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Cryogenic plans for ET-pathfinder

Cryogenic plans for ET-pathfinder. H.J. Bulten , for the ET-pathfinder collaboration and in collaboration with the EMS-group, TU Twente. GWADW 2019, Elba (May 22). ET pathfinder. ETpathfinder facility (Limburg) to study cryogenic optics and operations, under design. 37[m].

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Cryogenic plans for ET-pathfinder

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  1. Cryogenic plans for ET-pathfinder H.J. Bulten, for the ET-pathfinder collaboration and in collaboration with the EMS-group, TU Twente GWADW 2019, Elba (May 22)

  2. ET pathfinder ETpathfinder facility (Limburg) to study cryogenic optics and operations, under design 37[m] Cleanroom 8.2[m] high Towers max 6.5[m] high Class:10 000 ? Beam splitter tower Suspended bench End Mirror tower Mirror tower 25[m] 2.6 m Mirror tower End Mirror tower crane with double hook M. Doets Currently design is ongoing. ~15 m long sections can contain 2 FB-cavities with small optics or 1 cavity with large (ET-size) optics. Roof cleanroom is 8m high. At the start, we will employ cooling in 1 arm. • Cryogenic option for ET: silicon mirrors at 1.550 micrometer • Lots to be designed/developed (coatings, monolithic suspensions, mirrors, coolers) • We want help develop techniques for ET and test cryogenic operation in an interferometer • Test facility in Limburg, under design • Si mirrors (later monolithic suspension) • We want to be able to measure at room temperature, 123 K, ~ 10 K • We aim for ASD(x) < 10-18 m/sqrt(Hz) from 10 Hz upwards.

  3. ET (pathfinder) cryogenic challenges • Cool end mirrors in the arms to about 10 K • Without introducing vibrations (< 10-18 m/sqrt(Hz)) • Without freezing water on the monolithic suspension and mirror surface • With thermal shields with openings for laser beam and optical levers • Without use of MLI superinsulation in the primary vacuum (CH partial pressure should be minimal, pumping/baking should be possible) • Maintain good vacuum in arm and around the end mirrors • Be able to operate at room temperature and at cryogenic temperature (displacement control) • Control temperature-dependent changes in optical path length – control mirror temperature. Beam splitter tower, warm Injection tower, warm Arms may possibly be cooled to 80K with liquid N. Not needed with small optics. End mirrors. Cryogenic (10K)

  4. End mirror towers Start configuration: 2 FP cavities in 1 arm. Vibration-free cooler needed for mirror and suspension Primary vacuum: <10-7 mbar required (residual gas noise). No MLI allowed. Thermal shields Double-walled with holes for pumping and viewports. Should not vibrate too much (NN, scattered light) Mass cryogenic payload ~ 70 kg (Etpathfinder), ~1000 kg (ET). Mass inner cryogenic shield ~200 kg Mass liquid Nitrogen shields ~ 300 kg. Area shields ~ 5-10 m2 so incident radiation is at kW level. Liquid N cooling required to reduce load on inner shield. Liquid N cooling (~200W). Inside primary vacuum. Here: vessel connected via 192 Litze wires (60mm long, 8mm diameter; DT = 12 K

  5. ET cooling (conceptual design 2011) Vibration-free cooling links needed! Vibration isolation of thermal link to marionette. Heat link, complicated 50-75 MLI layers required! End mirrors need to be cooled to 10 K (mirror coating thermal noise peak at 18 K) Separate cooling for shields/beam pipe and marionette Marionette cold link is suspended. Mirror cooled to 10 K via conduction (monolithic silicon wire suspension). Mirror heat input load ~ <100 mW (thermal radiation and laser power) Thermal load cold shield (4K) ~1 W Intermediate shield ~ 50W Cold shield (blue) needs to be vacuum-tight and need conductance to pump before cooling. Initial cooling via contact gas.

  6. Pulse-tube cooler (Kagra) Gas injection at JT restriction of a pulse-tube cooler lead to vibrations Kagra, CQG 31, 224001 (Chen et al.) Projected vibrations due to pulsetube cooler KAGRA (red: off, black: on, blue: ground) Pulse-tube cryocooler: typically pressure spikes (few bar ripple on high pressure before JT restriction, several N force, repetition rate ~1.5Hz) Liquid He: complicated and vibrations from boiling From Ushiba(KAGRA), GW workshop Taiwan(2015)

  7. Preferred option: Sorption cooling Collaboration with EMS-Twente (group ter Brake) to develop a sorption-cooling strategy for ET compressor Technique developed in Twente and demonstrated in the METIS instrument of the ELT telescope. Wu et al., Physics Procedia 67 ( 2015 ) 411 – 416, Cryogenics 84 (2017) Tzabar & ter Brake, Adsorption 24 (2018) p325-332 Presented by ter Brake, ET-meeting Hannover, Oct 23, 2013 Compressor: (reversible) physisorption of gas on Carbon. Pumped by heating. No moving parts, no degradations. Sorption cooling: pressure ripple several orders of magnitude below that of a pulse-tube cooler. Accelerations due to gas in feedline are at least 10,000 times smaller than in commercial pulse tube cooler.

