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Challenges in Vacuum Design of the International Liner Collider

Challenges in Vacuum Design of the International Liner Collider. Dr. Oleg B. Malyshev ASTeC Daresbury Laboratory. International Linear Collider. BDS. International Linear Collider. BDS. International Linear Collider. BDS. International Linear Collider. BDS. Helical undulator. BDS.

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Challenges in Vacuum Design of the International Liner Collider

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  1. Challenges in Vacuum Design of the International Liner Collider Dr. Oleg B. Malyshev ASTeC Daresbury Laboratory IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden

  2. International Linear Collider BDS IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  3. International Linear Collider BDS IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  4. International Linear Collider BDS IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  5. International Linear Collider BDS IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  6. Helical undulator BDS IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  7. Helical Undulator • Aim: positron production • Generation of 10 MeV polarised photon by helical undulator: • Eb = 150 GeV • B = 0.8 T • ID = 4 mm to 6 mm – the most realistic vacuum chamber aperture • L = 200 m – total undulator length • Positrons will be produced via pair production in a metallic target hit by a photon drive beam. • Two possible design: • Room temperature • Cryogenic IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  8. Vacuum for helical undulator • Required vacuum: • 100 nTorr (3.31015 molecules/m3) • Helical magnet • no side access for pumping • ID = 4 mm to 6 mm – • very low vacuum conductance • Sum of pumping port lengths should be much less than magnet length – space limitation: • The pumping port for Seff = 20 l/s for CO: • ID = 20 mm and • L= 35 cm to 40 cm (including tapers). IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  9. Vacuum for helical undulator • Unlike undulator in SR sources some SR from the undulator strikes its vacuum chamber walls and must be considered in vacuum design IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  10. Photon flux vs. energy of photons hitting the undulator vacuum chamber walls Calculated by D.J. Scott (ASTeC, DL, UK) IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  11. Synchrotron radiation on the walls of the undulator • The photon flux and photon energy are increasing as a function of distance z from beginning oh the undulator • Desorption flux q [molecules/(sm)] where  [molecules/photon] is desorption yield, which depends on photon energy  and photon dose D, t is time;  [photons/m] is the photon flux; both D and  depend on distance z from beginning oh the undulator. IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  12. Equivalent critical energy of photons Photon along the helical undulator IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  13. Photon flux along helical undulator Photons with equivalent critical energy 10-MeV photons IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  14. Desorption yield as a function of photon energy CERN data: courtesy of O. Gröbner, N. Hilleret IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  15. Gas density at room temperature at z = 100 m IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  16. Gas density at cryogenic temperature • The gas density is initially low • Desorbed gas condenses on the walls • Gas density is growing with time due to re-cycling of condensed gas • Collimators every 7-15 m will help IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  17. Damping rings • E = 5 GeV • I = 0.4 A • Required residual gas pressure (after 100 Ahr) • Electron ring (to avoid fast ion instability) • 0.5 nTorr in arc • 2 nTorr in wiggler • 0.1 nTorr in straights • Positron ring (defined by SR and e-cloud) • ~1 nTorr IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  18. Vacuum required for ILC DRs • The need to avoid fast ion instability leads to very demanding specifications for the vacuum in the electron damping ring [Lanfa Wang, private communication]: • < 0.5 nTorr CO in the arc cell, • < 2 nTorr CO in the wiggler cell and • < 0.1 nTorr CO in the straight section • In the positrondamping ring required vacuum level was not specified and assumed as 1 nTorr (common figure for storage rings) IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  19. Critical energy of photons 2.4 keV from dipoles 26 keV from wigglers Photon flux on vacuum chamber walls (maximum) 2.23·1018 photons/(m·s) in dipole 2.43·1019 photons/(m·s) in wiggler Photon flux onto vacuum chamber walls along the dipoles and the arc cell straights SR in damping rings IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  20. Pressure along the arc: inside a stainless steel tube after 100 Ahr beam conditioning: Seff = 200 l/s every 5 m IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  21. Pressure along the arc: inside a NEG coated tube after 100 Ahr beam conditioning: Seff = 20 l/s every 30 m IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  22. Main results of vacuum modelling • NEG coating of vacuum chamber along both the arcs and the wigglers as well as a few tens meters downstream of both looks to be the only possible solution to fulfil vacuum requirement for the ILC dumping ring Ideal vacuum chamber for vacuum design: • Round or elliptical tube • Cheapest from technological point of view • No antechamber (except wigglers where SR power is very high) • Beam conditioning is most efficient • Easy geometry for TiZrV coating • NEG coated • Requires less number of pumps with less pumping speed • 180C for NEG activation instead of 250-300C bakeout • Choice of vacuum chamber material (stainless steel, copper and aluminium ) does not affect vacuum in this case • Residual gas CH4 and H2 (almost no CO and CO2) O. Malyshev. Vacuum Systems for the ILC Damping Rings. EUROTeV Report-2006-094. IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  23. Electron cloud in the positron DR • Electrons appear in vacuum chamber due to photoemission and electron from beam induced gas ionisation • They accelerated by a beam charge • These electrons may strike the vacuum chamber wall causing • Secondary electrons • Electron stimulated gas desorption e- e+ beam e- H2 CO e- CH4 CO2 IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  24. How the e-cloud affect vacuum • The electron flux ~1016 e-/(sm) with E200 eV (0.3 W) will desorb approximately the same gas flux as the photon flux of ~1018/(sm) from a DR dipole. • If the electron simulated desorption is larger than photon stimulated desorption, that should be considered in vacuum design and conditioning scenario. • E-cloud of DR is studied by many scientist around the globe. • The flux and energy of electron hitting vacuum chamber walls are needed to be found for a number of vacuum chamber designs in dipole, wigglers and straights. IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  25. If e-cloud is too large in a round tube • What is the main source of electrons: • Photo-electrons • Geometrical: antechamber or other means of reduction or localisation of direct and reflected photons • Surface treatment, conditioning, coating • Secondary electrons • TiZrV or TiN coating • Biased electrods • Gas ionisation • Surface treatment and conditioning • Low outgassing coating • Better pumping • A complex solution required: • Good solution against Photo-electrons or Secondary electrons might lead to higher gas density and higher gas ionisation, and vice versa. IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  26. E = 500-1000 GeV CM I = 9.0 mA average current during beam pulse L = 2.5 km  2 Beam delivery system • <- P= 50 nTorr | 1 nTorr near IP • (~500m) IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  27. Input for vacuum modelling: • Required vacuum along BDS parts: • 1 nTorr near IP (~500m), far 50 nTorr • Corresponding beam conditioning (A*hrs) – • 100 Ahr ? • Beam radial aperture varies • from 10mm to 200mm (in extraction lines) • SR absorbers • Material for vacuum chamber: Cu or Au coated SS chambers (option: aluminium alloy chamber) – impedance • TiZrV coating (?) • Compatibility to bakeout (180-250C) • SR flux [photons/m] and c [MeV] • Some flux of lost e+ or e- on collimators, spoilers, tapers • 4 full power (11/17MW) beam dumps IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  28. Vacuum modelling - unknowns: The critical energy of photon spectrum from dipoles at BDS c > ~1 MeV PSD yield at c~1 MeV is not well studied (LEP data only) • Beam conditioning studied at DCI at c =~20 keV • LEP data over lifetime (Al and SS) • Different materials for vacuum chamber ? • Coatings (Cu, Au, TiZrV)? IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  29. Ion induced pressure instability in BDS e+ beam where Q = gas desorption, Seff = effective pumping speed,  = ion induced desorption yield  = ionisation cross section, I = beam current. H2+ H2 CO CH4 CO2 IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  30. Critical current Critical current, Ic, is a current when pressure (or gas density) increases dramatically. Mathematically, if Ic IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  31. Ion energy at BDS and Ion induced desorption yields The energy of ions reached at BDS is: • From 100 keV to 200 keVto be double checked Ion induced desorption yields [molecule/ion]: for Cu for E=3 keV (CERN): for E= 100 MeV – 1 GEV (CERN, GSI, TSL): 100 – 104 molecule/ion IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  32. Pressure instability thresholds: What can be calculated for given beam parameters and vacuum chamber geometry • Ic – critical current • for Cu, Al or SS tube, pump every XX m • for NEG coated tube, pump every XX m • Lc – critical length between pumps • for Cu, Al or SS tube • for NEG coated tube • Sc – critical pumping speed • Hence, • For given parameters and large uncertainties, there is a possibility of ion induced pressure instability in BDS, if pumping is insufficient. • Further studies are needed IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  33. Conclusions Vacuum design is quite challenging in the key areas of ILC • Helical undulator • intensive SR irradiation of walls in a long narrow vacuum chamber • Damping rings • Positron damping ring – electron cloud and its effect on vacuum design • Electron damping ring – fast ion instability and demanding vacuum specification • Beam delivery system • Complex irregular design (i.e. no repeatable structures) • Variety of locations with high energy (c of up to a few MeV) intensive SR irradiation and narrow vacuum chamber • Ion induced pressure instability to be investigated IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

  34. IVC-17/ICSS-13/ICN+T2007 Stockholm, Sweden O.B. Malyshev

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