Cooling the ET payloads Fulvio Ricci

Talk outline Assumptions for cooling the LF Interferometer HF Interferometer the thermal input evaluation and wires for the mirror suspension Payload Material for the reaction mass for the e.m. actuators The thermal links : material geometry mechanical transfer function thermal resistance Cooling strategies LF : cryo-fluid vs. cryo-generator Mechanical vs boiling noise HF cooling or heating?

Assumptions Two independent interferometers Main advantage: commissioning and data taking activity in parallel Two kinds of attenuator chains Superattenuators with different performances Cryogenic solution also for the HF interferometer for Thermal lensing compensation Mirror and coating thermal noise

LF Int. and thermal noise

HF Int. and thermal noise

Payload mechanical Issues Large Masses: reduces the recoils (good for suspension thermal noise ) increases the violin modes (good for control) reduces the vertical modes (not good for control) excess thermal load Wires Length Increment reduces the pendulum frequencies (good for suspension thermal noise ) reduces the violin modes (not good for control) reduces the vertical modes (not good for control) Wires Diameter Increment: increment of the wire sections (good for cooling) reduces the violin mode frequencies (not good for control) reduces the dilution factor (not good for suspension thermal noise )

7 Material Properties

Potenza immessa Conducibilità termica PT Cryomech multistage sample CuBe 3

Old Silicon sample prepared by micropulling

Modes: pendulum 0.28 Hz, 0.36 Hz, 0.50Hz vertical 0.4 Hz (blades), 20 Hz, 26 Hz violins33 Hz, 67 Hz, 100 Hz, 200 Hz, … M1M1 M2M2 M3M3

Modes: pendulum 0.28 Hz, 0.36 Hz, 0.50Hz vertical 0.4 Hz (blades), 23 Hz, 62 Hz violins15.8 Hz, 31.6 Hz, 63.2 Hz, Hz, … M1M1 M2M2 M3M3

13

Marionette Epoglass G11 arms Body in amagnetic Steel (AISI316L) Tungsten ( or CuW) insert Epoglass arms G11 (suitable for cryo applications) Copper plate to clamp the suspension wires and the thermal links Tungsten Mass for balancing the marionette by an electric motor

Recoil Mass To act as cryo trap (T RR < T mirror ) To protect the mirror from shocks, from pollution and wire breaks ; To support the coils for mirror actuation Center of mass coincident with the mirror one; Suspension plane passing through the center of mass; 4 back coils, 1 lateral coil; Lateral (one side) holes for mirror position monitoring; Materilas: SS + Dielectric material for the coils (epoglass); Design main characteristics Function

Material for the recoil mass: HF Int. days Pressure [mbar] The evolution of vacuum into the VIRGO tower for old (purple) and new (black) payloads Old payload Al reaction mass New payload TekaPeek, a high vacuum compatible plastic Alternative suggestion: Vespel ® Polyimide, an ultra-high vacuum compatible, easily machined,and an excellent insulator from DuPont. Unfortunately Vespel outgassing ~5 times higher than that of peek

Approximate outgassing rates to use for choosing vacuum materials or calculating gas loads (All rates are for 1 hour of pumping) Vacuum Material Stainless Steel Aluminum Mild Steel BrassHigh Density Ceramic Pyrex Vacuum Material Viton (Unbaked) Viton (Baked) Outgassing Rate(torr liter/sec/cm2)6x10-9 7x10-9 5x10-6 4x10-6 3x10-9 8x10-9Outgassing Rate(torr liter/sec/linear cm)8 x x 10-8

18 Outside : Steel AISI316L Inside: High density cermics tungsten carbide (WC) ceramics with a density of 15.5 g/cm 3 Safety stops The length of the RM can be changed according to mirror dimensions Design for HF will integrate the TCS components Recoil Mass High-density ceramics and their manufacture Yamase O.: Fuel and Energy, 38 (issue 4), July 1997, pp (1)

19 Same kind of Coil - Magnet System used in Virgo: Nb-Ti wires embedded in a copper matrix Coil and Magnet Size can be changed according to constraints given by locking. Electrostatic actuators easily adapted Piero Rapagnani 3/11/2008 Electromagnetic actuators

