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Locking improvements after the end of VSR1

Locking improvements after the end of VSR1. Gabriele Vajente for the Locking Group 14 th ILIAS WG1 meeting Cascina – March 6 th 2008. The end of the story…. October 1 st 2007. The end of the story…. February 5 th 2008. And now the full story…. Better diagonalization of driving

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Locking improvements after the end of VSR1

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

  2. The end of the story… October 1st 2007

  3. The end of the story… February 5th 2008

  4. 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. Virgo DOFs, sensing and driving DARM = Differential long armCARM = Common long arm (not frequency)PRCL = Power recycling cavity lengthMICH = Short Michelson differential DARM, CARM DARM, CARM MICH MICH PRCL ITF reflection= B2 Dark fringe= B1 BS pick-off= B5

  6. 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 LOCKING WI BS PR Reference

  7. 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

  8. 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. 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. New control filters / more LF gain • Largely increased loop gain below 100 mHz • No evidence of up-conversion of noise MICH PRCL CARM

  11. 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. Low noise MICH filter • Developed a new filter, with more optimized high frequency roll-off

  13. 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. Performances • With alpha (MICH) we can attain a suppression of 1000 • Less stringent requirements on • beta (PRCL) ~ 10 • Gamma (CARM) ~ 100 ALPHA Measurement and fit Suppression predicted with different filter orders Suppression 1/1000

  15. 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. Actuation noise • Series resistor on L/R coils 4 times larger than on U/D UP x4 x4 LEFT RIGHT DOWN

  17. 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. Environmental noise Effect of switching central hall air conditioning off MICH err CALIBRATION LINES PRCL err CARM err

  19. 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 1st February 5th

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