1. Virgo – Injection SYStem
…Today and Tomorrow…
Olivier FRANÇOIS
(European Gravitational Observatory)

2. Current problems/limitations Next upgrades
Introduction
ISYS nowadays…
Current problems/limitations
Next upgrades

3. ISYS today…situation wrt the interferometer

4. ISYS today…the Injection Bench
Mode Cleaner reflection
Toward Mode Cleaner
Mode Cleaner transmission M4 M6 O S M2 N
From the Laser Lab E M1
Dps M3 M5
Toward the interferometer
Beam going through M1 and entering the RC
RC (reference cavity)

5. Current problems/limitations
Power Recycling back scattering and IMC instability
Injection Bench resonances
Mode Cleaner mirror defects
Injection System alignment
Monolithic Power Recycling mirror

6. PR backscattering & IMC instability
PR aligned
PR misaligned
IMC control loop for freq stab is perturbated

7. PR backscattering & IMC instability
Temporary solution:
decrease to 10% the amount of light transmitted to the ITF
(1% of the light is back-reflected by the PR into the MC)
Definitive solution:
Implement a Faraday isolator
Not enough room on the current IB>redesign

8. Finite element model analyis
IB resonances
IB internal modes
Susp. wires
Finite element model analyis

9. IB resonances
Higher frequency modes (dihedron, reference cavity, …) are more difficult to be identify

10. IB resonances
26/06 C4: control from the ground coils
Many attempt to explain how IB resonance noise enters in the interferometer
Local control noise enters through coil drivers noise
Coherence between coil driver output and B2_ACq confirms this hypothesis
Controlling from the marionetta improves only partially the noise
On the it has been understood that the modulation frequency of the MC was about 70 Hz away the MC length matching frequency
07/09 DT: control from the marionetta
Before frequency tuning
After frequency tuning

11. => The MC mirror will be changed
MC mirror defects
The source of a large part of the light scattering we have in the ISYS is due to the MC mirror
That scattering is only partially due to dust contamination
=> The MC mirror will be changed

12. Present Injection System alignment scheme
-- lock servo
MC auto-
alignment
-- alignment servo
lock MC length
on RFC LC
picomotors
piezos MC
RFC auto-
alignment ty tx
Laser M6 LC
Matrix
Matrix
ABP M3 M5 M2
MC lock
frequ. prestab. FF NF NF h h v
Ref. cav. h v FF

13. Injection system alignment
The stability and reproducibility of the IMC alignment is not completely reliable
Present Injection System is automatically aligned on the RC: RC transmission drifts less than the Mode Cleaner one
It would be better align separately IMC and RC

14. Power recycling mirror
PR is a composite mirror
It shows internal modes at low frequency (about 100 and 400Hz)
These frequencies are changing in time
They are making the locking more complex and the final noise higher

15. Next upgrades: IB redesign

16. Next upgrades: IB redesign

17. Next upgrades: IB redesign

18. Next upgrades: new alignment scheme
New automatic alignment system
New hardware is needed
Beam drift
control
-- alignment servo
picomotors
piezos MC
Laser M6 LC
ABP M3 M5
Matrix M2
ABP NF
IMC auto-
alignment
Ref. cav.
Matrix FF NF
Beam pre-
Alignment
(Frederic Cleva) FF
RFC auto-
alignment
Matrix

19. Next upgrades: new PR
In the last VSC it has been decided to make the PR flat and without wedge, like the Input Mirrors:
A substrate is ready. Only coating time needed
It allows thermal regulation of the PR cavity reflectivity
An off-axis parabolic mirrors for the telescope in the IB has to be assembled
IMC: waist 4.9 mm
Condensing telescope
FI: waist 2.65 mm
M5: f= 75 mm
M6: f = 600 mm
Ø  10 cm
to PR: waist=20 mm
d= 675 mm

20. Next upgrades: OAPM telescope
Thermal lensing effect [100m-10m] can be compensated (curved FI exit surface)
Not necessary to implement a FK51 compensator