1. Stefan Hild and Andreas Freise Advanced Virgo meeting, August 2008 Advanced Virgo beam size: Asymmetric ROCs and Coating Thermal Noise

2. S. HildAdvanced Virgo, 15th of August 2008 Slide 2 PLEASE NOTE: Everything presented here is … … just want to spread some (more or less) recent information …

3. S. HildAdvanced Virgo, 15th of August 2008 Slide 3 Previous statements: Symmetric ROCs give lowest Coating Thermal noise

4. S. HildAdvanced Virgo, 15th of August 2008 Slide 4 Current baseline of Advanced Virgo ROCs  Use identical ROCs at ITM and ETM.  Beam waist in the centre of the arm cavity.  Beamsize at all 4 cavity mirrors is the same  Therefore all 4 mirrors contribute in the same way to the Overall Coating Thermal noise  N_all = sqrt(IMX^2 + IMY^2 + EMX^2 + EMY^2)  BIG Question: Is it really correct that all 4 mirrors contribute in the same way ???

5. S. HildAdvanced Virgo, 15th of August 2008 Slide 5 Coating noise increases with the number of coating layers

6. S. HildAdvanced Virgo, 15th of August 2008 Slide 6 How many coating layer will there be at ETM and ITM ?  General rule of thumb: The higher the reflectivity the more coating layers are required!  Adv conceptual design transmittances:  ETM = 5 x 10 -6  ITM = 7 x 10 -3  We don’t have the final number for the Advanced Virgo coatings … thus we use the ALIGO values for the moment …

7. S. HildAdvanced Virgo, 15th of August 2008 Slide 7 ETMs have stronger Coating Thermal Noise than the ITMs  For identical beam size the ETMs contribute much stronger to overall coating noise than the ITMs.

8. S. HildAdvanced Virgo, 15th of August 2008 Slide 8 Optimal Coating thermal noise: Asymetric ROCs  In order to minimize the thermal noise we have make the beam larger on ETM and smaller on ITM.  Equivalent to moving the waist closer to ITM.  Nice side effect, the beam in the central central area would be slightly smaller !!  Not so nice side effect: ETM and ITM have different ROCs (more spares required?) ITM ETM Symmetric ROCs = non optimal Coating noise Asymmetric ROCs = optimal Coating noise

9. S. HildAdvanced Virgo, 15th of August 2008 Slide 9 Coating thermal noise for asymmetric ROCs  Did a scan over various ROCs for ITM and ETM.  For each combination of ROCs we ran GWINC to evaluate detector performance.  Color indicates the binary neutron star range [Mpc].  Contour indicates beam sizes at the test masses.

10. S. HildAdvanced Virgo, 15th of August 2008 Slide 10 Coating thermal noise for asymmetric ROCs  Illustrating example: Black arrow: Both beam sizes are 5.0 cm. The ROCs are about 1570m. The Coating noise is about 4.1e-24. Blue arrow: The same level of coating noise can be achieved with a beam of only 4.8cm at the ITM.

11. S. HildAdvanced Virgo, 15th of August 2008 Slide 11 Summary  It seems likely that there will be a revision of the beam sizes, mirror ROCs and waist position.  We need to collect (over the next month) all important information:  Sensitivity (Coating noise)  Cavity stability (ROCs, g-factors, polishing accuracy, …)  Diffraction losses  We will try to come up with a proposal for beam sizes and ROCs which will be presented to the VIRGO collaboration for discussion and review.

12. S. HildAdvanced Virgo, 15th of August 2008 Slide 12 E N D…