Review of Vacuum Technology Issues for Low- Emittance Rings R .Kersevan , CERN-TE-VSC-IVM

1. Review of Vacuum Technology Issues for Low-Emittance RingsR.Kersevan, CERN-TE-VSC-IVM Low-e Workshop, Oxford, 8-10 July 2013, J. Adams Inst. Accel. Studies

2. Review of Vacuum Technology Issues for Low-Emittance Rings - R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Agenda: Intro: Vacuum and Stored Beams, An Often Difficult Relationship… Challenging Issues (Poll Results) Conclusions

3. The residual gas composition and pressure inside an accelerator, and the vacuum chamber geometry, surfaces, and commissioning are among the most important parameters determining the properties of the beam, both directly (e.g. scattering) or via indirect effects related to vacuum and the specific choice of the vacuum system The direct effect (pressure, gas composition) generally involves scattering of the stored beam on both nuclei and electrons, leading to a reduction of the beam lifetime, increased radiation background, radiation damage, personal safety issues, etc… The indirect effect can be due to one or more of the following mechanisms or problems: Heating e.g. SR on uncooled component  additional thermal desorption  air/water leak HOM Trapping  additional thermal desorption, beam de-tuning, accelerated saturation of NEG pumps and coatings Electron-cloud  beam detuning, additional thermal desorption, additional load on cryogenic devices,… Increased SR desorption  e.g. due to saturation of pumps or coating  increased scattering Non-optimal cleaning/conditioning of the vacuum system  generation of high-Z gas species  increased scattering … and more… Modern vacuum science and technology has, during the past decades, always been able to find solutions to all of these problems, but the ”next generation” of accelerators always poses new challenges. Low-emittance storage rings are no exception to this rule, possibly requiring new tailored solutions Review of Vacuum Technology Issues for Low-Emittance Rings - R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Vacuum and Stored Beams: An Often Difficult Relationship…

4. I’ve asked 3 vacuum scientists/engineers in charge of 3 SR light source projects, which, in their opinion, had been up to now the most challenging issues they had to face I am deeply indebted with E. Al-Dmour (MAX-IV), R. Molina-Seraphim and T. Mendes da Rocha (SIRIUS), and H.(Dick) Hseuh (NSLS-II) for sharing with me their experiences Challenges: BPM location within the cells and BPM block/electrodes analysis, design and fabrication Very little space left for stand alone BPM units: extremely difficult to find space for bellows on both sides. Result is one bellow is on BPM block, the other is at other end of chamber welded onto BPM block. Danger of breaking the delicate BeCu sliding joint blade/fingers Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 2. Challenging Issues (Poll Results)

5. Impossible to design the vacuum system compatible with in-situ bake-out The compression range of the bellows is far too small for the DT needed by bake/NEG activation The radial gap between chambers and magnetic elements is too small to fit in a thermal insulation jacket This forces the vacuum engineer to design an ex-situ bake-out system (as done in DIAMOND and ALBA, for instance): in case of vacuum accident, at least 5 days of intervention are foreseen, with large implications on the experimental program if the accident happens in the middle of an experimental run Choosing copper as the material for the vacuum chamber extrusion forces the implementation of a stricter chemical cleaning procedure, employing non-environmental friendly substances (ammonium persulphate followed by chromic acid passivation) NEG-coating of narrow crotch areas is proving very challenging Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 2. Challenging Issues (Poll Results) Crotch area test: SIRIUS, e-beam welded, machined Cu blocks Note: 1 Euro is NOT the cost of such a component!

6. Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Source: E. Al-Dmour, SIRIUS Vacuum System Review Workshop, Mar 2013

7. Many simulation tools are needed, often for the design of components which are, strictly speaking, not vacuum components: BPMs, kickers, RF cavities, SC devices (undulators, magnets), etc… Although some industries have bought the NEG-coating license from CERN, the most difficult geometries still need direct CERN involvement and/or support/troubleshooting Wish: It would be nice to have a pooled-type approach to this (and related) sputtering technology, where several labs join together their efforts (manpower and financial resources) Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 2. Challenging Issues (Poll Results)

8. Purpose: remove the oxide layer (around 50um) of the surface of the tube. Total length of tubes to be etched = 890 m. 32 tubes/day R&D: Effect on tolerances. Effect on brazing joints. New support structure for mass cleaning. Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 MAX-IV: Etching of Cu tubes at CERN prior to NEG-coating

9. Degreasing: Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Ethanol/Acetone soaked rag cleaning, 3. Immersion in detergent solution with solution circulation for 5-20 minutes each tube and ultrasonic agitation (NGL 17.40 sp Alu III at 15 g/l at 55 °C), 2. High-pressure rinsing with demineralised water using rotary nozzle (Kärcher), 4. Rinsing with demineralised water and drying with N2. NEG-coating of MAX-IV chambers: Surface preparation at CERN 1. Etchingat room temp. for 30 minutes (100 g/l ammonium persulphate solution), rinsing, 2. Passivation at room temperature for 5 minutes (chromic acid solution at 80 g/l + 3 ml of Sulphuric acid 96%), rinsing with water and ethanol, drying.

