Click to edit Master text styles Second level Third level Fourth level Fifth level 1 CERN European Organization for Nuclear Research Graphic charter Fabienne Marcastel CERN DSU-CO 26 February ‘07 CARE-HHH Workshop, November 25 th, 2008 E-cloud mitigation: surface treatments Paolo Chiggiato TS-MME - Coatings, Chemistry and Surfaces- 1/20

GENERAL INTENTION Surface treatments aim at reducing the secondary electron yield of the beam pipe surfaces and, as a consequence, impede unduly electron multipacting. CONTENT  Starting point.  Bakeout temperature limitations.  Amorphous carbon and its relevant properties.  Vacuum issues for amorphous carbon.  Implementation of amorphous carbon in the SPS.  Final remarks. 2/20

Starting Point  LIM = 1.3  MAX = 2.1 SEY lower than 1.3 are needed to eradicate e-clouds in the SPS. For traditional beam pipe metals, after surface cleaning, SEY is higher than 2. 3/20 SEY can be reduce by: in situ bakeout (for T=300°C,  max of Cu: 2.3  1.5) increasing the electron impingement dose (conditioning): fully conditioned surface for C mm -2.

Starting Point Most of the Long Straight Sections of the LHC are coated with Ti-Zr-V :  more than 1200 vacuum chambers were produced;  about 15 Kg of Ti-V-Zr is spread over 6 Km of LHC beam pipe;  vacuum commissioning successfully concluded. Lower SEY are obtained for Ti-Zr-V coatings after heating in vacuum at a temperature as low as 180 °C. 4/20

Bakeout Temperature Limitations But, for new CERN projects, heating temperatures are limited severely :  SPS-Upgrade:  The vacuum chambers cannot be heated (embedded in the magnets).  CLIC positron damping ring:  heating temperatures shall be presumably limited to 150°C mainly because of SC wigglers and beam monitors.  PS-2:  Maximum heating temperature to be defined, but most likely as low as possible. Preliminary conclusion: when the vacuum system can be heated at temperatures higher than 180°C, the ecloud eradication solution exists (NEG coatings) with additional benefits of:  extremely high distributed pumping speed;  low photon, electron and ion desorption yields. 5/20

Bakeout Temperature Limitations Possible Solutions To abandon the search for low SEY surfaces and opt for clearing electrodes installed along the vacuum chambers. F. Caspers T. Kroyer To find out other thin films with an intrinsically low SEY. To render the surface rough enough to block secondary electrons. … or both combined Lower activation temperature NEG No need of heating once in vacuum By machiningBy chemical or electrochemical methods By coating 6/20

Bakeout Temperature Limitations N. Rey Whetten, J. Appl. Phy. 34(1963)771 The ideal film material :  has intrinsically low SEY;  is not prone to adsorb water vapor, oxygen and hydrocarbons;  can be easily deposited on stainless steel beam pipes;  is compact, smooth and not inclined to produce dust;  is UHV compatible;  has possibly low resistivity. Graphite could be a good compromise. ! 7/20

a-C Thin Films Goal: to produce low SEY carbon films, ideally graphite thin films. However, carbon films are neither pure graphite nor pure diamond. In general they are amorphous (lack of long-range order). Locally, the carbon orbital hybridization can be diamond-like (sp 3 ) or graphite-like (sp 2 ). Our aim consists in producing amorphous carbon (a-C) films with the highest fraction of sp 2 hybridization Source: Wikipedia ‘hybridization’ sp 3 sp 2 a-C films produced by magnetron sputtering are graphite-like. 8/20

a-C Thin Films: SEY Results SEY for a-C coatings does not depend on coating thickness in the range nm a-C film on copper deposited by magnetron sputtering (Ne discharge gas) CNe9B 9/20

a-C Thin Films: Adhesion Good adhesion, no loose particles. Measurements performed with an optical particle counter give the same indication 10 μm 2 μm Courtesy of S.Heikkinen TS-MME-MM a-C coatings are used in the Industry to produce very smooth surfaces 10/20

a-C Thin Films: Pumpdown Measured after 10h pumping (subsequent to 1h exposure to the air). Effect of discharge gas pressure. -Reduced nanoporosity at lower discharge gas pressure [F.Rossi et al., J.Appl. Phys. 75, 3121, 1994 ]. -Possibly because the growing film is bombarded by higher energy neutrals -The fine tuning of the sputtering parameters is in progress… Preliminary results Nominal sputtering parameters Discharge gas pressure 10 times lower Outgassing rates normalized to st. steel 11/20

a-C Thin Films: ESD e - current: 1 mA e - energy: 500 eV a-C coating Schematic view of the ESD system Electron stimulated desorption of a-C 12/20

a-C Thin Films: PSD Photon Induced Desorption, ESRF dedicated beamline, Ec=20.5 KeV Ongoing measurements Courtesy of R. Kersevan Preliminary results Critical Energy 20.5 KeV Angular acceptance mrad Photon Flux (E>10eV) 2.94x10 15 photons (s mA) -1 Beam Energy 6 GeV Typical Beam Current 185 mA Angle of incidence 25 mrad 31 13/20

a-C Thin Films: Measurements in the SPS - The a-C coating gives times current compared to SS; consistent with measured δmax. - same results obtained after 15 days exposure to the air (MD run w33) or 2 months in the SPS vacuum (MD run week 41) Set-up: a-C coated liner with a strip detector in dipole magnet with 1.2 KG field. Beam: 2-3 batches, 72 proton bunches, 25 ns spacing, 450 Gev/c, MD run w.28 St. steel: δmax=2.5 TiZrV coating activated δmax=1.1 a-CNe δmax= /20

a-C Thin Films: the Hot Topic  max SEY deterioration as a function of air exposure time Electron doses not cumulated -  max is below 1.3 for air exposures up to some 20 days. -The best coatings have about 1.1 after 40 days in air. -Maximum air exposure time for the application should be specified. max 1.4 min /20

