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Superattenuator for LF and HF interferometers. F. Frasconi – INFN Pisa (WP2 – 3 rd ET General Workshop) - Budapest – November 24, 2010. The ET Interferometers. Within the WG2 the presence of two detectors in the same tunnel has been considered; A Possible solution for ET is:
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Superattenuator for LF and HF interferometers F. Frasconi – INFN Pisa (WP2 – 3rd ET General Workshop) - Budapest – November 24, 2010
The ET Interferometers • Within the WG2 the presence of two detectors in the same tunnel has been considered; • A Possible solution for ET is: - LF Detector, Laser with low power, cryogenic Last Stage [TOP] - HF Detector, Laser with high power, room temperature Last Stage [BOTTOM] F. Frasconi - INFN Pisa
Detectors Accommodation • The LF Detector should be positioned on TOP of the HF Detector F. Frasconi - INFN Pisa
Why LF Detector on TOP • Two main reasons for having the LF Detector on TOP of the HF Detector: - Low power laser passing through mirror suspension wires (metallic). Safety situation for possible beam misalignment; - High power laser does not deteriorate the cryogenic environment (no relevant heat transfer within cryogenic chamber). F. Frasconi - INFN Pisa
Seismic Isolation System • Starting from the VIRGO Susperattenuator (SA), an evaluation of its performance to use it in the ET Project has been done; • Special care has been devoted to extend the detection bandwidth in the low frequency region taking into account: - a lower seismic noise level (underground detector – Kamioka reference xs≈ 5*10-9/f2) - different impact on the detector performance of the Newtonian noise [see Cella’s talk @ Erice]. F. Frasconi - INFN Pisa
Seismic Isolation Performance • A measurement campaign has been done to evaluate the passive attenuation performance of the VIRGO SA; • The suspension point has been excited at known frequencies and ITF has been used to measure the motion transmitted to the mirror (ITF dark port); • Noise lines at different frequencies have been injected through the coil magnets actuators. F. Frasconi - INFN Pisa
Displacement (m .Hz-1/2) Top frequency (Hz) Mirror The measurements with VIRGO ITF Attenuation factor > 1010 F. Frasconi - INFN Pisa
The VIRGO Suspension System • The Superattenuator (SA) is the mechanical system adopted to isolate the optical components from seismic activities (local disturbances). It is based on the working principle of a multistage pendulum. • Hybrid system: active control below 4 Hz and passive attenuation starting from 4 Hz. F. Frasconi - INFN Pisa
Measurements Results Red Vert. Exc. Bleu Hor. Exc. Thanks to S. Braccini F. Frasconi - INFN Pisa
Main features of SA • Above 3 Hz the present VIRGO SA is compliant with the ET requirements; • Minor changes are needed to extend the detection bandwidth in the low frequency region (below 3Hz); • The VIRGO SA is a very good candidate to be installed in the LF Detector: - well tested behavior over 10 years activity - well tested technology - vacuum compatibility of crucial elements (under vacuum for many years) - many mechanical and electro-mechanical elements compatible with cryogenic environment F. Frasconi - INFN Pisa
SA optimization for ET • A SA model for simulation purpose has been developed optimizing some parameters to extend the detection band in the low frequency region in accordance with ET requirements; • A comparison between the present VIRGO SA and an ideal SA for LF Detector has been performed changing: - weight of each stage (filter) - number of stages along the chain - distance between two consecutive stages - cut-off frequency in vertical direction (filter tuning) - total length of the chain F. Frasconi - INFN Pisa
Simulation results • Move the cross-over frequency (between SA transfer function and ET requirements) changing SA parameters F. Frasconi - INFN Pisa
Simulation results (cont.) F. Frasconi - INFN Pisa
LF Detector: seismic noise filtering • The LF Detector based on the VIRGO SA for seismic noise filtering can be developed. Minor changes are needed to extend the detection bandwidth in the low frequency region; • A SA 17 m tall with 6 stages (filters) in “equal-spaced” configuration and filter tuning with a vertical cut-off around 300 mHz is proposed; • With this set-up a suspension system with a conservative cross-over frequency around 1.8 Hz (compliant with ET requirements) can be developed. [Details on the simulation results are available in S. Braccini’s talk (previous meeting) and on ET Design Study (in progress)] F. Frasconi - INFN Pisa
SA upgrades: the SAFE Program • The SAFE Project is based on the upgrades devoted to improve the feed-back control performance of the SA; • A long VIRGO SA (with minor differences) to be used as a test bench has been assembled (filters not yet installed) on VIRGO site with: - more rigid pre-isolator stage (new IP with monolithic legs) - new Top Stage platform (no spurious resonances) - 3 PZT actuators (under the IP feet) for tilt control and vertical ID improvements. F. Frasconi - INFN Pisa
SAFE status: new IP and new Top Stage G. Balestri, A. Basti, F. Frasconi (INFN Pisa) F. Frasconi - INFN Pisa
SAFE Status: PZTs actuators F. Frasconi - INFN Pisa
The PIEZO actuators and Tilt-sensors • The IP has been designed to be controllable in all six degree of freedom (including the tilt control): the PIEZO actuator acts against the elastic element mounted on each IP foot; • The tilt control problem demands a very high sensitive tilt-sensor (10-8 rad/√Hz @ 50 mHz). So far the force balance sensor developed to this purpose did not reach the requirement (fragile flex joint); • A second generation tilt-sensor based on the same working principle but using a flex joint with thin wires has been built. A very preliminary characterization is available (A. Giazotto) F. Frasconi - INFN Pisa
2nd Generation Tilt-sensor Flex joint with thin wires (300 µm diameter) F. Frasconi - INFN Pisa
HF Detector for ET • The VIRGO SA could be used as seismic isolation system of the HF Detector; • The attenuation performance is compliant with the ET requirements; • The cross-over frequency (around 4 Hz) is good for this purpose; • The technology used (magnetic anti-spring filters) for its construction is well tested ; • All upgrades of the system are available also for this application. F. Frasconi - INFN Pisa
Final Considerations • LF Detector on TOP of the HF Detector seems to be the best solution for ET; • Above 3 Hz the VIRGO SA for seismic noise filtering is compliant with the ET requirements; • Below 3 Hz the detection bandwidth can be extended starting from the VIRGO SA and changing a few parameters; • A seismic isolation system 17 m tall with 6 filters (in “equal-spaced” configuration) tuned with vertical cut-off around 300 mHz, is proposed for the LF Detector. It will have the cross-over with ET requirements at 1. 8 Hz; • The HF Detector can be developed on the basis of the present VIRGO SA profiting by the upgrades in progress. F. Frasconi - INFN Pisa