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Damping and Tuning of Violin Modes in Monolithic Silica Suspensions

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  1. Damping and Tuning of Violin Modes in Monolithic Silica Suspensions Stefan Goßler for the GEO600 Team

  2. Motivation Damping of the violinmodes without degrading the pendulum Q To supress the influence of thermal noise, high Q suspensions areakey element of the advanced interferometric detectors But:High Q violinmodes may interferewith the interferometer length-control servo Example: GEO600 monolithic silica suspensions made in December ´02

  3. GEO Suspensions ... UpperMass Intermediate Mass Fibers Mirror 180mm Æ = 5.6kg Cantilever Springs Stack Isolator Coilholder for Local Control at the Upper mass

  4. ... with Feedback Magnet-Coil Actuator (0 – 10Hz) Electrostatic Actuator (10 – 100Hz) Cantilever Springs Coilholder Stack Isolator Upper Masses Cantilever springs Intermediate Mass Fibers 3mm Mirror

  5. Violinmodes Example: 3 108/(p 500Hz) = approx. 2,3 Days P. Willems et al.: Q = 5,2 108 f1 f0 Amplitude 1 1 500 Frequency [Hz] Q ~ Resonant Enhancement Q = p fres t , t = Decay Time for Factor 1/e

  6. How to keep the servos happy? 102 • f0: Q < 3*106 • f1: Q < 2*106 • Frequencies (each eight for f0 and f1) within 10% • Frequencies as high as possible (650Hz) Magnitude 1 10-2 100 1000 Frequency [Hz]

  7. Damping & Tuning... f0 f1 ...violin modes without degrading the pendulum Q: By coating sections of the fibers with lossy material (i.e, amorphous Teflon dissolved in fluorinert in our case)

  8. Damping & Tuning... f0 f1 Damping scales stronger with coating diameter than with coating length Frequency shift scales with massadded ...violin modes without degrading the pendulum Q: By coating sections of the fibers with lossy material (i.e, amorphous Teflon dissolved in fluorinert in our case)

  9. Measuring Violinmode Qs Elektrostatic Actuator Split-Photodiode Laser 1,4kg Spectrum An. Lock-in Ampl. Qmax = 3*107, Vacuum: 10-5 mbar 50kg Fused Silica Plate Isolation Fiber (Fused Silica) Fused Silica Fiber with GEO parameters: 280mm Length 220µm Diameter Fused Quarz Block (400g) Fused Quarz Block (with Steel Clamp)

  10. Measuring Violinmode Qs Elektrostatic Actuator 1,4kg Qmax = 3*107, Vacuum: 10-5 mbar

  11. Measuring Violinmode Qs Elektrostatic Actuator Split-Photodiode Laser 1,4kg Spectrum An. Lock-in Ampl. Qmax = 3*107, Vacuum: 10-5 mbar 50kg Fused Silica Plate Isolation Fiber (Fused Silica) Fused Silica Fiber with GEO parameters: 280mm Length 220µm Diameter Fused Quarz Block (400g) Fused Quarz Block (with Steel Clamp)

  12. „Reference Fiber“ Q1 = 5 105 Q0 = 2 106 f1 = 1313Hz f0 = 664Hz Coating length: 31mm @ 1/4 of the fiber length Fiber diam.: 210µm After coating: 340-370µm Coating length: 26mm @ 1/2 of the fiber length Fiber diam.: 240µm After coating: 300-325µm f0 f1 f0: 743Hz f1: 1451Hz Length: 284mm Diameter: 210 – 240µm A fiber with: Bounce mode: 14.9Hz

  13. Selective Tuning... After Initial coating After 1st corrective coating After 2nd corrective coating 665 1316 691 1325 678 1316 724 1490 676 1380 662 1353 660 1306 f0[Hz] f1[Hz] ... of the violin modes: Target: 664Hz 1313Hz Uncoated f0[Hz] f1[Hz] 741 1415

  14. Fiber Preparation for GEO f0 = 663Hz 0,48% f1 = 1314Hz 0,86% Before coating 3,5% Before coating 4,2% But: Welding and suspending increases the spread by not yet known amount Target: 664Hz 1313Hz 20 of 30 coated Fibers within:

  15. Tuning after welding(in situ)

  16. Tuning after welding (in situ) Two of eight fibers were treated with a corrective coating after welding Example: After welding: f0 = 669,5 Hz f1 = 1344 Hz After corrective coating: f0 = 654 Hz f1 = 1324 Hz

  17. Results: Frequencies & Qs f0 = 645,5Hz 3,86% Q0=6 105–2,8 106 . Target: Q < 3 106 Q1=5,4 105–1 106 f1 = 1287Hz4,86% 620 640 650 660 670 630 Frequenz [Hz] . Target: Q < 2 106 Amplitude [a.u.]

  18. Conclusion It is possible to tune the violin mode frequencies (downwards) almost independently to the desired values with great accuracy It is possible to bring the Qs to the desired levels, BUT it is NOT possible to measure them prior to final installation 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)