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Kazuhiro Yamamoto Institute for Cosmic Ray Research, the University of Tokyo

Overview of the 20K configuration. Kazuhiro Yamamoto Institute for Cosmic Ray Research, the University of Tokyo. 23 May 2016 Gravitational Wave Advanced Detector Workshop @ Hotel Hermitage, La Biodola, Isola d’Elba, Italy. 1. 1. 0. Abstract. KAGRA mirror : 20K ET-LF mirror : 10K

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Kazuhiro Yamamoto Institute for Cosmic Ray Research, the University of Tokyo

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  1. Overview of the 20K configuration Kazuhiro Yamamoto Institute for Cosmic Ray Research, the University of Tokyo 23 May 2016 Gravitational Wave Advanced Detector Workshop @ Hotel Hermitage, La Biodola, Isola d’Elba, Italy 1 1

  2. 0. Abstract KAGRA mirror : 20K ET-LF mirror : 10K (1)How much is the noise below 20K ? We expect smaller noise. (2)However, cooling below 20K is challenge. What are issues ?

  3. Contents • Introduction • Noise vs temperature • Challenges for cooling • Summary

  4. Introduction Motivation : Why the mirrors and suspension are cooled ? (1)Small thermal noise (2)Small thermal lens (3)Less serious parametric instability (4)Small cosmic ray effect Kenji Numata and Kazuhiro Yamamoto, ”Chapter 8. Cryogenics”, in ”Optical Coatings and Thermal Noise in Precision Measurement” Cambridge University Press (2012). T. Tomaru et al., Classical and Quantum Gravity 19 (2002) 2045. K. Yamamoto et al., Journal of Physics: Conference Series 122 (2008) 012015. K. Yamamoto et al., Physical Review D 78 (2008) 022004. 4 4 4 4

  5. Introduction Room temperature second generation interferometer Fused silica mirror suspended by fused silica fibers This suspension is not a good for cryogenics. Large mechanical dissipation and low thermal conductivity at low temperature Other material with small dissipation and high thermal conductivity Sapphire or Silicon 5 5 5 5

  6. 2. Noise vs Temperature Motivation : Why the mirrors and suspension are cooled ? (1)Small thermal noise Mirror thermal noise Substrate thermoelastic, Coating Brownian Pendulum thermal noise Wire thermoelastic, Wire Brownian Kenji Numata and Kazuhiro Yamamoto, ”Chapter 8. Cryogenics”, in ”Optical Coatings and Thermal Noise in Precision Measurement” Cambridge University Press (2012). 6 6 6 6

  7. 2. Noise vs Temperature Mirror substrate thermal noise (thermoelastic) Baseline is 3km. Beam radius is 35mm. Operation temperature must be below 20 K or around 120 K. Sensitivity of 2nd generation 7 7 7 7 7 7

  8. 2. Noise vs Temperature Coating thermal noise (Brownian) Coating loss angle is 5*10-4. Coating thermal noise does not limit sensitivity below 20 K. Sensitivity of 2nd generation 8 8 8 8 8 8

  9. 2. Noise vs Temperature Pendulum thermal noise (Thermoelastic) Mirror is 23kg. Thinnest wire (Wire strength limit) Sensitivity of 2nd generation AdLIGO fiber : 0.4mm Below 120K, thermoelastic noise does not matter. A.V. Cumming et al., Classical and Quantum Gravity 29 (2012) 035003. 9 9 9 9 9 9

  10. 2. Noise vs Temperature Pendulum thermal noise (Brownian) Sensitivity of 2nd generation T. Uchiyama et al., Physics Letters A 273 (2000) 310 (Sapphire). Wire Q is 5*106. Below 120K, Brownian noise dominates but it does not matter. 10 10 10 10 10 10

  11. 2. Noise vs Temperature Summary Any thermal noise is enough small below 20 K. Especially, thremoelastic noise reduces drastically. Other kinds of thermal noise is also small because thermal noise formula (Fluctuation-Dissipation Theorem) includes temperature (Power spectrum density is proportional to Temperature). 11 11 11 11 11 11

  12. 3. Challenges for cooling • How to keep temperature with heat load ? • (a) Is cooling power enough large ? • (b) Is heat path enough ? • 2. How to cool down as short as possible at initial ? 12 12 12 12 12 12

  13. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? Sapphire or Silicon fibers Pulse tube cryocoolers Absorption in mirror KAGRA : 1W ET-LF : 0.1W Mirror(below 20K) 13 13 13 13 13 13 13 13 13 13 13 13 13

  14. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (a)Cooling power of cryocoolers Y. Sakakibara et al., Classical and Quantum Gravity 31 (2014) 224003. According to KAGRA experiment, power is enough ! 14 14 14 14 14 14

  15. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Mirror is in vacuum. Radiation is about 1 mW at most. Thermal conduction is only solution. 15 15 15 15 15 15

