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Rüdiger Schmidt Bullay Oktober 20071 The LHC collider I Rüdiger Schmidt - CERN Klausurtagung Graduiertenkolleg Bullay Oktober 2007 Challenges LHC accelerator.

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Presentation on theme: "Rüdiger Schmidt Bullay Oktober 20071 The LHC collider I Rüdiger Schmidt - CERN Klausurtagung Graduiertenkolleg Bullay Oktober 2007 Challenges LHC accelerator."— Presentation transcript:

1 Rüdiger Schmidt Bullay Oktober The LHC collider I Rüdiger Schmidt - CERN Klausurtagung Graduiertenkolleg Bullay Oktober 2007 Challenges LHC accelerator physics LHC technology Operation and protection

2 Rüdiger Schmidt Bullay Oktober Energy and Luminosity l Particle physics requires an accelerator colliding beams with a centre-of-mass energy substantially exceeding 1TeV l In order to observe rare events, the luminosity should be in the order of [cm -2 s -1 ] (challenge for the LHC accelerator) l Event rate: l Assuming a total cross section of about 100 mbarn for pp collisions, the event rate for this luminosity is in the order of 10 9 events/second (challenge for the LHC experiments) l Nuclear and particle physics require heavy ion collisions in the LHC (quark-gluon plasma.... )

3 Rüdiger Schmidt Bullay Oktober ATLAS Detector

4 Rüdiger Schmidt Bullay Oktober events / second LHC simulated event

5 Rüdiger Schmidt Bullay Oktober CERN and the LHC

6 Rüdiger Schmidt Bullay Oktober CERN is the leading European institute for particle physics It is close to Geneva across the French Swiss border There are 20 CERN member states, ~7 observer states, and many other states participating in research LEP / LHC

7 Rüdiger Schmidt Bullay Oktober The CERN Beschleuniger Komplex LEP e+e- ( ) 104 GeV/c LHC pp and ions 7 TeV/c 26.8 km Circumference CERN Hauptgelände Schweiz Genfer See Frankreich LHC Beschleuniger (etwa 100m unter der Erde) SPS Beschleuniger CERN- Prevessin CMS ALICELHCbATLAS

8 Rüdiger Schmidt Bullay Oktober LHC: From first ideas to realisation 1982 : First studies for the LHC project 1983 : Z0 detected at SPS proton antiproton collider 1985 : Nobel Price for S. van der Meer and C. Rubbia 1989 : Start of LEP operation at 45 GeV (Z-factory) 1994 : Approval of the LHC by the CERN Council 1996 : Final decision to start the LHC construction 1996 : LEP operation at 100 GeV (W-factory) 2000 : End of LEP operation 2002 : LEP equipment removed 2003 : Start of the LHC installation 2005 : Start of system commissioning (hardware commissioning) 2008 : Commissioning with beam

9 Rüdiger Schmidt Bullay Oktober The LHC is the largest machine that has ever been built, and probably the most complex one To make the LHC a reality: Accelerators physics and.... l Electromagnetism und Relativity l Thermodynamics l Mechanics l Physics of nonlinear systems l Solid state physics und surface physics l Quantum mechanics l Particle physics and radiation physics l Vacuum physics + Engineering Mechanical, Cryogenics, Electrical, Automation, Computing, Civil Engineering

10 Rüdiger Schmidt Bullay Oktober l Accelerator Physics: An Introduction Why protons? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC layout and beam transport l The quest for high luminosity and the consequences l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Status of commissioning l Conclusions Outline

11 Rüdiger Schmidt Bullay Oktober l Accelerator Physics: An Introduction Why protons? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC layout and beam transport l The quest for high luminosity and the consequences l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Status of commissioning l Conclusions Outline

12 Rüdiger Schmidt Bullay Oktober Lorentz Force The force on a charged particle is proportional to the charge, the electric field, and the vector product of velocity and magnetic field: For an electron or proton the charge is: Acceleration (increase of energy) only by electrical fields – not by magnetic fields:

13 Rüdiger Schmidt Bullay Oktober Acceleration Acceleration of a particle by an electrical potential Energy gain given by the potential For an acceleration to 7 TeV a voltage of 7 TV is required

