces strong effects on the AA error signals up to 0.1 mrad The CP has to be misaligned more than 0.1 mrad with respect to the IMX front face

16. AdV 1st Project Review, Cascina - 3.11.2008 16 RH TM RM Shielded ring heater should be embedded in the reference mass. It is necessary to avoid any heat transfer between the ring heater and the RM. The heating element should have the highest emissivity, while its shield should have the lowest (gold coating). The heating element must be designed to avoid emitting any magnetic field that could couple with Advanced Virgo main beam or with local controls. Geometry, shielding, materials must be optimized Shielded ring heater design guidelines

17. AdV 1st Project Review, Cascina - 3.11.2008 17 Shielded ring heater design guidelines RH TM RH HR Surface Developing FEA to optimize the position/power of the heating ring. Result for a TM heated by a simple ring - no shielding included yet, lower power is expected to give the same ROC correction

18. AdV 1st Project Review, Cascina - 3.11.2008 18 The full TC System to be implemented on Advanced Virgo will comprehend sensors which sample the thermal wavefront distortions within the interferometer and the ROC, The TCS sensing scheme is to be defined Need to investigate a dedicated TCS wavefront sensor. At present the Hartmann sensor seems to be a promising solution. The University of Adelaide’s Hartmann sensor has been demonstrated to have a shot-to-shot reproducibility of λ/580 at 635 nm, which can be improved to λ/16000 with averaging, and with an overall accuracy of λ/6800. Hartmann sensor compliant with AdVirgo spec should be developed within one year. TCS sensing scheme

19. AdV 1st Project Review, Cascina - 3.11.2008 19 Status of the R&D The definition of the TCS of AdV can benefit of the many years of R&D already performed by LIGO Last spring, an R&D program has been proposed to EGO Stac, it has been evaluated as necessary for AdV. The budget for the first year will be allocated within 2008. The activity consists of development of FEA and measurements of thermal effects induced in the optics and their compensation: – FEA is used to determine the amount of thermal effects both on the TM and in the CP and the compensator parameters (such as dimensions of the CP, profile and power of the CO 2 beam, dimensions and power of the heating ring). The FEA modeling also serves as consistency check for the measurements results. This modeling effort is in a well advanced state and has already supplied useful indications to define the TCS parameters. – The efficiency of a scaled down prototype compensator (CP heated by CO 2 laser and shielded HR on TM) in correcting at the same time both wavefront distortions and thermo- elastic deformations will be measured. The possibility to monitor both thermal effects will give information for the development of a closed loop control scheme. In the frame of this R&D we also plan to define a dedicated wavefront sensor to be implemented on AdV.

20. AdV 1st Project Review, Cascina - 3.11.2008 20 Directional Radiative Cooling (DRC) Principle Establish thermal radiation heat exchange between a cold surface (masking the environment) and the mirror – Heat radiated from the spot is absorbed by the cold target – The cold target returns less heat to the laser spot The cold target could be a surface cooled with LN 2 or a cryocooler Simulations have been performed with target at 77K: – Highly efficient  99.6%, emits only 0.4% thermal radiation than a body at room temperature

21. AdV 1st Project Review, Cascina - 3.11.2008 21 DRC Issues Matched 10% larger 10% smaller Radiative cooling is effective in correcting thermal lensing and ROC at the same time High sensitivity to profile mismatches Scattered light: is the presence of large reflectors placed in front of the TM at small angle (~20 deg) compatible with AdV specifications? Since most of the apparatus lives in-vacuum, how to make it more flexible, to be easily upgraded as new understanding of the IFO is realized? What if the absorptions are non uniform? Methods to tune the cooling power must be investigated Need to investigate noiseless cooling systems Evaluate interactions with other subsystems Small R&D foreseen to evaluate the feasibility

22. AdV 1st Project Review, Cascina - 3.11.2008 22