Rüdiger Schmidt1 The LHC collider project I Rüdiger Schmidt - CERN SUSSP Sumer School St.Andrews Challenges LHC accelerator physics LHC technology Operation and protection

Rüdiger Schmidt2 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 -1 s -2 ] (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.... )

Rüdiger Schmidt events / second LHC Event

Rüdiger Schmidt4 CERN and the LHC

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, 5 observer states, and many other states participating in research LEP CMS

LEP: e+e- 104 GeV/c ( ) Circumference 26.8 km LHC proton-proton Collider 7 TeV/c in the LEP tunnel Injection from SPS at 450 GeV/c ATLAS CMS Auberge Communale Cessy

Rüdiger Schmidt7 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 (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 (second life for sc cavities ?) 2003 : Start of the LHC installation 2005 : Start of hardware commissioning 2007 : Commissioning with beam

Rüdiger Schmidt8 To make the LHC a reality: Accelerators physics and.... l Electromagnetism und Relativity l Thermodynamics l Mechanics l Quantum mechanics l Physics of nonlinear systems l Solid state physics und surface physics l Particle physics and radiation physics l Vacuum physics + Engineering Mechanical, Cryogenics, Electrical, Automation, Computing

l Accelerator Physics: An Introduction Why protons? Why in the LEP tunnel? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC Layout l The quest for high luminosity and the consequences l Beam-Beam interaction l Crossing angle and insertion layout l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Conclusions Outline

l Accelerator Physics: An Introduction Why protons? Why in the LEP tunnel? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC Layout l The quest for high luminosity and the consequences l Beam-Beam interaction l Crossing angle and insertion layout l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC Operation and machine protection l Conclusions Outline

Rüdiger Schmidt11 Lorentz Force The force on a charged particle is proportional to the charge, and to 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:

Rüdiger Schmidt12 Acceleration Acceleration of a particle by an electrical potential Energy gain given by the potential l For an acceleration to 7 TeV a voltage of 7 TV is required l The maximum electrical field in an accelerator is in the order of some 10 MV/m (superconducting RF cavities) l To accelerate to 7 TeV would require a linear accelerator with a length of about 350 km (assuming 20 MV/m)

Rüdiger Schmidt13 Acceleration in a cavity U = V d = 1 m q = e 0  E = 1 MeV +-+

Rüdiger Schmidt14 RF cavity g 2a z LHC frequency 400 MHz orthogonal

Rüdiger Schmidt15 RF systems: 400 MHz and possibly 200 MHz 400 MHz system: all 16 sc cavities (copper sputtered with niobium) for 16 MV/beam were built and assembled in four modules Power test of the first module 200 MHz warm system: if rquired, decision for implementation to be taken later - to ease the injection process

Rüdiger Schmidt16 How 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) LHC circular machine with energy gain per turn some MeV

Rüdiger Schmidt17 Deflection by magnetic fields For a charged particle moving perpendicular to the magnetic field the force is given by: z x s v B F The particle moves on a circle

Deflection by magnetic fields

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

Rüdiger Schmidt20 Energy loss for charged particles electrons / protons in LEP tunnel

Rüdiger Schmidt21...just assuming to accelerate electrons to 7 TeV...better to accelerate protons

l Accelerator Physics: An Introduction Why protons? Why in the LEP tunnel? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC Layout l The quest for high luminosity and the consequences l Beam-Beam interaction l Crossing angle and insertion layout l The CERN accelerator complex: injectors and transfer l Wrapping up: LHC Parameters l LHC technology l LHC operation and machine protection l Conclusions Outline

Rüdiger Schmidt23 LHC: eight arcs (approximatley circular) and eight long straight section (about 700 m long) Momentum Cleaning Betatron Cleaning Beam dump system RF + Beam instrumentation CMS ATLAS LHC-B ALICE

Rüdiger Schmidt24 Layout of the LHC ring: 8 arcs, and 8 long straight sections Momentum Cleaning Betatron Cleaning Beam dump system RF + Beam instrumentation One sector = 1/8 Injection

