Stefan Hild and Andreas Freise

Advanced Virgo meeting, August 2008

Advanced Virgo beam size: Asymmetric ROCs and Coating

Thermal Noise

S. Hild Advanced Virgo, 15th of August 2008 Slide 2

PLEASE NOTE: Everything presented here is …

… just want to spread some (more or less) recent information …

S. Hild Advanced Virgo, 15th of August 2008 Slide 3

Previous statements: Symmetric ROCs give lowest Coating Thermal noise

S. Hild Advanced 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 ???

S. Hild Advanced Virgo, 15th of August 2008 Slide 5

Coating noise increases with the number of coating layers

S. Hild Advanced 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 …

S. Hild Advanced 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.

S. Hild Advanced 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

ITM

ETM

ETM

Symmetric ROCs = non optimal Coating noise

Asymmetric ROCs = optimal Coating noise

S. Hild Advanced 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.

S. Hild Advanced 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.

S. Hild Advanced 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.

S. Hild Advanced Virgo, 15th of August 2008 Slide 12

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