Lhc construction & operation

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Lhc construction & operation

  1. 1. LHC : construction and operation Jö Wenninger rg CERN Beams Department / Operation group LNF Spring School Bruno Touschek - May 2010 Part 1: •Introduction to accelerator physics •LHC magnet and layout •Luminosity and interaction regions •Injection and filling schemesJ. Wenninger LNF Spring School, May 2010 1
  2. 2. Outline • The LHC challenges • Introduction to magnets and particle focusing • LHC magnets and arc layout Part 1 • LHC luminosity and interaction regions • Injection and filling schemes • Machine protection Part 2 • Incident 19th Sept. 2008 and consequences • LHC operationJ. Wenninger LNF Spring School, May 2010 2
  3. 3. LHC History 1982 : First studies for the LHC project 1983 : Z0/W discovered at SPS proton antiproton collider (SppbarS) 1989 : Start of LEP operation (Z/W boson-factory) 1994 : Approval of the LHC by the CERN Council 1996 : Final decision to start the LHC construction 2000 : Last year of LEP operation above 100 GeV 2002 : LEP equipment removed 2003 : Start of LHC installation 2005 : Start of LHC hardware commissioning 2008 : Start of (short) beam commissioning Powering incident on 19th Sept. 2009 : Repair, re-commissioning and beam commissioning 2010 : Start of a 2 year run at 3.5 TeV/beamJ. Wenninger LNF Spring School, May 2010 3
  4. 4. The Large Hadron Collider LHCInstalled in the 26.7 km LEP tunnelDepth of 70-140 m Lake of Geneva g C rin LH CMS, Totem LHCb Control Room SP S r ing ATLAS, LHCf ALICE 417.03.2010 Der LHC
  5. 5. Tunnel circumference 26.7 km, tunnel diameter 3.8 m Depth : ~ 70-140 m – tunnel is inclined by ~ 1.4%J. Wenninger LNF Spring School, May 2010 5
  6. 6. LHC Layout8 arcs. IR5:CMS8 straight sections (LSS), 2 Beam dump m ea blocks ~ 700 m long. B 1The beams exchange their e am B IR6: Beampositions (inside/outside) in 4points to ensure that both rings IR4: RF + Beam dumping systemhave the same circumference ! instrumentation IR3: Momentum IR7: Betatron collimation (normal collimation (normal conducting magnets) conducting magnets) IR8: LHC-B IR2:ALICE IR1: ATLAS Injection ring 1 Injection ring 2J. Wenninger LNF Spring School, May 2010 6
  7. 7. LHC – yet another collider? The LHC surpasses existing accelerators/colliders in 2 aspects :  The energy of the beam of 7 TeV that is achieved within the size constraints of the existing 26.7 km LEP tunnel. LHC dipole field 8.3 T A factor 2 in field HERA/Tevatron ~4 T A factor 4 in size  The luminosity of the collider that will reach unprecedented values for a hadron machine: LHC pp ~ 1034 cm-2 s-1 A factor 30 Tevatron pp 3x10 cm s 32 -2 -1 in luminosity SppbarS pp 6x1030 cm-2 s-1 The combination of very high field magnets and very high beam intensities required to reach the luminosity targets makes operation of the LHC a great challenge !J. Wenninger LNF Spring School, May 2010 7
  8. 8. Luminosity challenges The event rate N for a physics process with cross-section σ is proprotional to the collider Luminosity L: N = Lσ k = number of bunches = 2808 N = no. protons per bunch = 1.15×10 11 kN 2 f f = revolution frequency = 11.25 kHz L= σ * x, σ * y = beam sizes at collision point (hor./vert.) = 16 µ m 4πσ xσ * * y To maximize L: High beam “brillance” N/ ε • Many bunches (k) (particles per phase space • Many protons per bunch (N) volume) • A small beam size σ * u = ( β * ε ) 1/2  Injector chain performance ! Small envelope β * : the beam envelope (optics) Optics  Strong focusing ! ε : is the phase space volume occupied property Beam property by the beam (constant along the ring).J. Wenninger LNF Spring School, May 2010 8
  9. 9. Basics of accelerator physicsJ. Wenninger LNF Spring School, May 2010 9
