Piero Rapagnani I.N.F.N. Sezione di Roma

1. Cryogenic payloads and cooling systems (towards a third generation interferometer)part I: An Interferometer at Cryogenic Temperatures Piero Rapagnani I.N.F.N. Sezione di Roma Piero Rapagnani – INFN Roma ILIAS

2. Why cool the mirrors? • Test masses and suspensions thermal noise reduces at low temperature: • Thermoelastic noise both of the mirror substrates and coatings decrease: • Thermal expansion rate a decreases at low temperature; • Mechanical Q of some materials increases at low temperature @ w << wint • Thermal lensing: • Thermal conductivity increases and consequently reduces thermal gradients on the coating; • Refraction index variation with temperature is very small at low temperature; Piero Rapagnani – INFN Roma ILIAS

3. R&D on Cryogenics Liquid helium Refrigerators Hybrid system 1) Study of the refrigeration system - noise - refrigeration power 2) Suspension compatibility: thermal conduction and acoustic quality factor Q measurements 3) Sensors at low temperatures - accelerometers and position sensing devices - actuators Piero Rapagnani – INFN Roma ILIAS

4. Issues to cool the mirrors • Refrigeration system: • The injected mechanical noise must be negligible, the sensitivity must be preserved: • Þ Good mechanical isolation between the mirror and the cooling system; • Cooling time of the mirror as low as possible: • Þ Good thermal couplings; • Þ High refrigeration power; • Suspension system compatible with good mechanical and thermal couplings: • Thermal conductivities change with temperature; • Mechanical quality factor Q; Piero Rapagnani – INFN Roma ILIAS

5. No excess noise Cryogenic fluids and G.W. Detectors The first cryogenic antenna in the world 1974-1980: M=20 kg, T =4 K , n ~ 5 kHz Piero Rapagnani – INFN Roma ILIAS

6. Piero Rapagnani – INFN Roma ILIAS

7. The second cryogenic antenna of the Rome group -1978: M~ 400 kg, T =4 K , n ~ 1.8 kHz Excess noise in the first phase of operation: Due to suspension system!! Piero Rapagnani – INFN Roma ILIAS

8. Advantage of the superfluid liquid Helium: the  transition He phase transition to superfluid Data from the Antenna EXPLORER installed at CERN Piero Rapagnani – INFN Roma ILIAS

9. VIRGO • The current technique to cool down a Resonant Antenna requires “Heavy Work” and several weeks • Detector duty cycle: less than 1 month. • For an interferometric antenna 6 masses to be cooled. • To preserve the duty cycle this “heavy work” must be done in parallel..... Piero Rapagnani – INFN Roma ILIAS

10. In a BIG Laboratory,large Cryogenic Facilities are possible The example of LHC at CERN: The Cryogenic Distribution Line (QRL) for the LHC(Large Hadron Collider). Each of the eight ~3.2 km QRL sectors is feeding Helium at different temperatures and pressures to the local cooling loops of the strings of superconducting magnets operating in superfluid helium below 2 K. With an overall length of 25.8km the QRL has a very critical cost to performance ratio. Technologies are available, but are VERY expensive and require extensive manpower Piero Rapagnani – INFN Roma ILIAS

11. An alternative way to cool down without liquid helium: the new generation of Cryocoolers • A Pulse Tube Refrigerator (PTR) or "G-M style" pulse tube cryocooler, is a variant of a Gifford-McMahon (GM) cryocooler. • PTR operate at low frequencies, typically <5 Hz. • Used a conventional oil-flooded G-M compressor and a valve set near the cold head to convert the continuous flow of helium to a low frequency pressure wave. First stage Second stage • Suitable for applications that require efficient operation: • No moving parts in cold head. Minimal vibration, low acoustic noise, reliability. • High efficiency: 2 to 3 times higher efficiency than GM cryocoolers for loads temperatures between 55 and 120 K. Piero Rapagnani – INFN Roma ILIAS

12. A possible solution • Passive vibrational isolation system for the heat link • Long heat link • Part of the refrigerating power absorbed by the isolators • Attenuation of the refrigerating power Piero Rapagnani – INFN Roma ILIAS

13. Our solution Active vibration isolation system for the heat link • Shorter heat link • Refrigerating power preserved Piero Rapagnani – INFN Roma ILIAS

14. Q from refrigerator Vacuum Chamber and Cryostat Thermal Shields Marionetta Reaction Mass: Thermal Shield at ~ 4K High Efficiency Thermal Links Silicon Monolithic Wire Mirror Reaction Mass: Thermal Shield at ~ 4K Q from laser beam Piero Rapagnani – INFN Roma ILIAS

15. Q from refrigerator Vacuum Chamber and Cryostat Thermal Shields Marionetta Reaction Mass: Thermal Shield at ~ 4K High Efficiency Thermal Links Silicon Monolithic Wire Mirror Reaction Mass: Thermal Shield at ~ 4K Q from laser beam Rough Estimates give Tmirror ~ 10 K Piero Rapagnani – INFN Roma ILIAS

16. Q from Superfluid Helium Reservoir A hybrid system using Superfluid Helium could allow to reach T ~ 1.5 K Vacuum Chamber and Cryostat Thermal Shields Marionetta Reaction Mass: Thermal Shield at ~ 1.5 K High Efficiency Thermal Links Silicon Monolithic Wire Mirror Reaction Mass: Thermal Shield at ~ 1.5 K Q from laser beam Piero Rapagnani – INFN Roma ILIAS

17. Thermal Links: Many Materials and Composites available Thermal behavior at low temperatures must be tested Piero Rapagnani – INFN Roma ILIAS

18. The short/medium term future:The Cryogenic Suspension Test Facility Still non investigated Problems: Cryogenic (T~ 50 K) Suspension Elements Thermal link (T ~ 4 K) Piero Rapagnani – INFN Roma ILIAS