AdV Thermal Compensation SystemViviana FafoneAdV/aLIGO joint technical meeting, February 4, 2004
Stable recycling cavities ? Good • More stable RF and audio-GW sidebands (better signals for control and better interferometer sensitivity) • Smaller beam at the input and output port • Easier extraction of pick-off from recycling cavity Bad • Multi-mirror superattenuators • Astigmatism (folded cavities) • Alignment signals reduced Baseline:stable cavities but re-evaluation on going
Re-evaluation of marginally stable cavities • Re-design of the suspensions is very complex and costly • The question: the marginally stable cavities are really so bad? • Crucial role of the thermal compensation system: • Which can be the exact degree of compensation?
Thermal Compensation System 5 • Design guidelines: • TCS must reduce thermal effects to a level which allows AdV to acquire the lock and such that the sensitivity of the detector is not spoiled. • TCS should provide as much flexibility as possible for corrections, to help in case some optics should not meet the specifications (mirror radius errors, higher or non-uniform absorptions). • Based on the experience of Virgo/Virgo+, TCS should be designed so that most of the apparatus lives outside vacuum and can be easily upgraded as new understanding of the IFO is realized. LIGO-Virgo TCS meeting 4.02.2010
6 TCS baseline design (same as Advanced LIGO) Compensation plates shined with CO2 laser will correct thermal effects in the PRC Shielded ring heaters will compensate HR surface deformations Green dots: shielded heating ring Blue rectangles: CP Compensation plates are suspended from the SA. Constraints from SAT and PAY must be taken into account in the design (geometry/position) of CP and RH Design of the new payload with CP and RH is in progress. LIGO-Virgo TCS meeting 4.02.2010
TCS performances investigation LIGO-Virgo TCS meeting 4.02.2010 • We simulated TCS performances with different CP (28 cm diameter) set-ups: • Changed position of the CPs: close and far from the TM • Changed thickness of the CPs: 3.5cm, 6.5cm and 10cm (with 3.5 cm the lower resonance is at 3 kHz) • Heating patterns generated by an AXICON based telescope as in Virgo+ • RH is always ON to correct ROC • Results given in terms of coupling losses, mismatch of the FP cavity beam with the RC beam, OPL, residual focal length.
TCS performances investigation 8 LIGO-Virgo TCS meeting 4.02.2010 • Effect of the absorptions on ITM with: • 0.6ppm (coating) and 0.3ppm/cm (substrate). From Advanced Virgo Conceptual VIR‐042A‐07 absorptions are for substrate and coating are 0.2‐0.3 ppm/cm and 0.3‐0.4 ppm respectively. Recently: from Pinard VIR-0555A-09, AdV substrates technical readiness review, values given were: less than 0.3ppm/cm and 0.6-0.7ppm, “state of the art at the present moment”. • FP cavity power ~ 800 kW (F=885, Pin=125W, Grec=23.5) • Absorbed power ~ 0.5 W • Beam size on ITM = 56 mm
Effect of CP thickness TM+RH HR face From a quadratic fit with Gaussian weights, the residual focal length is 320 km RH at 45 mm from the AR face. RH power need to recover the cold ROC (1416m) is 16.5 W. OPL corresponding to minimum L LIGO-Virgo TCS meeting 4.02.2010 CP “far” from the TM CP thickness 10cm, 6.5 cm and 3.5cm
CP close to the TM TM heated by radiation from the CP TM Tmap • CP radiates heat towards the TM • This heat escapes from the TM lateral surface • This radial gradient adds to that due to YAG absorption • Thermal lensing is increased and ROC is reduced • ROC depends also in the CO2 power • Less RH power is needed to keep the cold ROC • RH and CO2 power are strongly coupled • Thermo-structural analysis to find what is the parameters set (RH power, CO2 power and heating profile) that minimizes coupling losses and restore the cold ROC. HR face Residual focal length = 256 km CP - ITM distance presently fixed at 20 cm LIGO-Virgo TCS meeting 4.02.2010 CP thickness fixed at 3.5 cm
Shielded ring heater design guidelines Shielded ring heater is embedded in the reference mass. It is necessary to avoid any heat transfer between the ring heater and the RM. The heating element should have the highest emissivity, while its shield should have the lowest (e.g. gold coating). The heating element must be designed to avoid emitting any magnetic field that could couple with Advanced Virgo main beam or with local controls. Geometry, shielding, materials are being optimized HR Surface Developing FEA to optimize the position/power of the heating ring. Result for a TM heated by a simple ring - no shielding included yet, lower power is expected to give the same ROC correction. RH TM RH LIGO-Virgo TCS meeting 4.02.2010
Coating absorption increased to 1.2ppm (a factor of 2 wrt specs.) 12 Coating absorption increased to 2ppm (~ a factor of 4 wrt specs.) • Power absorbed by the TM ~ 2W • RH power is increased to 48W • RH temperature increases from 360K to 420K • Minimum coupling losses are1.3·104ppm Phase maps are being used in FFT optical simulations to investigate impact of thermal effects for different optical layouts Power absorbed by the TM ~ 1W RH power is increased to 28W RH temperature increases from 360K to 384K Minimum coupling losses are4.6·103ppm LIGO-Virgo TCS meeting 4.02.2010
TCS sensing • TCS sensing concept: same as Advanced LIGO. • Wavefront sensors will probe the input test masses individually while all TMs will be probed in reflection for change of the ROCs. • Degree of aberration will be manifest also in ITF channels, as it is in Virgo. • To sense the spatial structure of the cavity mode, phase cameras will sample the ITF beam. • The final TCS control will likely adopt a blend of these sensors as inputs LIGO-Virgo TCS meeting 4.02.2010
Comparing ANSYS results and measurements with phase camera – ITF input power 13W RED: thermo-optic+thermo-elastic BLUE: thermo-optic Gives a measurement of the power absorbed by the test mass: f = 405[m·W]/Pabs[W] Coupling losses reach a minimum for about 4.75W, compatible with the measured level of compensation: f3W=(15.8±0.6)km f4W=(24±2)km 14 LIGO-Virgo TCS meeting 4.02.2010
Comparing ANSYS results and measurements with phase camera – ITF input power 17W For 6.5W of TCS power, a minimum of the coupling losses is reached, compatible with optimal compensation, if we allow NI absorptions to be 30% less than on the WI f6.5W=(40±8)km f4W=(12.3±0.5)km f6.5W=(40±8)km f4W=50km Equal absorptions 30% less on the NI 15 LIGO-Virgo TCS meeting 4.02.2010