Locking improvements after the end of VSR1 Gabriele Vajente for the Locking Group 14 th ILIAS WG1...
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Transcript of Locking improvements after the end of VSR1 Gabriele Vajente for the Locking Group 14 th ILIAS WG1...
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