  10. 10. Accelerator concept Charged particles are accelerated, guided and confined by electromagnetic fields. - Bending: Dipole magnets - Focusing: Quadrupole magnets - Acceleration: RF cavities In synchrotrons, they are ramped together synchronously to match beam energy. - Chromatic aberration: Sextupole magnetsJ. Wenninger LNF Spring School, May 2010 10
  11. 11. Bending → → → → Force Lorentz force Magnetic rigidity LHC: ρ = 2.8 km given by LEP tunnel! To reach p = 7 TeV/c given a bending radius of ρ = 2805 m:  Bending field : B = 8.33 Tesla  Superconducting magnets To collide two counter-rotating proton beams, the beams must be in separate vaccum chambers (in the bending sections) with opposite B field direction.  There are actually 2 LHCs and the magnets have a 2-magnets-in-one design!J. Wenninger LNF Spring School, May 2010 11
  12. 12. Bending Fields I II B B field p B F force F p Two-in-one magnet designJ. Wenninger LNF Spring School, May 2010 12
  13. 13. Focusing N S F By y F x S N Transverse focusing is achieved with quadrupole x magnets, which act on the beam like an optical lens. Linear increase of the magnetic field along the axes (no effect on particles on axis). y Focusing in one plane, de-focusing in the other!J. Wenninger LNF Spring School, May 2010 13
  14. 14. Accelerator lattice horizontal plane Focusing in both planes is achieved by a succession of focusing and de-focusing quadrupole magnets : The FODO structure vertical plane 14
  15. 15. Alternating gradient lattice One can find an arrangement of quadrupole magnets that provides net focusing in both planes (“strong focusing”). Dipole magnets keep the particles on the circular orbit. Quadrupole magnets focus alternatively in both planes. The lattice effectively constitutes a s particle trap!y xJ. Wenninger LNF Spring School, May 2010 15
  16. 16. LHC arc lattice QF dipole decapole QD sextupole QF magnets magnets magnets small sextupole corrector magnets LHC Cell - Length about 110 m (schematic layout)  Dipole- und Quadrupol magnets – Provide a stable trajectory for particles with nominal momentum.  Sextupole magnets – Correct the trajectories for off momentum particles (‚chromatic‘ errors).  Multipole-corrector magnets – Sextupole - and decapole corrector magnets at end of dipoles – Used to compensate field imperfections if the dipole magnets. To stabilize trajectories for particles at larger amplitudes – beam lifetime ! One rarely talks about the multi-pole magnets, but they are essential for good machine performance !J. Wenninger LNF Spring School, May 2010 16
  17. 17. Beam envelope  The focusing structure (mostly defined by the quadrupoles: gradient, length, number, distance) defines the transverse beam envelope.  The function that describes the beam envelope is the so-called ‘β’-function (betatron function): • In the LHC arcs the optics follows a regular pattern – regular FODO structure. • In the long straight sections, the betatron function is less regular to fulfill various constraints: injection, collision point focusing… QF QD QF QD QF QD QF The envelope peaks in the focusing elements ! Vertical Horizontal Betatron functions in a simple FODO cellJ. Wenninger LNF Spring School, May 2010 17
  18. 18. Beam emittance and beam size  For an ensemble of particles: The transverse emittance , ε, is the area of the phase-space ellipse. Beam size = projection on X (Y) axis.  The beam size σ at any point along the accelerator is given by (neglecting the contribution from energy spread): σ = Envelope × Emittance = β ε For unperturbed proton beams, the normalized emittance ε n is conserved: ε n = εγ = constant γ = Lorentz factor The beam size shrinks with β εn 1 energy: σ= ∝ γ γJ. Wenninger LNF Spring School, May 2010 18
