Surface treatments for vacuum applications at CERN

Transcription

1 Surface treatments for vacuum applications at CERN Paolo Chiggiato Technology Department Vacuum, Surfaces & Coatings JRC-CERN collaboration workshop, CERN, January 27 th, 2014

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3 Outlook Vacuum materials require surface treatments Chemical surface treatments Coatings 4 Future developments 3

4 1 Vacuum materials require surface treatments Low degassing Low secondary electron yield Low or selected surface resistance Achievable only by appropriate surface treatments Surface treatments Any deliberate modification of the surface properties of vacuum materials to cope with the accelerators requirements. 4

5 1 Surface treatments are strategic for accelerator operation and new projects CERN investment: new building for chemical surface treatments (and special printed circuits) 5

6 1 Vacuum materials require surface treatments Oils, dirt, silicone, C x H y, H 2 O, Cl,, Me x O y Gross Contamination Sorption Layer (~nm) Oxide Layer (1 10 nm) Solvent or detergent cleaning Chemical pickling Dislocations, voids, inclusions Damaged Layer ( mm) Etching or electropolishing Coating Undamaged Metal 6

7 2 RFQ of the Linac4 7

8 2 Radio Frequency Quadrupoles (RFQ) for the Linac4 8

9 2 LHC beam screens with cooling capillaries 9

10 Electropolishing of Cu 2 Chemical polishing of Cu LHC RF cavities HIE-ISOLDE RF cavities Courtesy of Serge Forel 10

11 3 Coatings modify the surface properties Removal of charges from insulators Reduction of friction Lower surface impedance Coatings Lower SEY Deposition techniques: Electroplating. Physical Vapour Deposition. Lower desorption CVD and ALD: studies ongoing. Modified optical properties Distributed pumping 11

12 3 Coatings: Cu electroplating Electroless deposition of LHC s VCTYB 12

13 3 Coatings: Cu electroplating Electroless deposition of LHC s VCTYB 13

14 4 Coatings: sputtered Ti-Zr-V Ti Distributed pumping speed Nominal composition: Ti 30 Zr 30 V 40 V Zr Activation temperatures: 180 C for 24 h 14

15 3 Coatings: sputtered Ti-Zr-V 15

16 LHC Long Straight Sections MAX IV at Lund (S) 3 11 th June 2013 MAX IV - 3 GeV storage ring: Circumference 528 m, 20 sectors. Courtesy of Marek Grabski A B m C Photon beam A C Electron beam B Ø22 7 Ø

17 4 SEY Coatings: sputtered Ti-Zr-V Primary electron energy [ev] 17

18 3 Coating: Sputtered Carbon Courtesy of Pedro Costa Pinto Coating by hollow cathode sputtering

19 3 Coating: Niobium for superconducting RF cavities LEP 350 MHz > 250 pieces: Industrial collaboration LHC 400 MHz 30 pieces Industrial collaboration DC Magnetron, Nb (1-2 µm) Purity (vacuum!) and substrate preparation (electro polishing or chemical polishing) are crucial. Developed at CERN in the 80 s, presently applied to HIE-ISOLDE cavities 09 January 2014 Surface Chemistry and Coating team 19

20 3 Coating: Niobium for superconducting RF cavities HIE-ISOLDE 100 MHz 20 pieces high beta, production started 12 pieces low beta, design phase 09 January 2014 Surface Chemistry and Coating team 20

21 4 Future developments Less harmful chemical products for cleaning and electropolishing. NEG coatings in confined structures (we have requests for coatings of 4-mm diameter beam pipes). Understanding of the role of coating parameters on the SEY of carbon films. Applications of alternative coating techniques: ALD and HiPIMS. 21

22 Acknowledgements I wish to thank all my colleagues of the Vacuum, Surfaces and Coatings group of the Technology Department at CERN, including students and visitors. The VSC group has become a centre of excellence in Europe encompassing all aspects of vacuum technology, from surface treatments to computation, operation and control of unique facilities. 22

