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VAC – Vacuum system modifications

ERC telecon, 24Mar09A.Pasqualetti for the VAC team

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SUMMARY

• Task list

• Main changes: cryotraps

scientific motivations

vacuum performances

cryostat design

LN2 plant layout

thermal effects and diffused light issue

construction plan

• Other modifications

• Construction planning

• Manpower

• Costs estimate

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Vacuum upgrade: The improvement of the Virgo sensitivity by a factor of 10 needs an upgrade of the present vacuum level to lower the phase noise due to residual pressure statistical fluctuations.The solution is to install 4 cryogenic traps, the associated tasks to be undertaken are:Task 1.Cryotraps 4 cryogenic traps (Nikhef)Task 2.LN2 plant liquid nitrogen plant for the 4 traps (InfnGE)Task 3.Vacuum accessories sensors, pumps, valves (EGO)Task 4.Thermal effects on TMs study of the induced effects on optics (Roma2)

Tasks 1-3 includes design, procurement and installation. The installation work is shared with EGO.

Present link diameter (400mm with 250mm valves) is not fitting with the increased beam radius and with secondary beams transmitted between towers. A baseline design exists, with a cost estimate, and will be finalized together with optical layout. The work can be considered straightforward. Task 5.Enlarged Links tube links between central towers (EGO/tbc)

TASK LIST 1/2

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AdV optical configuration and control strategies require changes of up to about 1m in the position of the principal mirrors. Task 6.Towers displacement displacement of selected towers as required (EGO)

The contamination reduction during the payload integration phase within the vacuum chamber is a crucial item. A system to direct a quasi-laminar air flow within the vacuum chamber shall be realized making use of the present clean air apparatus, with the addition of metallic gratings inside each tower. Task 7.UHV clean air flux improvement of the air flux inside mirror towers (LAPP)

The SR tower has to grow by 3 vertical rings to host a full size superattenuator. Also the separating roof is included. All this will be a simple replica of existing objects. IB and DT towers will grow by one ring, to allow longer SA with improved performances.Task 8.Towers upgrade upgrade of SR tower and rings for IB,DT (LAPP)

Vacuum control system requires to be rebuilt, since designed more than 10 years ago , based on components no more available on the market. This procedure is already started and has to be strongly boosted for sake of AdV. It will be necessary to upgrade also the control logic, both hardware and software, to include new devices.Task 9.Control System upgrade integration of new vacuum equipment

renewal of the SW&HW obsolete parts (LAL)

TASK LIST 2/2

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Cryogenic traps, Vacuum system main change

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MOTIVATIONS for UPGRADE

Design sensitivity of Advanced Virgo (green) modified by present excess gas noise (dotted), (residual gas pressure at 10-7 mbar, dominated by water).AdV would be limited at about 1.10-23 Hz -0.5

Proposal: keep residual gas noise at least a factor of three below the AdV design sensitivity.• safely below the noise due to more fundamental sources • improvement of vacuum level by a factor 100

Gas species Pressure (mbar) Noise (Hz -0.5)

Hydrogen 10-9 2.1 10-25

Water 10-9 7.2 10-25

Air 5 10-10 6.5 10-25

Hydrocarbons 10-13 3.5 10-25

Total 2.5 10-9 1.0 10-24

Goal pressures and phase noise:

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•Bake-out apparatus is a present feature of Virgo, already tested and operated in the past. Here the total pressure evolution and a check of the outer insulation by thermocamera

•It has been decided that baking of ‘towers ‘ is not acceptable because too risky and excessive time consuming.• Cryogenic traps are the classical solution for the wanted change: they are cryostats cooled by liquid nitrogen, installed between ‘towers’ and ‘tubes’.

CRYOTRAP DESIGN

Proposed goal can be met installing cryogenic traps at the tube extremities and baking the tubes

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

0 24 48 72 96 120 144 168 192

time (hr)

pre

ssu

re (

torr

)

bottle tube

1st section bake 16-24 May 2001total pressure evolution

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The preferred position is between towers and large valves, by far the more easy and less risky position to install the trap.

•The Internal Review Committee has examined the trap design and endorsed the proposed choice

CRYOTRAP DESIGN

•Cryotraps will also provide an increased pumping speed for condensable gases (contaminants) improving the environment for mirrors. The original way to get a lower contaminants pressure (tower baking) is not feasible.

trap position

The performances in other positions are comparable , and the difference in sensitivity is negligible.

