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Cryogenic requirements an overview

Cryogenic requirements an overview. Gravitational waves. Dynamical structure of space-time. A new window to the universe. Tiny changes O(. Einstein telescope, next generation detector. Gravitational waves can be detected using laser interferometers.

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Cryogenic requirements an overview

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  1. Cryogenic requirementsan overview

  2. Gravitational waves Dynamical structure of space-time. A new window to the universe. Tiny changes O(

  3. Einstein telescope, next generation detector Gravitational waves can be detected using laser interferometers Detector with higher sensitivity and starting at lower frequencies - underground to reduce seismic noise - longer arms (higher sensitivity) and more power in cavities (lower quantum noise) - Cryogenic mirrors (lower thermal noise) - 3 interferometers (measure all GW-polarizations) Arm length 4 km. h ~ 10^-22 Arial view Ligo/Virgo. Sensitivity curve

  4. Einstein telescope noise limits Thermal noise limits the sensitivity in mid-frequency range. Radiation pressure Shot noise Thermal noise dominant 10-200 Hz. Low frequencies: Quantum noise: heavy mirrors, 18 kW beam power, squeezing. Seismic noise: underground. Thermal noise: cryogenic. High-frequencies: shot noise limited. Room temperature, 3 MW beam power in arms.

  5. Cryogenic Mirrors: suspension Mirror surface should be defined to < 10-20 m/sqrt(Hz) Equilibruim The Virgo superattenuator suspension. Pendula for horizontal/angular d.o.f. and geometric anti-spring (GAS) filters for vertical isolation. Horizontal isolation: (inverted) pendula. Vertical isolation: geometric anti-springs. Geometric Anti-spring filter stages need to stay at room temperature! Eliminate external forces on mirror – vacuum better than 10-10 mbar, low acoustic noise in surroundings, jellyfish connections of wires for actuators and cooling.

  6. Einstein telescope mirror suspension Suspension chain of 17 m limits seismic noise above 2 Hz. Thermal noise is critical in last elements of the chain. Last stage suspension: 422 kg Marionetta with 211 kg Silicon mirror (M2) and 211 kg reaction mass (M3). Monolitic suspension (crystalline Si) wires/bondings are required to obtain small loss factors. Transfer function of the proposed super attenuator chain for ET (ET design report 2011), a 17-m long chain. Red shows the performance of the current Virgo SA (9 m chain). Seismic motion is reduced with 9 orders of magnitude above 2 Hz. Thermal noise of the suspension calculated for ET, assuming monolithic suspension (Si with loss factor 10-8, marionetta from TiAlV alloy (10-5), mirror at 10 K and marionette at 2K, 2 m wires 3mm diameter, ET design report)

  7. Thermal noise contributions Noise coatings is dominant • Thermal noise in the cryogenic interferometers limit the performance in several ways: • Bulk mass mirror. Silicon has a very high quality factor exceeding 108 , so the thermal noise generated in the bulk mass of the mirror can be concentrated in very sharp peaks (drum modes of the mirror). For these modes, the noise is 8 orders of magnitude above the background but the line width is also in the mHz-microHz range. • Suspension • Above the frequencies of interest, the suspensions filter seismic motion to below detection limit. The noise of the last stage is the dominant one; this is the noise in the wires between marionetta and reaction mass/mirror. Using monolithic suspensions (pure Si wires), bonded monolithically to the mirror, the quality factor can be chosen such that also this noise is concentrated in sharp lines. Requirements on the suspension of the mirror are very stringent: only material with high quality factor can be used and one should aim for simple structures so that there are not a lot of mechanical resonances that can be excited. For ET the wire diameters and lengths must be chosen such that the resonant frequencies limit the sensitivity as little as possible. • Mirror coating • To obtain high enough reflectivity and small absorption, mirror coatings need to be applied (also with sub-nanometer thickness fluctuations: atomically flat!) Typically these coatings contain Tantalum. The thermal noise in these coatings is the dominant thermal noise source, since a) there will be some laser light absorbed in the coating (< 1ppm), heating it and b) quality factor of the coating can never be as high as that of the bulk material.

  8. ET cooling Vibration-free links needed. Vibration isolation of thermal link to marionetta. End mirrors need to be cooled to 10 K (mirror coating thermal noise peak at 18 K) Separate cooling for shields/beam pipe and marionetta Marionetta cold link suspended. Mirror cooled to 10 K via conduction silicon wires. Mirror heat 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.

