Requirements and options for PS2 vacuum system E. Mahner * thanks to W. Bartmann, M. Benedikt, S. Calatroni, P. Chiggiato, P. Cruikshank J.M. Jimenez, G. Rumolo, M. Taborelli What can we learn from the PS – Vacuum system layout history/today – Performance with beams (p, Pb ions) – What we need for PS2 PS2 dynamic vacuum challenge & options – Electron cloud build-up, SEY – Electron cloud mitigation techniques Grooves, clearing electrodes, coatings (a-C, TiZrV) Advantages & disadvantages – Space requirements for vacuum system Magnets, drifts, LHC warm modules Conclusions PS2 meeting, Edgar Mahner

PS vacuum system – a short history PS history – Designed and built in the mid 50’s, started operation in Some 100 pumping groups (rotary & oil diffusion pumps), elastomer seals, reached mbar, high pressure + hydrocarbons detected. – Modifications Upgrade 1: installation of getter ion pumps in the 60’s, but hydrocarbons remained. Upgrade 2: only in mid 80’s all 100 dipoles received new vacuum chambers (fired 316LN stainless steel), metal seals, static pressure of mbar achieved with ion pumps alone (80 x 200 l/s + 40 x 400 l/s). Upgrade 3: anticipated around 1987/88 in view of ion operation, introduction of Ti-sublimation pumps (TSP), 1991: mbar (static) reached on a test stand (PS dipole with ion pump + sublimation pump) PS upgrade/renovation during 90’s. No need for cryo-pumping (to reduce pressure in high-outgassing areas), a more economic approach was followed: cleaner beam tubes and cleaner equipment in tanks. More pumping speed (TSP’s) was added, new gauges, tanks etc. Similar upgrades done in parallel for the PSB. For heavy-ion operation the conclusion was that one would not need to rebuild the complete vacuum system, a bakeout was considered as not necessary (right) PS2 meeting, Edgar Mahner

PS vacuum system – situation today Characteristics – A rather "new" stainless steel vacuum system, unbaked, 10 sectors – Dipole vacuum chamber apertures Type 1: ≈70% (146 x 70 mm), L = 4661 mm, 2 mm wall thickness, 316 LN Type 2: ≈20% (178 x 68 mm), L = 4661 mm, 2.5 mm wall thickness, Inconel Specials: ≈10% (injection, extraction) + kicker, septa, cavities Main magnet vacuum chambers contain – sputter ion pump, sublimation pump, pressure gauge, two bellows Total pumping speed – 147 ion pumps (200 l/s, 400 l/s) – 106 sublimation pumps (nominal 1200 l/s) – ≈ 13 pumping groups – 48 pressure gauges PS2 meeting, Edgar Mahner Type 1 Type 2

PS performance with beams (1) Protons – Lifetime measurements No recent lifetime measurements available, dynamic vacuum is not a limiting factor for protons.  p >> 3BP = 3.6 s (BP: basic period of 1.2s) PS2 beam lifetime should be about the same – Dynamic vacuum of p ≤ mbar is sufficient for reliable operation Even a local pressure bump, as observed in 2006 with a high pressure in straight section 48 (the PS internal dump started leaking), did not affect proton operation, but largely degraded the ion run… – Beam related pressure rise Detailed studies made in 2007/08 with PS electron cloud setups installed in SS98 and SS84. Only observed for nominal LHC beam, clear footprints: fast vacuum pressure rise, characteristic signal on shielded button pickups, current on clearing electrode. Developed only during last ms before extraction towards SPS, no pressure run-away due to low duty cycle in the PS. Electron cloud is no operational problem in the PS (for vacuum) PS2 meeting, Edgar Mahner

PS performance with beams (2) Heavy ions – LEIR delivery: Pb54+, 72 MeV/u – Measured lifetime:  >> 700 ms, considered as excellent – PS dynamic vacuum Is much more critical for heavy ions than for protons, needs conditioning of all PS sublimation pumps prior to ion operation. Very sensitive to a local pressure bump, as observed in – Desorption yields  Very high yields of 10 4 – 10 5 molec./Pb53+ion measured for various bare st.st. vacuum chambers at LINAC 3 (4.2 MeV/u), confirmed at other labs (GSI, RHIC, LBNL). Very sensitive to chamber surface properties,  scales with (d E e/d x ) 2-3, in favour for high energy. LEIR vacuum system: bakeable UHV with TiZrV (NEG) coated chambers wherever possible, low desorption collimators (Au/st.st.) to intercept charge exchanged ion (Pb53+) under perpendicular impact. – Global requirement for PS2 low losses…low losses…low losses, injection is critical (as in SIS18), high distributed pumping speed, SIS18 (for FAIR) needed upgrade from baked st.st. to NEG coated st.st. chambers plus ion collimation (catcher) system. Clear impact on performance, cost, and time! Guideline (C. Carli, ) of 10 s vacuum lifetime for 9 x 10 8 Pb54+ ions seems "over-ambitious“. PS2 meeting, Edgar Mahner

PS2 electron cloud build-up: a challenge for the dynamic vacuum – Simulations by G. Rumolo, build-up over 2 turns in PS2 PS2 meeting, Edgar Mahner 6/3 - 4 cm aperture gives a low electron cloud threshold value. 7.5/5 cm apertures, as presented in last PS2 meeting, for a 4.5  envelope must be reviewed. Giovanni started new simulations for these apertures. First results ( ): Situation improves a bit at top energy, but becomes worse at injection energy (longer and fatter bunches). With a bigger chamber the SEY threshold decreases to 1.1, calculated for LHC25 & FT beams. SEY threshold <1.2 !!

