Thermal Compensation System

Roma Tor Vergata

Outline

• Updates

– TCS positioning – TC system – Status of the quotations requests – Installation planning – Laser power stabilization

• Referee comments

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

After last Virgo Week, a meeting with all the people (software, electronics, infrastructure and vacuum) involved in TCS installation and the Detector Coordinator on September 17

th

.

•UHV ZnSe window •Vacuum cabling and feed-through •Control software for stepping motors •Electronic boards and cabling for stepping motors •Laser chiller •Acoustic enclosure •Defined position of the optical benches

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Optical benches positioning A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review Oven

This allowed a final design of the Optical Imaging System

Definition of the optical components completed last Friday (October 12) Defined list of lenses E. Genin A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

UHV ZnSe viewport DN100CF Clear aperture 90mm Size of the first in-vac steering mirror increased from 140 to 190mm A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Dimensions of the optical tables defined (E. Genin, A. Pasqualetti)

750mm 1800mm A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

NOT IN SCALE CO 2 Laser Beam Dump AOM All quotes in millimeters l /2 waveplate Polarizer L7 L8 492 810 (762) Beam Dump Power Meter 537.5

Components in the blue box are used for laser power stabilization, will be installed by mid 2008 198 L1 AXICON L2M L4 HeNe Aiming Laser A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review LX To in-vacumm steering mirror

Two more CO

2

ULO Optics optics companies contacted: Laser Research Optics

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Provisional installation planning

Starting date depends on arrival time of components Duration of the single tasks agreed with all the people involved Duration of the whole TCS installation approx 3 weeks Manpower: A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

CO

2

laser power stabilization

(TCS phase 2 – Mid 2008)

25W CO 2 Laser PD PD AOM

Critical components

RIN



P Area inc



D

* A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Referees’ comments (R. Flaminio)

Referee: “The second important element of the discussion is the system foreseen to provide the required thermal compensation. Originally two systems had been considered: CO2 laser and heating ring. The main reason for choosing the laser system were two: 1) it is more flexible i.e. it is possible to heat the mirror not only at the border but also in the centre.

2) it is 'easier' to implement as all the components stay in air.

In practice both these reasons do not apply completely. First according to the simulation I had the impression that we need to heat the border of the mirror not the centre.

Second, as it is described in the document, the implementation of the laser system is rather complex; two remotely controlled relatively large mirrors need to be installed inside the vacuum system as no window are available to look at the mirrors. Two optical benches (2mx0.75m) need to be installed outside of the ovens. The beam has to be sent from these benches into the ovens and then into vacuum chamber. As one wants to have power fluctuations at a relatively small level both the bench and the optical path need to be protected with an appropriate acoustic isolation.

Defending the idea that this is easier than installing a ring heater inside the vacuum chamber is a challenge.”

We agree with the referee in considering the installation of the CO 2 laser TCS more complicated than it has been on LIGO. We considered the possibility to use a ring heater, as it seems a much easier task. However, there are a couple of considerations that make this solution impractical on Virgo/Virgo+ A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Shields Ring heater Markers A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

To solve the previous problem, the heating ring could be embedded inside the reference mass, but...

…this would require a deep investigation: geometry of the Reference Mass itself; the path of the thick cables carrying the ring heater high power supply; the behaviour of the Tekapeek CF30 under such high thermal load (plastic deformation, melting, creep, emission of unwanted pollutants…) A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Ring heater

Referee: “It is said that only 100 mW are required in LIGO because the mirror are precurved. what are the numbers ? What is the LIGO nominal curvature?

What was the pre-curvature? How much power was expected to be absorbed in the LIGO input mirrors? How much was actually found to be absorbed?

I'm asking these questions because some time ago some persons of the LIGO lab told me that the precurvature was wrong as the absorption was considerably different than those initially expected. Can the authors clarify these points?”