  19. 19. Why does the transverse emittance shrink?  The acceleration is purely longitudinal, i.e the transverse momentum is not affected: pt = constant  The emittance is nothing but a measure of <pt>.  To maintain the focusing strength, all magnetic fields are kept proportional to E (γ), including the quadrupole gradients.  Withconstant <pt> and increasing quadrupole gradients, the transverse excursion of the particles becomes smaller and smaller !J. Wenninger LNF Spring School, May 2010 19
  20. 20. LHC beam sizes  Beta-function at the LHC β = 0.5 ÷ 5000 m β = 30 ÷ 180 m ARC  Nominal LHC normalized emittance : εn = εγ = 3.5 µm Example LHC arc, peak β = 180 m Energy ε (nm) σ (mm) (GeV) 450 7.2 1.14 3500 0.93 0.41 7000 0.47 0.29J. Wenninger LNF Spring School, May 2010 20
  21. 21. Acceleration  Acceleration is performed with electric fields fed into Radio-Frequency (RF) cavities. RF cavities are basically resonators tuned to a selected frequency.  To accelerate a proton to 7 TeV, a 7 TV potential must be provided to the beam:  In circular accelerators the acceleration is done in small steps, turn after turn.  At the LHC the acceleration from 450 GeV to 7 TeV lasts ~20 minutes, with an average energy gain of ~0.5 MeV on each turn.  E(t ) s 21J. Wenninger LNF Spring School, May 2010
  22. 22. LHC RF system  The LHC RF system operates at 400 MHz.  It is composed of 16 superconducting cavities, 8 per beam.  Peak accelerating voltage of 16 MV/beam. For LEP at 104 GeV : 3600 MV/beam ! Synchrotron radiation loss LHC @ 3.5 TeV 0.42 keV/turn LHC @ 7 TeV 6.7 keV /turn LEP @ 104 GeV ~3 GeV /turn The nominal LHC beam radiates a sufficient amount of visible photons to be actually observable ! (total power ~ 0.2 W/m)J. Wenninger LNF Spring School, May 2010 22
  23. 23. Visible protons !  Some of the energy radiation by the LHC protons is emitted as visible light. It can be extracted with a set of mirrors to image the beams in real time.  This is a powerful tool to understand the beam size evolution. Protons are very sensitive to perturbations, keeping their emittance small is always a challenge. Flying wire LHC Synch. light Flying wire SPS (injector)J. Wenninger LNF Spring School, May 2010 23
  24. 24. Cavities in the tunnelJ. Wenninger LNF Spring School, May 2010 24
  25. 25. RF buckets and bunches The particles oscillate back The particles are trapped in the RF voltage: RF Voltage and forth in this gives the bunch structure time/energy 2.5 ns time ∆E LHC bunch spacing = 25 ns = 10 buckets ⇔ 7.5 m RF bucket time 2.5 ns 450 GeV 3.5 TeV RMS bunch length 12.8 cm 5.8 cm RMS energy spread 0.031% 0.02%J. Wenninger LNF Spring School, May 2010 25
  26. 26. Magnets & TunnelJ. Wenninger LNF Spring School, May 2010 26
  27. 27. Superconductivity  The very high DIPOLE field of 8.3 Tesla required to achieve 7 TeV/c can only be obtained with superconducting magnets !  The material determines: Tc critical temperature Bc critical field  The cable production determines: Jc critical current density  Lower temperature ⇒ increased current density ⇒ higher fields. Bc Applied field [T]  Typical for NbTi @ 4.2 K Normal state 2000 A/mm2 @ 6T  To reach 8-10 T, the temperature must be lowered Superconducting to 1.9 K – superfluid Helium ! state Tc Temperature [K]J. Wenninger LNF Spring School, May 2010 27
  28. 28. The superconducting cable ∅6 µm ∅1 mm A.Verweij Typical value for operation at 8T and 1.9 K: 800 A width 15 mm Rutherford cable A.VerweijJ. Wenninger LNF Spring School, May 2010 28
  29. 29. Coils for dipoles Dipole length 15 m I = 11’800 A @ 8.3 T The coils must be aligned very precisely to ensure a good field quality (i.e. ‘pure’ dipole)J. Wenninger LNF Spring School, May 2010 29
