1. Experimental Test of Radiative Cooling Roberto Passaquieti Università di Pisa and INFN-Pisa V. Fafone, Y. Minenkov, I. Modena, A. Rocchi Università di Roma Tor Vergata and INFN Advanced Virgo - Cascina, Feb. 3rd, 2009

2. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 2 Radiative Cooling Motivations The present solution for the TCS makes use of a compensation plate to correct the lensing effect in the input test mass and of a ring heater to correct the radius of curvature of the input and the end test mass A possible alternative solution would be to eliminate the heat deposited by the laser beam at the source (the stored beam spot) before it has a chance to deform the mirror. In advanced detectors almost a MW standing power will impinge the HR coating of FP mirrors over a gaussian spot of ~6 cm radius:  Expected heating power up to ~ 0.5 W For a 6 cm radius spot over the mirror surface at room temperature (293 K): –Fused silica emissivity  0.93, ( Wien law  λ  10  m ) –Emissive power E =  T 4  389 W/m 2  Mirror spot emispheric emitted power P  4.4 W At equilibrium the same amount is absorbed from the environmental thermal bath

3. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 3 Principle of Directional Radiative Cooling (DRC) Establish thermal radiation heat exchange between a cold surface (masking partially the environment to the mirror) and the mirror hot spot surface The cold target could be a Li-N 2 surface: –higly efficient  99.6% –emits only 0.4% thermal radiation than a room temperature body A solid angle coverage of a just a quarter of one steradian is sufficient to collect the expected power absorbed by the TM (~0.5 W).

4. First Experimental Results Measurement of the cooling power of a LN 2 cold target focused on a linear array of temperature probes in air (August 2008 - Caltech Lab.)

5. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 5 Experimental Apparatus: Schematics J. Kamp, H. Kawamura, R. Passaquieti, and R. DeSalvo: Radiative cooling TCS, LIGO-G080414-00-R Pasadena 12 August 2008 (article inpreparation ) 0.55 m Thermometer array (n. 8 - 2.5 cm sp.- LM 19 ) 62 mm aperture

6. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 6 Power Deposition/Extraction: Results Result: measured cooling power  155 ±78 ±39 mW (average over 6 meas.) (max theor. cooling power  260 mW) 40W lamp

7. A Preliminary Case Study: Design and Simulation Mirror DRC focused system dimensionally compatible with the actual Virgo vacuum chamber

8. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 8 Model: Implementation of Parabolic Collectors Cold Target Parabolic Collector ParabolicReflector

9. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 9 Model Geometry

10. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 10 Model Implementation in Virgo (4/5)

11. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 11 ZEMAX IR Simulation Set Up Source: Disc radius=6 cm T=293 K (λ  10  m)  =0.93 Flux= 4.4 W (100000 rays) Angular cosine distribution: I=I 0 cos(θ) Target : BB Disc Detector (diam.= 5 cm)

12. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 12 Target Geometry Optimization

13. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 13 Detected Power on the Targets Total detected power from 6cm radius spot:  170 mW each P d  0.7 W !! Assuming: 0.97 reflectivity of Au plated surfaces target emissivity  0.8 (mirror 0.93 already considered) Effective detected power P= P d X 0.97 2 x0.8  0.5 W

14. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 14 Simulation Summary Thermal Radiation exchanges: –Thermal radiation from 6 cm radius mirror spot to 4 targets (5 cm diam) ~ 0.5 W –Thermal radiation from 4 cold targets (5 cm diam) to mirror: 77 K : ~ 4 mW 200 K: ~ 200 mW –Net radiation flux from mirror 6cm radius spot to targets at 77 K: ~ 500 mW Use of LN 2 or low noise refrigerators (pulse-tube) –Net radiation flux from mirror 6cm radius spot to targets at 200 K: ~ 300 mW Possible use of peltier cells ( multilayer ΔT  90 K)

15. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 15 DRC Control Methods Iris control: –An iris placed in front of each target tuning the sink surface –Require remote adjustment and moving parts in vacuum Target temperature control –A resistor heater (C) tuning the target (D) temperature Reaction time depends on target heat capac. Hot resistor power balance –Shielded resistor heater and cold target both focussed on the mirror Fast Useful during unlock Remote driving of peltier cells (if implementable)

16. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 16 DRC Issues Radiative cooling is effective in correcting thermal lensing and ROC at the same time with high sensitivity to beam profile mismatches (See A. Rocchi presentation AdVirgo Biweekly Meeting - 18.12.2008 ) 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

17. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 17 Radiative Cooling Experimental Test The aim is to investigate the capability of DRC to generate proper cooling profiles. This measurement can benefit of the cryogenic facilities already present in the Tor Vergata Laboratories, that will be made available for this research program This activity aim to give answers to some important issues: –how to produce a Gaussian profile exactly matching the heating profile –what if the absorptions in TM are not uniform –how to efficiently tune the cooling power without introducing noise Investigate the possible implementation of peltier cells It is mandatory to investigate also those issues directly related to the AdVirgo requirements: –Scattered light (see presentation by J.Y. Vinet) –Cooling system noise

18. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 18 The DRC test structure has been simulated and designed: A system of 5 parabolic reflectors and collectors arranged into a cylindrical structure with an outer diameter of 35 cm an an hight of about 75 cm. Reflectors and collectors will be obtained by warming up a plexiglass substrate over metallic supports of appropriate shape Al or Au spattered coating will be applied to achieve the desired surface reflectivity In a first attempt the dummy TM will be made of dieletric material (glass or plexiglass) and will be equipped with high precision Pt-thermometers having a resolution of 0.001 0 C with long term accuracy better than 0.01 0 C The cold target will be directly cooled down by a LN 2 flux and also their temperature will be monitored together with that of the vessel The system will operate in vacuum The expected cooling power of this device should be of about 150 mW Experimental Apparatus Cold spot Parabolic Collector Parabolic Reflector Target

19. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 19 Experimenthal Plan The first step will be to perform a measurement of the cooling profile of the dummy TM surface facing the reflectors From the experimental result the process of system characterization and optimization will follow: –Adjustment of the geometric set-up –Tuning of the simulation parameters The information acquired will give an answer about the coling efficiency and the sensitivity of the thermal power distribution to geometric parameters The following step will be to perform thermal compensation experiments trying to balance the thermal power input from an hot source impinging directly over the TM surface –In this case the thermal profile obtained from the thermometer array over the hot surface will give us informations about the capability of this DRC system to match the hot spot profile

20. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 20 Timeline Measurements are foreseen to start by April 2009 An intermediate status report is foreseen by the end of April 2009 close to the start-up A status report is foreseen by end of June with first consistent results expected

21. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 21 Participants

22. Cascina 03 Feb. 2009 Experimental Test of Radiative Cooling 22 Budget