10-07-08 ILIAS WG1 meeting - Birmingham 1

Cryogenic Trap Vs Brewster window& noise coupling through windows

J. Marque

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Motivations

The Brewster window was supposed to be the main coupling path to the dark fringe for acoustic and seismic noises in the central building.

Below 100Hz Above 100Hz

AC off test

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Cryo Trap benefit (1)

Removed some Brewster characteristical structures

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Cryo Trap benefit (2)

Highly improved stationarity respect to alignment fluctuations / bad weather

Before

After

Before

After

Before After

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Cryo Trap isolation performances (1)

Cryo Trap Residual Gas Analyser

SR valve DT valve

-196 degrees (at this temperature hydrocarbons will condensate when hitting the tube)

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Cryo Trap isolation performances (2)

DT contamination level

DT RGA (blue) vs Cryo Trap RGA (black)larger by > 10 timesbefore DT opening

after DT opening

CryoTrap RGA: no increase of contamination on SR/BS side after opening to DT

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Cryo Trap isolation performances (3)

after DT openingafter SR/BS opening

CryoTrap RGA: no significant increase of contamination on SR/BS side after opening to SR

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Cryo Trap maintenance requirements

• LN2 consumption : 2 refill / week (Friday-Tuesday), tolerating a 3-4°C of warmup of the upper side

• LN2 delivery every 2 weeks on tuesday: small truck at central building, ‘lift’ needed, time of arrival difficult to be programmed

• ‘regeneration’ (needed every ? Months) maybe take more than 1 day. To be studied

• LN2 supply for V+ to be discussed, solution possibly implemented during the summer stop

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What was the noise coupling mechanism?

Would have been interesting to understand exactly what was the coupling mechanism not to repeat this mistake in the future…

We have still windows: input/output tank windows, Injection Brewster windows.

- Elasto-optic effect (seismic/acoustic produce a variable stress on the window. The elasto-optic effect translates that into index variation, which modulate the phase of the beam)?- Back-scattered light (some light will recombine to the main beam phase modulated by the scattering object motion)?- Clipping / scattering on tubes can be excluded (many diaphragm baffles have been installed before and after the Brewster without any improvement)

…

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Concrete example: the detection output window (1)

After first relock with the cryo trap: noticed coherence with detection tower accelerometer attached close to the output window.

Cryogenic trap requires the detection turbo pump to be always on (was always off with the Brewster window)

Config: Cryotrap and turbopump on

Config: Brewster and turbopump off

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Detection turbo pump problem “fixed” with bellow installation (2)

Installation of a bellow between detection tower and turbo pump. Turbo pump clipped to the platform.

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Detection turbo pump problem “fixed” with bellow installation (3)

Turbo on

Turbo off

Comparison between:SR+DT turbo pumps offSR on + DT offSR on + DT on with bellow

Remaining coherence: there is still something!!!!

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A tentative projection of the noise coupling through the detection output window (1)

Assumptions:- the turbo pump noise path to dark fringe was all through the DET window, and not for example through the detection bench. This assumption is supported by the fact that the seismic noise produced by the pump is larger on the flange than on the bench.

- the Em_SETODE01 accelerometer is actually measuring the vibration of the DET window. This sensor is attached to the tower flange very close to the DET window, and perpendicular to it. The coherence is however not so good: it is between 0.1 and 0.4.- the noise coupling is direct, not through upconversion (this is supported by the fact that there is some coherence).

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A tentative projection of the noise coupling through the detection output window (2)

Transfer function, in units of strain/m, between the sensitivity and the seismometer. Indicated in red are the coherent frequencies (coherence > 0.1). That is were the measurement makes sense, elsewere it is just an upper limit of the real TF.

Projection of the detection flange seismic noise. The upper plot shows how the projection reproduces the dark fringe noise structures at 100-200Hz and 350Hz before the DET turbo pump was damped. The bottom plot is the projection after pump was damped. In both cases the red line is an upper limit of the noise.Good reason for putting the diodes under vacuum

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Noise coupling through the end towers output windows

We have no measurement… But since we know that here the problem can only be backreflected or backscattered light AND since we have to replace this window (damaged by the hublot implosion), it is time to speak about requirements.

1) Backreflected light: the mechanical mount of the window will be changed so that the window will be tilted by a few degrees. The window will be AR-coated in LMA (<100ppm).

2) Backscattered light: here are the requirements for the polishing and corresponding amount of scattered light.

- Cosmetics 20/10 => TIS=150ppm- Roughness: <0.5nm RMS => TIS=35ppm

Theoretical (Parks, 1980), on 100mm diameter

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Conclusion

The cryotrap fixed the Brewster link noise coupling.But, so far, we are not sure about the coupling mechanism.

- The detection output window is the new dominating coupling path => diodes into vacuum.- The end towers output window will be carefully analysed.- What about the injection Brewster? The input window?