1. Dust issue of Vacuum System Notes for discussion 31 Jan 2017

2. Virgo a Gravitational Waves Detector
Firenze-Urbino
Genova
Napoli
Perugia
Pisa
Roma
Trento
Centre
National de la
Recherche
Scientifique
Annecy
Lyon
Nice
Orsay
Paris
Istituto
Nazionale di
Fisica
Nucleare
200 Researchers and Technicians
Construction,
Commissioning
… Physics
from Amsterdam
from Budapest
Warszawa

3. It is located at Cascina, Pisa
EGO – the European Gravitational Observatory – is a consortium created by the VIRGO funding institutions (CNRS for France and INFN for Italy)

4. SUMMARY VIRGO/EGO (presentation, AdV) Vacuum System overview
Fibers breaking events
Tests done (summary)
Modifications overview (see slides from last VW)
New design details
New Pump experience
Venting equipment, materials

5. VIRGO VACUUM CHAMBERS ‘Towers’ 7+3 chambers (core optics)
Large valves up to 1m diameter, to isolate and access each ‘tower’
high or low vacuum 3+5 chambers for optical benches and other apparatuses
Large 77K cryostats to pump water vapor
‘Tubes’
Contain just the laser beam
Lenght = 3000 m, Ø = 1.2 m,

6. Vacuum System West Tube 3 km North Tube 3 km Total volume of 7000 m3
Two long tubes Ø=1.2m below 1E-8 mbar
10 vertical chambers (towers) containing the core optics (in yellow) …

7. The Central Hall

8. ‘Towers’ (lower part) during installation
‘lower’ part of a tower chamber
raw material = 304L air-baked at 400°C

9. Core optics inside Towers
Dust and contamination would increase scattering and absorption by optical surfaces . When vented, ‘towers’ become a white room (class 100 or better)

10. The payload = the core optic with last stage of seismic attenuator mechanics.
Last suspension section is realized with fibers made of fused silica (0.4mm diameter).
During the past months we had failures of fibers when in service under vacuum

11. V+ Beam Splitter mirror inside the ‘tower’
Carlo Bradaschia
31/10/2002
V+ Beam Splitter mirror inside the ‘tower’

12. Ø = 2 m
6/7

13. Fused silica fibers failures
Strength is due to defects in the material: the larger the flaw the lower the strength.
A crack can be produced on fiber surface when hit by a (≈ 10um) dust particle travelling at some speed; crack will then propagate and failure will occur after time (a few sec or hours/days depending on crack characteristics and stress field )
No other failure circumstances experienced in test conditions
Total failures = cases directly related to venting operations.

14. Vacuum Investigations
Pumping/Vent line
Vent
Scroll pump
A weak point vs dust for pumping/venting pipe

15. Vacuum Investigations
Tests at 1500W vacuum chamber.
The chamber was equipped with a test fiber with nominal load (+ shadowmeter and 120 fps camera).
We reproduced the failures during venting process using the same piping circuit of the towers.
Dust was collected on Si-wafers for analysis

16. Vacuum Investigations
F. Sorrentino
Dust on Si Wafer from test chamber
Dust on Si Wafer n.4 in NI tower
Piece #5
Piece #2
Piece #4
Piece #3
Piece #1
Raman spectra anlysis : No conclusive indication on the origin of the particles is found.
Further analysis are underway.
Dust abundance on 1500W smallest chamner :
~100 particles on a surface of 2 mm X 75 mm
=> estrapolated ~ 3000 on the disk
=> ~0.7 particles/mm2

17. Vibrations induced by venting
The signals show that the fiber modes are excited immediately at beginning of the venting. While the mode at 3 Hz (rocking mode) looks loud during the entire venting process, the violin modes (first around 440 Hz) calm down just few seconds after the venting starting. During the process the violin mode excitation amplitude was about a factor 3 higher than the previous hammer test (‘moderate hit’ by hand of the vacuum chamber base)
Note that fiber mount used in the test chamber is slightly different (no anchors) and venting circuit is more exasperated than towers one

