1. Virgo Vacuum System – 3G challenges
Workshop on Large UHV Systems
for Frontier Scientific Research
LIGO – LLO, January 29-31, 2019
C.Bradaschia, A.Buggiani , A.Pasqualetti, D.Sentenac, T.Zelenova on behalf of
Virgo collaboration / EGO – Vacuum team

2. VIRGO experiment
a Gravitational Waves Detector designed, built and operated by a collaboration made up of 20 laboratories in 6 countries.
200+ Researchers and Technicians
Construction,
Commissioning
V, V+
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3. SUMMARY Virgo vacuum system Considerations about 3G arm tubes
General description of the apparatus
‘3 km tubes’ realization and costs
Experience, selected issues
Considerations about 3G arm tubes
LIGO VACUUM WSHOP - Jan '19

4. VIRGO SITE
2 UHV arms x 3 km
Large chambers and cryogenics at extremities
ET - Hannover Jun '18

5. VIRGO SITE LOCATION

6. VIRGO VACUUM CHAMBERS Main chambers one per core optics
Large Vacuum valves up to ø= 1 m
77K cryostats to pump water vapor
Arm tubes
LIGO VACUUM WSHOP - Jan '19

7. SNAPSHOT of the vacuum level
4.0-E09 mbar
arms at ∽ low E-9 mbar
towers at ∽ low E-8 mbar
H2O + H2 + Air
5.0-E09 mbar
2.0-E09 mbar
1.0-E08 mbar

8. ‘Towers’ chambers Ø = 2m, up to 11m high, 20-30 ton. Unbaked.
House crucial optics and complex mechanics (stiff frequency response).
Neet to allow clean and easy access. Metal seals or single o-rings.
View of central towers
Optics degradation risks
Heavy organics: materials screening and pre-baking . PHC< 1E-13 mbar (theoretical ).
Cleanroom class 100 (even if w/out strictly laminar flow).
Electropolishing, baking in situ before optics inserption .
New req.t for 3G: particles concentration on chamber walls .
Clean rooms under floor
LIGO VACUUM WSHOP - Jan '19

9. Towers: two (three) vacuum levels
Upper Part
10-6 mbar
Differential pumping
Lower part
10-8 mbar

10. PUMPING SYSTEM Statistic (not updated) 22 Roughing/backing dry pumps
Main requirements: oil free pumps against contamination risk, low acoustic / seismic / magnetic emissions , long maintenance intervals to care for duty cycle.
Gauges .
Statistic (not updated)
Roughing/backing dry pumps
Turbo-molecular pumps
Ion pumps
Titanium sublimation pumps
Residual gas analyzers
221 Angle valves
Gate valves
Large valve ø=1000 mm
Gauges
Control Rack and its operation panel for a ‘tower’ pumping station
Tower synoptic

11. LARGE CRYOGENIC PUMPS
Installed between unbaked ‘towers’ and baked ‘tubes’
Added in a second stage
COLD BORE
300 l capacity, aluminum, 2 m long , 0.6/0.9 m inner aperture
Function is to condense water vapor coming from towers (escaping fraction = 3%).

12. 77K CRYOGENIC EQUIPMENT
Refill operations during weekly ‘maintenance breaks’ (3 ‘horizontal tanks, 50 m3 total capacity). This procedure ‘influence’ the general schedule.

13. 3 km UHV TUBES
LIGO VACUUM WSHOP - Jan '19

14. They contain ‘only’ optical baffles and the laser beam
3 km UHV TUBES
They contain ‘only’ optical baffles and the laser beam
Ø = 1.2 m
LIGO VACUUM WSHOP - Jan '19

15. TUBE DESIGN: THE MODULE
Raw material 304L (1.4306), plain wall 4 mm thick with stiffeners every 1.2 m
Prefabricated modules to be joined by welding
Length = 15 m balancing costs of fabrication, transportation, installation
Bellows to allow heating up to 150°C (80 kgf/mm)
Metal sealed flanges, a few on 3 km

16. TUBE MODULES JOINTS TIG Welding, 2 mm depth (no backside purge gas)
Nibbling, 3 mm OFF
20.5 mm, N. 3 trims
module 1
module 2
2 mm
LIGO VACUUM WSHOP - Jan '19