14 Rüdiger Schmidt Bullay Oktober Acceleration with RF fields U = V d = 1 m q = e 0  E = 1 MeV U = V

15 Rüdiger Schmidt Bullay Oktober RF buckets and bunches EE time RF Voltage time LHC bunch spacing = 25 ns = 10 buckets  7.5 m 2.5 ns The particles are trapped in the RF voltage: this gives the bunch structure RMS bunch length 11.2 cm 7.6 cm RMS energy spread 0.031%0.011% 450 GeV 7 TeV The particles oscillate back and forth in time/energy RF bucket 2.5 ns

16 Rüdiger Schmidt Bullay Oktober orthogonal g 2a z LHC RF frequency 400 MHz Revolution frequency Hz RF cavity

17 Rüdiger Schmidt Bullay Oktober RF systems: 400 MHz 400 MHz system: 16 sc cavities (copper sputtered with niobium) for 16 MV/beam were built and assembled in four modules

18 Rüdiger Schmidt Bullay Oktober To get to 7 TeV: Synchrotron – circular accelerator and many passages in RF cavities LINAC (planned for several hundred GeV - but not above 1 TeV, e.g ILC) LHC circular machine with energy gain per turn ~0.5 MeV acceleration from 450 GeV to 7 TeV takes about 20 minutes....requires deflecting magnets (dipoles)

19 Rüdiger Schmidt Bullay Oktober Particle deflection: Lorentz Force The force on a charged particle is proportional to the charge, the electric field, and the vector product of velocity and magnetic field: z x s v B F Maximum momentum 7000 GeV/c Radius 2805 m fixed by LEP tunnel Magnetic field B = 8.33 Tesla Iron magnets limited to 2 Tesla, therefore superconducting magnets are required Deflecting magnetic fields for two beams in opposite directions

20 Rüdiger Schmidt Bullay Oktober Radius Lorenz Force = accelerating force Particle trajectory Radiation field charged particle Figure from K.Wille Energy loss for charged particles by synchrotron radiation

21 Rüdiger Schmidt Bullay Oktober Energy loss for charged particles electrons / protons in LEP tunnel

22 Rüdiger Schmidt Bullay Oktober l Accelerator Physics: An Introduction Why protons? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC layout and beam transport l The quest for high luminosity and the consequences l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Status of commissioning l Conclusions Outline

23 Rüdiger Schmidt Bullay Oktober LHC Layout eight sectors eight arcs eight long straight sections (insertions) about 700 m long IR6: Beam dumping system IR4: RF + Beam instrumentation IR5: CMS IR1: ATLAS IR8: LHC-B IR2: ALICE Injection IR3: Momentum Beam Cleaning (warm) IR7: Betatron Beam Cleaning (warm) Beam dump blocks Main dipole magnets: making the circle

24 Rüdiger Schmidt Bullay Oktober Beam transport Need for getting protons on a circle: dipole magnets Need for focusing the beams: l Particles with different injection parameters (angle, position) separate with time Assuming an angle difference of rad, two particles would separate by 1 m after 10 6 m. At the LHC, with a length of m, this would be the case after 50 turns (5 ms !) l Particles would „drop“ due to gravitation l The beam size must be well controlled At the collision point the beam size must be tiny l Particles with (slightly) different energies should stay together

25 Rüdiger Schmidt Bullay Oktober Focusing using lenses as for light f1f1 x x Quadrupolemagnet – B-field zero in centre, linear increase (as an optical lense) Dipolemagnet – B-field in aperture constant z z

26 Rüdiger Schmidt Bullay Oktober Assuming proton runs along s (=y), perpendicular to x and z z x x z s z s x Side view focusing Looking along proton trajectory Top view defocusing From Maxwell equations:

27 Rüdiger Schmidt Bullay Oktober Focusing of a system of two lenses for both planes d = 50 m horizontal plane vertical plane To focuse the beams in both planes, a succession of focusing and defocusing quadrupole magnets is required: FODO structure

28 R.Schmidt28 A cell in the LHC arcs SSS quadrupole MQF sextupole corrector (MCS) decapole octupole corrector (MCDO) lattice sextupole (MS) lattice sextupole (MS) lattice sextupole (MS) orbit corrector special corrector (MQS) special corrector (MO) quadrupole MQD quadrupole MQF main dipole MB orbit corrector main dipole MB main dipole MB main dipole MB main dipole MB main dipole MB LHC Cell - Length about 110 m (schematic layout) Vertical / Horizontal plane (QF / QD) Quadrupole magnets controlling the beam size „to keep protons together“ (similar to optical lenses)

