111 Kazuhiro Yamamoto Institute for Cosmic Ray Research, the University of Tokyo Cryogenic interferometer technologies 19 May 2014 Gravitational Wave Advanced Detector Alyeska Resort, Girdwood, Alaska, U.S.A.

2 0. Abstract Cryogenic technologies (1)How much target of mirror temperature in interferometer operation ? What we must take care of ? (2)Initial cooling time is an issue. How can we reduce ? (3)What are large open issues for cryogenic suspension ?

3 Contents 1.Introduction 2.Operative temperature 3.Initial cooling time 4.Status of the art and needed developments 5.Summary

44444 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 1.Introduction T. Tomaru et al., Classical and Quantum Gravity 19 (2002) K. Yamamoto et al., Journal of Physics: Conference Series 122 (2008) K. Yamamoto et al., Physical Review D 78 (2008) Kenji Numata and Kazuhiro Yamamoto, ”Chapter 8. Cryogenics”, in ”Optical Coatings and Thermal Noise in Precision Measurement” Cambridge University Press (2012).

55555 Room temperature second generation interferometer Fused silica mirror suspended by fused silica fibers 1.Introduction

66666 Room temperature second generation interferometer Fused silica mirror suspended by fused silica fibers 1.Introduction 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

Operative temperature 7 In principle, lower temperature is better. Sapphire > 50K Constant <20K Enough small

Operative temperature 8 In principle, lower temperature is better. But there is an exception. Silicon 120K Thermoelastic noise vanishes. Temperature control is necessary. Silicon

Operative temperature 9 In principle, lower temperature is better. But there is an exception. Silicon 120K Thermoelastic noise vanishes. Coating noise must be small ! Silicon

10 2. Operative temperature 10 20K operation (KAGRA) Heat absorbed in mirror : about 1 W Heat extraction : Conduction in fibers Sapphire or silicon has extremely high conductivity (5000 W/m/K, size limit). Thicker fiber for heat conduction (1.6 mm in diameter). Lower violin mode (about 200 Hz).

11 2. Operative temperature K operation (LIGO Voyager) Heat absorbed in mirror : Several W Heat extraction : Radiation, 13 W/m 2 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).

12 2. Operative temperature 12 At any operation temperature … Heat absorbed in mirror should be checked carefully. In actual, safety margin is not so large ! J. Degallaix’s talk: Optical Absorption on substrates

13 3. Cooling time 13 Initial cooling time is always serious issue in cryogenic experiment. (1)Initial cooling of radiation shield Otherwise, mirror can not be cooled ! (2)Initial cooling of mirror and its suspension (cryogenic payload) It is isolated thermally ! (3) How to reduce cooling time

14 3. Cooling time 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 100 W, the cooling time is 14 days.

15 3. Cooling time 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)

16 3. Cooling time Initial cooling of radiation shield Experiment showed that it takes 15 days to cool the radiation shield. Y. Sakakibara et al., Classical and Quantum Gravity 31(2014)

17 3. Cooling time Initial cooling of cryogenic payload Payload is isolated thermally. KAGRA : Total mass of payload is 200 kg. Typical specific heat : 1000 J/Kg/K at 300K Dulong-Petit law When power of heat extraction is 100W, the cooling time is 4 days. Power for initial cooling must be larger than that to keep operation temperature (mirror heat absorption is on the order of 1 W).

18 3. Cooling time Initial cooling of cryogenic payload Cooling payload above 100 K Black body radiation KAGRA : Total surface area of payload is 1 m 2. Radiation power is 460W at most (black coating is necessary). Thus, temperature of payload is comparable with that of radiation shield while the payload temperature above 100K. Radiation does not work below 100K.

19 3. Cooling time Initial cooling of cryogenic payload KAGRA : Temperature of payload is comparable with that of radiation shield. Y. Sakakibara et al., Classical and Quantum Gravity 31(2014)

20 3. Cooling time Initial cooling of cryogenic payload Cooling payload below 100 K Heat links (Al): Thermal conductivity is 10000W/m/K. KAGRA : Heat link design is based on the assumption that mirror heat absorption is 1 W. Specific heat is smaller at lower temperature. Debye model (Specific heat is proportional to cubic of temperature.) Typical specific heat : 300 J/Kg/K at 100K KAGRA : Total mass of suspension is 200 kg.

