1. Stefan Hild and Andreas Freise University of Birmingham Advanced Virgo telecon, June 2008 Beam sizes and mirror curvatures for Advanced Virgo

2. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 2 Introduction  Advanced Virgo conceptual shows an inconsisten- cy regarding the beam size:  The optics section uses 3.5cm radius  The sensitivities have been computed with 6.0cm radius  We compiled a preliminary set of consistent beam sizes and mirror curvatures.  “ Advanced Virgo design: Preliminary Revision of the Beam Size and the Test Mass Curvatures ”, S. Hild and A. Freise, available from Virgo codifier: VIR-038A-08

3. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 3 Beam size versus mirror size (general approach)  Mirror weight:  Go as heavy as you can/want.  Aspect ratio:  To optimize thermal noise, use a mirror diameter to mirror thickness ratio of 7:4.  Coating area:  Try to make it as large as possible  Limitation: Flats, rim.  Beam size:  Make it as large as the coating allow  Keep an eye on the clipping-losses

4. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 4 Clipping losses  Why are clipping losses a problem?  Reduced power buildup.  Scattered light noise.  In the ideal case a factor 2.5 (beam radius to mirror radius) seems to be fine = clipping loss of only a few ppm  In Reality:  Mirror imperfections  Miscentering  Residual alignment fluctuations  A safty factor might be beneficial = factor 3 beam radius to mirror radius

5. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 5 Revision of the beam size for Advanced Virgo  We considered only the main arm cavities.  Actuators might introduce different constraints.  However, type and geometry of the actuators are unknown at the moment.  We considered flats of heights between 5 and 10 cm.  We considered a coating of maximum size:  Circular geometry  Minimum separation from the rim of 5mm

6. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 6 Maximum Coating Diameter  Now we can calculate for the ideal case possible beam radii:  Depending on the flat height  For beam size mirror size ratios of 2.5 and 3.0.  Beam radii between 5.5 and 6.5 cm seem to be reasonable

7. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 7 Resulting mirror curvatures  For a given beam radius we can calculate the curvature of the corresponding mirrors:  Please note: A curvature deviate of 1% will cause a beam size change of 10% !!!  This will give strict requirements for Polishing and Coating as well as for the performance of the TCS.

8. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 8 Steps towards the final beam size for advanced Virgo  The analysis presented in this talk should be seen as an intermediate step.  For the final analysis following steps are required:  Take clipping from actuators into account (FFT simulations required)  Consider influence of thermal effects onto curvatures  Estimate the expected noise from clipped/scattered light  CAREFUL TRADE OFF PROCESS REQUIRED:  Detector sensitivity  Power buildup  Scattered light noise

9. S. Hild and A. FreiseAdvanced Virgo, 3rd of June 2008 Slide 9 E N D