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24 Additional slides 24

25 2 Cleaning Procedure Yes Removal of gross contamination Solvent cleaning Heavy contamination? Yes Rinsing in tap water Pickling Rinsing in tap water No CERN standard: Ultrasonic cleaning in detergent Solvents perchlorethylene vapour (121 C) Heavy oxidation? No Detergent Stainless steel: P3-ALMECO 18 (ph=9.7) at 65 C Al and Cu: NGL sp ALU at 45 C Rinsing with deionised water jet Rinsing by immersion in deion. water Pickling St. steel: HF+HNO 3 solut. Drying in a filtered air oven at 150 C St. Steel Deionised water resistivity 1 M cm. 25

26 2 O 2 Plasma Cleaning Courtesy of Pedro Costa Pinto and Sergio Calatroni - Applied at CERN for hazardous materials: beryllium. In situ pure O 2 plasma removes carbon from the substrate without hazardous waste. + - Standard magnetron-sputtering assembling Injection of O 2 (about 10-2 Torr) C CO The central wire is polarized positive relative to the chamber Reverse sputtering is started CO O O + 2 O C -> CO + CO 2 Key issues: Chemical sputtering yield >> physical sputtering yield maximize evacuation of CO and CO 2 O 2 + energy about 100eV Successfully applied to the LHCb Be chamber CO 26

27 2 Methods for the evaluation of surface cleanliness Wettability test Surface sensitive techniques (for example AES, XPS, FTIR ) Radiotracers Functional characterization: outgassing rate, ESD yields and SEY In-situ measurements 27

28 3 Removal of the Damaged Layer Dislocations, voids, inclusions Damaged Layer ( mm) Etching or electropolishing Undamaged Metal The damaged layer is removed to: Reduce surface roughness Eliminate inclusions and embedded contaminations. Two methods: Electropolishing Chemical attack or chemical polishing 28

29 Preparation for Electropolishing 3 Removal of the Damaged Layer: Cu for SC RF Cavities Degreasing by detergent Sulphamic acid pickling Pickling: HCl SUBU5 ( ** ) : 40 min, 20 μm x2 Passivation: Chromic Acid Rinsing Electropolishing ( * ) : 60 μm (1h) Removal of lump particles x3 Sulphamic acid pickling Rinsing with demi water and alcohol Nb coating High Pressure Water Rinsing Courtesy of Serge Forel Welding ( ** ) ( * ) H 2 SO 4 (96% mol) : 90 % vol. HF (40% mol) : 10 % vol. Sulfamic acid (H 3 NSO 3 ): 5g/l Hydrogen peroxide (H 2 O 2 ): 5% vol. Butanol (C 4 H 9 OH): 5% vol. Ammonium citrate: 1g/l 29

30 4 Pickling Rinsing Electroplating Degreasing 30

31 4 Effective desorption yield [molecules photon -1 ] Coatings: sputtered Ti-Zr-V Dose [photons m -1 ] Activation (2.3x10-3 ) = 0,38 D -1 = 4,2 x 10-5 D -0,38 Photon desorption at the ESRF (6.1x10-6 ) Dose [ma h] 31

32 4 Ti-Zr-V in Future Circular Collider Extruded Al Beampipe Lead Shielding Preliminary study: FCC in 80-km or 100-Km tunnel. NEG Coating Cooling Channels NEG coating production rate: distributed in three years, 100 chambers per week, was 20 during LHC production, feasible with industrial cooperation (ex. LEP RF cavities, European XFEL, ) 32

33 4 Coating: Sputtered Carbon Ti-Zr-V film coating have d max 1.1 after activation at temperature higher than 180 C (24h). But they cannot be applied to unbaked vacuum chambers like those of the LHC s injectors and LHC s beam pipes at cryogenic temperatures. Carbon coatings deposited by sputtering are a valid solution. 0.2 μm 33