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Damping and Tuning of Damping and Tuning of Violin Modes in Violin Modes in Monolithic Silica Monolithic Silica
SuspensionsSuspensions
Stefan Goßlerfor the
GEO 600 Team
MotivationMotivation
Example: GEO 600 monolithic silica suspensionsmade in December ´02
But: High Q violinmodes may interfere with the interferometer length-control servo
To supress the influence of thermal noise, high Q suspensions are a key element of the advanced interferometric detectors
Damping of the violinmodeswithout degrading the pendulum Q
GEO Suspensions ...GEO Suspensions ...
Cantilever Springs
StackIsolator
Coilholder forLocal Controlat the Upper mass
Fibers
Upper Mass
IntermediateMass
Mirror
5.6 kg180 mm
... with Feedback... with Feedback
3 mm
Cantilever Springs
StackIsolator
Mirror
Upper Masses
Coilholder
Fibers
IntermediateMass
Cantilever springs
ElectrostaticActuator(10 – 100 Hz)
Magnet-CoilActuator(0 – 10 Hz)
ViolinmodesViolinmodes
Q ~ Resonant Enhancement
Q = fres , = Decay Time for Factor 1/e
Am
plit
ude
1
Frequency [Hz]1 500
f0f1
Example: 3 108/(500 Hz) = approx. 2,3 Days
P. Willems et al.: Q = 5,2 108
How to keep the servos happy?How to keep the servos happy?
• f0: Q < 3*106
• f1: Q < 2*106
• Frequencies (each eight for f0 and f1) within 10%
• Frequencies as high as possible (650 Hz)
Frequency [Hz]100 1000
Mag
nitu
de
10-2
1
102
f0
Damping & TuningDamping & Tuning......
f1By coating sections of the fibers with lossy material(i.e, amorphous Teflon dissolved in fluorinertin our case)
...violin modes without degrading the pendulum Q:
Damping & TuningDamping & Tuning......
f0 f1
Damping scales stronger with coating diameter than with coating length
Frequency shift scales with mass added
...violin modes without degrading the pendulum Q:
By coating sections of the fibers with lossy material(i.e, amorphous Teflon dissolved in fluorinertin our case)
Measuring Violinmode Measuring Violinmode QQss50 kg
Isolation Fiber(Fused Silica) Fused Silica Fiber
with GEO parameters:280 mm Length220 µm Diameter
1,4 kg
Lock-in Ampl.
Spectrum An.
Laser Split-Photodiode
Elektrostatic ActuatorFused Silica Plate
Fused Quarz Block(400 g)
Fused Quarz Block (withSteel Clamp)
Vacuum: 10-5 mbarQmax = 3*107 ,
Measuring Violinmode Measuring Violinmode QQss
1,4
kg
Elektrostatic Actuator
Vacuum: 10-5 mbarQmax = 3*107 ,
Measuring Violinmode Measuring Violinmode QQss50 kg
Isolation Fiber(Fused Silica) Fused Silica Fiber
with GEO parameters:280 mm Length220 µm Diameter
1,4 kg
Lock-in Ampl.
Spectrum An.
Laser Split-Photodiode
Elektrostatic ActuatorFused Silica Plate
Fused Quarz Block(400 g)
Fused Quarz Block (withSteel Clamp)
Vacuum: 10-5 mbarQmax = 3*107 ,
„„Reference Fiber“Reference Fiber“
A fiber with:Length: 284 mmDiameter: 210 – 240 µm
f0 = 664 Hz
Q0 = 2 106
Coating length: 26 mm@ 1/2 of the fiber lengthFiber diam.: 240 µmAfter coating: 300-325 µm
f0
Bounce mode: 14.9 Hz
Q1 = 5 105
Coating length: 31 mm @ 1/4 of the fiber lengthFiber diam.: 210 µmAfter coating: 340-370 µm
f1 = 1313 Hz
f1
f0: 743 Hzf1: 1451 Hz
Selective Tuning...Selective Tuning...
f0 [Hz]
f1 [Hz]
... of the violin modes:
741
1415
660
1306
UncoatedAfterInitial coating
691
1325
After 1st corrective coating
678
1316
After 2nd
corrective coating
665
1316
724
1490
676
1380
662
1353
f0 [Hz]
f1 [Hz]
Target: 664 Hz 1313 Hz
Fiber Preparation for Fiber Preparation for GEOGEO
20 of 30 coated Fibers within:
Before coating 3,5%
f0 = 663 Hz 0,48%Before coating 4,2%
f1 = 1314 Hz 0,86%
But: Welding and suspending increases the spread by not yet known amount
Target: 664 Hz 1313 Hz
TTununing after weldinging after welding ((in situin situ))
Tuning after welding (Tuning after welding (in situin situ))
After welding:
f0 = 669,5 Hzf1 = 1344 Hz
After correctivecoating:
f0 = 654 Hzf1 = 1324 Hz
Two of eight fibers were treated with a corrective coating after welding
Example:
ResultsResults: : FrequenciesFrequencies & & QQss
Am
plit
ude
[a.u
.]
Frequenz [Hz]620 640 660630 650 670
f0 = 645,5 Hz 3,86%
Q0= 6 105 – 2,8 106
Q1= 5,4 105 –1 106
f1 = 1287 Hz 4,86%
Target: Q < 2 106 .
Target: Q < 3 106 .
ConclusionConclusion
It is possible to tune the violin mode frequencies (downwards) almost independently to the desired values with great accuracy
In December ´02 we successfully suspended two GEO mirrors with damped and tuned violin modes (And the beam splitter only with slightly damped violin modes)
It is possible to bring the Qs to the desired levels, BUT it is NOT possible to measure them prior to final installation
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