TCS update

TCS update

INFN Roma Tor Vergata

Advanced Virgo Meeting - 16.07.08 2

Thermal Lensing Effects in Advanced Virgo• Thermal compensation for Advanced Virgo, must take into

account the much greater circulating laser power (~800kW) • In Advanced Virgo, thermal effects will create distortions both

in the recycling and in the Fabry-Perot cavity:– Wavefront distortion in the PRC– HR surface elastic deformation

•The cavity becomes less concentric, and the spot sizes at the mirrors will shrink (increase of thermal noise and reduction of arm cavity coupling to the input beam)•Necessity to control the radii of curvature of all test masses, TCS will have to act on both ITM and ETM.

From S.Chelkowski presentation at 1.7.08 beweekly meeting


Advanced LIGO and Advanced Virgo share the same thermal issues. A possible TC System has already been proposed in the LIGO Collaboration

Red dots: shielded heating ringGreen rays: CO2 heating beamsBlue rays: sensing beams

Compensation plates will correct thermal effects in the PRCShielded heating rings will compensate HR surface deformationsHartmann sensors will be used to monitor TM and CP phase profile

Advanced LIGO TCS scheme

In the case of Advanced Virgo,The positioning of CPs in the PRC is a delicate issue due to its strong impact with the suspension systems


Simulation of thermal effects in Advanced Virgo Test Masses

In the simulation we used the following parameters:• 125W of input ITF power, beam radius on TM = 6 cm• FP cavity Finesse=885• gain of the SRC=23.5• absorptions of the coating and substrate respectively 0.5ppm

and 2ppm/cm• Total absorbed YAG power ~0.5W• Test Mass diameter: 350 mm• Test Mass thickness: 200 mm• Test Mass ROC (cold): 1530 m



Effect of the Yag on ITM



Effect of the Yag on ITM- Thermoelastic deformation

Thermo-elastic deformation of the HR surface

Change in the ROC



Effect of the Yag on ITM- Lensing effect (thermooptic 95% + thermoelastic 5%)

Optical path length increase in the substrate.The focal length of the equivalent lens is 4.5 km



Optical path length for different CO2 powers

Is it possible to correct both effects acting with a CO2 beam on the HR face of the ITM?

- heating profile due to AXICON- internal radius = 6 cm- external radius = 16 cm } Not optimized



Temperature map for 9.7W of CO2

Is it possible to correct both effect acting with a CO2 beam on the HR face of the ITM?


HR face displacement for 9.7W of CO2



ROC for different CO2 powers

ROC=1530 m

ROC=1538 m

ROC=1556 m

ROC=1567 m

Is it possible to correct both effect acting with a CO2 beam on the HR face of the ITM?



Since the ROC increases when heating the HR face, we tried acting with a CO2 beam on the AR face of the ITM. Same profile as HR heating.


ROC=1530 m

ROC=1538 m

ROC=1518 m

The lensing effect is corrected at (almost) the same level as when acting on the HR face.What about the radius of curvature?

ROC for different CO2 powers


From the simulations it follows:


•Front heating increases ROC (~3.1 m·W)

•Back heating decreases ROC (~2 m·W)

Need to find the correct balance between front and back heating


Starting from these evaluations we heated the TM on both faces, with the same profile, but different powers: 2.5 W on the HR face, 7.5 W on the AR face.


Effect on the ROC

CO2 heating on both faces



Effect on the OPL

CO2 heating on both faces

Temperature map

OPL

Advanced Virgo Meeting - 16.07.08 15LIGO Laboratory 15

TCS Noise Coupling Mechanisms• Thermoelastic (TE)- fluctuations in locally

deposited heat cause fluctuations in local thermal expansion

• Thermorefractive (TR)- fluctuations in locally deposited heat cause fluctuations in local refractive index

• Flexure (F)- fluctuations in locally deposited heat cause fluctuations in global shape of optic


Flexure Noise- A Simple Model

probe beam

heating

heating

slat mirror

CM lineA very skinny mirror with ‘annular’ heating

The probe beam sees the mirror move at the center due to wiggling far from center


Ad Virgo TCS noise

17

Heating profile from Zemax fitted with:

W/m2

w1=0.099 and w2=0.0048

Zemax profileFit

This profile has been fed to a structural simulation in COMSOL to calculate thedisplacement of the HR face wrt the CM

Evaluation of the flexural noise

H(r) 64 10 5 r2

w12w2

4w2

2 r2 2e

2r 2

w12

0 0.05 0.1 0.15 0.20

5

10

15

20


Ad Virgo TCS noise

18

In case the CO2 is applied on the HR face,to have TCS noise 10 times smaller than AdVirgo sensitivity @ 50 Hz RIN must be 5·10-9 1/Hz

Flat RIN over the entire frequency band

No relevant change with TM 30 cm thick

In case of heating on both sides of the TM, the noise model must be reviewed to take into account possible cancellation mechanisms.

A cross check between the model with experimental data (Edwige from TCS commissioning) and analytical calculations (S. Ballmer) is in progress

10 100 1 103 1 10

41 10

26

1 1025

1 1024

1 1023

1 1022

1 1021

TCS noise totalFL noiseTR + TE noiseRad pressure noiseAdVirgo sensitivity


We started to investigate the behavior of heating ring, from a question done by Andreas:An interesting idea could be to optimize the ROC for a normal TEM00 modes then at a later stage use the TCS to change the ROC in order to adapt the cavities for higher order modes. Do you have any idea of how much static change we could get, say, from a ring heater without creating other problems? I am thinking of changing a ROC of maybe 1530m to 1600m or 1630m or so.


Heating ring actuation

Geometry description: axisymmetric model

Model components: mirror with a cold ROC = 1530 m + ring heater placed in front of the HR face of the mirror at a distance of 5.5 cm.


Temperature distribution in the mirror (cold mirror at T= 295 K)



Displacement of the HR face

160 W delivered by the Heating ring. Temperature of the HR = 530 K




160 W delivered by the Heating ring. Temperature of the HR = 530 KDetailed calculations not yet performed (need to model the LG33 effect).Thermal lensing is probably overcorrected! This is possible only on ETM

Effect on the ROC


Next steps in simulations• Check of the noise model• Investigate the possibility to relax the RIN requirements with

both faces heating• Possibility to replace the CO2 beam on the back with a heating

ring• Evaluate the noise introduced by the heating ring• Study the case of HOLM:

– possibility to change statically the ROC to optimize it for the HOLM– determine the TCS requirements

• Determine the requirements of a possible CP• Study interaction of the TCS with nearby optics and hardware

TCS update

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Transcript of TCS update