Rüdiger Schmidt25 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

Rüdiger Schmidt26 Focusing using lenses as for light f1f1 z x z x Quadrupolemagnet – B-field zero in centre, linear increase (as a lense) Dipolemagnet – B-field in aperture constant

Rüdiger Schmidt27 Assuming proton runs along s into the screen, 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:

Rüdiger Schmidt28 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

Rüdiger Schmidt29 The LHC arcs: FODO cells u Dipole- und Quadrupol magnets –Particle trajectory stable for particles with nominal momentum u Sextupole magnets –To correct the trajectories for off momentum particles –Particle trajectories stable for small amplitudes (about 10 mm) u Multipole-corrector magnets –Sextupole - and decapole corrector magnets at end of dipoles –Particle trajectories can become instable after many turns (even after, say, 10 6 turns)

Rüdiger Schmidt30 Particle stability and supraconducting 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

Rüdiger Schmidt31 Dynamic aperture and magnet imperfections l Particles with small amplitudes are in general 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 error - 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 and on the sextupole magnets for correction of chromatic effects

l Accelerator Physics: An Introduction Why protons? Why in the LEP tunnel? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC Layout l The quest for high luminosity and the consequences l Beam-Beam interaction l Crossing angle and insertion layout l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Conclusions Outline

Rüdiger Schmidt33 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 -1 s -2 ] LEP (e+e-) : [cm -1 s -2 ] Tevatron (p-pbar) : [cm -1 s -2 ] B-Factories: [cm -1 s -2 ] 40 m in straight section IP

Rüdiger Schmidt34 Luminosity parameters What happens with one particle experiencing the force of the em-fields or protons in the other beam during the collision ?

l Accelerator Physics: An Introduction Why protons? Why in the LEP tunnel? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC Layout l The quest for high luminosity and the consequences l Beam-Beam interaction l Crossing angle and insertion layout l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Conclusions Outline

Rüdiger Schmidt36 Limitation: beam-beam interaction

Rüdiger Schmidt37 Electromagnetic force on a particle in the counterrotating beam Bunch intensity limited due to this strong non- linearity to about N = Optimising luminosity by increasing N

Rüdiger Schmidt38 Beam beam interaction determines parameters Beam size 16  m, for  = 0.5 m 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 ]

Rüdiger Schmidt39 Large number of bunches N = bunches - spacing of about 25 ns Minimum beam size at IP of 16  m

l Accelerator Physics: An Introduction Why protons? Why in the LEP tunnel? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC Layout l The quest for high luminosity and the consequences l Beam-Beam interaction l Crossing angle and insertion layout l Wrapping up: LHC Parameters l The CERN accelerator complex: injectors and transfer l LHC technology l LHC operation and machine protection l Conclusions Outline

Rüdiger Schmidt41 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 Vacuum system: photo electrons

Rüdiger Schmidt42 Large number of bunches IP l Crossing angle to avoid long range beam beam interaction l Interaction Region quadrupoles with gradient of 250 T/m and 70 mm aperture

Rüdiger Schmidt43 u Focusing quadrupole for beam 1, defocusing for beam 2 u High gradient quadrupole magnets with large aperture (US-JAPAN)  Total crossing angle of 300  rad  Beam size at IP 16  m, in arcs about 1 mm Crossing angle for multibunch operation

Rüdiger Schmidt44 Layout of insertion for ATLAS and CMS

l Accelerator Physics: An Introduction Why protons? Why in the LEP tunnel? Why superconducting magnets? Why “two” accelerators in one tunnel? l LHC Layout l The quest for high luminosity and the consequences l Beam-Beam interaction l Crossing angle and insertion layout l The CERN accelerator complex: injectors and transfer l Wrapping up: LHC Parameters l LHC technology l LHC operation and machine protection l Conclusions Outline

Rüdiger Schmidt46 Very high beam current Many bunches and high energy - Energy in one beam about 330 MJ l Dumping the beam in a safe way l Beam induced quenches (when of beam hits magnet at 7 TeV) l Beam stability and magnet field quality l Beam cleaning (Betatron and momentum cleaning) 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

Rüdiger Schmidt47 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)

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 350 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

Rüdiger Schmidt49 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 : 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