17. TUBE FLOOR (TUNNEL)
Discontinue floor slabs supported by pillars (2 x ø=500mm every 15 m).
Ground subsidence (relative movements).
LIGO VACUUM WSHOP - Jan '19

18. TUBE SUPPORTS A complex component:
allow longitudinal displacement (module thermal espansion)
electrically isolated (to allow tube heating by DC current)
allow vertical (and transversal) position adjustment to cope with ground subsidence
stiff enough to not transfer mechanical stress onto ‘lip’ joint
feature fiducial marks for tube alignment
allow ‘fine’ adjustment if feet position for module installation
significant impact on cost
LIGO VACUUM WSHOP - Jan '19

19. AIR-BAKE OUT
Base material conditioning was required to meet vacuum goals (24000 m2 walls). baking at ∼ 400°C in air involved a ‘’simple’’ oven and reduced the walls outgassing by a factor ∼ 100; our result: q(H2) ≤ 3E-14 mbar l 20°C Original studies pointed to ‘bulk ordinary sites role’, not conclusive (effects of high energy sites, oxide layer, recomb. kinetics …) (*) Initial hydrogen content ≤ 2 ppm wt CONTRACTUAL
Applied to finished modules
Electrical oven, ‘sealed’ modules with inner air flushing
410°C +20/-10 , plateau 72h
Hot air purge 8 m3h-1
5 days long cycle
(*) Advances reported in literature possibly could bring to lower the engineering specification of h2 outgassing for the future case.
We shall set up a more accurate meas. of hydrogen outgassing of both arms by a set of calibrated gauges and RGAs
LIGO VACUUM WSHOP - Jan '19

20. TUBE MANUFACTURING
AISI304L cold rolled sheets / solution annealed - surface finish 2B (supplied by Avesta)
Conventional’ industrial tools, Rate = 1 module / day (it took about 2 years).
- modules realized in 3 consecutive cylinders plus the hydroformed bellows.
- UHV recipes (qualified machining oil, dirt free rolling, separated halls and tools, ...)
Circumferential + longitudinal plasma welds
LIGO VACUUM WSHOP - Jan '19

21. TUBE BAFFLES made out by air-baked 304L 160 baffles per arm
make the modules non-interchangeable
LIGO VACUUM WSHOP - Jan '19

22. DUST CONTROLLED WORKSHOP
Cleaning bath room (hot alcaline sol. and deminer. rinsing )
Oven (4 modules)
Post baking geometry II
Test bench: He leak detection
(3E-10 mbar.l/s) + RGA (Σ>44)
- useful , for instance, to monitor the effectiveness of the rinsing process -

23. TUBE LOGISTIC
Transportation (resp. transfer), sealed packaging and modules respiration along the trip, storage needs, thermal insulation installation. Rate up to 30 m / day (2 modules)
LIGO VACUUM WSHOP - Jan '19

24. TUBE ON-SITE INSTALLATION
Rate = 1 to 2 modules / day
yard arranged in line: civil works, geometry, module positioning, welding, vacuum tests
Installation inside ‘tunnel’
Theodolite + GPS positioning
Supports are pre-installed every 15 m
final tube position accuracy is  5 mm
Joint by automatic orbital welding:
Gap between modules ‘lips’ must be within 0.5 mm
LIGO VACUUM WSHOP - Jan '19

25. ARM TUBES: BAKE-OUT in situ
Chamber at 150°C uniform and at a controlled rate ( 1 week for SAT stage)
1 Mwatt to heat one tube (15 cm thick thermal insulation)
Joule effect: 2000 A flowing through tube walls
diesel generators:  10^5 litres of fuel to bake one tube
electrical distribution, fumes and heat exhaust to be addressed
Normally to be performed just one time.
ET - Hannover Jun '18

26. ARM TUBES: Assembly strategy
Module #0 : .....……...first produced module tested for ougassing to qualify the industrial process
Sections : .....… tube was assembled in separated ‘sections’ (300 m to 1500 m long) which were vacuum tested to verify the assembly work .
3 km Tube : once completed, it remained to leak test only the
few joints lastly done between ‘sections’
The entire 3 km tube was not baked (not yet)
2 leaks / 400 joints [contractual, LR < 3E-10 mbar.l/s]
≈10-7 mbar for early Virgo
≈10-9 mbar for Advanced Virgo unbaked (H2, H2O)

