1. AdV Thermal Compensation System Viviana Fafone AdV/aLIGO joint technical meeting, February 4, 2004

2. Stable recycling cavities ? Good  More stable RF and audio-GW sidebands (better signals for control and better interferometer sensitivity)  Smaller beam at the input and output port  Easier extraction of pick-off from recycling cavity Bad  Multi-mirror superattenuators  Astigmatism (folded cavities)  Alignment signals reduced Baseline:stable cavities but re-evaluation on going

3. Advanced Virgo baseline

4. Re-evaluation of marginally stable cavities  Re-design of the suspensions is very complex and costly  The question: the marginally stable cavities are really so bad?  Crucial role of the thermal compensation system:  Which can be the exact degree of compensation?

5. LIGO-Virgo TCS meeting 4.02.2010 5 Thermal Compensation System 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.

6. LIGO-Virgo TCS meeting 4.02.2010 6 TCS baseline design (same as Advanced LIGO) Green dots: shielded heating ring Blue rectangles: CP Compensation plates shined with CO 2 laser will correct thermal effects in the PRC Shielded ring heaters will compensate HR surface deformations Compensation plates are suspended from the SA. Constraints from SAT and PAY must be taken into account in the design (geometry/position) of CP and RH Design of the new payload with CP and RH is in progress.

7. LIGO-Virgo TCS meeting 4.02.2010 TCS performances investigation We simulated TCS performances with different CP (28 cm diameter) set-ups: – Changed position of the CPs: close and far from the TM – Changed thickness of the CPs: 3.5cm, 6.5cm and 10cm (with 3.5 cm the lower resonance is at 3 kHz) 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, OPL, residual focal length. 7

8. LIGO-Virgo TCS meeting 4.02.2010 TCS performances investigation Effect of the absorptions on ITM with: 0.6ppm (coating) and 0.3ppm/cm (substrate). From Advanced Virgo Conceptual VIR‐042A‐07 absorptions are for substrate and coating are 0.2‐0.3 ppm/cm and 0.3‐0.4 ppm respectively. Recently: from Pinard VIR-0555A-09, AdV substrates technical readiness review, values given were: less than 0.3ppm/cm and 0.6-0.7ppm, “state of the art at the present moment”. FP cavity power ~ 800 kW (F=885, P in =125W, G rec =23.5)  Absorbed power ~ 0.5 W  Beam size on ITM = 56 mm 8

9. LIGO-Virgo TCS meeting 4.02.2010 Effect of CP thickness CP “far” from the TM CP thickness 10cm, 6.5 cm and 3.5cm TM+RH HR face RH at 45 mm from the AR face. RH power need to recover the cold ROC (1416m) is 16.5 W. 9 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 From a quadratic fit with Gaussian weights, the residual focal length is 320 km

10. LIGO-Virgo TCS meeting 4.02.2010 CP close to the TM 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 Thermo-structural analysis to find what is the parameters set (RH power, CO2 power and heating profile) that minimizes coupling losses and restore the cold ROC. TM heated by radiation from the CP TM Tmap 10 HR face CP thickness fixed at 3.5 cm CP-TM distanceRH power (W)dROC/dP CO2 (m/W)Minimum L (ppm) 1cm2-0.91.3·10 4 10cm8-0.56.2·10 3 20cm12-0.262.2·10 3 Residual focal length = 256 km CP - ITM distance presently fixed at 20 cm

11. LIGO-Virgo TCS meeting 4.02.2010 11 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 (e.g. 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.

12. LIGO-Virgo TCS meeting 4.02.2010 Coating absorption increased to 1.2ppm (a factor of 2 wrt specs.) Power absorbed by the TM ~ 1W RH power is increased to 28W RH temperature increases from 360K to 384K Minimum coupling losses are 4.6·10 3 ppm 12 Coating absorption increased to 2ppm (~ a factor of 4 wrt specs.) Power absorbed by the TM ~ 2W RH power is increased to 48W RH temperature increases from 360K to 420K Minimum coupling losses are 1.3·10 4 ppm Phase maps are being used in FFT optical simulations to investigate impact of thermal effects for different optical layouts

13. LIGO-Virgo TCS meeting 4.02.2010 13 TCS sensing TCS sensing concept: same as Advanced LIGO. Wavefront sensors will probe the input test masses individually while all TMs will be probed in reflection for change of the ROCs. Degree of aberration will be manifest also in ITF channels, as it is in Virgo. To sense the spatial structure of the cavity mode, phase cameras will sample the ITF beam. The final TCS control will likely adopt a blend of these sensors as inputs

14. LIGO-Virgo TCS meeting 4.02.2010 14 Comparing ANSYS results and measurements with phase camera – ITF input power 13W Coupling losses reach a minimum for about 4.75W, compatible with the measured level of compensation: f 3W =(15.8±0.6)km f 4W =(24±2)km Gives a measurement of the power absorbed by the test mass: f = 405[m·W]/Pabs[W] RED: thermo-optic+thermo-elastic BLUE: thermo-optic

15. LIGO-Virgo TCS meeting 4.02.2010 15 Comparing ANSYS results and measurements with phase camera – ITF input power 17W For 6.5W of TCS power, a minimum of the coupling losses is reached, compatible with optimal compensation, if we allow NI absorptions to be 30% less than on the WI f 6.5W =(40±8)km f 4W =(12.3±0.5)km f 6.5W =(40±8)km f 4W =50km Equal absorptions30% less on the NI