  30. 30. Ferromagnetic iron Non-magnetic collars Superconducting coil Beam tube Steel cylinder for Helium Insulation vacuum Vacuum tank Supports Weight (magnet + cryostat) ~ 30 tons, length 15 mJ. Wenninger LNF Spring School, May 2010 30 Rüdiger Schmidt 30
  31. 31. Regular arc: Magnets 1232 main dipoles + 3700 multipole 392 main quadrupoles + corrector 2500 corrector magnets magnets (dipole, sextupole, octupole) (sextupole, octupole,J. Wenninger LNF Spring School, May 2010 decapole) J. Wenninger - ETHZ - December 2005 31 31
  32. 32. Regular arc: Connection via service module and Cryogenics jumper Static bath of superfluid Supply and recovery of helium at 1.9 K in cooling helium with 26 km long loops of 110 m length cryogenic distribution lineJ. Wenninger LNF Spring School, May 2010 J. Wenninger - ETHZ - December 2005 32 32
  33. 33. Regular arc: Beam vacuum for Beam 1 + Beam Vacuum 2 Insulation vacuum for the Insulation vacuum for magnet cryostats the cryogenic distribution lineJ. Wenninger LNF Spring School, May 2010 J. Wenninger - ETHZ - December 2005 33 33
  34. 34. Tunnel view (1)J. Wenninger LNF Spring School, May 2010 34
  35. 35. Tunnel view (2)J. Wenninger LNF Spring School, May 2010 35
  36. 36. Complex interconnects Many complex connections of super-conducting cable that will be buried in a cryostat once the work is finished. This SC cable carries 12’000 A for the main quadrupole magnetsJ. Wenninger LNF Spring School, May 2010 CERN visit McEwen 36
  37. 37. Magnet cooling scheme 10000 SOLID 1000 CRITICAL POINT λ line HeII HeI P [kPa] 100 Pressurized He II GAS 10 Saturated He II 1 1 10 T [K]  He II: super-fluid o Very low viscosity o Very high thermal conductivity Courtesy S. ClaudetJ. Wenninger LNF Spring School, May 2010 37
  38. 38. Cryogenics Pt 5 Pt 4 Pt 6 8 x 18kW @ 4.5 K 1’800 SC magnets Pt 3 24 km & 20 Distribution Cryoplant kW @ 1.8 K Pt 7 Present Version 36’000 t @ 1.9K 130 t He inventory Pt 2 Pt 8 Pt 1.8 Pt 1 Cryogenic plant Courtesy S. Claudet Grid power ~32 MWJ. Wenninger LNF Spring School, May 2010 38
  39. 39. Cool down First cool-down of LHC sectors Cool-down time to 1.9 K is nowadays ~4 weeks/sector [sector = 1/8 LHC] 300 250 Temperature [K] 200 150 100 50 0 12- 10- 07- 04- 03- 31- 28- 26- 23- 21-Jul- 18- 15- Nov- Dec- Jan- Feb- Mar- Mar- Apr- May- Jun- 2008 Aug- Sep- 2007 2007 2008 2008 2008 2008 2008 2008 2008 2008 2008 ARC56_MAGS_TTAVG.POSST ARC78_MAGS_TTAVG.POSST ARC81_MAGS_TTAVG.POSST ARC23_MAGS_TTAVG.POSST ARC67_MAGS_TTAVG.POSST ARC34_MAGS_TTAVG.POSST ARC12_MAGS_TTAVG.POSST ARC45_MAGS_TTAVG.POSSTJ. Wenninger LNF Spring School, May 2010 39
  40. 40. Vacuum chamber  The beams circulate in two ultra-high vacuum chambers, P ~ 10-10 mbar. 50 mm  A Copper beam screen protects the bore of the magnet from heat deposition due to image currents, synchrotron light etc from the beam.  The beam screen is cooled to T = 4-20 K. 36 mm Beam screen Magnet bore Cooling channel (Helium) Beam envel (± 4 σ) ~ 1.8 mm @ 7 TeVJ. Wenninger LNF Spring School, May 2010 40
  41. 41. Luminosity and interaction regionsJ. Wenninger LNF Spring School, May 2010 41
  42. 42. Luminosity Let us look at the different factors in this formula, and what we can do to maximize L, and what limitations we may encounter !! kN 2 f L= 4πσ xσ * * y  f : the revolution frequency is given by the circumference, f=11.246 kHz.  N : the bunch population – N=1.15x1011 protons - Injectors (brighter beams) - Collective interactions of the particles - Beam encounters  k : the number of bunches – k=2808 For k = 1: - Injectors (more beam) L = 3.5 × 1030 cm −2 s −1 - Collective interactions of the particles - Interaction regions - Beam encounters  σ* : the size at the collision point – σ*y=σ*x=16 µm - Injectors (brighter beams) - More focusing – stronger quadrupolesJ. Wenninger LNF Spring School, May 2010 42
