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Thermal effects and their Thermal effects and their compensation in Advanced Virgocompensation in Advanced Virgo
E. Coccia1,2, M. Di Paolo Emilio2, V. Fafone1,2, V. Malvezzi2, Y. Minenkov2, A. Rocchi2, L. Sperandio1,2.
1University of Tor Vergata2INFN Roma Tor Vergata
46th Rencontres de Moriond46th Rencontres de Moriond26.03.201126.03.2011
Thermal effectsBrief introduction
Compensation principleAn example: Virgo+ TCS
Thermal Compensation System for AdVLayoutHeating pattern optimizationRing heater developmentWave-front sensing
26/03/2011 A. Rocchi - Rencontres de Moriond 2011- La Thuile 2
Thermal effects: introduction
A. Rocchi - Rencontres de Moriond 2011- La Thuile 3
z
TE dzzrTdT
dnrS ),()(
High Reflectivity coating and substrate of the Test Masses absorb some (O(ppm)) power stored in the Fabry-Perot and recycling cavities.
Due to the low thermal conductivity of SiO2, a thermal gradient is established in the substrate.
Thermal lensing: The refraction index is temperature dependent
(dn/dT≠0, O(ppm/K)). The optical path inside the substrate of the TMs
is not uniform. This is equivalent to putting a lens in the
substrate of the ITMs. Thermo-elastic deformation of the HR surface
of all the TMs.
26/03/2011
Thermal effects: consequences Thermal lensing: wave-front distortions of the
fields in the SRC and PRC cavities. Decrease of the optical gain in the RCs; If lensing is too strong, ITF cannot acquire the lock. FOM: coupling losses.
A. Rocchi - Rencontres de Moriond 2011- La Thuile 4
Thermo-elastic deformation: changes the RoC in both ITMs and ETMs, affects the FP cavity. This effect is negligible in current detectors, but becomes relevant in advanced IFOs. Decrease of the spot size on TMs increase of thermal noise of about 15%.
26/03/2011
2( ) ( )r OPL r
( )20 0 0 ( )
00 0 0 0 0
( ) 2i r a
i rE E E e ES E r e rdr
E E E E
*1L S S [ppm]
A. Rocchi - Rencontres de Moriond 2011- La Thuile 5
Thermal lensing is only due to the temperature gradient along the radial direction.
So, we can heat the peripheral of the test mass to flatten the gradient and, thus, the optical path length.
Working principle of TCS
z
TE dzzrTdT
dnrS ),()(
26/03/2011
A. Rocchi - Rencontres de Moriond 2011- La Thuile 6
Current compensation systems
All use CO2 (=10.6m) lasers to heat the peripheral of the input test masses.
CO2 Laser
Annular heating Gold star mask
AXICON
Initial LIGOEnhanced LIGO and Virgo+
26/03/2011
A. Rocchi - Rencontres de Moriond 2011- La Thuile
Virgo+ scheme
Mirror B
Mirror A
Single AXICON used to convert a Gaussian beam into an annular beam. Size of the annulus hole can be set by moving L3
Half wave plate and fixed polarizer are used for DC power control. This system does not deviate the beam impinging on the AXICON
To monitor the CO2 beam quality, an infrared camera has been installed on each bench.
726/03/2011
Noise from TCS TCS can inject displacement noise into the detector. Coupling mechanisms:
Thermo-elastic (TE) - fluctuations in locally deposited heat cause fluctuations in local thermal expansion
Thermo-refractive (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 Radiation pressure (negligible)
A. Rocchi - Rencontres de Moriond 2011- La Thuile
TE TR F
Present detectors already require intensity stabilization of the CO2 laser.
Achievable level of intensity stabilization (10-7/√Hz) not enough to heat with CO2 directly the TM in advanced detectors (10-9/√Hz needed) compensation plates required.
826/03/2011
RIN = relative intensity noise
9
Green dots: heating ringsBlue rectangles: CPs
Compensation plates shined with CO2 laser will correct thermal effects in the RCs
Ring heaters will compensate HR surface deformations
This set up allows to control independently the thermal lensing and the ROCs
A. Rocchi - Rencontres de Moriond 2011- La Thuile
TCS in Advanced detectorsPower absorbed by TMs is about 0.5W, wrt ~20mW in initial detectors
26/03/2011
Some details• RH at 5 cm from the anti-reflecting face of the TM to maximize the efficiency• CP diameter: 350 mm (same as TMs)• CP thickness = 35 mm• CP must be “far” from the TM. Distance fixed at 200 mm
Sym
me
try
axi
s
Heat escaping from the barrel of the CP
10A. Rocchi - Rencontres de Moriond 2011- La Thuile
TM heated by radiation fromthe CP if CP-TM distance = 1 cm
TM Tmap
HR face
Sym
me
try
axi
s
26/03/2011
CP heating pattern
A. Rocchi - Rencontres de Moriond 2011- La Thuile
The heating profile must be much more precise than in present detectors
Simple system like an axicon (Virgo-like) is not enough Too high content of higher order modes in RF sidebands
Necessity to optimize the CP heating pattern Linear iterative optimization process based on FEM developed, to
take into account radiative coupling btw TM and CP and the presence of the RH. Uses the OPL increase as error signal.