21 3. Cooling time Initial cooling of cryogenic payload Cooling payload below 100 K When power of heat extraction is 1W, the cooling time is 15 days. Y. Sakakibara et al., CEC/ICMC2013, 2EOrD4-03, Anchorage, USA (2013).

22 3. Cooling time How to reduce cooling time (1)Short cooling of radiation shield Payload temperature can follow shield temperature above 100 K owing to radiation. (2)Short cooling of payload below 100K It is not necessary in the case of 120K. Y. Sakakibara, Ph.D. thesis (2015).

23 3. Cooling time How to reduce cooling time (1)Short cooling of radiation shield Much more cryocoolers ? (KAGRA : 2 cryocoolers for inner shield 2 cryocoolers for payload) Geometrical constrain, vibration More powerful cryocoolers ? Development, vibration

24 3. Cooling time 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.

25 3. Cooling time How to reduce cooling time (2)Short cooling of payload below 100K Thicker heat link should be an issue because of external vibration transmission. Extra vibration isolation system for heat links is necessary. D. Chen et al., Classical and Quantum Gravity 31(2014) Extra pendulum for vibration isolation

26 3. Cooling time How to reduce cooling time (2)Short cooling of payload below 100K Heat path with thermal switch Gas : Radiation shield should keep gas. All holes for laser beam and bar from room temperature part are closed. During cooling, main laser beam can not observe drift of mirror. Super insulator absorbs helium gas. It takes longer time to evacuate helium gas.

27 3. Cooling time How to reduce cooling time (2)Short cooling of payload below 100K Heat path with thermal switch Mechanical thermal switch between payload and crycoolers Large heat contact during initial cooling (force) Large thermal isolation after initial cooling (soft metal for contact disturbs detachment) Thermal switch on suspended object Direction and position of mirror should be changed after this switch is turned off. Mechanism must work at cryogenic temperature.

28 3. Cooling time How to reduce cooling time (2)Short cooling of payload below 100K Other method ? Optical cooling (so far, above 100K) Near field radiation Small gap (  m) enhance heat transmission of radiation (photon tunneling). Does it work below 100 K ?

29 4. Status of the art and needed developments 29 Open issues for cryogenic payload (1)Sapphire (or silicon) suspension (mainly) Mirror and fibers which suspend mirror KAGRA sapphire suspension as example (2)Vibration isolation at cryogenic temperature

30 4. Status of the art and needed developments 30 “Sapphire monolithic lop-eared suspension” One of the most important parts of KAGRA : Main sapphire mirrors are included. All parts are made from sapphire.

31 4. Status of the art and needed developments 31 A. Hagiwara Ear Nail head of fiber Fiber with nail head on both ends Blade spring (compensation of fiber length difference) Mirror Flat cut

32 4. Status of the art and needed developments 32 Hydroxide Catalysis Bonding D. Chen fiber break Bonding between sapphire parts

33 4. Status of the art and needed developments 33 Current status of sapphire suspension Concerns of components are almost fixed (except for blade spring). Measurement of Strength (after thermal cycles) Thermal conductivity Mechanical loss Calculation of thermal noise (ELiTES support: Glasgow, Jena, Rome) D. Chen, 3 rd ELiTES meeting (

34 4. Status of the art and needed developments 34 Next important step : Full size prototype of sapphire suspension Room temperature : fused silica suspension Amorphous : Welding Sapphire (crystal): Welding is not easy. Two large challenges in assembly or trouble shooting. No experience even in AdLIGO and AdVirgo

35 4. Status of the art and needed developments 35 Next important step : Full size prototype of sapphire suspension Two large challenges (a)Assembly of thick (1.6 mm diameter) fibers and blade springs (Part size should be precise. Otherwise, fibers can be broken !). (b)How to apply and remove indium

36 4. Status of the art and needed developments Vertical isolation at cryogenic temperature D. Chen et al., Classical and Quantum Gravity 31(2014) Cryogenic soft spring Extra pendulum for vibration isolation Softer spring have larger thermal drift.

37 5. Summary 1.Mirror temperature in interferometer operation 20K : Thick fibers with high thermal conductivity is necessary. 120K : Temperature control and black coating on mirror is necessary. Largest issues Heat in mirror should be investigated carefully !