18. Indirect test: clean venting
Aim of the present test is to verify the fiber integrity in presence of several evacuation/venting cycles once removed the suspected failure mechanism = the scroll pump operating on the same venting line .
21/12 evacuation
Note: slight contact
27/12 12:16:00 LT vent safe mode ( with 'shielded' inlet port): contact removed
27/12 14:54:50 LT start evacuation
28/12 16:07:15 LT vent through test port ( = without scroll pump, with new bellows)
29/12 10:17:00LT restart pumping
29/12 15:03:30 LT vent through test port
29/12 17:46 LT restart pumping
30/12 10:28:52 LT vent through test port
30/12 12:09 LT restart evacuation (with turbo)
02/01 10:06:28 LT vent through test port …
STOP/VENT on 09/01 (then breaking load test)

19. Hints about vacuum effects
Further tests undergoing to explain all cases of fibers failure (= not including venting propulsion)
Particles bursts from scroll pumps being checked with ‘velocimeter’
Then same by valves motion and turbomolecular pump (inhaled or backstreamed?)
Shock wave effects at opening/closing of (small) valves at the end of rough phase (with low pressure differences , < 1 mbar)
Electrostatic effects
Others
Small cracks produced during in air phase can evolve and propagate faster once under vacuum?
Other potential effects due to vacuum? (Coefficient of friction typically increase -> 1)

20. ‘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
Speed?
Valves operation?

21. dust accumulation example
‘Tower’ chamber
dust accumulation example
Dust collected on a wafer put inside WI tower (V+) after some operation cycles, showing high content of large size particles

22. Cleaning process
Aim: remove particles that would migrate with cycles, in particular the larger ones (10 µm). Some stages:
strip coating material ‘first contact’ applied on critical optical surfaces (or removed, difficult for larger parts…)
Blow CO2 snow / N2 ionized gas on walls and parts surfaces (many insulators) in presence of clean air flow ≈ 0.5 m/s
Smaller particles likely difficult to be reduced (higher adhesion forces, electrostatic effects)
Series of evacuation/venting cycles monitoring dust on Si-wafers to verify that migration is effectively reduced (for larger particles)
Solvent wiping not foreseen (not easily applied in situ)

23. Dust migration effects enhanced by electrical forces?
Simulation of electrostatic field on the fibers, showing that it could increase the effective cross section of the fibers for incoming particles.
Particles falling from top
This effect could influence also mirror contamination.
A. Chincarini

24. Velocimeter test ‘Test Venting’ Through pumping pipe
‘Clean Venting’ (separate pipe)
G. Pillant

25. Particles speed meter first look
Example

26. UHV towers venting-pumping duct sketch
Present (V+ and initial AdV)
Scroll
TMP
Pumping/Vent line
Vent
Scroll
TMP

27. UHV towers venting-pumping duct sketch
Present (V+ and initial AdV) pumping
Scroll
TMP
Pumping/Vent line
Vent
Scroll
TMP

28. UHV towers venting-pumping duct sketch
Present (V+ and initial AdV) venting
Scroll
TMP
Pumping/Vent line
Vent
Scroll
TMP

29. UHV towers venting-pumping duct sketch1
separated Vent
Scroll
TMP
Remote
Pumping
(scroll + TMP)
filter 2 3 4
Scroll or Multiroot
TMP 29

30. Towers apparatus modifications
separation of venting/pumping duct to avoid injection mechanism
in vacuum dust filters integrated in pumping line to block dust backstreaming from pump
shield / porous diffusers in the venting duct to disperse flow and avoid ballistic trajectories of dust
substitution of scroll pumps with another model not producing dust to cancel one source of dust
setup of a permanent diagnostic system (Si wafers and vacuum pipes dust monitor)

31. Scroll pump
Towers are pumped with ‘Scroll’ pumps, both for rough evacuation and for backing turbo-molecular pumps.
They produce dust particles by wearing of seals and other parts.
Dust can migrate in the pumping duct (against the flow direction due to vibrations …) 31

32. Scroll pump - substitute pump
Experience on small size dry pumps, multiroots Pfeiffer ACP as example (or Ebara PDV / Kashiyama NeoDry)
EXAMPLE

33. Notes for discussion
Dust particles management during pumping/venting processes
Dust migration experience (10um > 1m/s more relevant in our case), maximum speeds requirements , design criteria
Diffusers
In vacuum dust measurements methods
In situ cleaning methods
Example of dust contamination req.ts on vac chambers? …

36. Venting - new equipment
Design and equipment choice to be finalized:
Shields at ports outlets to disperse flow, limiting the migration of particles .
(for venting pipes, not for rough pumping or turbo-pumping …). Examples?
Gas diffusers ? Setup and equipment choices for Virgo needs?