Transcript Mirror 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
Radiative Cooling Motivations
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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
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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 = T4  389 W/m2
Mirror spot emispheric emitted power P  4.4 W
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At equilibrium the same amount is absorbed from the environmental
thermal bath
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Principle of Directional Radiative Cooling (DRC)
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Establish thermal radiation heat exchange between a cold surface (masking
partially the environment to the mirror) and the mirror hot spot surface
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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 quarter
of one steradian is sufficient to collect the
expected power absorbed by the TM (~0.5
W).
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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.)
0.55 m
Experimental Apparatus: Schematics
62 mm aperture
Thermometer array
(n. 8 - 2.5 cm sp.- LM 19 )
J. Kamp , H. Kawamura, R. Passaquieti, and R. DeSalvo:
Radiative cooling TCS , LIGO-G080414-00-R Pasadena 12 August 2008 (article inpreparation)
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Power Deposition/Extraction: Results
40W lamp
Result:
measured cooling power 155 ±78 ±39 mW (average over 6 meas.)
(max theor. cooling power  260 mW)
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A Preliminary Case Study: Design and Simulation
Mirror DRC focused system dimensionally compatible with the
actual Virgo vacuum chamber
Model: Implementation of Parabolic Collectors
ParabolicReflector
Cold Target
Parabolic Collector
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Model Geometry
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Model Implementation in Virgo (4/5)
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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=I0 cos(θ)
Target :
BB Disc Detector (diam.= 5 cm)
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Target Geometry Optimization
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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 power
P= Pd X 0.972x0.8  0.5 W
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Simulation Summary
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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)
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DRC Control Methods
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Iris control:
– An iris placed in front of each target tuning the sink
surface
– Require remote adjustment and moving parts in vacuum
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Target temperature control
– A resistor heater (C) tuning the target (D) temperature
• Reaction time depends on target heat capac.
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Hot resistor power balance
– Shielded resistor heater and cold target both focussed
on the mirror
• Fast
• Useful during unlock
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Remote driving of peltier cells (if implementable)
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DRC Issues
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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 )
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Scattered light: is the presence of large reflectors placed in front of the TM at small
angle (~20 deg) compatible with AdV specifications?
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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?
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Methods to tune the cooling power must be investigated
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Need to investigate noiseless cooling systems
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Evaluate interactions with other subsystems
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Radiative Cooling Experimental Test
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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
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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
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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
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Experimental Apparatus
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. Collector
Parabolic
• Reflectors and collectors will be obtained by warming up a plexiglass substrate
over metallic supports of appropriate shape
Cold spot
• 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
Target
•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
Parabolic Reflector
•The expected cooling power of this device should be of about 150 mW
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Experimenthal Plan
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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
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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
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Participants
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Budget
Budget
Mechanical machining
Temperature analyzer system
Thermometers
Connectors and cabli ng
Consumables
YEAR 1
9.5 kΫ
12 kΫ
1.9 kΫ
1.5 kΫ
3 kΫ
TOTAL
27.9 kΫ
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