1. Some considerations on radiative cooling V. Fafone, Y. Minenkov, I. Modena, A. Rocchi INFN Roma Tor Vergata

2. Advanced Virgo Meeting - 18.09.082 Working principle see 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 centre of the test mass is cooled

3. Advanced Virgo Meeting - 18.09.083 Simulation of radiative cooling effects in Advanced Virgo Test Masses In the simulation we used the following parameters: 125W of input ITF power, beam radius on TM = 6 cm FP cavity Finesse=885 gain of the SRC=23.5 absorptions of the coating and substrate respectively 0.5ppm and 2ppm/cm Total absorbed YAG power ~0.5W Test Mass diameter: 350 mm Test Mass thickness: 200 mm Test Mass ROC (cold): 1530 m Simulated a “perfect” system: Capable of extracting (from the HR surface) all the absorbed power from the test mass (0.4W in the coating plus 0.1W in the substrate) Cooling profile identical to the heating profile, Gaussian

4. Advanced Virgo Meeting - 18.09.084 Simulation of radiative cooling effects in Advanced Virgo Test Masses Effect of the Yag + ideal radiative cooling system on TM

5. Advanced Virgo Meeting - 18.09.085 Simulation of thermal effects in Advanced Virgo Test Masses Effect of the Yag + ideal radiative cooling system on TM - Lensing effect (thermooptic 95% + thermoelastic 5%) Optical path length. The strength of the residual lens is 3.5·10 -5 dioptres

6. Advanced Virgo Meeting - 18.09.086 Simulation of radiative cooling effects in Advanced Virgo Test Masses Effect of the Yag + ideal radiative cooling system on TM - Thermoelastic deformation Thermo-elastic deformation of the HR surface  Change in the ROC ROC 0 Compensated Uncompensated

7. Advanced Virgo Meeting - 18.09.087 Simulation of radiative cooling effects in Advanced Virgo Test Masses What if the cooling profile is non-ideal? -Cooling profile is Gaussian, but its waist is 10% larger/smaller than heating profile waist -Radiated power equals absorbed power ROC 0 Ideal 10% larger 10% smaller Change in the ROC 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 Ideal 10% larger 10% smaller It is important to control the cooling profile

8. Advanced Virgo Meeting - 18.09.088 Comments Radiative cooling is effective in correcting thermal lensing and ROC at the same time High sensitivity to profile mismatches Precise evaluation of the required profile and power is mandatory Most of the equipment lives in vacuum What if the absorptions are non uniform? Methods to tune the cooling power must be investigated Need to evaluate the possible noise introduced by scattered light (large mirrors at small angles) Need to investigate noiseless cooling systems Evaluate interactions with other subsystems Different systems are under investigation Cold surface Test mass