1. Superattenuator for LF and HF interferometers
F. Frasconi – INFN Pisa
(WP2 – 3rd ET General Workshop)
- Budapest – November 24, 2010

2. 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]
November 24, 2010
F. Frasconi - INFN Pisa

3. Detectors Accommodation
The LF Detector should be positioned on TOP of the HF Detector
November 24, 2010
F. Frasconi - INFN Pisa

4. 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).
November 24, 2010
F. Frasconi - INFN Pisa

5. 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 Erice].
November 24, 2010
F. Frasconi - INFN Pisa

6. 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.
November 24, 2010
F. Frasconi - INFN Pisa

7. The measurements with VIRGO ITF
Attenuation factor > 1010
frequency (Hz)
Displacement (m .Hz-1/2)
Top
Mirror
November 24, 2010
F. Frasconi - INFN Pisa

8. 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.
November 24, 2010
F. Frasconi - INFN Pisa

9. Measurements Results Thanks to S. Braccini Red Vert. Exc.
Bleu Hor. Exc.
Thanks to S. Braccini
November 24, 2010
F. Frasconi - INFN Pisa

10. 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
November 24, 2010
F. Frasconi - INFN Pisa

11. 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
November 24, 2010
F. Frasconi - INFN Pisa

12. Simulation results
Move the cross-over frequency (between SA transfer function and ET requirements) changing SA parameters
November 24, 2010
F. Frasconi - INFN Pisa

13. Simulation results (cont.)
November 24, 2010
F. Frasconi - INFN Pisa

14. 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)]
November 24, 2010
F. Frasconi - INFN Pisa

15. 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.
November 24, 2010
F. Frasconi - INFN Pisa

16. SAFE status: new IP and new Top Stage
G. Balestri, A. Basti, F. Frasconi (INFN Pisa)
November 24, 2010
F. Frasconi - INFN Pisa

17. SAFE Status: PZTs actuators
November 24, 2010
F. Frasconi - INFN Pisa

18. 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 ( 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)
November 24, 2010
F. Frasconi - INFN Pisa

19. 2nd Generation Tilt-sensor
Flex joint with thin wires (300 µm diameter)
November 24, 2010
F. Frasconi - INFN Pisa

20. 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.
November 24, 2010
F. Frasconi - INFN Pisa

21. 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.
November 24, 2010
F. Frasconi - INFN Pisa