  43. 43. Collective (in-)stability  The electromagnetic fields of a bunch interact with the vacuum chamber walls (finite resistivity !), cavities, discontinuities etc that it encounters:  The fields act back on the bunch itself or on following bunches.  Since the fields induced by of a bunch increase with bunch intensity, the bunches may become COLLECTIVELY unstable beyond a certain intensity, leading to poor lifetime or massive looses intensity loss.  Such effects can be very strong in the LHC injectors, and they will also affect the LHC – in particular because we have a lot of carbon collimators (see later) that have a very bad influence on beam stability !  limits the intensity per bunch and per beam !J. Wenninger LNF Spring School, May 2010 43
  44. 44. ‘Beam-beam’ interaction Y  When a particle of one beam encounters the opposing beam at the collision point, it senses the fields of the opposing beam.  Due to the typically Gaussian shape of the F o rc e beams in the transverse direction, the field (force) on this particle is non-linear, in particular at large amplitudes. Quadrupole e lens focal length depends on amplitude ! Q u a d r u p o le L n s e  The effect of the non-linear fields can become Y so strong (when the beams are intense) that large amplitude particles become unstable and are lost from the machine:  poor lifetime F o rc e  background THE INTERACTION OF THE BEAMS SETS A LIMIT ON THE BUNCH INTENSITY! Beam(-beam) n lens B eam - B eam Le seJ. Wenninger LNF Spring School, May 2010 44
  45. 45. From arc to collision point CMS collision point ARC cells ARC cells Fits through the hole of a needle!  Collision point size @ 7 TeV, β* = 0.5 m (= β-function at the collision point): CMS & ATLAS : 16 µm  Collision point size @ 3.5 TeV, β* = 2 m: All points : 45 µmJ. Wenninger LNF Spring School, May 2010 45
  46. 46. Limits to β*  The more one squeezes the beam at the IP (smaller β*) the larger it becomes in the surrounding quadrupoles (‘triplets’): Small size Smaller the size at IP: Huge size !!  Larger divergence (phase space conservation !) Huge size !!  Faster beam size growth in the space from IP to first quadrupole ! Aperture in the ‘triplet’ quadrupoles around the IR limits the focusing !J. Wenninger LNF Spring School, May 2010 46
  47. 47. Combining the beams for collisions quadrupole quadrupole Q4 Q4 quadrupole recombination separation inner quadrupole inner quadrupole separation recombination quadrupole Q5 dipole dipole (warm) triplet triplet dipole dipole Q5 beam II ATLAS or CMSbeamdistance194 mm beam I collision point 24 m 200 m Example for an LHC insertion with ATLAS or CMS  The 2 LHC beams must be brought together to collide.  Over ~260 m, the beams circulate in the same vacuum chamber. They are ~120 long distance beam encounters in total in the 4 IRs.J. Wenninger LNF Spring School, May 2010 47
  48. 48. Crossing angles  Since every collision adds to our ‘Beam-beam budget’ we must avoid un-necessary direct beam encounters where the beams share a common vacuum: COLLIDE WITH A CROSSING ANGLE IN ONE PLANE !  There is a price to pay - a reduction of the luminosity due to the finite bunch length and the non-head on collisions: L reduction of ~17% IP Crossing planes & angles •ATLAS Vertical 280 µrad 7.5 m •CMS Horizontal 280 µrad •LHCb Horizontal 300 µrad •ALICE Vertical 400 µradJ. Wenninger LNF Spring School, May 2010 48
  49. 49. Separation and crossing : example of ATLAS Horizontal plane: the beams are combined and then separated 194 mm ATLAS IP ~ 260 m Common vacuum chamber Vertical plane: the beams are deflected to produce a crossing angle at the IP Not to scale ! ~ 7 mmJ. Wenninger LNF Spring School, May 2010 49
