VAC – Vacuum system
modifications
ERC telecon, 24Mar09
A.Pasqualetti for the VAC team
1
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
2
TASK LIST 1/2
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)
3
TASK LIST 2/2
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)
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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
Goal pressures and phase noise:
Gas species
Hydrogen
Water
Air
Hydrocarbons
Total
Pressure (mbar)
Noise (Hz -0.5)
10-9
10-9
5 10-10
10-13
2.1 10-25
7.2 10-25
6.5 10-25
3.5 10-25
2.5 10-9
1.0 10-24
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CRYOTRAP DESIGN
Proposed goal can be met installing cryogenic traps at the tube extremities
and baking the tubes
•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
1.00E-04
1st section bake 16-24 May 2001
total pressure evolution
1.00E-05
pressure (torr)
1.00E-06
1.00E-07
1.00E-08
1.00E-09
bottle
tube
1.00E-10
0
24
48
72
96
120
144
168
192
time (hr)
•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’.
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CRYOTRAP DESIGN
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
The performances in other positions are comparable ,
and the difference in sensitivity is negligible.
Gas
species
Water
Total
proposed
proposed
Large trap Large trap
trap pressure trap noise
pressure
Noise (Hz -0.5)
(mbar)
(Hz -0.5)
(mbar)
-10
-25
2 10
3.2 10
7.10-11
1.9 10-25
1.7 10-9
8.3 10-25
1.6 10-9
7.9 10-25
‘large option’
trap position
•Cryotraps will also provide an increased pumping speed for condensable gases
(contaminants) improving the environment for mirrors. The original way to get a lower
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contaminants pressure (tower baking) is not feasible.
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). 10-9
Calculations with MonteCarlo
methods and F.E.M. models
(Comsol),
Water profile along the tube, torr
Trap 2.0m long, 1m inner diameter, 0.6m baffles
Load from towers 5E-4 mbar.l/s
Average water pressure~2E-10 mbar
Traps are calculated to maintain
the goal pressure (1E-9mbar) when
loaded by towers recently vented
(two days of turbo-pumping, as at
present).
-10
10
Tube lenght, m
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CRYOTRAP DESIGN
‘tower’
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
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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 frequently11
DIFFUSED LIGHT ISSUE
AdV follows the Virgo prescriptions against diffused light at arm tube ends
the 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
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•Detailed evaluations are in progress by J.Y.Vinet (OCA Nice)
THERMAL EFFECTS
The cryo-surface will induce thermal effects on the Test Masses by radiative heat exchange
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.
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ΔT is approximately 0.4K
TRAPS CONSTRUCTION (and OPERATION)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Design finalization, call for tender
Design finalization
technical specifications:
Call for tender call for tender
Contract assignment
approval of the executive design (prepared by Contractor)
Approval of theand
executive
start ofdesign
the production
16 17 18 19 20 21 22 23 24 25 26 27
begin 2010
within 2010
= 1 Month
within 2010
Production of thefirst trap
Production
Qualification
testsestimate of 14 months (8 months for the first trap , as qualification )
st
First trap on site 1 trap qualified in factory
call for
tender
mid 2010
remaining three ones
ready
Production of 3 traps
1st trap qualified in factory mid 2011
Acceptance
all traps ready
begin 2012
mid 2011
begin 2012
Transport on site
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
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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.
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