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

2. Overview Payload local sensing Payload Actuation Options in ET
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 Sensing General 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 Sensing Virgo 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 Sensing Virgo 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 Sensing Virgo Scheme
Virgo alignment is engaged “almost fully” by using local controls
=> over-performance of the Local Controls

7. Payload Local Sensing Constraints 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 Sensing Constraints for ET – Related R&D
ET: very preliminary hints (based on present 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

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

10. Payload Local Sensing Constraints for ET – Related R&D
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.
Pulse-tube facility with active vibration attenuation (44 dB)
Compact vertical accelerometer using CuW25/75 mass (under construction)

11. Payload Local Sensing Options for ET
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;
Vacuum feedthroughs with standard ACP connectors
Vacuum compatible fibers and collimators

12. Payload Local Sensing Options for ET -1
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
Same performances as in Virgo
Cold Shield
Vacuum Fiber
PSD and uncoupling optics 12

13. Payload Local Sensing Options 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 Sensing Options for ET
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 l

15. Payload Local Sensing Options for ET -3
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;
Same performances as in Virgo
Quad Fiber Tapers
Vacuum Fiber 15

16. Payload Local Sensing Options 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; 16

17. Payload Local Sensing Options for ET
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
Fiber coupled S-LED 17

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

19. Payload Actuation Option for ET General 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; 19

20. Payload Actuation Option for ET Actuation 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; 20

21. Payload Actuation Option for ET Actuation on the Test Mass – Option 1
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;
Marionetta
Rigid connection
Electrostatic actuator
Suspended Mirror 21

22. Payload Actuation Option for ET Actuation on the Test Mass – Option 2
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;
Marionetta
Electrostatic actuator
Suspended Mirror
(Motorized ?) Actuator holder
Ground 22

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; 23