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

Review of Vacuum Technology Issues for Low-Emittance Rings - R.Kersevan, Low-e Workshop, Oxford, 8-10July2013

Agenda:

1. Intro: Vacuum and Stored Beams, An Often Difficult Relationship…

2. Challenging Issues (Poll Results)3. Conclusions


1. Vacuum and Stored Beams: An Often Difficult Relationship…• 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


• 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 faceI 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

2. Challenging Issues (Poll Results)


• 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


Crotch area test:SIRIUS, e-beam welded,

machined Cu blocksNote: 1 Euro is NOT the

cost of such a component!


Min. clearance with the iron 0.5 mm

Min. clearance with the coils 2 mm

Source: E. Al-Dmour, SIRIUS Vacuum System Review Workshop, Mar 2013


• 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)



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.

MAX-IV: Etching of Cu tubes at CERN prior to NEG-coating


Degreasing:

1. 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), 4. Rinsing with demineralised

water and drying with N2.

1. Etching at 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.

2. High-pressure rinsing with demineralised water using rotary nozzle (Kärcher),

NEG-coating of MAX-IV chambers: Surface preparation at CERN


• 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!

2. Challenging Issues (Poll Results)Area for fast corrector

Example: absorber for ID SR.

Temperature (°C)

Stress (MPa)


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

Power and Flux Ray-Tracing on +/- 5 mrad Screen Perpendicular to Beam Direction

Flux(red) is flatter and broader than Power (blue) distribution;

Solid lines: HorizontalDashed lines: Vertical

10 units ~ 1 mrad

CAREFUL RAY-TRACING WITH REAL MAGNETIC FIELD MAPPING IS NECESSARY FOR HIGH-POWER IDs !!


A more orthodox approach: NSLS-II (courtesy H. Hseuh)

Vacuum chambers made from Al extrusionLow 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

• P(avg) < 1x10-9 Torr (> 80% H2, < 20% H2O, CO, CO2, CH4, …)• (beam-gas) > 30 hrՇ (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

Storage Ring Vacuum Systems

.Beam

ID fans

E beam



Multipole cross sectionShort chamber cross section

DM

M

M

D

Dipole cross section

Cell Vacuum Chambers and Cross Sections

BLW

BLW

GV

GV

BLWBLW

• ~ 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….

ID beam line

BM beam line

SS

Front End


• 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

Chamber Extrusions and Bending

Extrusion Press

Spacerblocks Shims

Bending at BNL

Bended extrusion



Extrusion Machining at Vendors

Stick absorber

Pump port Pump port

Multipole Dipole

3m long, 6o bend, ρ = 25 m

Crotch absorber

Machined Machined

• Machine the magnet cut-outs• Machine the ends• Machine the side and pump

ports

< 2 mm clearance



Machining of Extrusions by 3 US VendorsHigh precision machining using 3-axis, 7m NC machines

Multipole Extrusion Machining Dipole Extrusion Machining



• 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

Note: Pictures taken at Argonne National Laboratory

Chamber Welding at APS / ANL

full fusion weld w/o step

4mm

Weld side portsWeld end jointsExit port in S6




Be-Cu RF shield with 50% openings

ABS

Chamber Assembly at BNL

BPMBPM

BPM

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

Welded chambers are measured for dimensional accuracy


NEG Strip Assembly in Ante-Chamber

NEG carriers and mounting

NEG strips in antechamber

Two NEG strips (top/bottom) in ante-chamber

Riveted mounting on stainless carrier plates every 10 cm with ceramic bushings

Activation at ~ 4500C x 20 min (30 A thru NEG strips)



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


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 , cern-te-vsc-ivm

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