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Advanced Virgo beam size: Asymmetric ROCs and Coating Thermal Noise

Advanced Virgo beam size: Asymmetric ROCs and Coating Thermal Noise. Stefan Hild and Andreas Freise Advanced Virgo meeting, August 2008. PLEASE NOTE: Everything presented here is …. Preliminary. … just want to spread some (more or less) recent information ….

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Advanced Virgo beam size: Asymmetric ROCs and Coating Thermal Noise

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

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

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

  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 ??? Advanced Virgo, 15th of August 2008

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

  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 … Advanced Virgo, 15th of August 2008

  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. Advanced Virgo, 15th of August 2008

  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?) Symmetric ROCs = non optimal Coating noise ITM ETM ITM ETM Asymmetric ROCs = optimal Coating noise Advanced Virgo, 15th of August 2008

  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. Advanced Virgo, 15th of August 2008

  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. Advanced Virgo, 15th of August 2008

  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. Advanced Virgo, 15th of August 2008

  12. E N D… Advanced Virgo, 15th of August 2008

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