11
with single axicon = 2200 ppm
Optimal heating patternOptimal heating pattern Resulting Optical Path LengthResulting Optical Path Length
Residual coupling losses= 26 ppm
Coupling losses = superposition integral between “unperturbed” beam and single pass through thermal lensing.
26/03/2011
No TCS = 5·105 ppm
Solution using known technology:modulate rings dimensions by changing distances between lenses and axicons and modulate power in each ring (Double Axicon System)
-With DAS, residual coupling lossesaround 102 ppm.Residual focal length O(103 km)-With single axicon, residual coupling losses around 2000 ppmResidual focal length O(102 km)
Heating pattern generation: DAS
12A. Rocchi - Rencontres de Moriond 2011- La Thuile26/03/2011
The beam is split by polarization and recombined with round polarizer at 45 deg AOI. The optical layout is very flexible! Allows to change independently the two rings:
Inner and outer diameters of each ring Power density of each ring
Heating profile simulated with Zemax gives 200 ppm coupling losses Experimental DAS gives 210 ppm
A. Rocchi - Rencontres de Moriond 2011- La Thuile 13
Experiment vs. simulation
Heating pattern generated by the DAS
26/03/2011
Main concerns: Stray magnetic field: AdV will use electro-magnetic actuators to control the TMs; Vacuum compatibility.
Down-scaled prototypes with different geometries and materials realized: Annular winding with counter-flux coils; Toroidal winding:
○ Single coil;○ Double coil (counter-flux).
Magnetic field measurements: Detailed maps as a function of the position; Frequency domain transfer function; 3D magnetic field simulations.
Annular winding Toroidal winding
Ring heater development
A. Rocchi - Rencontres de Moriond 2011- La Thuile 14
x
y
z
Measured Simulated
26/03/2011
Noise projection• Simulated a RH of 400 mm diameter and considered the magnetic field and gradient in the position of the magnets• Assumed a flat current noise of 1nA/Hz
A. Rocchi - Rencontres de Moriond 2011- La Thuile 15
z
B
Lm
gh zz 2
LI
BDgh yx
yx 2
,,
g = magnets symmetry = magnetic momentm = mirror massI = moment of inertiaL = arm cavity lengthD = beam displacement
B
D
26/03/2011
Materials optimization
A. Rocchi - Rencontres de Moriond 2011- La Thuile 16
To avoid contamination from the insulation of the wire we have chosen a design with bare wire on insulated support.
Under investigation:glass corealuminum core with insulating coating
Aluminum samples machined and coated with:inorganic paint plasma spray with aluminadiamond like coating
26/03/2011
The full TC System to be implemented on Advanced Virgo will comprehend sensors which sample the thermal wave-front distortions within the interferometer, either from samples of the IFO beam picked off at appropriate points, or directly from the optics themselves using dedicated probe beams and servo electronics that will derive control signals from the sensors and feed them back to the heater elements.
Advanced Virgo will use a Hartmann sensor developed by the University of Adelaide, that has demonstrated to have a shot-to-shot reproducibility of λ/1450 at 820 nm, which can be improved to λ/15500 with averaging (Opt. Express 15 (16), 10370-10375, 2007).
TCS sensing scheme
A. Rocchi - Rencontres de Moriond 2011- La Thuile 17
Measurement taken at the Gingin HOPTF
L
x
x
W
26/03/2011
A. Rocchi - Rencontres de Moriond 2011- La Thuile 18
Thermal compensation systems proven to be efficient and reliable in initial detectors: recovered 85% of sidebands recycling gain.
In advanced detectors, thermal effects must be corrected with higher precision: More power in the cavities; Larger spot size on ITMs; One more effect to take care of: TMs RoC.
Good experimental results from the DAS: Good agreement with simulations; Easy to assemble; High degree of flexibility.
Ring heater design is progressing. High sensitivity wave-front sensor identified for advanced
detectors.
26/03/2011