  16. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Which do we need (crystal or pure metal) ? Both of them ! 16 16 16 16 16 16

  17. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Sapphire or Silicon fibers (High Q values) Pure metal (Soft) Mirror 17 17 17 17 17 17

  18. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path 8 K Mirror 10 K - 20 K Intermediate mass How much is temperature ? 18 18 18 18 18 18

  19. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Intermediate mass temprature Too high : Sapphire fibers must be too thick. (Large thermal noise) Too low : Heat links must be too thick. (Large external vibration) We must face trade off. 19 19 19 19 19 19

  20. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Intermediate mass temperature is close to mirror temperature. Constant T3 20 20 20 20 20 20

  21. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path 8 K Intermediate mass 10K-16K Mirror 10K-20 K Absorption in mirror 0.1W – 1W 21 21 21 21 21 21

  22. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Diameter of sapphire fiber Size limit : Upper limit of thermal conductivity of fiber, which is proportional to diameter. T. Tomaru et al., Physics Letters A 301 (2002) 215. Heat absorption in mirror : 0.1 W - 1 W Temperature of mirror : 13K-23K Here we adopt 1.6mm diameter fibers (350mm length) Larger pendulum thermal noise and lower violin mode frequencies 22 22 22 22 22 22

  23. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Pendulum thermal noise (Thermoelastic) Below 80K, thermoelastic noise does not matter. Sensitivity of 2nd generation 23 23 23 23 23 23

  24. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Sensitivity of 2nd generation Pendulum thermal noise (Brownian) Below 30K, thermal noise is enough small. 24 24 24 24 24 24

  25. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Violin modes Thin wire (0.6mm) Thick wire (1.6mm) Sapphire Silicon Sapphire Silicon 343Hz 448Hz 175Hz 191Hz 698Hz 908Hz 410Hz 413Hz 1075Hz 1392Hz 732Hz 684Hz First violin mode might be too low. AdLIGO : 511Hz A.V. Cumming et al., Classical and Quantum Gravity 29 (2012) 035003. 25 25 25 25 25 25

  26. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Violin modes Thin wire (0.6mm) Thick wire (1.6mm) Sapphire Silicon Sapphire Silicon 343Hz 448Hz 175Hz 191Hz 698Hz 908Hz 410Hz 413Hz 1075Hz 1392Hz 732Hz 684Hz First violin mode might be too low. Solution : Larger mirror with smaller absorption In any case, larger mirror is necessary for 3rd generation to reduce standard quantum limit. 26 26 26 26 26 26

  27. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path For vibration isolation, they must be thin. 8 K Intermediate mass 10K-16K Mirror 13 K - 23 K Absorption in mirror 0.1W – 1W 27 27 27 27 27 27

  28. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path A : Cross section l : Length 15 K (9.75K) A/l = 10-5 m (4*10-6 m) A/l = 10-4 m (3*10-5 m) 8 K Intermediate mass 16K(10K) Mirror 23 K(13K) Absorption in mirror 0.1W – 1W 28 28 28 28 28 28

  29. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Thick heat link (cross section could be on the order of cm2) should be an issue. Solution : Larger mirror with smaller absorption In any case, larger mirror is necessary for 3rd generation to reduce standard quantum limit. 29 29 29 29 29 29

  30. 3. Challenges for cooling 3-1. How to keep temperature with heat load ? (b)Heat path Thick heat link (cross section could be on the order of cm2) should be an issue. (1)Many thin fibers (softer) (2)Soft vertical spring and extra vibration isolation system for heat links is necessary. Extra pendulum for vibration isolation Soft vertical spring Investigation is in progress. D. Chen et al., Classical and Quantum Gravity 31(2014)224001. 30 30 30 30 30 30

  31. 3. Challenges for cooling 3-2. Short initial cooling time Since we discuss difference between 20K and 120K, here, we consider initial cooling time below 120K. Y. Sakakibara et al., CEC/ICMC2013, 2EOrD4-03, Anchorage, USA (2013). 20 days 31 31 31 31 31 31 31 31 31 31 31 31 31

  32. 3. Challenges for cooling 3-2. Short initial cooling time A : Cross section l : Length Bottle neck ! 15 K (9.75K) A/l = 10-5 m (4*10-6 m) A/l = 10-4 m (3*10-5 m) 8 K Intermediate mass 16K(10K) Mirror 23 K(13K) Absorption in mirror 0.1W – 1W 32 32 32 32 32 32

  33. 3. Challenges for cooling 3-2. Short initial cooling time Some kinds of tricks are necessary. (a)Mechanical thermal switch between payload and crycoolers (b)Softer vertical spring or thicker heat links with better vibration isolation systems Softer vibration isolation Better vibration isolation system Thicker heat links 33 33 33 33 33 33

  34. 4. Summary Operation below 20K (1)How much is the noise below 20K ? All kinds of thermal noise is smaller than goal sensitivity of 2nd generation.

