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

2. ERC Meeting, Cascina - 27.04.2009 2 Thermal Compensation System Scope of the TCS subsystem: Design, construction, installation and commissioning of a system that corrects thermal effects in core optics. Design guidelines: TCS must reduce thermal effects to a level which allows AdV to acquire the lock and such that the sensitivity of the detector is not spoiled. TCS should provide as much flexibility as possible for corrections, to help in case some optics should not meet the specifications (mirror radius errors, higher or non-uniform absorptions). Based on the experience of Virgo/Virgo+, TCS should be designed so that most of the apparatus lives outside vacuum and can be easily upgraded as new understanding of the IFO is realized.

3. ERC Meeting, Cascina - 27.04.2009 3 Progress summary First presentation to the ERC on the 03/11/08: preliminary design has been proposed, based on the Advanced LIGO solution. Suggestion from the ERC: do not limit your considerations to the Advanced LIGO design but compare different options. Activity during the last 5 months was dedicated to consolidate the reference solution by: performing more detailed analyses of the baseline design. checking in detail for consistency with other subsystems (PAY, OSD, SAT). An activity to evaluate alternative designs has started (more details in the extra-slides). TCS internal review from February 24 th to April 1 st. The main target of the review has been to prepare a reference design as much as possible complete and coherent Discussions of thermal effects during OSD weekly teleconferences TCS dedicated biweekly meeting on March 12 th.

4. ERC Meeting, Cascina - 27.04.2009 4 TCS reference solution Green dots: shielded heating ring Blue rectangles: CP Compensation plates shined with CO2 laser will correct thermal effects in the PRC Shielded heating rings will compensate HR surface deformations

5. ERC Meeting, Cascina - 27.04.2009 5 Compensation plate Transmissive optic that lives entirely in the recycling cavity; TCS noise coupling is much lower: CO 2 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 – Wedge/tilt of the CP will be defined within the general optical layout. – If the CP is wedged, there will be sensitivity to transverse motion. We considerer a maximum wedge of 17 mrad (1 degree). safety factor of 10 wrt the AdV sensitivity

6. ERC Meeting, Cascina - 27.04.2009 6 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 about 280 mm for the CP is foreseen.

7. ERC Meeting, Cascina - 27.04.2009 7 CP effect on Longitudinal Sensing and Control CP modeled as a mirror with 100ppm reflection added behind IMY, inside PRC, no wedge (worst case) Transfer function from CP motion to all longitudinal signals simulated with Optickle 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 (F. Bondu) 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.

8. ERC Meeting, Cascina - 27.04.2009 8 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 The CP etalon tuning produces an optical gain variation of the order of 1‰ on the AA error signals The misalignment of the CP front face produces a big effect on the alignment error signals (1.5 nrad of corresponding EMX misalignment for a CP misalignment of 10  rad), the effect disappears for misalignment above 0.1 mrad. Thermal effects included in the CP: lens with ROC of about 2700m (IFO at full power). No relevant changes from the CP plane configuration CP misalignment effects on the AA control loop For more detail on CP effect on LSC and AA see M.Mantovani and G. VajenteM.Mantovani and G. Vajente

9. ERC Meeting, Cascina - 27.04.2009 TCS performances We simulated TCS performances with different CP set-ups: – Changed position of the CPs: close and far from the TM – Changed thickness of the CPs: 3.5cm, 6.5cm and 10cm Heating patterns generated by an AXICON based telescope as in Virgo+ RH is always ON to correct ROC Results given in terms of coupling losses, mismatch of the FP cavity beam with the RC beam, and OPL Effect of the absorptions on ITM with: 0.5ppm (coating, foreseen absorption by MIR is 0.3-0.4 ppm) 2ppm/cm (substrate). FP cavity power ~ 800 kW  Absorbed power ~ 0.5 W 9