  8. Sorption coolers cold tip temperature stabilized by PID controller and heater Technique, demonstrated in satellite missions (DARWIN and ATHENA) and ground-based( METIS/ELT) METIS instrument: 3 heat inputs: LM-band 40K, 1.4W, N-band detector 25K, 1.1W, N-band imager 8K, 0.4W. 4 sorption compressors used (1 Neon, 2 hydrogen, 1 He) for optimal performance. Heat exchangers at different temperatures used. The He sorption cooler efficiency is strongly dependend on the sink temperature: for ET-pathfinder we consider using a pulse-tube cooler to precool the gas for the He sorption cooler. DARWIN cooler: extreme temperature stability required Ter Brake, TU Twente

  9. Sorption coolers Vibration measurements for the DARWIN cooler, slide from ter Brake, ET-meeting Hannover, Oct. 23, 2013 Exerted forces by the gas are negligible.

  10. Cryogenic shields around mirror (Preliminary) design cryogenic shields (M. Doets) 300-K shield, stabilize filter temperature Floating reflective shields to reduce radiation load on first shield. Outer (80 K) shields (liquid nitrogen) Inner thermal shield, < 40 K. Pipes with baffles to limit view factor of mirror- tower Jellyfish wires (ultra-pure aluminum) to cool marionette and limit vibrations Vibration-free cold finger for cooling mirror (JT restriction sorption cooler) Shields should reduce thermal radiation, but have holes to be pumped out. Holes through all 6 shields for optical levers and for the laser beam (shielded with pipes). Scattered light and thermal radiation must be absorbed somewhere.

  11. Shield modeling, vacuum Raytracing code to calculate vacuum performance, scattered-light absorption, and temperature gradients Water monolayer, baking required. • For the design of the sorption cooler we need to be able to calculate the conductive and radiative heat loads in the system. • Vacuum conductance and scattered-light contribution of shields is modeled too. • Water sticking time to SS/Al halves for every ~5.5 deg. C heating. • In order to avoid freezing water on the mirror/monolithic suspension, we need baking OR cooling down the liquid Nitrogen shield while heating the mirror and inner thermal shield.

  12. Shield modeling, thermal equilibrium 116 K (inside) Shields all low-emissivity (0.1) except for baffles. Assumed conical (45 deg) baffles with 15 mm diameter at top opening. Detailed studies (diffuse/specular scattering, size holes, baffles, etc) starting now. 300 K 117 K, 12.6W 269K (inner) 120 K, 21+14 W 292K 285 K 132 K, 7 W 30.6K, 0.24W 30.6K, 1.0W 30.8K, 0.2W Coldhead 10K, 50 mW 13K, 10mW 85 K, 180W 30K, 3.1 W

  13. Liquid Nitrogen cooling Inlet bellow D10[mm] Liquid Nitrogen cooling Outlet bellows D65[mm] Shields Total volume 40 liter SS and Kapton flexures to accommodate shrinkage when cooling Slotted inlet pipes to avoid bubbles in inlet. Wide vessel to reduce vibrations from boiling. Vessel decoupled from 80-K shield with 192 litze braids (8mm diameter, 60mm length – may be replaced). Outer thermal shield supports the inner thermal shield. Flexible joints to accommodate shrinkage when cooling down.

  14. Link between cryogenic coldhead and mirror suspension We intend to produce a jellyfish connection and measure the transfer function. Indium foils To coldhead. Slot for suspension wire Purple wires to marionette, blue to reaction chain Jellyfish connections: introduce minimal vibrations Ultrapure Al : up to 15000 W/m/K at 10 K (bulk) Monolithic silicon suspension: flexible, >2 kW/m/K

  15. Summary Design studies for ET-pathfinder are in progress • Baseline design for mechanics, vacuum, cooling is well underway • We aim for liquid-N cooling to reduce thermal radiation and to define a point where the water vapor might be frozen – Else we need to bake for a week after every vacuum breach. • We aim for sorption cooling for the cryogenic part • Well-tested technique, both in space and ground-based astrophysical detectors • Coolants and design sorption cooler depend on requirements, which depend on the thermal shields (study ongoing) • We will fix the thermal shields to the ground. Baffles on the reaction chain of the mirror should define the beam profile (shields should have zero scattered light contribution) • We intend to both build 1 sorption cooler and test vibrations of the cold mass and build a jellyfish transition to measure mechanical transfer of vibrations; this depends on available time and extra funding.

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