1. Thermal effects and their compensation in Advanced Virgo E. Coccia 1,2, M. Di Paolo Emilio 2, V. Fafone 1,2, V. Malvezzi 2, Y. Minenkov 2, A. Rocchi 2, L. Sperandio 1,2. 1 University of Tor Vergata 2 INFN Roma Tor Vergata 46th Rencontres de Moriond 26.03.2011

2.  Thermal effects Brief introduction  Compensation principle An example: Virgo+ TCS  Thermal Compensation System for AdV Layout Heating pattern optimization Ring heater development Wave-front sensing 26/03/2011A. Rocchi - Rencontres de Moriond 2011- La Thuile2

3. Thermal effects: introduction A. Rocchi - Rencontres de Moriond 2011- La Thuile3  High Reflectivity coating and substrate of the Test Masses absorb some (O(ppm)) power stored in the Fabry-Perot and recycling cavities.  Due to the low thermal conductivity of SiO 2, a thermal gradient is established in the substrate.  Thermal lensing: The refraction index is temperature dependent (dn/dT≠0, O(ppm/K)). The optical path inside the substrate of the TMs is not uniform. This is equivalent to putting a lens in the substrate of the ITMs.  Thermo-elastic deformation of the HR surface of all the TMs. 26/03/2011

4. Thermal effects: consequences  Thermal lensing: wave-front distortions of the fields in the SRC and PRC cavities. Decrease of the optical gain in the RCs; If lensing is too strong, ITF cannot acquire the lock. FOM: coupling losses. A. Rocchi - Rencontres de Moriond 2011- La Thuile4  Thermo-elastic deformation: changes the RoC in both ITMs and ETMs, affects the FP cavity. This effect is negligible in current detectors, but becomes relevant in advanced IFOs. Decrease of the spot size on TMs  increase of thermal noise of about 15%. 26/03/2011 [ppm]

5. A. Rocchi - Rencontres de Moriond 2011- La Thuile5 Thermal lensing is only due to the temperature gradient along the radial direction. So, we can heat the peripheral of the test mass to flatten the gradient and, thus, the optical path length. Working principle of TCS 26/03/2011

6. A. Rocchi - Rencontres de Moriond 2011- La Thuile6 Current compensation systems All use CO 2 ( =10.6  m) lasers to heat the peripheral of the input test masses. CO 2 Laser Annular heating Gold star mask AXICON Initial LIGO Enhanced LIGO and Virgo+ 26/03/2011

7. A. Rocchi - Rencontres de Moriond 2011- La Thuile 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. 726/03/2011

8. Noise from TCS  TCS can inject displacement noise into the detector.  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 (negligible) A. Rocchi - Rencontres de Moriond 2011- La Thuile TETRF Present detectors already require intensity stabilization of the CO 2 laser. Achievable level of intensity stabilization (10 -7 /√Hz) not enough to heat with CO 2 directly the TM in advanced detectors (10 -9 /√Hz needed)  compensation plates required. 826/03/2011 RIN = relative intensity noise

9. 9 Green dots: heating rings Blue rectangles: CPs Compensation plates shined with CO 2 laser will correct thermal effects in the RCs Ring heaters will compensate HR surface deformations This set up allows to control independently the thermal lensing and the ROCs A. Rocchi - Rencontres de Moriond 2011- La Thuile TCS in Advanced detectors Power absorbed by TMs is about 0.5W, wrt ~20mW in initial detectors 26/03/2011

10. Some details RH at 5 cm from the anti-reflecting face of the TM to maximize the efficiency CP diameter: 350 mm (same as TMs) CP thickness = 35 mm CP must be “far” from the TM. Distance fixed at 200 mm Symmetry axis Heat escaping from the barrel of the CP 10A. Rocchi - Rencontres de Moriond 2011- La Thuile TM heated by radiation from the CP if CP-TM distance = 1 cm TM Tmap HR face Symmetry axis 26/03/2011

