Nawrodt 05/2010 Thermal noise and material issues for ET Ronny Nawrodt Matt Abernathy, Nicola Beveridge, Alan Cumming, Liam Cunningham, Giles Hammond, Daniel Heinert, Jim Hough, Iain Martin, Peter Murray, Stuart Reid, Sheila Rowan, Christian Schwarz, Paul Seidel, Marielle van Veggel GWADW2010 Meeting, Kyoto 20/05/2010 Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena Sonderforschungsbereich Transregio 7 „Gravitationswellenastronomie“ Institute for Gravitational Research, University of Glasgow Einstein Telescope Design Study, WP2 „Suspension“
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Nawrodt 05/2010 Thermal noise and material issues for ET Ronny Nawrodt Matt Abernathy, Nicola Beveridge, Alan Cumming, Liam Cunningham, Giles Hammond,
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Nawrodt 05/2010
Thermal noise and material issues for ET
Ronny NawrodtMatt Abernathy, Nicola Beveridge, Alan Cumming, Liam Cunningham, Giles
Hammond, Daniel Heinert, Jim Hough, Iain Martin, Peter Murray, Stuart Reid, Sheila Rowan, Christian Schwarz, Paul Seidel, Marielle van Veggel
GWADW2010 Meeting, Kyoto 20/05/2010
Institut für Festkörperphysik, Friedrich-Schiller-Universität JenaSonderforschungsbereich Transregio 7 „Gravitationswellenastronomie“
Institute for Gravitational Research, University of GlasgowEinstein Telescope Design Study, WP2 „Suspension“
Nawrodt 05/2010
Overview
• Motivation
• Material Properties
– thermal properties– mechanical properties
• Thermal Noise Issues for ET
• Summary
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Motivation
• ET will need a radical change in the materials in order to achieve the sensitivity goals:
– suspensions,– test mass materials,– coatings,– optical materials
• Additionally, going towards cryogenics temperatures will dramatically change material properties additional degree of freedom.
• The new material has to be compared to the best optical material currently available at room temperture!
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Material Properties – Thermal Conductivity in Crystals
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• typically 3 zones:
– higher temperatures: TC is limited by phonon-phonon scattering– lower temperatures: mean free path of phonons increases,
scattering at impurities becomes important
– high purity samples: at very low temperatures the sample geometry becomes important (scattering of phonons at the sample surface limitation of TC)
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Material Properties – Thermal conductivity of Silicon
experimental results (double-log scale!):
“recommended curve”(< 1014 cm-3 boron, approx.1 mg B in 1 t Si)
increasing impurity concentration (scatteringof phonons on impurities)
smaller structures + impurities(~ 1/L term)
see Callaway 1961 or Casimir 1938
[Touloukian]
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• in high purity silicon the different silicon isotopes take the role as scatter centers (-> impurities)
• natural Si has 3 stable isotopes:– 92% Si-28– 5% Si-29– 3% Si-30
• they cause small local changes in the lattice due to their different atom masses effect is small
• however, concentration is very large compared to typical impurity concentrations (ppm range)
Material Properties – Thermal conductivity of Silicon
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• it is possible to enrich/purify silicon
• isotopic pure silicon shows a much larger thermal conductivity in the peak region compared to standard semiconductor grade silicon
sensitivity goal can be reached, additional „help“ is needed at low frequencies (artificial lowering of pendulum frequency needed – actively/passivly)
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Cooling Issues through the suspension
• cooling through fibre• target temperature: below 22 K
• thermal bath:
– technically limited to 2-5 K – no huge advantage to go for 2 K from a thermal conductivity point
of view (limitation through geometry, low thermal conductivity at T < 10 K)
– however, 2 K allows use of suprafluid helium with much reduced mechanical disturbances
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Cooling Issues through the suspension
• maximum cooling power is very low (L = 1m, Tbath = 2 K, 4 fibres)
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Tmirror [K] Diameter [mm] Pmax [mW]
20 3 480
5 1300
8 3400
15 3 270
5 740
8 1900
10 3 100
5 270
8 690
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Cooling Issues through the suspension
• highest possible thermal conductivity needed
• investigation optical absorption in silicon (at 1550 nm unknown)
• strong reduction of introduced thermal load needed
– reduction of incident laser power(Xylophon concept, 2 detectors, low frequency detector with
low laser power e.g. 18 kW)
– very carefull dealing with scattered light needed (additional heating of test masses)
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[Hild et al. 2010]
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Conclusion
• crystalline materials are candidate materials for 3rd generation detectors
• cooling necessary to reduce thermo-elastic noise
• high thermal conductivity is used to extract heat, however minimum thermal load should have very high priority (scatter!)
• thermal noise can be reduced below the requirements with reasonable materials (silicon) and R&D (loss measurments, optics absorption, coating research,…)