10. Area for fast corrector SIRIUS and MAX-IV envisage 1 mm-thick Cu chambers almost everywhere in the achromat sections, with very tight tolerances on the whole process of manufacturing, NEG-deposition, testing, storage and installation in the tunnel Connection/integration of ceramic inserts for fast orbit correctors is difficult (>1kHz). No space for flanges, direct integration into the Cu chamber, and needed coatings (Ti for electrical conductivity, NEG). Impedance requirements stemming from very short bunch lengths mandates smaller and smaller radial gaps between chambers and components. Custom-designed ConFlat (or other) seals are necessary, calling for extensive prototyping and testing Very high SR power densities at some absorbers: tight space between dipoles and following magnetic element makes removal of power a difficult task, and localized lumped pumping difficult to implement. Somewhat challenging design of crotch absorbers Careful analysis and design of absorbers and crotches is mandatory when IDs with large horizontal (elliptical) radiation fan are installed. Even if made out of copper, 1 mm thick-chamber wouldn’t survive a prolonged exposure to such high power densities. Calculate carefully the tails of the transversal power AND FLUX distribution, to avoid localized pressure bursts. Remember: flux distributions ≠ power distributions! Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Example: absorber for ID SR. Temperature (°C) 2. Challenging Issues (Poll Results) Stress (MPa)

11. Flux(red) is flatter and broader than Power (blue) distribution; Solid lines: Horizontal Dashed lines: Vertical 10 units ~ 1 mrad Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 SYNRAD+ simulation of NSLS-II DW90 radiation (cosine approx.) 40x 9-cm, Bmax=1.85 T periods; K=15.55; qmax=K/g=2.65 mrad CAREFUL RAY-TRACING WITH REAL MAGNETIC FIELD MAPPING IS NECESSARY FOR HIGH-POWER IDs !! Power and Flux Ray-Tracing on +/- 5 mrad Screen Perpendicular to Beam Direction

12. Storage Ring Vacuum Systems • Vacuum chambers made from Al extrusion • Low dipole field of 0.4 T (ρ = 25 m) ➾ narrow BM photon fan ➾narrower chambers • ➾ ➾extrusion possible • Adequate beam aperture – 25 mm (V) x 76 mm (H) • With ante-chamber for pumping and for photon extraction • Smooth cross section changes with tapers: ≤ 50 mr • Minimum steps or cavities < 1 mm • Surface roughness < 1 μm Beam . Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 ID fans • P(avg) < 1x10-9Torr(> 80% H2, < 20% H2O, CO, CO2, CH4, …) • Շ (beam-gas) > 30 hr(inelastic scattering) • σZ2 – minimize high Z gases such as hydrocarbon • Fast conditioning after intervention • In-situ baking for all components • All metal, radiation compatible • TSP/IP at photon absorbers; NEG strips for linear pumping • NEG cartridges at DW ABS and IVU A more orthodox approach: NSLS-II (courtesy H. Hseuh) E beam

13. A more orthodox approach: NSLS-II (courtesy H. Hseuh) Cell Vacuum Chambers and Cross Sections Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Short chamber cross section Dipole cross section Multipole cross section M D GV Front End ID beam line S S BLW M BLW BM beam line BLW D BLW M • ~ 220 long aluminum chambers, 3 – 5.5 m long • > 12 types (length, magnet cutouts, diag. etc.) • > 60 short Al chambers, 0.5 – 2.5 m long • 90 inconel chambers for fast correctors • ~ 10 narrow/long insertion device chambers…. GV

14. A more orthodox approach: NSLS-II (courtesy H. Hseuh) Chamber Extrusions and Bending • 2 vendors for • > 180 multipole extrusions (Taiwan) • > 150 dipole extrusions (US) • Incoming extrusions were measured to • ±1 mm / 5 m • Dipole extrusions were bent at BNL • 6o bend (ρ = 25m) • Thermal cycled twice at 180oC • Re-measured to <1.5 mm/3m Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Extrusion Press Bending at BNL Spacer blocks Shims Bended extrusion

