Dust issue of Vacuum System Notes for discussion 31 Jan 2017

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 1997 - 2003 Construction, 2004 - 2006 Commissioning 2007 - … Physics from 2006 Amsterdam from 2009 Budapest Warszawa

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)

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

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,

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) …

The Central Hall

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

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)

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

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

Ø = 2 m 6/7

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 = 7. 3 cases directly related to venting operations.

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

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

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

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

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)

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)

‘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?

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

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)

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

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

Particles speed meter first look Example http://www.virgo-gw.eu/advirgo/protected/monolithic/Repository/VCI-Vacuum%20Investigations/dust%20velocimeter/1st_trial/apertura%20flusso.wav

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

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

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

UHV towers venting-pumping duct sketch 1 separated Vent Scroll TMP Remote Pumping (scroll + TMP) filter 2 3 4 Scroll or Multiroot TMP 29

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)

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

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

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? …

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?