Experimental Test of Radiative Cooling

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

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 10m )

– Emissive power E = T4 389 W/m2

Mirror spot emispheric emitted power P 4.4 W

• At equilibrium the same amount is absorbed from the environmental thermal bath


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-N2 surface:

– higly efficient 99.6%

– emits only 0.4% thermal radiation than a room temperature body

A solid angle coverage of a just a quarterof one steradian is sufficient to collect the expected power absorbed by the TM (~0.5 W).

First Experimental Results Measurement of the cooling power of a LN2 cold target focused on a linear array of temperature probes in air(August 2008 - Caltech Lab.)


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


Power Deposition/Extraction: Results

Result: measured cooling power 155 ±78 ±39 mW (average over 6 meas.)(max theor. cooling power 260 mW)

40W lamp

A Preliminary Case Study: Design and Simulation

Mirror DRC focused system dimensionally compatible with the actual Virgo vacuum chamber


Model: Implementation of Parabolic Collectors

Cold Target

Parabolic Collector

ParabolicReflector


Model Geometry


Model Implementation in Virgo (4/5)


ZEMAX IR Simulation Set Up

Source:•Disc radius=6 cm•T=293 K (λ10 m) =0.93•Flux= 4.4 W (100000 rays)Angular cosine distribution:I=I0 cos(θ)

Target :BB Disc Detector (diam.= 5 cm)


Target Geometry Optimization


Detected Power on the Targets

Total detected power from 6cm radius spot: 170 mW each

Pd 0.7 W !!Assuming:• 0.97 reflectivity of Au plated surfaces• target emissivity 0.8• (mirror 0.93 already considered)

Effective detected powerP= Pd X 0.972x0.8 0.5 W


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 LN2 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)


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)


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


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


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 0C with long term accuracy better than 0.01 0C

•The cold target will be directly cooled down by a LN2 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


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


Timeline

Measurements are foreseen to start by April 2009An intermediate status report is foreseen by the end of April 2009 close to the start-upA status report is foreseen by end of June with first consistent results expected


Participants


Budget

Budget YEAR 1 Mechanical machining 9.5 k€ Temperature analyzer system 12 k€ Thermometers 1.9 k€ Connectors and cabling 1.5 k€ Consumables 3 k€ TOTAL 27.9 k€

Experimental Test of Radiative Cooling

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