  50. 50. TevatronJ. Wenninger LNF Spring School, May 2010 50
  51. 51. Tevatron CDF D0
  52. 52. Tevatron I  The Tevatron is presently the ‘energy frontier’ collider in operation at FNAL, with a beam energy of 980 GeV and a size of ~ ¼LHC (about same size than SPS).  It is the first super-conducting collider ever build.  It collides proton and anti-proton bunches that circulate in opposite directions in the SAME vacuum chamber.  One of the problems at the TEVATRON are the long-distance encounters of the bunches in the arc sections. A complicated separation scheme with electrostatic elements has to be used: Tricky to operate !! E E 52J. Wenninger LNF Spring School, May 2010
  53. 53. Tevatron II  The Tevatron has undergone a number of remarkable upgrades and it presently collides 36 proton with 36 anti-proton bunches (k=36), with bunch populations (N) similar to the ones of the LHC (but there are always fewer anti-protons !).  Compare LHC and Tevatron: kN 2 f L= 4πσ xσ * * y fTevatron ≈ 4 fLHC Tevatron gets a factor 4 ‘for free’ due to ring size !! kLHC ≈ 100 kTevatron LLHC ≈ 30 LTevatron N2/(σx σy) ~ equal Luminosity gain of LHC comes basically from the number of bunches (k) !! 53J. Wenninger LNF Spring School, May 2010
  54. 54. Injection and injector complexJ. Wenninger LNF Spring School, May 2010 54
  55. 55. Beam 2 5 Beam 1 4 LHC 6 7 3 TI8 2 SPS 8 TI2 protons Booster 1 LINACS Top energy/GeV Circumference/m CPS Ions Linac 0.05 30 PSB 1.4 157 CPS 26 628 = 4 PSB SPS 450 6’911 = 11 x PS LEIR LHC 7000 26’657 = 27/7 x SPS Note the energy gain/machine of 10 to 20. The gain is typical for the useful range of magnets.J. Wenninger LNF Spring School, May 2010 55
  56. 56. Principle of injector cycling The beams are handed from one accel. to the next or used for its own customers ! B field SPS top energy, prepare for SPS transfer … Beam transfer ramp SPS waits at injection to be filled by PS SPSB field time PS B time PS Booster timeJ. Wenninger LNF Spring School, May 2010 56
  57. 57. Principle of injection (and extraction) Circulating beam Kicker B-field Injected beam Injecte d beam Septum magnet B time Kicker magnet B Circulating beam Kicker magnet  A septum dipole magnet (with thin coil) is used to bring the injected beam close to the circulating beam.  A fast pulsing dipole magnet (‘kicker’) is fired synchronously with the arrival of the injected beam: deflects the injected beam onto the circulating beam path.  ‘Stack’ the injected beams one behind the other.  At the LHC the septum deflects in the horizontal plane, the kicker in the vertical plane (to fit to the geometry of the tunnels).  Extraction is identical, but the process is reversed !J. Wenninger LNF Spring School, May 2010 57
  58. 58. Linac2Radio-frequency Alvarez’s drift-tubequadrupole (RFQ) Delivered beam current: ~150mA Beam energy: 90 keV (source) → 750 keV (RFQ) → 50 MeV Repetition rate: 1 Hz Radio-frequency system: 202 MHzJ. Wenninger LNF Spring School, May 2010 58
  59. 59. PS Booster  Constructed in the 70ies to increase the intensity into the PS  Made of four stacked rings  Acceleration to E kin=1.4 GeV  Intensities > 10 13 protons per ring.J. Wenninger LNF Spring School, May 2010 59
  60. 60. Filling the PS with LHC beams  Rings 2,3 & 4 are filled with 2 bunches per ring.  The 6 bunches are transferred to the PS. x3J. Wenninger LNF Spring School, May 2010 60
  61. 61. Proton Synchrotron Recently celebrated its first 50 years!!J. Wenninger LNF Spring School, May 2010 61
