1. LHC : construction and operation
Jörg Wenninger
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 schemes
J. Wenninger LNF Spring School, May 2010

2. Outline The LHC challenges
Introduction to magnets and particle focusing
LHC magnets and arc layout
LHC luminosity and interaction regions
Injection and filling schemes
Machine protection
Incident 19th Sept and consequences
LHC operation
Part 1
Part 2
J. Wenninger LNF Spring School, May 2010

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/beam
J. Wenninger LNF Spring School, May 2010

4. The Large Hadron Collider LHC
Installed in the 26.7 km LEP tunnel
Depth of m
Lake of Geneva
LHC ring
CMS, Totem
LHCb
Control Room
SPS ring
ATLAS, LHCf
ALICE
Der LHC Beschleuniger - DPG - Bonn

5. Tunnel circumference 26.7 km, tunnel diameter 3.8 m
Depth : ~ m – tunnel is inclined by ~ 1.4%
J. Wenninger LNF Spring School, May 2010

6. LHC Layout 8 arcs. 8 straight sections (LSS), ~ 700 m long.
The beams exchange their positions (inside/outside) in 4 points to ensure that both rings have the same circumference !
IR5:CMS
Beam dump blocks
Beam2
Beam1
IR6: Beam dumping system
IR4: RF + Beam instrumentation
IR3: Momentum collimation (normal conducting magnets)
IR7: Betatron collimation (normal conducting magnets)
IR8: LHC-B
IR2:ALICE
IR1: ATLAS
Injection ring 1
Injection ring 2
J. Wenninger LNF Spring School, May 2010

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
HERA/Tevatron ~4 T
The luminosity of the collider that will reach unprecedented values for a hadron machine:
LHC pp ~ 1034 cm-2 s-1
Tevatron pp 3x1032 cm-2 s-1
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 !
A factor 2 in field
A factor 4 in size
A factor 30
in luminosity
J. Wenninger LNF Spring School, May 2010

8. Luminosity challenges
The event rate N for a physics process with cross-section s is proprotional to the collider Luminosity L:
k = number of bunches = 2808
N = no. protons per bunch = 1.15×1011
f = revolution frequency = kHz
s*x,s*y = beam sizes at collision point (hor./vert.) = 16 mm
To maximize L:
Many bunches (k)
Many protons per bunch (N)
A small beam size s*u = (b *e)1/2
b * : the beam envelope (optics)
e : is the phase space volume occupied by the beam (constant along the ring).
High beam “brillance” N/e
(particles per phase space volume)
 Injector chain performance !
Small envelope
 Strong focusing !
Optics property
Beam property
J. Wenninger LNF Spring School, May 2010

9. Basics of accelerator physics
J. Wenninger LNF Spring School, May 2010

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 magnets
J. Wenninger LNF Spring School, May 2010

11. Bending → Lorentz force Magnetic rigidity
LHC: ρ = 2.8 km given by LEP tunnel! →
To reach p = 7 TeV/c given a bending radius of r = 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

12. B F p Bending Fields B field F force Two-in-one magnet design I B
J. Wenninger LNF Spring School, May 2010

13. Focusing N S By
F x
F y
Transverse focusing is achieved with quadrupole 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).
Focusing in one plane, de-focusing in the other! x y
J. Wenninger LNF Spring School, May 2010
1313

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

15. Alternating gradient lattices x y
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 particle trap!
J. Wenninger LNF Spring School, May 2010

16. LHC arc lattice Dipole- und Quadrupol magnets Sextupole 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

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 ‘b’-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…
Betatron functions in a simple FODO cell QF QD
The envelope peaks in the focusing elements !
Vertical
Horizontal
J. Wenninger LNF Spring School, May 2010

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 s at any point along the accelerator is given by (neglecting the contribution from energy spread):
For unperturbed proton beams, the normalized emittance en is conserved:
g = Lorentz factor
The beam size shrinks with energy:
J. Wenninger LNF Spring School, May 2010

19. Why does the transverse emittance shrink?
The acceleration is purely longitudinal, i.e the transverse momentum is not affected:
The emittance is nothing but a measure of <pt>.
To maintain the focusing strength, all magnetic fields are kept proportional to E (g), including the quadrupole gradients.
With constant <pt> and increasing quadrupole gradients, the transverse excursion of the particles becomes smaller and smaller !
J. Wenninger LNF Spring School, May 2010

20. LHC beam sizes Beta-function at the LHC ARC
Nominal LHC normalized emittance :
Example LHC arc, peak b = 180 m
Energy (GeV)
e (nm)
s (mm)
450
7.2
1.14
3500
0.93
0.41
7000
0.47
0.29
J. Wenninger LNF Spring School, May 2010

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. s
J. Wenninger LNF Spring School, May 2010

22. Synchrotron radiation loss
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
3.5 TeV
0.42 keV/turn
7 TeV
6.7 keV /turn
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

