Mirror Cleaning

1. Mirror Cleaning Michele Punturo INFN Perugia

2. Thermal Transient • A large thermal transient affects the Virgo operation Sidebands amplitude Locking steps

3. Mirror Absorption • We attributed to the excess of absorption in the Virgo input mirrors the responsibility of this thermal transient • This has been possible thanks to the mirror thermal mode technique We measure the mirror temperature increase during the lock procedure We compare it with a FEM analysis, miming also the power injected during all the locking procedure We adjust the hypothetical absorption of the mirror, to match the measured temperature increase

4. Resonant mode technique • Obviously the resonant frequencies of a body depend on the temperature of the body • For a Virgo mirror we evaluated this dependence with a ANSYS based FEM (2004, F.Travasso PhD thesis) Drum mode: 0.61 Hz/K Butterfly mode(s): 0.28 Hz/K

5. Resonant mode “calibration” • To increase our confidence in this method we cross-checked the mirror temperature increase (due to the environmental fluctuation) measured through the resonant mode technique with the Super Attenuator temperature, measured by traditional thermometers • This is just an example: the filters T variation could be both smaller and larger than the mirror fluctuation (different height, presence of the separating roof), but it is always similar 1 day

6. 80 rings Expected T increase • The expected temperature increase, due to the beam illumination, is evaluated through a Matlab based FEM (confirmed by ANSYS & Comsol FEMs) • Hypothesis: • We know the laser power injected in the ITF (and we rescale it with the power read by B5) • We believe to the 0.7ppm/cm of substrate absorption (cross-checked with a measurement in Lyon on a spare substrate) • We concentrate all our “ignorance” on the surface (coating) absorption (expected to be 1.3ppm) • The environmental temperature is constant during the locking procedure

7. Measured excess • NI: (ppm) 4.5 ±0.50 (stat) ± 0.23 (laser power) +- 0.15 (finesse fluctuation) +- 0.38 (calibration) • WI: Measurement difficulties: results ranging from 7ppm to 16ppm

8. Increase our confidence • The confidence in the resonant mode is critical in this evaluation. • Could we find an independent confirmation? • Etalon effect in the input mirrors B7/B8 phd AR coating DP/P DT

9. … Increase our confidence • dn/dT depends on the particular kind of FS, but the order of magnitude is correct (0.8-1 × 10-5) • An incertitude of 15% is directly transferred in a similar error in the temperature evaluation. • Finally, we can confirm that an excess of absorption in the input mirrors is present (and that the evaluation of the absorbed ppm is reliable). • Possible sources of absorption: • Substrate • Coating • Pollution

10. How to clean the mirrors? • The “natural” way should be to dismount the mirrors • Rejected, because we cannot accept the dead time (~1 month) for the commissioning • First contact polymer • http://www.photoniccleaning.com/ Credit to L.Pinard (LMA) Applying First Contact Polymer with a Brush Clear Polymer Applied with A Pump Spray

11. Cleaning performance demonstration Dirty Telescope Mirror Polymer Sprayed Right Over The Contaminates No Prior Cleaning of Any Kind. Credit to L.Pinard (LMA) Telescope mirror After removing the film

12. “Quantitative” evaluation of the performances 50 mm mirror after classical cleaning Scattering 6 ppm Absorption : 1.91 ppm Credit to L.Pinard (LMA)

13. ...“Quantitative” evaluation of the performances 50 mm mirror, with particles Scattering 25 ppm Absorption : 5 ppm Credit to L.Pinard (LMA)

14. ...“Quantitative” evaluation of the performances 50 mm mirror, after puting and removing a film of ‘First Contact' Scattering 6-6.5 ppm - idem after cleaning Absorption : 1.73 ppm The Film has ’cleaned’ the mirror Credit to L.Pinard (LMA)

15. Virgo Mirrors Cleaning http://virgo.pg.infn.it/~punturo/#2007

16. Effects of the cleaning • No bad effects due to the cleaning procedure • Mirrors surface looks really cleaner • Any more translucent reflection • But no substantial improvement (except a 25% increase of the sidebands amplitude)

17. Thermal transient Credits: G.Vajente

18. And what about the absorption?

19. T instability and absorption measurement • Mirror T instability obstacle the measurement of the mirror absorption • Zoom of the previous plot

20. T instability and absorption measurement • Mirror T instability obstacle the measurement of the mirror absorption • Zoom of the slide #8 plot

21. Locking warming up • In this “general” T fluctuation of the order of 0.5K at the mirror level, we must extract the warming up due to the mirror due to the laser, that should be of the order of 10-40mK

22. Locking warming up extraction • Qualitatively is easy, but the numerical evaluation can be affected by a large error • Long relaxation time after the perturbation:

23. Matching with the FEM • No real access to the overall Temperature trend • FEM can be run just with the Tambient=constant hypothesis • Matching with the real data performed lock-by-lock Pre-VSR1 evaluation: 6ppm VSR1 evaluation: NI (ppm): 4.5 ±0.50 (stat) ± 0.23 (laser power) ± 0.15 (finesse fluctuation) ± 0.38 (calibration) 5.4ppm 5.4ppm 5.4ppm Error to be defined!

24. Matching with the FEM: WI Pre-VSR1 evaluation: 16ppm VSR1 evaluation: WI (ppm): 7? ± ?? (difficulties to measure it… same difficulty now) 11ppm 3.4ppm (T decrease during the lock) 11ppm

25. Conclusions • Despite the better look of the mirror surface, no substantial decrease of the thermal transient and of the absorption occurred • What kind of pollution (if any) contaminated the mirror? • What other investigations cn be performed to understand better? • What to do if TCS is not enough?