27. TUBE PUMPING STATIONS
Every 600 m - Turbomolecular pump 1000 ls-1 for the intermediate stage and baking - TSP/IP pumps 2000 ls-1 H2 for the permanent phase - Gauges (separated chamber for easy maintenance, pros and cons) - Valves: “Viton” for the gates (‘evacuated’ to limit permeation). Maintenance may be difficult (venting). Ti Ti

28. LARGE VACUUM VALVES
4 DN1000 Valves to isolate the ‘tubes’ from the ‘towers’
Stainless steel body, air-baked (*)
Metal sealed
Viton o-ring on the gate (single)
Bakeable at 150°C
Tested for outgassing
Cost: important
7 DN650/400 Valves to isolate each ‘tower’

29. TUBES MAINTENANCE
Due to ground subsidence tube foundations are sinking up to 1 mm/month. Tubes are surveyed and periodically realigned to limit mechanical stress.
External cleaning of ‘tunnel’ building is performed once per year.
LIGO VACUUM WSHOP - Jan '19

30. TUBES – SOME WEAK POINTS
1 ) Operation: the relative low capacity of installed TSP (N2) +
2) the coming air from ‘towers’ (recharged at every venting) involves frequent (>= weekly) regeneration.
3) Operation: valves permeation (gate seal compression set?): to be evacuated/blanked OFF.
4) Leak risk: Vulnerability of pumping ports to leaking risks (for instance due to baking cycles of ancillary chambers of gauges).
5) Leak risk: IP HV ceramic connectors (cables) exposed to breakage.
Foto ports tubo

31. VIRGO APPARATUS COSTS Construction schedule
... Early studies and prototypes
1994 – Virgo project official startup
1996 – Design finalization
1997 – Tube contracts setup
– Towers construction
1999 – CB towers site qualification tests
– begin of tube construction
2001 – begin of tube installation
2002 – completion of vacuum enclosures
2003 – full ITF
Construction Costs (apparatus)
75 M€ - Nov.2000 Council report
Vacuum
Others
(tunnel = 9 M€)
LIGO VACUUM WSHOP - Jan '19

32. VACUUM CONSTRUCTION COSTS
Tube Inst.
10.1 M€
Tube Manuf.
Nov.2000 report
LIGO VACUUM WSHOP - Jan '19

33. ENCOUNTERED ISSUES
LIGO VACUUM WSHOP - Jan '19

34. ‘WINDOW’ SEPARATIONS
Vacuum tight glass separation are in use to allow beam passage between chambers at different vacuum levels. May become limiting for the experiment (or for vacuum if removed). Not a valid solution for critical areas.
LIGO VACUUM WSHOP - Jan '19

35. VIEWPORT RISKS
Order of 70 viewports needed, mostly standard ones. Dedicated policy against breaking risks in force. Further mitigation actions shall be implemented (external screen).
Breaking event of a viewport, 2008
Risk = defect + stress x time
Glass/KOVAR joint design was the origin of the stress (SSV et al.)
LIGO VACUUM WSHOP - Jan '19

36. LN2 BUBBLING
LN2 boiling inside cryostats is a possible source of noise (mechanical vibrations): accurate design to avoid ‘heat concentration spots’ , seismic isolation of the cryostat, large walls opening.
Frequency , Hz
Increase of the eismic vibrations of cryostat walls due to LN2 bubbling
LIGO VACUUM WSHOP - Jan '19

37. DUST PARTICLES vs QUARTZ FIBERS
Dust particles of a few µm and travelling at some m/s inside chambers has been recognized as the main cause of failure of quartz fibers
Venting circuit re-designed
Primary pumps (scroll type) replaced
Guards added to fibers
failure test: few µm size particles projected against a fiber
0.4 mm
Electrostatic forces may play a role…

38. MIRRORS CHARGING
It is the present issue, we are studying the LIGO experience.
LIGO VACUUM WSHOP - Jan '19

39. CONSIDERATIONS about 3G UHV TUBES
LIGO VACUUM WSHOP - Jan '19

40. 3G TUBES THOUGHTS The ‘extended 2G technology’ as baseline solution:
Estimate for 40+2 km, ø = 0.9 m / 0.75 m (based on enquiries to 2 large companies + 2G experience) 95 M€ - possibly optimistic (based on not compelling quotations, 2G style ‘not-inclusive’ contracts).
Raw material incidence < 20% of the installed pipe cost (estimate).
Lifetime / Structural aging: normally > 2G one
Scientific obsolescence: ultimate vacuum performances (3G+ ?)
Mandatory to explore existing technologies in the widest prospect.
LIGO VACUUM WSHOP - Jan '19

