1. Locking improvements after the end of VSR1 Gabriele Vajente for the Locking Group 14 th ILIAS WG1 meeting Cascina – March 6 th 2008

2. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina2 The end of the story… October 1 st 2007

3. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina3 The end of the story… February 5 th 2008

4. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina4 And now the full story… Better diagonalization of driving New sensing strategy Modulation at 8 MHz New filtering algorithms Better speed performances Much more flexible Improved control filters More low frequency gain Less noise re-introduction Improved noise subtraction Alpha, beta, gamma techniques Beam jitter subtraction Reduced actuator noise Locking with lateral coils

5. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina5 Virgo DOFs, sensing and driving DARM = Differential long arm CARM = Common long arm (not frequency) PRCL = Power recycling cavity length MICH = Short Michelson differential ITF reflection = B2 Dark fringe = B1 BS pick-off = B5 DARM, CARM PRCL MICH

6. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina6 Better diagonalization of MICH-PRCL MICH correction is sent both to BS and PR Measured the relative actuation gain in simple configuration PR – BS – WI cavity Gained a better de-coupling of MICH and PRCL loops PR BS WI LOCKING Reference

7. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina7 New sensing strategy Main modulation 6 MHz Additional modulation at 8 MHz, phase-locked Not resonant in PRC or arm cavities B2 beam (ITF reflection) demodulated at 8 MHz Gives good signal for central cavity control Old (VSR1) configurationNew sensing scheme MICH Controlled with B5_ACq + B2_ACp_mix * UGF @ 15 Hz PRCL Controlled with B2_ACp_mix * UGF @ 40 Hz Sidebands Balanced with an offset on B2_3f_ACp and/or B5_ACq MICH Controlled with B2_8MHz_ACp UGF @ 15 Hz PRCL Controlled with B5_ACq UGF @ 80 Hz Sidebands Balanced with an offset on MICH error signal added in Gc

8. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina8 Performances of new sensing 8 MHz demodulated signal is much less noisy than 6 MHz one if used to control MICH Large reduction of control noise at low frequency

9. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina9 New Global Control filtering algorithm Just a “software change” Reduce computation overhead Allow to redefine control filters while locked But a significant improvement in terms of Robustness Flexibility Allowed a much faster development and test of new control filters

10. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina10 New control filters / more LF gain MICHPRCLCARM Largely increased loop gain below 100 mHz No evidence of up-conversion of noise

11. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina11 New DARM filter DARM loop had low phase margin (13 degrees) When cavity poles move (etalon) the DARM gain peaking changes a lot (30- 40 %) This directly affects the dark fringe calibration New filter for more margin: calibration changes by 2% at most

12. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina12 Low noise MICH filter Developed a new filter, with more optimized high frequency roll-off

13. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina13 Alpha, beta, gamma techniques Auxiliary loop noises couples strongly to dark fringe Noise subtraction techniques The MICH, PRCL, CARM corrections are filtered and added to DARM correction, to cancel noise (B. Swinkels)

14. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina14 Performances With alpha (MICH) we can attain a suppression of 1000 Less stringent requirements on beta (PRCL) ~ 10 Gamma (CARM) ~ 100 Suppression 1/1000 Suppression predicted with different filter orders Measurement and fit ALPHA

15. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina15 Beam jitter noise subtraction MICH error signal (ITF reflection demod at 8 MHz) is dominated by input beam jitter noise Part of this is visible on BMS signals It can be subtracted with a simple filter Black: B2 8MHz Red: Subtracted

16. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina16 Actuation noise UP DOWN RIGHTLEFT x4 Series resistor on L/R coils 4 times larger than on U/D

17. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina17 Locking with Left + Right coils Need to accurately measure and balance actuation gains If coils are not well tuned, alpha filter changes significantly over time and during different locks We suspect very small alignment drifts Solved developing a technique to measure coil gains with ITF in low noise state Now this is the standard configuration (V. Dattilo)

18. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina18 Environmental noise Effect of switching central hall air conditioning off CALIBRATION LINES PRCL err MICH err CARM err

19. Gabriele Vajente, 14 th ILIAS WG1 meeting – Cascina19 Conclusions Lot of work after the end of VSR1 Improved locking robustness Longitudinal noise is no more limiting at any frequency Error signals are dominated by environmental noise With AC off, locking is compliant with design Developed many techniques to characterize the ITF October 1 st February 5 th