11. CP heating pattern A. Rocchi - Rencontres de Moriond 2011- La Thuile  The heating profile must be much more precise than in present detectors Simple system like an axicon (Virgo-like) is not enough Too high content of higher order modes in RF sidebands  Necessity to optimize the CP heating pattern Linear iterative optimization process based on FEM developed, to take into account radiative coupling btw TM and CP and the presence of the RH. Uses the OPL increase as error signal. 11 with single axicon = 2200 ppm Optimal heating pattern Resulting Optical Path Length Residual coupling losses= 26 ppm Coupling losses = superposition integral between “unperturbed” beam and single pass through thermal lensing. 26/03/2011 No TCS = 5·10 5 ppm

12. Solution using known technology: modulate rings dimensions by changing distances between lenses and axicons and modulate power in each ring (Double Axicon System) -With DAS, residual coupling losses around 10 2 ppm. Residual focal length O(10 3 km) -With single axicon, residual coupling losses around 2000 ppm Residual focal length O(10 2 km) Heating pattern generation: DAS 12A. Rocchi - Rencontres de Moriond 2011- La Thuile26/03/2011

13.  The beam is split by polarization and recombined with round polarizer at 45 deg AOI.  The optical layout is very flexible! Allows to change independently the two rings: Inner and outer diameters of each ring Power density of each ring  Heating profile simulated with Zemax gives 200 ppm coupling losses  Experimental DAS gives 210 ppm A. Rocchi - Rencontres de Moriond 2011- La Thuile13 Experiment vs. simulation Heating pattern generated by the DAS 26/03/2011

14.  Main concerns: Stray magnetic field: AdV will use electro-magnetic actuators to control the TMs; Vacuum compatibility.  Down-scaled prototypes with different geometries and materials realized: Annular winding with counter-flux coils; Toroidal winding: ○ Single coil; ○ Double coil (counter-flux).  Magnetic field measurements: Detailed maps as a function of the position; Frequency domain transfer function; 3D magnetic field simulations. Annular windingToroidal winding Ring heater development A. Rocchi - Rencontres de Moriond 2011- La Thuile14 x y z MeasuredSimulated 26/03/2011

15. Noise projection Simulated a RH of 400 mm diameter and considered the magnetic field and gradient in the position of the magnets Assumed a flat current noise of 1nA/  Hz A. Rocchi - Rencontres de Moriond 2011- La Thuile15 g = magnets symmetry  = magnetic moment m = mirror mass I = moment of inertia L = arm cavity length D = beam displacement B D 26/03/2011

16. Materials optimization A. Rocchi - Rencontres de Moriond 2011- La Thuile16 To avoid contamination from the insulation of the wire we have chosen a design with bare wire on insulated support. Under investigation: glass core aluminum core with insulating coating Aluminum samples machined and coated with: inorganic paint plasma spray with alumina diamond like coating 26/03/2011

17. The full TC System to be implemented on Advanced Virgo will comprehend sensors which sample the thermal wave-front distortions within the interferometer, either from samples of the IFO beam picked off at appropriate points, or directly from the optics themselves using dedicated probe beams and servo electronics that will derive control signals from the sensors and feed them back to the heater elements. Advanced Virgo will use a Hartmann sensor developed by the University of Adelaide, that has demonstrated to have a shot-to-shot reproducibility of λ/1450 at 820 nm, which can be improved to λ/15500 with averaging (Opt. Express 15 (16), 10370-10375, 2007). TCS sensing scheme A. Rocchi - Rencontres de Moriond 2011- La Thuile17 Measurement taken at the Gingin HOPTF 26/03/2011

18. A. Rocchi - Rencontres de Moriond 2011- La Thuile18  Thermal compensation systems proven to be efficient and reliable in initial detectors: recovered 85% of sidebands recycling gain.  In advanced detectors, thermal effects must be corrected with higher precision: More power in the cavities; Larger spot size on ITMs; One more effect to take care of: TMs RoC.  Good experimental results from the DAS: Good agreement with simulations; Easy to assemble; High degree of flexibility.  Ring heater design is progressing.  High sensitivity wave-front sensor identified for advanced detectors. 26/03/2011