41. 3G TUBES THOUGHTS
A possible contribution about ‘extended 2G technology’ as baseline solution: Study of the industrialization of the ‘2G tube’ in collaboration with Pisa University / Engineering companies (local experience on submarine gas pipeline). The ‘proposed’ tube diameter would be the first input needed.
Cheaper methods should be available for ø ≈ 0.7 m (‘belt’ process)
Tube profile
Given beam profile (not in scale)
Example (R.De Salvo)
Arm lenght [m]
LIGO VACUUM WSHOP - Jan '19

42. End
LIGO VACUUM WSHOP - Jan '19

43. PROPOSAL PROPOSAL for a NEXT MEETING at EGO supported by S. Katsanevas
LIGO VACUUM WSHOP - Jan '19

44. CONTROL SYSTEM One ‘control rack’ per ‘station’
Logic of operation is managed by a PLC
SW interlock loops by Supervisor
Data fully integrated in Virgo DAQ Hz
Flux ∽20 kb/s (< 0.1 % of the total)

45. SIMPLIFIED P&I SKETCH
LIGO VACUUM WSHOP - Jan '19

46. ORIGINAL DESIGN PRESSURES
UHV region
2 tubes+7base towers
H < 1E-9 mbar
H2O+N2+… < 1E-10 mbar
HC < 1E-13 mbar
230m3 HV region
6 upper towers + DET/INJ/MC
Total < 1E-6 mbar
HC < 1E-8 mbar
LIGO VACUUM WSHOP - Jan '19

47. Pressure trend follows TSP regeneration (film capacity for nitrogen).
TSP OPERATION
Pressure trend follows TSP regeneration (film capacity for nitrogen).
Residual ‘Air’ shall decrease (progressive implementation of metal seals)
RGA trend (28), uncal

48. OPENING ARMS at ITF RESTART
(RGA uncal)

49. ASSEMBLY YARD at MIDDLE ARM
EXTERNAL CONSULTANTS SUPPORT for CONTRACTS MANAGEMENT
CENTRAL AREA
MODULES ASSEMBLY PATH
TUNNEL
DOWNLOAD 3%
CLEANING
38 x 24 m
PROVISIONAL STORAGE AREA 7%

50. TUBE MODULE FABRICATION

51. SOME DATA of PRE-BAKED TOWERS

52. ARM TUBES: PUMPING STATIONS
Distant pumps is the 2G solution to exploit tube conductance and low walls outg. rates (every ≥ 300m: intermediate stage & baking, permanent service) .
Redundancy : help for operation, maintenance and running efficiency
Distributed pumping: an option to be explored.
normally needed to push the vacuum limits
implementation in a ‘big’ GW pipe is to be studied.
Virgo – active stations
In Virgo si ottiene circa 1E-9 (N2 eq.) unbaked, a torri isolate, staz. attive circa ogni 1000m
speed vs distance
(Optical baffles not accounted)

53. ET -L (credits GL et al. Symp.18)
ET Vacuum – ‘size’
ET triangular - DS
3 x (HF+LF) 10 km arms
(+ filter cavities)
Virgo
3 km arms
(+ 300m FC)
ET -L (credits GL et al. Symp.18)
1 x (HF+LF) 15 km arms
(+ filter cavities)

54. ‘Tower’ chamber contamination phases
HEPA FLUSH
HEPA FLUSH
Operation
<1E-7 mbar
Venting
Control speed and shock waves
Maintenance
Particle counter, up to class 10000
Cleaning
Gas jets, class 0 if without personnel
Roughing
Slow speed.
Valves operation?

55. SNAPSHOT at CLOSED ARMS
‘Closed arms’ (+ winter)
2.0 E-10 mbar
3.0 E-10 mbar
3.0 E-10 mbar

56. DUST PARTICLES vs QUARTZ FIBERS
The last stage of seismic attenuator is realized with fibers made of fused silica (0.4 mm diameter).

57. VIEWPORT STUDY (SSV)