38 5. Summary 2. Initial cooling time (1)Short cooling of radiation shield Much more cryocoolers More powerful cyrocoolers Cooling bath, … Vibration is an issue. (2)Short cooling of payload below 100K Thick heat links, Gas, Mechanical thermal switch, Others (optical cooling, near field radiation)… Issues; Vibration, complicate system, …

39 5. Summary 3. Cryogenic payload (1)Sapphire (Silicon) suspension Assembly with thick fiber How to apply and remove indium for bonding ? (2)Vertical isolation at cryogenic temperature Thermal drift is an issue.

40 Thank you for your attention !

41 Note: (1)Kazuhiro Yamamoto’s naive idea and Kentaro Somiya’s hope (they could be changed or wrong) (2)Triggers for discussion about details K. Yamamoto (1)Full size prototype of sapphire suspension (2)Coating mechanical loss (3)Heat absorption in sapphire (4)Other topics for cryogenic payload K. Somiya (5)Development of Output Mode Cleaner 4. Ideas for 2015

42 (4)Other topics for cryogenic payload Vertical isolation at cryogenic temperature Cryogenic experiments for components Measurement of Q-value, thermal resistance, emissivity.. Control and adjustment system (not only cryogenic but also room temperature) 4. Ideas for 2015

43 In principle, KAGRA sensitivity is not limited by thermal noise. However, (1)There is not so large margin for pendulum around 10 Hz. (2)The 1 st violin mode is around 220 Hz. Advanced LIGO : 1 st violin around 500 Hz Thick fiber to transfer heat ! 3. Suspension

44 3. Cooling time 44 Current status (KAGRA case) Radiation Payload with black coating Follows shield. Y. Sakakibara et al., Classical and Quantum Gravity 31(2014) Heat link When thicker ones are adopted, Vibration transmission should be an issue. Cooling radiation is consistent with Calculation.

45 Thick fiber to transfer heat ! Elasticity is not perturbation in KAGRA. (1)Thin wire (only tension) : Violin mode of thicker wire is lower. KAGRA : Violin mode of thicker fiber is higher owing to elasticity (although it is not strong dependence). 3. Suspension

46 Thick fiber to transfer heat ! Elasticity is not perturbation in KAGRA. (2)Frequency of the first violin mode When we do not take elasticity into account, 1 st violin mode is 139 Hz ! Actually, the 1 st violin mode is 220 Hz. 3. Suspension

47 Thick fiber to transfer heat ! Elasticity is not perturbation in KAGRA. (3)Frequencies of higher violin modes Thin wire (only tension) : n-th violin frequency is n times higher than fundamental frequency. Thick wire (only elasticity) : n-th violin frequency is n 2 times higher. In KAGRA, both of tension and elasticity must be considered. 3. Suspension

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

49 Thick fiber to transfer heat ! Elasticity is not perturbation in KAGRA. (3)Frequencies of higher violin modes Thin wire (only tension) : n-th violin frequency is n times higher than fundamental frequency. Thick wire (only elasticity) : n-th violin frequency is n 2 times higher. In KAGRA, both of tension and elasticity must be considered. 3. Suspension

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

51 Thick fiber to transfer heat ! Elasticity is not perturbation in KAGRA. (3)Frequencies of higher violin modes Thin wire (only tension) : n-th violin frequency is n times higher than fundamental frequency. Thick wire (only elasticity) : n-th violin frequency is n 2 times higher. In KAGRA, both of tension and elasticity must be considered. 3. Suspension

52 Higher violin modes ? Thicker fiber : It is not so effective. (2.2 mm diameter, 245 Hz) Shorter fiber : Pendulum mode thermal noise is larger. Mirror diameter is 220 mm. Current design (300mm) fiber is not so enough long … 3. Suspension

53 4. Challenges for cryogenic 1. Issues of cooling : Reduction of heat load (Absorption in mirror) In order to keep mirror temperature … Absorption in mirror : less than 1 W Coating : 0.4 W (1 ppm) Substrate : 0.6 W (50 ppm/cm) Our target of substrate : 20 ppm/cm

54 Sensitivity of KAGRA Thermal noise Assumption (1) : Upper ends of fibers are fixed rigidly. Resonant frequencies (except for violin modes) are different from the actual system. However, the thermal noise above the resonant frequency is the same. Horizontal motion along optical axis Pendulum and violin modes Loss dilution by tension (gravity) must be taken into account. Assumption (2): Number of fiber : 4 Fiber length : 0.3 m Fiber diameter : 0.16 mm Q-values of sapphire fibers : 5*10 6