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Review of Vacuum Technology Issues for Low- Emittance Rings R .Kersevan , CERN-TE-VSC-IVM. Low- e Workshop , Oxford, 8-10 July 2013, J. Adams Inst. Accel . Studies. Review of Vacuum Technology Issues for Low- Emittance Rings - R.Kersevan , Low- e Workshop, Oxford, 8-10July2013. Agenda:

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review of vacuum technology issues for low emittance rings r kersevan cern te vsc ivm

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

slide2

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

slide3

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…

slide4

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)

slide5

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!

slide6

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

slide7

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)

slide8

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

slide9

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.

slide10

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)

slide11

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

slide12

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

slide13

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

slide14

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

slide15

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

slide16

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

slide17

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

slide18

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

slide19

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)

slide20

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

slide21

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