29 Rüdiger Schmidt Bullay Oktober Magnets and beam stability l Dipole magnets To make a circle around LHC l Quadrupol magnets To keep beam particles together Particle trajectory stable for particles with nominal momentum l Sextupole magnets To correct the trajectories for off momentum particles Particle trajectories stable for small amplitudes (about 10 mm) l Multipole-corrector magnets Sextupole - and decapole corrector magnets at end of dipoles l Particle trajectories can become instable after many turns (even after, say, 10 6 turns)

30 Rüdiger Schmidt Bullay Oktober Particle stability and superconducting magnets - Quadrupolar- and multipolar fields Particle oscillations in quadrupole field (small amplitude) Harmonic oscillation after coordinate transformation Circular movement in phase space Particle oscillation assuming non-linear fields, large amplitude Amplitude grows until particle is lost (touches aperture) No circular movement in phasespace

31 Rüdiger Schmidt Bullay Oktober Dynamic aperture and magnet imperfections l Particles with small amplitudes are stable l Particles with large amplitudes are not stable l The dynamic aperture is the limit of the stability region l The dynamic aperture depends on field errors - without any field errors, the dynamic aperture would be large l The magnets should be made such as the dynamic aperture is not too small (say, 10  the amplitude of a one sigma particle, assuming Gaussian distribution) l The dynamic aperture depends also on the working point (number of oscillations per turn) and on the sextupole magnets for correction of chromatic effects

32 Rüdiger Schmidt Bullay Oktober l Accelerator Physics: An Introduction Why protons? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC layout and beam transport l The quest for high luminosity and the consequences l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Status of commissioning l Conclusions Outline

33 Rüdiger Schmidt Bullay Oktober High luminosity by colliding trains of bunches Number of „New Particles“ per unit of time: The objective for the LHC as proton – proton collider is a luminosity of about [cm -2 s -1 ] LEP (e+e-) : [cm -2 s -1 ] Tevatron (p-pbar) : some [cm -2 s -1 ] B-Factories: > [cm -2 s -1 ]

34 Rüdiger Schmidt Bullay Oktober Luminosity parameters What happens with one particle experiencing the force of the em-fields of protons in the other beam during the collision ?

35 Rüdiger Schmidt Bullay Oktober Limitation: beam-beam interaction

36 Rüdiger Schmidt Bullay Oktober Electromagnetic force on a particle in the counterrotating beam Bunch intensity limited due to this strong non- linear field to about N = Optimising luminosity by increasing N

37 Rüdiger Schmidt Bullay Oktober Beam beam interaction determines parameters Beam size 16  m, for  = 0.5 m (  is a function of the lattice) f = Hz Beam size given by injectors and by space in vacuum chamber Number of protons per bunch limited to about L = N 2 f n b / 4   x  y = [cm -2 s -1 ] with one bunch with 2808 bunches (every 25 ns one bunch) L = [cm -2 s -1 ]

38 Rüdiger Schmidt Bullay Oktober Large number of bunches IP Bunch structure with 25 ns spacing Experiments: more than 1 event / collision, but should not exceed a number in the order of Limit number of collision points as far as possible Vacuum system: photo electrons

39 Rüdiger Schmidt Bullay Oktober Large number of bunches IP l Crossing angle to avoid beam beam interaction (only long range beam beam interaction present) l Interaction Region quadrupoles with gradient of 250 T/m and 70 mm aperture

40 Rüdiger Schmidt Bullay Oktober u Focusing quadrupole for beam 1, defocusing for beam 2 u High gradient quadrupole magnet triplet with large aperture (US-JAPAN)  Total crossing angle of 300  rad  Beam size at interaction point 16  m, in arcs about 0.3 mm Crossing angle for multibunch operation distance about 100 m Interaction point QFQDQFQDQFQD Experiment

41 Rüdiger Schmidt Bullay Oktober Layout of insertion for ATLAS and CMS

42 Rüdiger Schmidt Bullay Oktober l Accelerator Physics: An Introduction Why protons? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC layout and beam transport l The quest for high luminosity and the consequences l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Status of commissioning l Conclusions Outline

43 Rüdiger Schmidt Bullay Oktober Very high beam current Many bunches and high energy - Energy in one beam about 360 MJ l Dumping the beam in a safe way l Beam induced quenches (when of beam hits magnet at 7 TeV) l Beam cleaning (Betatron and momentum cleaning) l Beam stability and magnet field quality l Synchrotron radiation - power to cryogenic system l Radiation, in particular in experimental areas from beam collisions (beam lifetime is dominated by this effect) l Photo electrons - accelerated by the following bunches