  35. 4. Summary Operation below 20K (2)However, cooling below 20K is challenge. Heat path is an issue. We need “thick” heat path. It implies low violin modes and large external vibration effect. Solution Larger mirror with smaller absorption Vibration isolation Thermal switch

  36. Thank you for your attention !

  37. 2. Noise vs Temperature Motivation : Why the mirrors and suspension are cooled ? (1)Small thermal noise (2)Small thermal lens (3)Less serious parametric instability (4)Small cosmic ray effect Kenji Numata and Kazuhiro Yamamoto, ”Chapter 8. Cryogenics”, in ”Optical Coatings and Thermal Noise in Precision Measurement” Cambridge University Press (2012). T. Tomaru et al., Classical and Quantum Gravity 19 (2002) 2045. K. Yamamoto et al., Journal of Physics: Conference Series 122 (2008) 012015. K. Yamamoto et al., Physical Review D 78 (2008) 022004. 37 37 37 37

  38. 2. Noise vs Temperature Motivation : Why the mirrors and suspension are cooled ? (2)Small thermal lens Below 20 K : Several orders of magnitude smaller (Higher thermal conductivity and smaller temperature coefficient of refractive index) T. Tomaru et al., Classical and Quantum Gravity 19 (2002) 2045. 38 38 38 38

  39. 2. Noise vs Temperature Motivation : Why the mirrors and suspension are cooled ? (2)Small thermal lens 120K : 10times smaller Thermal conductivity is 10times larger. Temperature coefficient of refractive index is almost independent of temperature between 120K and 300K. But it does not matter ? J. Komma et al., Applied Physics Letters 101 (2012) 041905. J. Degallaix,”Cryogenic interfetometers”, in ”Advanced Gravitational Wave Detectors” Cambridge University Press (2012). 39 39 39 39

  40. 2. Noise vs Temperature Motivation : Why the mirrors and suspension are cooled ? (3)Less serious parametric instability If larger beam is adopted to reduce thermal noise, number of instability modes increases. K. Yamamoto et al., Journal of Physics: Conference Series 122 (2008) 012015. 40 40 40 40

  41. 2. Noise vs Temperature Motivation : Why the mirrors and suspension are cooled ? (4)Small cosmic ray effect K. Yamamoto et al., Physical Review D 78 (2008) 022004. Below 20K, this decay time is much shorter (than that above 120K). 41 41 41 41

  42. 2. Noise vs Temperature Motivation : Why the mirrors and suspension are cooled ? (4)Small cosmic ray effect K. Yamamoto et al., Physical Review D 78 (2008) 022004. Around 120K (silicon), this amplitude is smaller. 42 42 42 42

  43. 2. Operative temperature In principle, lower temperature is better. > 50K Constant <20K Enough small Sapphire 43 43 43 43 43 43

  44. 2. Operative temperature In principle, lower temperature is better. But there is an exception. Silicon 120K Thermoelastic noise vanishes. Temperature control is necessary. Silicon 44 44 44 44 44 44

  45. 2. Operative temperature 120K operation (LIGO Voyager) Heat absorbed in mirror : Several W Heat extraction : Radiation, 13 W/m2 at most Black coating on mirror is necessary. It should have small mechanical dissipation and so on. Fibers has lower conductivity (1000W/m/K). Diameter must be a few times larger at least (a few mm). 45 45 45 45 45 45

  46. 3. Suspension T. Sekiguchi, K. Somiya, K, Yamamoto 46 46 46 46

  47. 3. Cooling time 3-1 Initial cooling of radiation shield KAGRA : Total mass of inner radiation shield is about 700 kg. C. Tokoku et al., CEC/ICMC2013, 2EPoE1-03, Anchorage, USA (2013). Typical specific heat : 1000 J/Kg/K at 300K Dulong-Petit law When power of heat extraction is 100W, the cooling time is 14 days. 47 47 47 47 47 47

  48. 3. Cooling time 3-1 Initial cooling of radiation shield Heat extraction power Pulse tube cryocooler : 100 W (above 60K) in KAGRA (two cryocoolers). C. Tokoku et al., CEC/ICMC2013, 2EPoE1-03, Anchorage, USA (2013). Experiment showed that it takes 15 days to cool the radiation shield. Y. Sakakibara et al., Classical and Quantum Gravity 31(2014)224003. 48 48 48 48 48 48

  49. 3. Cooling time 3-3 How to reduce cooling time (1)Short cooling of radiation shield Cooling bath (liquid nitrogen and helium): Latent heat of liquid nitrogen is 200J/g. 700kg (900l)liquid nitrogen is necessary for KAGRA. Complicate circulation system is also necessary and it could be path of vibration. 49 49 49 49 49 49

  50. 3. Challenges for cooling • How to keep temperature with heat load ? • If we use liquid helium …, 48g liquid helium evaporated (1W heat load) ! 50 50 50 50 50 50

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