Locking improvements after the end of VSR1

Gabriele Vajente for the Locking Group

14th ILIAS WG1 meeting

Cascina – March 6th 2008

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 2

The end of the story…

October 1st 2007

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 3

The end of the story…

February 5th 2008

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 4

And now the full story…

Better diagonalization of drivingNew sensing strategy

Modulation at 8 MHz

New filtering algorithmsBetter speed performancesMuch more flexible

Improved control filtersMore low frequency gainLess noise re-introduction

Improved noise subtractionAlpha, beta, gamma techniquesBeam jitter subtraction

Reduced actuator noiseLocking with lateral coils

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 5

Virgo DOFs, sensing and driving

DARM = Differential long armCARM = Common long arm (not frequency)PRCL = Power recycling cavity lengthMICH = Short Michelson differential

ITF reflection=

B2

Dark fringe=

B1

BS pick-off=

B5

DARM, CARMDARM, CARM

PRCLMICHMICH

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 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

PR

BS

WILOCKING

Reference

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 7

New sensing strategy

Main modulation 6 MHz

Additional modulation at 8 MHz, phase-lockedNot resonant in PRC or arm cavities

B2 beam (ITF reflection) demodulated at 8 MHzGives good signal for central cavity control

Old (VSR1) configuration New sensing schemeMICH

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

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 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

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 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 ofRobustness

Flexibility

Allowed a much faster development and test of new control filters

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 10

New control filters / more LF gain

MICH PRCL CARM

Largely increased loop gain below 100 mHz

No evidence of up-conversion of noise

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 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 calibrationNew filter for more margin: calibration changes by 2% at most

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 12

Low noise MICH filter

Developed a new filter, with more optimized high frequency roll-off

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 13

Alpha, beta, gamma techniques

Auxiliary loop noises couples strongly to dark fringeNoise subtraction techniques

The MICH, PRCL, CARM corrections are filtered and added to DARM correction, to cancel noise

(B. Swinkels)

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 14

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

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 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 filterBlack: B2 8MHzRed: Subtracted

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 16

Actuation noise

UP

DOWN

RIGHTLEFTx4

x4

Series resistor on L/R coils 4 times larger than on U/D

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 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 locksWe suspect very small alignment driftsSolved developing a technique to measure coil gains with ITF in low noise state

Now this is the standard configuration (V. Dattilo)

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 18

Environmental noiseEffect of switching central hall air conditioning off

CALIBRATION LINES

PR

CL

err

MIC

H e

rrC

AR

M e

rr

Gabriele Vajente, 14th ILIAS WG1 meeting – Cascina 19

Conclusions

Lot of work after the end of VSR1Improved 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