Originally the LIGO 1 Interferometer was designed to be a point design with respect to input power. The radius of curvature of the Recycling Mirror was ground to be to concave to match the effective curvature of the Input Test Masses when the instrument was hot. It was anticipated that power absorbed in the ITMs would create a thermal lens that would alter the effective radius of curvature of the ITM as seen from the power recycling cavity side such that it matched the recycling mirror. This relied on accurate knowledge of the absorption coefficients of the ITM substrates and coatings. These are poorly controlled parameters. The Hanford 4K interferometer has been shown to absorb excess power in the substrates and hence achieves an optimum power-recycling cavity at only 2.5 Watts of input power. The Hanford 2K and Livingston Interferometers do not reach the optimum operating point with the designed six watts of input power.

For this reason, LIGO people faced two different problems: on some of the optics they needed central heating; other ITMs showed an excess of absorption (about a factor of five for the worst case) and needed annular heating.

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Referee: “It is said that the losses (better to say absorption) "resulted to be about a factor 6 and a factor 2 higher than the nominal value ..".

Would it be possible to have the actual numbers?”

According to our simulations, the absorptions of the Virgo ITMs are 7.7ppm and 2.3ppm for the West and North input mirrors respectively. We must not forget that all the simulations based on the frequency shift method done so far rely on the knowledge of the elastic parameter dependency on temperature.

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Referee: “It is written that the power needed to compensate for Virgo is 2W. the comparison with the 100 mW needed for LIGO is impressive. Especially if one considers that the finesse of the LIGO cavities is 4.5 times higher than Virgo (should make the thermal problem in LIGO worst). Maybe the answer to question 1) and 2) will clarify also this point.”

Besides the above considerations, there is a mere geometrical factor to take into account. There is an empirical formula to determine roughly the power needed to compensate, given a certain amount of absorbed power:

P TCS



P abs

  

R ITM s

  2  1 .

4 Where P abs is the power absorbed by the optics, R ITM is the radius of the mirror and s is the beam spot size on the ITM and 1.4 is a “refinement” factor found with the simulations. In the case of Virgo the numbers are: P abs =36mW, R ITM =0.175m and s=0.03m, thus P TCS ~2W. While in LIGO, R ITM =0.125m and s=0.04m, so for the same amount of absorbed power, P of 3.5 lower than the Virgo case.

TCS is ~0.6W, a factor A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Referee: “It is said that the power stabilization needed for Virgo+ is 3·10 -7 at 30 Hz. It might be interesting to give the specification as a function of the frequency (or a reference to a document containing this graph). what is the security factor taken in giving this specification?”

  1  10 22  ztot   z1   z   zrad  1  10 23 Sh fv )   24 1  10 24 10 10 100      fv

Black

: Virgo+ sensitivity (with monolithic suspensions)

Red

: total TCS noise

Blue

: TCS flexural noise

Green

: Thermoelastic+ Thermorefractive

Pink

: TCS rad.pressure

1 10 1 10 3 When performing noise calculations, the RIN has been considered to be flat over the whole frequency range of interest (10-1000 Hz). In this case, the security factor is about 2 @ 30Hz and about 3 @ 100Hz. This is why we need a power stabilization level better than 3·10 -7 /√Hz @ 30Hz.

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Referee: “About the TCS servo it is said that two bull eye detectors have been implemented in LIGO. Even if their experience is that one is sufficient, this may depend on the differences between the losses in the two input mirrors. I would recommend implementing two readout systems as they did.”

We agree with the referee. Besides the bull’s eye detectors, LIGO uses the so called “pick-off port”, placed inside the recycling cavity, as the second error signal. In Virgo there is no such port. We are investigating the possibility to use the signal from B2.

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

Referee: “The description of the commissioning activity looks like 'once the TCS is installed, the interferometer is locked and the CO2 is switched on ... then we will try". I agree that things will certainly goes like that.

On the other hand one can try to foresee some commissioning time required to characterize the TCS (e.g. measure the time response of the system, ..).”

Together with E. Calloni (responsible for the TCS Servo Task), we agreed in the following commissioning provisional planning: around two weeks will be necessary to characterize the actuator, i.e. to measure the open loop transfer function (apply some TCS power and measure the resulting change in ITM ROC and time constant). One week is needed to set up the error signal. Finally two more weeks are necessary to develop and implement the control loop.

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review

A. Rocchi - 16.10.2007 - 2 nd Virgo+ Review