First clues on ET payload sensing and control

1. First clues on ET payload sensing and control INFN – Roma1, INFN – Roma2, INFN – Napoli (R. De Rosa)

2. Overview • Payload local sensing • Local sensing in VIRGO • Options for Local sensing in ET • Payload Actuation Options in ET • Actuation on the test mass • Actuation on the “marionetta”

3. Payload Local SensingGeneral Idea • The local sensing system is used to measure the payload position respect to a local reference system • It can be used for different tasks: • Reduce the mirror motion amplitude to easily engage the ITF locking; • Recover the mirror angular position to the last “aligned” condition after an unlock; • Since such sensing is performed respect to the ground, the sensitivity is intrinsically limited by the seismic noise.

4. Payload Local SensingVirgo Scheme • Local control system acts within the suspension but its readout is ground-based. • Several read-out components are engaged hierarchically • Components for each suspension: • CCD camera (Mirror level); • 2 optical levers (Mirror and Marionetta levels) • The system’s target is to steer and drive the ITF mirrors (BW~2Hz) to the interference by slowing their speed within the dynamics of global controls: v ~ 0.5 m/s and by aligning them within the range of the linear alignment: ~ 1 rad

5. Payload Local SensingVirgo Scheme • Optical uncoupling of the levers among translation and tilt around 1%; • Range and resolution of the optical levers: • 10 mm => 0.1 m rms; • 50 mrad => 50 nrad rms;

6. Payload Local SensingVirgo Scheme Virgo alignment is engaged “almost fully” by using local controls => over-performance of the Local Controls

7. Payload Local SensingConstraints for ET • There are some constraints for the implementation in ET of a similar optical read-out: • Cryogenic payload; • Underground operation; • Limit as much as possible the number of optical windows to enhance the effectiveness of the cooling system: • Change the optical scheme; • Seismic ground noise is expected to be about 10 times lower respect to the surface: • In principle this allows to increase the sensitivity of the local sensing system, but the current performances of the VIRGO sensing system are already under the limit;

8. Payload Local SensingConstraints for ET – Related R&D • Cryo-compatible mechanics, sensing and actuation in the superattenuator design • Cooling system: cold finger on the last stage • Pulse-tube technology: use of active vibration reduction • Thermal shields and extension of mirror suspension cryostat along the beam axis Tanks to R. Passaquieti ET: very preliminary hints (based on present R&D)

9. Payload Local SensingConstraints for ET – Related R&D A completely new payload “cryo-compliant” (basic elements and technologies under test in Cascina)

10. Payload Local SensingConstraints for ET – Related R&D Pulse-tube facility with active vibration attenuation (44 dB) Compact vertical accelerometer using CuW25/75 mass (under construction) LVDTs recently tested at Low Temperature (Coll. Roma-Pisa) Commercial accelerometers are not suitable for LT. Development (within VFC project) of cryogenic accelerometer derived from Virgo.

11. Payload Local SensingOptions for ET Vacuum feedthroughs with standard ACP connectors Vacuum compatible fibers and collimators • Common characteristics • Use of hermetic feedthroughs for injecting the probe beams; • Use of vacuum compatible optical fibers and collimators inside the vacuum chamber; • Some small apertures have to be foreseen on the cold shields around the payload to allow the beam to propagate; • Sensing both the test mass and the marionetta; • Use of PSD or quadrants for sensing; • No component inside the cold shields;

12. Payload Local SensingOptions for ET -1 Same performances as in Virgo Cold Shield • The most simple option is to put inside the vacuum chamber the detection components: • Uncoupling lenses • PSD • Advantages: • Reduced number of beams inside the chamber; • Reduced number of apertures on the cold shields; • Disadvantages: • Space for optical uncoupling equipments inside the vacuum chamber; • Photodiodes need to stay in (almost) room temperature place Vacuum Fiber PSD and uncoupling optics