- Advantage: low SEY, lower deterioration in air - Drawbacks: it requires 2 subsequent coatings + higher outgassing a-C/Zr a-C Thin Films: a-C on rough coatings  max as received 2h in air after coating after 4 months exposure to the air /20

 Length 6.5 metres.  Weight ~10 tons.  Radioactive.  About 700 magnets to be coated.  Use the dipole’s magnetic field for the magnetron effect.  Foreseen coating time: three annual shutdowns (two arcs per year). a-C Thin Films: Implementation the SPS Magnets Preliminary target: 3 magnets ready for test in the SPS on March /20

The SPS magnets coating system: designed by Giuseppe Foffano TS-MME a-C Thin Films: Implementation the SPS Magnets 18/20

Vacuum Chamber vacuum chamber Graphite Cathode Cathodes Insertion Mechanism a-C Thin Films: Implementation the SPS Magnets The SPS magnets coating system: designed by Giuseppe Foffano TS-MME 19/20

Final Remarks Ecloud effects can be mitigated by TiZrV coating if the vacuum chamber can be baked at temperature higher than 180°C. Their properties depend on the sputtering parameters. At the present, an optimization is in progress to reduce their  max, outgassing rates and deterioration provoked by long lasting exposure to the air. A prototype of the coating bench for the about 700 vacuum chambers of the SPS arcs is being produced and assembled. Three vacuum chambers are to be coated and installed in March ’09 for tests. I warmly thank the support of the SPS-U team, in particular Elena Shaposhnikova and Roland Garoby, and the member of the CCS section for their enthusiasm and invaluable ability. In case of heating temperature limitations, as for the SPS, sputtered a-C films are a potential solution. The first CERN a-C coating was produced last year, on November 13th. The first tests with a-C coated liner in the SPS are encouraging. 20/20

THE END

Starting Point Fully conditioned surfaces N. Hilleret, EPAC 2000 Effect of bakeout SEY can be reduce by IN SITU high temperature treatments (BAKEOUT) and increasing the electron impingement dose (CONDITIONING).

Starting Point 3mm wires of Ti, Zr and V

Bakeout Temperature Limitations  SEY high for insulators (for example MgO, CsI, Al 2 O 3 and diamond)  SEY low for light metals (Ti) and graphite  Strongly dependent on surface cleanliness, oxidation and roughness  Strongly dependent on impinging electron dose. Many atomically clean elements and their compounds fulfill the  max <1.3 imposed by the SPS.  But when exposed to the air their SEY increases steeply, resulting in  max higher than about 1.5: oxides formation and water adsorption.  An additional and progressive rise is recorded during months of stay in the air, eventually leading to  max higher than 2: airborne hydrocarbon adsorption. N. Hilleret, EPAC 2000 proceedings Ding, Tang, and Shimizu, J. Appl. Phys. 89 (2001) 718

a-C films produced by magnetron sputtering are graphite-like. Sputtered C atoms have an energy of few eV. Magnetron sputtering does not provide enough energy to the deposited C atoms to produce a highly packed diamond-like film: the displacement energy of C atoms in graphite is about 20 eV. beam pipe wall graphite rod + + -U discharge gas sputtered C atoms B E a-C Thin Films: Deposition Technique Magnetron sputtering is the ideal deposition technique for long beam pipes.

a-C Thin Films: Implementation the SPS Magnets Coating pace: about 90 chambers per month, namely 4 to 5 per day. On the coating bench, a batch of 5 chambers remains 2 days: first day in the afternoon for installation and pumping overnight; second day for coating, third day in the morning for disassembling. Two coating benches are necessary! ECX5 cavern in the SPS underground is available.The cavern is a 20- meter diameter cylinder. Room for two coating benches and a storage area have to be fitted in it.

We can profit of the experience acquired during the refurbishment of the cooling circuits of 255 dipole magnets, running over three years : Repairing pace: 4 to 5 magnets per day Buffer of 10 magnets in ECX5. Handling. Synchronized logistic. Courtesy of S. Sgobba TS-MME-MM a-C Thin Films: Implementation the SPS Magnets

a-C Thin Films: Dust Production Two identical SS tubes measured in clean room TUBE + REF TUBE coated with carbon REF remains in the coating lab. (closed with plastic covers) TUBE + REF measured again in the clean room Dust: not an issue Measured with an optical particle counter - Same result for size above 5 µm; - No increase after shaking and gentle hammering of the chamber; - No increase for a chamber left in the air for months. intake cycles Number of particles Particle counting measurement