10. Effect of CP thickness CP “far” from the TM CP thickness 10cm, 6.5 cm and 3.5cm ERC Meeting, Cascina - 27.04.2009 TM+RH HR face RH at 45 mm from the AR face. RH power need to recover the cold ROC is 16.5 W. 10 Thickness (cm)CP mass (kg)RH power (W) Minimum L (ppm) 1013.516.53000 6.58.816.51300 3.54.716.5300 OPL corresponding to minimum L

11. CP close to the TM ERC Meeting, Cascina - 27.04.2009 CP radiates heat towards the TM This heat escapes from the TM lateral surface This radial gradient adds to that due to YAG absorption Thermal lensing is increased and ROC is reduced ROC depends also in the CO 2 power Less RH power is needed to keep the cold ROC RH and CO 2 power are strongly coupled TM heated by radiation from the CP TM Tmap 11 HR face dROC/dP CO2 = -0.9m/W

12. CP thickness fixed at 3.5cm ERC Meeting, Cascina - 27.04.2009 12 Effect of the CP-TM distance CP positionRH power (W)dROC/dP CO2 (m/W)Minimum L (ppm) On the RM (1cm)2-0.91.3·10 4 On the RM (10cm)8-0.56.2·10 3 On the RM (20cm)12-0.262.2·10 3 These numbers could improve using a radiative shield around the TM. -Modeling of the shield is in progress -Investigation with PAY for a possible implementation In the design of the Reference Mass a distance of about 20 cm is considered.

13. ERC Meeting, Cascina - 27.04.2009 13 Compensation plate – Reference Mass 2D drawing by PAY. Ring Heater Compensation Plate Detail of the clamping are under investigation with PAY (thermal noise, birefringence)

14. ERC Meeting, Cascina - 27.04.2009 14 Shielded ring heater is 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 are being optimized Shielded ring heater design guidelines RHTMRH 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

15. ERC Meeting, Cascina - 27.04.2009 15 The full TC System to be implemented on Advanced Virgo will comprehend sensors which sample the thermal wavefront distortions within the interferometer and the change of ROC. The HR face of each test mass could be monitored in reflection for deformation (optical levers or spot size measurement). At present the Hartmann sensor seems to be a promising solution for wavefront sensing. A Hartmann sensor is being developed at the University of Adelaide (T.L. Kelly, et al., Appl. Opt., 46(6), 861-866 (2007)). A version compatible with Advanced IFOs should be developed within one year. Contacts with the Adelaide group are active. The TCS sensing is being studied in the frame of the ongoing TCS studies. TCS sensing

16. Investigation on different TCS schemes Investigate different heating profiles Investigate different materials for the CP – So far, the CPs have been thought being made of fused silica. Actually, different optical materials could be in principle used, optimizing the value of dn/dT, thermal expansion coefficient and thermal conductivity, still matching the optical requirements. The right choice of these parameters could increase the CO 2 efficiency (dOPL/dP CO2 ), so that less CO 2 power is needed to compensate thermal lensing, thus reducing the radiative effects between CP and TM. This line of research is also being investigated within Advanced LIGO (LIGO-G0900115) Different TCS strategies – A different suggested TCS scheme (R. Flaminio, J. Marque), does not use CPs to correct thermal lensing. Instead, TCS would dynamically change PRMX and SRMX ROCs to match the RC beam to the FP cavity beam. This scheme is viable only in case NDRC design 4 is chosen. Some preliminary simulations have been performed to check TCS efficiency when the recycling mirrors are radiated by a ring heater. From (Granata, 2009) the variation of the ROC given by a RH would not be enough to compensate. Acting on PRMX with a CO 2 laser could increase the TCS efficiency (in progress). – Better performances could be obtained by putting the HR surface behind the mirror substrate and take advantage also of the dn/dT, which, in the case of SiO 2, has an effect 20 times larger than the thermal expansion coefficient. Radiative cooling – A completely different TCS strategy relies on the radiative cooling of the TM (Kamp, 2008; Passaquieti 2008). Some experimental test to check if this method is able to provide cooling patterns with the necessary precision is ongoing. ERC Meeting, Cascina - 27.04.2009 16

17. ERC Meeting, Cascina - 27.04.2009 17