  9. ET pathfinder ETpathfinder facility (Limburg) to study cryogenic optics, under design 37[m] Cleanroom 8.2[m] high Towers max 6.5[m] high Class:10 000 ? Beam splitter tower Suspended table Mirror tower Mirror tower 25[m] 2.6 m Mirror tower Mirror tower crane with double hook M. Doets Currently design is ongoing. ~15 m long sections can contain 2 arms with small optics or 1 arm with large optics. Roof cleanroom is 8m high. Optionally we can cool only 1 arm to 10K and the other to 125K/room temperature. • Cryogenic option for ET: silicon mirrors at 1.550 micrometer • Optical absorption properties of Silicon • Development of optical coatings need study • Thermal lensing: depends on thermal properties of the coating. • Cryogenic operation needs to be explored. • Test facility in Limburg, under design

  10. Central tower contains the beam splitter in the center-of-mass. The end towers in the arms can contain either 2 Fabry-perot cavities or a single cavity. The mirrors and Marionetta need to be cooled, viewports to room-temperature beam splitter and detection bench are needed as well as small holes for the wires that support the marionetta and the mirror control signals. The GAS filters need to stay at room temperature. Filter Filter Filter IP Filter Marionetta Marionetta D2.5[m] H6.5[m] Optical Bench Mirror (pitch 620[mm]) Mirror Tower Central Tower M Doets/M Kraan-Nikhef

  11. Cryogenic shields around mirror (Preliminary) design study for the cryogenic cooling of the optics (M. Doets) 300-K shield, stabilize filter temperature Passive reflective shield to reduce radiation load on first shield. 80-K shield (liquid nitrogen? First stage pulse-tube/sorption cooler? Second thermal shield, 10K. 40-mm diameter holes, viewports laserlight (extend with ~ 1m pipe, black on the inside, for radiation absorption) Jellyfish wires (ultra-pure aluminum) to cool marionetta and limit vibrations Volume inner shield ~ 0.2 cubic meters, surface ~2 square meters. Vacuum conductance should be > 100 l/s to be able to pump down in a reasonable time (extra slots needed). Ring to cool Marionetta should not vibrate (<femtorads/sqrt(Hz)). Heat load 10K-part in the order of 1 W (mainly radiation)

  12. Requirements cryogenics Temperature, vacuum, optical viewports • Goal: study cryogenic interferometry: we should be able to measure the performance and reach the ET design limits: • Thermal noise coating • Thermal noise suspensions/bulk mass • There are viewports for the beam at both sides of the mirror: there is a small solid angle with direct 300-K thermal radiation. This should be limited in size: pipes with baffles to absorb scattered radiation/light needed. • Scattered light interferes in the wrong manner: a few photons ending up in the dark fringe already spoil the sensitivity. Should be suppressed at all costs. ET pathfinder: restrictions are further away than in ET, Virgo, Ligo (about 20 beam radii instead of >6) so probably we do better. • The mirror should reach 10K • Peak in thermal noise coatings expected around 18 K! • The mirror surface is extremely clean: we should not freeze stuff on it! • This implies that we have to pump down before cooling to about 10-7 mbar, else the water vapor pressure is still too high and layers of water build up on the shields too soon. • High pumping speed needed; the vacuum conductance of the shield should be OK • We cannot use superinsulation in the vacuum system; multiple layers of DAK foil cannot be pumped down. • We should keep the mirror warm while the shields around it cool down, so that the water first freezes on the shields. This may be complicated; we cannot use a single pulse-tube cooler in which the cold head cools the mirror and the first stage the 80-K shield.

  13. Requirements and open issues cryogenic system Vibrations and thermal conductances • The thermal shields have feed-throughs: e.g. the actuators between mirror and recoil mass; maybe some displacement sensing device, the wire suspending the marionetta. • Small holes needed and thermal leaks introduced • The vibration of the marionetta needs to be limited to (?? femtometer/femtorad level ??) above 5 Hz: the cold head cooling the marionetta itself must probably be suspended and connections made via jellyfish wires (ultrapure aluminum, ~ 0.1 mm radius) • Actual limitiations on vibrations need to be calculated still. • Cold mass: sorption cooling scheme possible? pulse-tube cooling, liquid helium? • Mass of marionetta, mirror and reaction mass will be in the order of 20 kg (for the light optics). To reach a temperature of 10 K of the mirror will be very challenging. • We assume that a coldhead with ~ 1W cooling power at 10 K suffices to cool marionetta/mirror. Thermal conductance of pure silicon wires is excellent (> 1kW/mK at 10 K) so a small thermal drop will occur over the wires. A system that can cool a coldhead to ~5K and has 1W cooling power left at ~10 K typically consume more than 5 kW electric power. • It would be nice if a single system can cool both the inner shield and the marionetta; probably we need 2 per tower. • Comparing with KAGRA: • They reach 20K mirror temperature in about 1 month; they have 4 pulse-tube coolers with 0.9 W at 5K per cold mirror; 2 for the shield and 2 for the marionetta and mirror/reaction mass. • The inner 10-K shield weighs 530 kg. • They use pulsetube coolers also for the 80-K shield for safety reasons; we can use liquid nitrogen instead and have much more cooling power there.

  14. Current design shields Not indicated: Recoil masses to act on ,mirrors, marionette, filter (coil magnet) Mirror safety structure. Top platform LVDT`s and actuators. Optical Levers : 3 Per string at 60deg angle. 2 mirror option ,openings in shields and vacuum vessel. 1mirror option , openings in the shields and vessel are in other places. Top stage 10k shields Support for upper vessel 80k shields floating shields Intermediate filter IP`s standing on the floor Vessel decoupled (bellows) M Doets/ Nikhef

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