  62. 62. Bunch Splitting at the PS  The bunch splitting in the PS is probably the most delicate manipulation for the production of LHC beams – multiple RF systems with different frequencies: from 6 injected to 72 extracted bunches  The quality of the splitting is critical for the LHC (uniform intensity in all bunches…). PS ejection: 320 ns beam gap 72 bunches 72 bunches on h=84 in 1 turn Quadruple splitting at 25 GeV Acceleration 18 bunches to 25 GeV on h=21 Triple splitting at 1.4 GeV PS injection: 6 bunches 2+4 bunches on h=7 bucket Empty in 2 batchesJ. Wenninger LNF Spring School, May 2010 62
  63. 63. Super-Proton SynchrotronJ. Wenninger LNF Spring School, May 2010 63
  64. 64. SPS-to-LHC transfer lines Courtesy of J. UythovenJ. Wenninger LNF Spring School, May 2010 64
  65. 65. Collision schemes  The 400 MHz RF system provides 35’640 possible bunch positions (buckets) at a distance of 2.5 ns along the LHC circumference.  A priori any of those positions could be filled with a bunch…  The smallest bunch-to-bunch distance is fixed to 25 ns, which is also the nominal distance: max. number of bunches is 3564. 2.5 ns … 25 ns = filled position = bunch position  In practice there are fewer bunches because holes must be provided for the fast pulsed magnets (kickers) used for injection and dump.  But the LHC and its injectors are very flexible and can operate with many bunch patterns: from isolated bunches to trains.J. Wenninger LNF Spring School, May 2010 65
  66. 66. Collision point symmetry = collision point CMS Symmetry  ATLAS, ALICE and CMS are positioned axis on the LEP symmetry axis (8 fold sym.)  LHCb is displaced from the symmetry axis by 11.25 m <<-->> 37.5 ns. LHC  For filling patterns with many bunches this is not an issue, but it becomes a bit tricky with few bunches. LHCb by m d .25 Alice ce la 11 sp = Atlas di ns Cb 7.5 LH x 3 cJ. Wenninger LNF Spring School, May 2010 66
  67. 67. Filling pattern example: 1x1 CMS  With 1 bunch/beam, there are 2 collision points at opposite sides of the ring.  Depending on their position along the circumference, the 2 bunches can be made to collide: in ATLAS and CMS, LHC OR in ALICE, OR in LHCb, LHCb but never in all experiments at the same time !! Alice AtlasJ. Wenninger LNF Spring School, May 2010 67
  68. 68. (Some) LHC filling patterns Schema Nominal bunch No. bunches Comment distance (ns) 43x43 2025 43 No crossing angle required 156x156 525 156 No crossing angle required 25 ns 25 2808 Nominal p filling 50 ns 50 1404 2010-2011 run target Ion nominal 100 592 Nominal ion filling Ion early 1350 62 No crossing angle required  With 43x43 and 156x156, some bunches are displaced (distance ≠ nominal) to balance the ALICE and LHCb luminosities.  In the multi-bunch schemes (25, 50, 100 ns) there are larger gaps to accommodate fast injection magnets (‘kickers’) rise times.  There is always a ≥ 3 µs long particle free gap for the beam dump kicker.J. Wenninger LNF Spring School, May 2010 68
  69. 69. Nominal filling pattern  The nominal pattern consists of 39 groups of 72 bunches (spaced by 25 ns), with variable spacing to accommodate the rise times of the injection and extraction magnets (‘kickers’). 72 bunches τ5 τ3 b=bunch, e=empty τ2 τ1 69J. Wenninger LNF Spring School, May 2010
  70. 70. Spare slidesJ. Wenninger LNF Spring School, May 2010 70
  71. 71. PS - bunch splittingJ. Wenninger LNF Spring School, May 2010 71
  72. 72. Injection elements 12 mrad TED 0.8 mrad TED From the LHC Page1J. Wenninger LNF Spring School, May 2010 72
  73. 73. Role of the TDI collimator The TDI is one of the key injection protection collimators: Protects the machine in case of (1) missing kicks on injected beam and (2) asynchronous kicker firing on the circulating beam. It must be closed around the circulating beam trajectory when the kicker is ON.J. Wenninger LNF Spring School, May 2010 73

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