44 Rüdiger Schmidt Bullay Oktober Challenges: Energy stored in the beam courtesy R.Assmann Momentum [GeV/c] Energy stored in the beam [MJ] Transverse energy density: even a factor of 1000 larger x 200 x One beam, nominal intensity (corresponds to an energy that melts 500 kg of copper)

45 Momentum at collision 7 TeV/c Momentum at injection 450 GeV/c Dipole field at 7 TeV 8.33 Tesla Circumference26658m Luminosity cm -2 s -1 Number of bunches 2808 Particles per bunch 1.1  DC beam current 0.56 A Stored energy per beam 360 MJ Normalised emittance3.75 µm Beam size at IP / 7 TeV15.9µm Beam size in arcs (rms)300µm Arcs: Counter-rotating proton beams in two- in-one magnets Magnet coil inner diameter 56 mm Distance between beams 194 mm High beam energy in LEP tunnel superconducting NbTi magnets at 1.9 K High luminosity at 7 TeV very high energy stored in the beam Beam power concentrated in small area Limited investment small aperture for beams

46 Rüdiger Schmidt Bullay Oktober summarising the constraints…. Centre-of-mass energy must well exceed 1 TeV, LHC installed into LEP tunnel l Colliding protons (and heavy ions) l Magnetic field of 8.3 T with superconducting magnets Luminosity of cm -2 s -1 l Need for “two accelerators” in one tunnel with beam parameters pushed to the extreme – with opposite magnetic field Economical constraints and limited space l Two-in-one superconducting magnets

47 Rüdiger Schmidt Bullay Oktober l Accelerator Physics: An Introduction Why protons? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC layout and beam transport l The quest for high luminosity and the consequences l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Status of commissioning l Conclusions Outline

48 Rüdiger Schmidt Bullay Oktober LHC injector complex High intensity beam from the SPS into LHC at 450 GeV via TI2 and TI8 LHC accelerates to 7 TeV LEIR CPS SPS Booster LINACS LHC TI8 TI2 Ions protons Beam 1 Beam 2 Beam size of protons decreases with energy:  2 = 1 / E Beam size large at injection Beam fills vacuum chamber at 450 GeV

49 Rüdiger Schmidt Bullay Oktober Getting beam into the LHC Beam size of protons decreases with energy:  2 = 1 / E l Beam size large at injection l Beam “fills” vacuum chamber at 450 GeV If the injector energy would be lower... l larger vacuum chamber and larger magnets – increased cost l magnets and power converter limitations (dynamic effects, stability, …) l issues of beam stability Injection from the SPS at 450 GeV, via two transfer lines, into the LHC

50 Rüdiger Schmidt Bullay Oktober Injector Complex l Pre-injectors: Linac, PS Booster and Proton Synchrotron deliver protons at 26 GeV to the SPS l The SPS accelerates protons from 26 GeV to 450 GeV l Both, the pre-injectors and the SPS were upgraded for the operation with nominal LHC beam parameters l Already today beams are available close to the nominal beam parameters required for the LHC l The TI8 injection line has been commissioned, TI2 commissioning started

51 Rüdiger Schmidt Bullay Oktober Results of Transfer Line TI8 test

52 Rüdiger Schmidt Bullay Oktober End

53 Rüdiger Schmidt Bullay Oktober Spare slides

54 HCSS - J. Wenninger54 Collective (in-)stability 54  The electromagnetic field of a bunch interacts with the 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 !

55 HCSS - J. Wenninger55 Electron clouds… … affect high intensity beams with positive charge and closely spaced bunches.  Electrons are generated at the vacuum chamber surface by beam impact, photons…  If the probability to emit secondary e- is high (enough), more e- are produced and accelerated by the field of a following bunch(es) and multiplication start…  The cloud of e- that may build up can drive the beam unstable, and at the LHC, overload the cryogenic system by the heat they deposit on the chamber walls !  This effect depends strongly on surface conditions, simulations are tricky because they are very sensitive to very low energy (~ eV) electrons. The latest simulation indicate that the problem may be less severe than initially anticipated but …  The cloud can ‘cure itself’ because the impact of all those electrons cleans the surface, reduces the electron emission probability and eventually the cloud disappears ! N e- N+1 e- N+2 e- Bunch N liberates an e- Bunch N+1 accelerates the e-, multiplication at impact Bunch N+2 accelerates the e-, more multiplication…


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