20 Virgo o F7 legs and coils: 84 kg o Marionette (AISI316L): 100 kg o Reaction Mass(Al6063): 60 kg o Mirror (Suprasil): 21 kg o Overall payload weight: 181 kg ET o Marionette (AISI316L+Tungsten+epoglass): 400 kg o Reaction Mass (AISI316L+Peek): 140 kg o Mirror (Suprasil for HF, Silicon for LF ): 110 kg o Overall payload weight: 650 kg Comparing the Payloads

21 Design of the cooling system Tanks to R. Passaquieti 3 The upper part is thermally insulated by thermal screens Cryo-Compatible Superattenuator design

Thermal Links I Geometries A corona of thin beamsLong Braids

Thermal Links II: mechanical transfer function measurements at low temperature Evidence of a negligible influence of braids in the case of the torsion degrees of freedom

Pure Materials as aluminum and copper RRR = room temperature / o where o resid. resist. at T~0 K Thermal Links III

Solution for the stationary state Use of a high purity material k~2000 W/m/K in the range 1-10 K Thermal link length 20 m Thermal difference at the link ends ~ 1K HF Int. ~ 10 W: ~60 wires r~1 mm LF Int. ~200 mW ~8 wires r~1 mm Thermal Links IV

Epoglass LVDT at low temp Supercondcting wires Solutions from the previous ILIAS experience Elastic support Piezo actuators

27 Reducing the vibration Cooling mirrors reduces all those noises temperature dependent. Cooling mirrors reduces all those noises temperature dependent. Vibration noise of the refrigetation system (~ mm/(Hz) 1/2 ) kept under control. Vibration noise of the refrigetation system (~ mm/(Hz) 1/2 ) kept under control. Improved attenuation is possible by controlling other degrees of freedom and adding a Pt which o of phase Improved attenuation is possible by controlling other degrees of freedom and adding a Pt which o of phase The upper part is thermally insulated by thermal screens Cryo-Compatible Mirror suspension design

Evaluation for the thermal inputs (Order of magnitude ) Payload chamber: φ 1.5 m h~3 m -4 K shield (25 layers s.u.) ~ 0.4 W - 77 K shield (75 layers s.u.)~35 W Auxiliary tower: φ 1 m h~2 m -4 K shield (25 layers s.u.) ~ 0.3 W -77 K shield(75 layers s.u.) ~ 27 W Cryo trap: φ 1.2 m L4K~ 100 m (L77K > L4K) - 4 K shield (25 layers s.u.) ~ 10 W -77 K shield (75 layers s.u.) ~ 1 kW ( relaxing the thermal input requirement from the hot hole we can assume L4K~ 50 m) In the cryotrap case the cryofluid solution seems unavoidable

For each test mass we need 2 towers and 2 cryostats : Assuming a mirror of t~300 mm f~ 450 mm ( 400 is available already but soon we can hope in silicon slabs of 450 mm in diameter ) m~ 110kg The test mass is hosted in an inner cylindrical vacuum chamber f~ 1.5 m h ~ 4 m external cryostat f~ 2 m h ~ 4.5 m Cold element tower which includes filters f~ 1.5 m h ~ 4.5 m Cold box Vac. Tube Mirror 4 K cryo trap ~ 100m ~ 2 m ~ 1.5m Cryotraps for the vacuum tubes and test mass cryostat 300 K

Cryofluid solution : the boiling problem ( not present in the superfluid case) Displacement amplitude and frequency spectrum shape depend on the tank material and geometry: typical pressure fluctuation 20 dBa Pa. For example in the case of the GW resonant antenna Explorer x rms ~ K with an evaporation rate of a liquid Helium ~2 lt/h Example of the noise characteristics of a boiling fluid in cylindrical container

Open points for the discussion Do we agree to assume still that HF is a cryo detector? -If yes, the operating temperature is defined mainly by the optimization of the heat extraction from the mirror ( max thermal conductivity) -If not, we have to review the thermal noise contribution on the ET-HF sensitivity curve The cooling time - We need to reduce it ( up to 1 week per mirror ) - use of the He gas exchange, a complex solution in a real GW interferometer - Use a telescopic system to transmit the refr. power via solid