15. A more orthodox approach: NSLS-II (courtesy H. Hseuh) Extrusion Machining at Vendors Dipole Multipole • Machine the magnet cut-outs • Machine the ends • Machine the side and pump ports Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Machined Machined Crotch absorber Stick absorber Pump port Pump port < 2 mm clearance 3m long, 6o bend, ρ = 25 m

16. A more orthodox approach: NSLS-II (courtesy H. Hseuh) Machining of Extrusions by 3 US Vendors High precision machining using 3-axis, 7m NC machines Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Dipole Extrusion Machining Multipole Extrusion Machining

17. A more orthodox approach: NSLS-II (courtesy H. Hseuh) Chamber Welding at APS / ANL • Weld bi-metallic flanges to adapter plates • Weld adaptors to extrusion ends. • Weld side port and pump port flanges • Rout the beam channel weld beads • Bake out and vacuum certification Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 full fusion weld w/o step 4mm Exit port in S6 Weld side ports Weld end joints Note: Pictures taken at Argonne National Laboratory

18. A more orthodox approach: NSLS-II (courtesy H. Hseuh) Chamber Assembly at BNL Welded chambers are measured for dimensional accuracy • Assemble in a clean room with • 4 Beam position monitors(BPM) • Be-Cu RF shield • NEG strip assembly • Photon absorbers(ABS) • Cal rod heaters (~1kW/m) • Ion pump, TSP, CCG and RGA Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 Be-Cu RF shield with 50% openings BPM ABS BPM BPM

19. A more orthodox approach: NSLS-II (courtesy H. Hseuh) NEG Strip Assembly in Ante-Chamber Two NEG strips (top/bottom) in ante-chamber Riveted mounting on stainless carrier plates every 10 cm with ceramic bushings Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013 NEG carriers and mounting NEG strips in antechamber Activation at ~ 4500C x 20 min (30 A thru NEG strips)

20. Features and challenges of NSLS-II’s vacuum system Low field BM and large bending radius resulted in narrower SR fan, therefore chambers can be extruded with not-so-wide cross section Tolerance of extruded cross section is ±1mm and need good machining to achieve tight tolerance required for SR fan and BPM mounting Limited vertical opening with extruded profile, therefore very compact ABS dimension inserted from the side, all SR fan is on normal incidence instead of complicated ABS geometry Alignment of BPM to 100 μm is a challenge, so are others imposed by Diagnostics such as shielding of ante chamber… AP group requested larger active interlock envelope for e beam caused (still causes) a lot of grief, especially after most hardware is made already Fabrication of ABS has a slow learning curve. Most available vendors will not warrant the quality of brazed joints, we ended up doing/learning brazing ourselves for the difficult ones Quality (leaks) of bi-metal flanges at explosive bonding from ATLAS caused up a lot of headache, spray-seal, scraped chambers… No problem with NEG mounting and activation, 2 failures out of > 600 activations, which were caused by wrong hole patterns Some RGAs produce so much back peaks, even after very careful degassing, made diagnostics not easy at UHV Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013

21. Conclusions: The most prominent features of the design, fabrication and installation of 3 low-emittance machines have been highlighed: MAX-IV, SIRIUS, NSLS-II Two radically different concepts for both vacuum chamber geometry and pumping have been chosen: MAX-IV and SIRIUS have adopted a “100% NEG-coating”philosophy”, with thin, round copper chambers almost everywhere, distributed and lumped SR absorbers, few ion-pumps and mostly NEG-coating pumping and reduced photodesorption yield; No need for using Damping Wigglers for emittance reduction  no need to deal with additional desorption generated by extremely high-power and photon fluxes NSLS-II has stuck to a more “canonical” design (“à-la-APS”), with a chamber-antechamber solution, distributed NEG-strip pumping, and 100% lumped SR absorbers (but stray photons will hit un-coated Al surfaces, with attendant high photodesorption yield!); Also, the emittance is strongly reduced by a massive use of powerful Damping Wigglers  Additional complication of the design of the vacuum system and pumping These differences entail different costs, different risks, different operational scenarios, but most likely the same result: low-Z residual gas allowing a long beam lifetime MAX-IV and NSLS-II have employed industrial partners for the fabrication of their vacuum chambers, while SIRIUS has adopted (for a number of reasons) a “do-it-yourself” approach (practically everything’s done at LNLS in Campinas!) Only time will tell which solution will be the winning one in terms of fast commissioning, ease of operation, efficiency in recovering from accidents, operational efficiency, etc… Other types of low-emittance machines, e.g. Damping Rings for linear colliders, or extremely-high energy machines like TLEP, would deserve another talk… they have been left out due to time limitations. Thanks for your attention!  Review of Vacuum Technology Issues for Low-Emittance Rings- R.Kersevan, Low-e Workshop, Oxford, 8-10July2013