‘large option’

Gas species

proposed trap pressure (mbar)

proposed trap noise (Hz -0.5)

Large trap pressure (mbar)

Large trap Noise (Hz -0.5)

Water 2 10-10 3.2 10-25 7.10-11 1.9 10-25 Total 1.7 10-9 8.3 10-25 1.6 10-9 7.9 10-25

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CRYOTRAP DESIGN

Dimensions of the cold surface: length 2.0m, inner diameter 1.0m. Baffles of aperture 0.6m will be added for diffused light and thermal mitigation. Main parameters are the transmitted fraction of molecules, (of the order of 1-2%, function of the aperture/length ratio) and the pumping speed (proportional to the aperture).

Traps are calculated to maintain the goal pressure (1E-9mbar) when loaded by towers recently vented (two days of turbo-pumping, as at present).

Water profile along the tube, torrTrap 2.0m long, 1m inner diameter, 0.6m bafflesLoad from towers 5E-4 mbar.l/sAverage water pressure~2E-10 mbar

Tube lenght, m

Calculations with MonteCarlo methods and F.E.M. models (Comsol),

10-10

10-9

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CRYOTRAP DESIGN

Reference mechanical design consists of a stainless steel cryostat, containing about 200 liters of liquid nitrogen.•Lenght=2.02m, Inner diameter=1.0m•MLI is employed in the ‘isolation vacuum’ compartment. ‘Consumption’ is about 300watt. •LN2 is continuously fed and level is automatically adjusted •Special design options are taken to reduce the bubbling noise

‘tower’

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LN2 SUPPLY PLANT

LN2 reservoirs, 10000 liters capacity Superinsulated lines are 8-9m long(Single reservoir possible)A ‘phase separator’ shall be included in the transfer line

Overall consumption ~1400 liters/day (4 complete systems)Running cost ~0.2 eur/liter (+ reservoirs rent)

Refill every 2 weeks or less frequently

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DIFFUSED LIGHT ISSUE

AdV follows the Virgo prescriptions against diffused light at arm tube endsthe minimum free aperture radius is about 5 times larger than the average beam radius; clipped intensity fraction: 10-22

•any discontinuity (potential reflecting spot) of the vacuum enclosure is hidden by suitable absorbing glass baffles, with respect to the beam spot on any mirror

•no point of the smooth surface of the vacuum enclosure can be seen contemporarily by the beam spots on two facing mirrors

•Detailed evaluations are in progress by J.Y.Vinet (OCA Nice)

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THERMAL EFFECTS

Two independent fem analyses agree that the thermal and structural effects of the cryotrap on the TMs are negligible. (the change in the radius of curvature is of the order of 2 m, to be compared with an absolute value of about 1500m)Moreover, these effects have opposite sign with respect to those given by the YAG power absorbed by the TM, thus giving a small help to TCS.

TM temperature map obtained with FEM thermal simulation. ΔT is approximately 0.4K

The cryo-surface will induce thermal effects on the Test Masses by radiative heat exchange

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Design finalization, call for tender

technical specifications: begin 2010 call for tender within 2010approval of the executive design (prepared by Contractor)

and start of the production within 2010

Production estimate of 14 months (8 months for the first trap , as qualification ) 1st trap qualified in factory mid 2011 remaining three ones ready begin 2012

Installation 1.5 months/trap , 6 months in total. Work shared between responsible lab and EGO

Bake-out will take 1 month per arm. It shall be performed when needed for sensitivity reasons or convenient for commissioning, not necessarily immediately after the traps installation.

Operation and maintenance‘regeneration’ of traps involve a stop of the interferometer:Regenerations are expected once per year or less frequently.1 week shall be sufficient to regenerate 1 trap, conservatively. Normally, all traps shall be regenerated together to save ITF time

TRAPS CONSTRUCTION (and OPERATION)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Design finalization = 1 Month

Call for tender

Contract assignment

Approval of the executive design

Production of thefirst trap

Qualification tests

First trap on site

Production of 3 traps

Acceptance

Transport on site

call for tender mid 20101st trap qualified in factory mid 2011all traps ready begin 2012

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ENLARGED LINKS

Other modifications of the vacuum system are required by a change of the optical design:

Enlarged diameter pipes are needed between the ‘towers’ of the central area (‘links’). A vacuum valve of diameter 650mm (or 800mm) shall be installed to isolate towers during ventings.

The link design shall be finalized once selected the optical configuration.