Parameters for electron cloud build-up Beam parameters – bunch intensity: threshold effect, for SPS: 2-3 x p/bunch (dipoles) for SPS: 5 x p/bunch (field free) – bunch spacing: threshold effect for SPS: occurs for bunch spacing <75 ns – bunch pattern: surviving electrons Low-energy electrons (<5eV) are lost in missing bunches (gaps). For SPS: >225 ns required between bunch trains (batches) Vacuum chamber surface characteristics – Secondary Electron Yield is a key parameter Number of emitted secondary electrons by a primary electron Depends on: material (bare, coated), surface composition (morphology, oxide thickness, contaminations), energy & impact angle of the primary electron – Surface characteristics (in presence of beam) Surface cleaning: is the removal of physisorbed and chemisorbed gas from the vacuum chamber wall induced by impinging electrons from the cloud. This cleaning will improve the dynamic pressure with time Beam conditioning: is the decrease of the SEY with time resulting from the electron bombardment of the cloud PS2 meeting, Edgar Mahner Main parameter SPS data from J.M. Jimenez, CARE-HHH-APD Beam’07

PS2 electron cloud mitigation Mitigation methods – Adapted choice of base material, consider coatings, bakeouts, glow discharges, surface geometry (grooves), clearing electrodes, new techniques… – An ideal vacuum chamber surface would have the following characteristics: is UHV compatible and bakeable (if required) has an intrinsically low SEY, shows no (or little) ageing, withstands accelerator environment (venting, shutdown interventions) a coating can be deposited on standard beam pipe materials (st.st., Cu, Al) manufacture, cleaning, handling, transport should be "easy “ (no laboratory) has a low resistivity (impedance) – What are the options for PS2? Review possibilities: grooves, clearing electrodes, bare & coated surfaces, don’t rule out any option! PS2 meeting, Edgar Mahner

SEY of bare metals, technical surfaces SEY for unbaked materials – st.st., Cu, Al, Ti, Be –  max >2.0 SEY for baked Cu – Influence of 24 h in situ heating –  max ≈ 1.4 after 350  C PS2 meeting, Edgar Mahner The yields of bare vacuum chamber materials, as received and baked, are too high for the PS2 vacuum system. Look for mitigation techniques. N. Hilleret et al., EPAC 2000, p.217 I. Bojko et al., J. Vac. Sci. Technol. A18(3), 972 (2000)

PS2 ecloud mitigation: grooves option PS2 meeting, Edgar Mahner – Low SEY (  max ≤1.1) can in principle be achieved (simulation results with B-field) SEY measurements w/o B-field confirm the yield reduction by the expected factor – Advantages: passive device, no bakeout, no ageing – Disadvantages: impedance, aperture issue, high aspect ratio (difficult to manufacture), needs a coating to be effective M. Pivi - SLAC (2008)

Full electron cloud suppression achieved in the PS for positive/negative bias voltages, for st.st. and enamel electrodes machine impedance (invisible clearing electrodes)? PS2 ecloud mitigation: clearing electrodes option PS2 meeting, Edgar Mahner – Advantages: very effective ecloud suppression, no bakeout, no ageing – Disadvantages: impedance, aperture, active device (reliability, hardware) E.M. et al. PRST-AB 11, (2008) 440 mm In KEKB (LER) also very good results Performance ranking measured: Feedthrough modifications necessary, discharges decreased insulation resistivity from 2 M  to several 10 k  Electrode >> TiN+Groove >> TiN(Flat) > Cu(Flat)  10  5  10  2  3 Thin electrode: 100  m tungsten/200  m Al2O3 with thermal spray, size 440 x 40 mm 2 K. Shibata, 14 th KEKB Review, 2009

PS2 ecloud mitigation: amorphous carbon (a-C) coating option PS2 meeting, Edgar Mahner – Low SEY (<1) without baking deposited by magnetron sputtering (Ne) on copper, no dependence on coating thickness ( nm) good adhesion, no loose particles, dust creation is no issue, beam impact and desorption? – Differences of ageing in air under investigation, substrate roughness, long term SEY not measured – Important to specify a maximum air exposure time for the application (SPSU, PS2) first accelerator experience from coated SPS magnets (in 2009) – Advantages: very low SEY, no aperture issue, no impedance issue (for SPS), no bakeout – Disadvantages: ageing in air not known (under study for SPSU), no distributed pumping, up to 10 x higher outgassing rate than st.st. (under study), little experience at CERN (activities started in 12/2007) data from M. Taborelli