13. Payload Local SensingOptions for ET - 2 PSD outside the vacuum chamber: 2 angle + 1 longitudinal translation per periscope unit optical diagonalization 1% target performance 50 nrad, 100 nm rms light gathering through a small aperture (~1“) pre-aligned periscope, adjustable as a single block testable using the cryo facility in Cascina (vibration)

14. Payload Local SensingOptions for ET http://www.us.schott.com/fiberoptics/english/ Fast technology improvements Vacuum and inaccessible apparatuses: High resolution 12 or 24 m pixel Over acceptable section size 3 or 6 mm Endoscopy: High resolution 1.5 m pixel Over section size 1.5 mm http://www.fujikura.co.uk/fibre_optics/products/speciality_fibre/image_fibre.htm l

15. Payload Local SensingOptions for ET -3 Same performances as in Virgo Quad Fiber Tapers • No optical uncoupling at all • Use of suitable quadruple-square fiber tapers as “Quadrants” • 2 angle + 1 longitudinal translation in this configuration • Advantages: • Simpler setup inside the vacuum chamber; • Disadvantages: • Double the number of the probe beams; • More aperture in the shields; Vacuum Fiber

16. Payload Local SensingOptions for ET -3 Fiber Tapers Fiber with different section at the large and small end; Several possible design and dimensions; Need to be coupled with 4 different external photodiodes; http://www.us.schott.com/fiberoptics/english/

17. Payload Local SensingOptions for ET Fiber coupled S-LED To avoid any interference phenomenon inside the optical guides, the use of a non coherent source is highly recommended; S-LEDs are a very effective source for this application

18. Payload Local SensingOptions for ET UHV Operation with vacuum compatible fibers - Detectors in vacuum. Fiber coupled S-LED 4 – Masses torsion pendulum (Trento) Performances of one angular DOF S-LED based optical levers, with quadrant photodiode detectors, were used for the ground testing of an Optical Read-Out system for LISA

19. Payload Actuation Option for ETGeneral Idea • The actuation system for the payload is composed, as usual, by: • An upper level actuation system, acting on the marionetta; • A lower level system, acting directly on the mirror; • Starting idea: • Coil magnets pairs for upper control; • Electrostatic actuator for test mass control; • Try to “switch off” completely the lower control to reduce the effect of the control noises in the measurement band;

20. Payload Actuation Option for ETActuation on the Test Mass • Since the goal is to remove, in standard conditions, the control from the test mass, an electrostatic actuator is better suitable: • No need to use magnets on the test mass; • Better immunity from EM noise; • Action completely “off”; • Action only during the locking acquisition; • Poor requirements on the injected control noise; • To better check with some model; • Control from the upper stage requires no recoil mass;

21. Payload Actuation Option for ETActuation on the Test Mass – Option 1 Marionetta Rigid connection Electrostatic actuator Suspended Mirror The actuator can be rigidly connected to the marionetta; The distance respect to the mass can be carefully fixed during the mounting; The average relative distance between the actuator and the mirror does not change during mirror steering from the marionetta;

22. Payload Actuation Option for ETActuation on the Test Mass – Option 2 Marionetta Electrostatic actuator Suspended Mirror (Motorized ?) Actuator holder Ground An alternative choice, to simplify the suspension design, is to fix the actuator directly on the ground; Also in this case, the noise injected on the test mass during the lock acquisition, does not represent a problem; BUT, in this case the change of mirror position become a problem since the EA is sensitive to distance changes;

23. Discussion A payload sensing system based on the current VIRGO design represents a good starting point; The required performances are easy to achieve; Some changes have to be done to allow cryogenic operation of the payload; Preliminary design of possible implementation were presented; The proposed actuation system is based on the GEO design: coil-magnets on the marionetta and electrostatic actuators for the test mass; Lower stage actuation engaged only during the lock acquisition phase; No recoil mass to help control from the upper stage after the lock acquisition; Some work to do to estimate the requirements for the actuation forces (at different levels) and noise contributions;