PS2 ecloud mitigation: TiZrV (NEG) coating option PS2 meeting, Edgar Mahner – High SEY as received, low SEY (  max =1.1) after baking (fully activated) deposited by magnetron sputtering (Kr) on copper, 1  m thickness good adhesion, no loose particles, dust creation is no issue – Full vacuum characterization available, SEY evolution only known for some venting/activation cycles – Important to specify a maximum air exposure time for the application – Advantages: low SEY after activation, provides distributed pumping, no aperture issue, no impedance issue, a lot of CERN experience (LHC-LSS, LEIR) – Disadvantages: bakeout, SEY ageing to be studied for many activation/venting cycles B. Henrist et al. Appl. Surf. Sci. 172, 95 (2001) Thermal activation is necessary: 200  C or 180  C About 6 km of LHC long straight sections are coated with NEG (more than 1000 vacuum chambers) to provide pumping

Space requirements for PS2 vacuum system List of machine characteristics PS2 parameters – Beam (M. Benedikt PS2 status, ) 4 GeV injection energy,  50 GeV extraction energy 4x10 11 ppb (LHC25), 1.2x10 14 ppb(FT). Present PS: 1.7x10 11 ppb (LHC25), 3.3x10 13 ppb (FT) 25 ns bunch spacing 20 ns bunch length at injection, 4 ns bunch length at extraction Optimum length: m – Apertures, status on PS2 meeting, Edgar Mahner Calculation, see talk "PS2 Aperture Model " by J. Barranco, ; values give the half radius of the inner vacuum chamber All 1.2m long drifts include either a corrector + BPM or a sextupole

PS2 space requirements inside magnets Magnets – The primary requirements are given by the chosen beam envelope, 4.5  or less ?! – The necessary space for the vacuum system depends on – Installation of bakeout equipment Need 4 – 7 mm on radius for temperatures up to 300  C, according to specialist – depends on choice, to be decided for PS2 Tolerance to install/remove chamber with heating jackets, 1.5 mm on radius (tbc) – Vacuum chambers Material and wall thickness, PS case: st.st. 316LN (2mm), Inconel (2.5 mm) – to be studied and decided for PS2 Manufacturing tolerances, straightness of 0.2 mm/m (tbc) PS, SPS cases: captive in dipoles, not captive in others Pay attention with magnet tolerances and alignment possibilities Consequences for a 4.5  beam envelope – Dipoles (166 x 3.8m) 75/50 mm mm (wall) + 7 mm (bake) +1.5 mm (install) -> 86/61 mm half apertures ! – Quadrupoles 124 x (0.6 – 2.8 m) 80/80 mm mm (wall) + 7 mm (bake) +1.5 mm (install) -> 91/91 mm half apertures ! – Other magnets, same as quads – If magnets can be opened for chamber insertion, space can be saved. – Try to stick on standard UHV Conflat® flanges PS2 meeting, Edgar Mahner This would result in very large vacuum chambers with large DN 200 CF flanges !

Standard CF flange dimensions PS2 meeting, Edgar Mahner Vacuum tube outer diameters source: Material & machining cost, quantity of nuts/bolts, installation & dismantling time (ALARA), dimensions & weight (radioactive waste): all increase with flange size!

PS2 space requirements for drifts Drifts (straight sections) space between magnets Space is necessary for following items: – Valves for vacuum sectorization Size & price depend on flange size – Bellows, RF shieldings, Conflat ® flange sizes Important: ensure a smooth transition between different cross sections, which needs space, limit the number of different variants, flange sizes given by magnet vacuum chamber – Vacuum pumping, instrumentation, heating equipment, supports Mobile pumping groups, ion pumps, sublimation pumps (?), gauges, gas analyzers – Standardization for PS2 Make maximum use of modules developed/built for LHC (integration worked) Integrates all equipment mentioned above, module space requirement depends on type. Reduce the number of variants, saves time & cost, improves spare situation PS2 meeting, Edgar Mahner

Modules developed for LHC – use for PS2 PS2 meeting, Edgar Mahner

PS2 vacuum system design – conclusions Dynamic Vacuum – The PS2 requirements are demanding due a very low threshold for the electron cloud effect. The critical SEY value, calculated for the presently proposed apertures, is  max < 1.2. This is considered as a challenge. – For electron cloud mitigation, none of the known options (grooves, clearing electrodes, a-C and TiZrV coatings) should be completely ruled out. Presently, machine impedance arguments favor a bakeable vacuum system with coated vacuum chambers. – Based on today knowledge, the electron cloud suppression and dynamic vacuum requirements for proton and heavy-ion operation imply a baked UHV system to ensure vacuum stability. – Long term properties, especially the ageing (change of the SEY with time) of a-C and TiZrV, must be investigated before a final choice on the type of coating can be made. – Next issues to address: fix apertures, vacuum chamber materials, vacuum system layout & element integration (space), coating studies for PS2. PS2 meeting, Edgar Mahner