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

24. Cavities in the tunnel
J. Wenninger LNF Spring School, May 2010

25. The particles oscillate back and forth in time/energy
RF buckets and bunches
The particles oscillate back and forth in time/energy
The particles are trapped in the RF voltage:
this gives the bunch structure
RF Voltage
2.5 ns
time E
LHC bunch spacing = 25 ns = 10 buckets  7.5 m
RF bucket
time
2.5 ns
450 GeV TeV
RMS bunch length cm cm
RMS energy spread % 0.02%
J. Wenninger LNF Spring School, May 2010

26. Magnets & Tunnel
J. Wenninger LNF Spring School, May 2010

27. Superconductivity Bc Tc
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.
Typical for 4.2 K
2000 6T
To reach 8-10 T, the temperature must be lowered to 1.9 K – superfluid Helium ! Bc Tc
J. Wenninger LNF Spring School, May 2010

28. The superconducting cable
6 m
1 mm
A.Verweij
Typical value for operation at 8T and 1.9 K: 800 A
width 15 mm
Rutherford cable
A.Verweij
J. Wenninger LNF Spring School, May 2010

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

30. Steel cylinder for Helium
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 m
J. Wenninger LNF Spring School, May 2010
Rüdiger Schmidt 30

31. Regular arc: Magnets 1232 main dipoles +
3700 multipole corrector magnets
(sextupole, octupole, decapole)
392 main quadrupoles +
2500 corrector magnets (dipole, sextupole, octupole)
J. Wenninger LNF Spring School, May 2010
J. Wenninger - ETHZ - December 2005 31

32. Regular arc: Cryogenics Connection via service module and jumper
Static bath of superfluid helium at 1.9 K in cooling loops of 110 m length
Supply and recovery of helium with 26 km long cryogenic distribution line
J. Wenninger LNF Spring School, May 2010
J. Wenninger - ETHZ - December 2005 32

33. Regular arc: Vacuum Beam vacuum for Beam 1 + Beam 2
Insulation vacuum for the magnet cryostats
Insulation vacuum for the cryogenic distribution line
J. Wenninger LNF Spring School, May 2010
J. Wenninger - ETHZ - December 2005 33

34. Tunnel view (1)
J. Wenninger LNF Spring School, May 2010

35. Tunnel view (2)
J. Wenninger LNF Spring School, May 2010

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 magnets
J. Wenninger LNF Spring School, May 2010
CERN visit McEwen

37. Magnet cooling scheme He II: super-fluid Very low viscosity
Very high thermal conductivity
Courtesy S. Claudet
J. Wenninger LNF Spring School, May 2010

38. Cryogenics 8 x 18kW @ 4.5 K 1’800 SC magnets 24 km & 20 kW @ 1.8 K
36’ K
130 t He inventory
Courtesy S. Claudet
Grid power ~32 MW
J. Wenninger LNF Spring School, May 2010

39. Cool-down time to 1.9 K is nowadays ~4 weeks/sector
[sector = 1/8 LHC]
J. Wenninger LNF Spring School, May 2010

40. Vacuum chamber
The beams circulate in two ultra-high vacuum chambers, P ~ mbar.
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.
50 mm
36 mm
Beam screen
Magnet bore
Cooling channel (Helium)
Beam envel ( 4 s)
~ TeV
J. Wenninger LNF Spring School, May 2010

41. Luminosity and interaction regions
J. Wenninger LNF Spring School, May 2010

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 !!
f : the revolution frequency is given by the circumference, f= 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
- Injectors (more beam)
- Collective interactions of the particles
- Interaction regions
- Beam encounters
s* : the size at the collision point – s*y=s*x=16 mm
- Injectors (brighter beams)
- More focusing – stronger quadrupoles
For k = 1:
J. Wenninger LNF Spring School, May 2010

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

44. ‘Beam-beam’ interaction
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 beams in the transverse direction, the field (force) on this particle is non-linear, in particular at large amplitudes.
focal length depends on amplitude !
The effect of the non-linear fields can become so strong (when the beams are intense) that large amplitude particles become unstable and are lost from the machine:
 poor lifetime
 background
THE INTERACTION OF THE BEAMS SETS A LIMIT ON THE BUNCH INTENSITY!
Quadrupole lens
Beam(-beam) lens
J. Wenninger LNF Spring School, May 2010

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, b* = 0.5 m (= b-function at the collision point):
CMS & ATLAS : 16 mm
Collision point size @ 3.5 TeV, b* = 2 m:
All points : 45 mm
J. Wenninger LNF Spring School, May 2010

46. Limits to b*
The more one squeezes the beam at the IP (smaller b*) the larger it becomes in the surrounding quadrupoles (‘triplets’):
Small size
Huge size !!
Smaller the size at IP:
 Larger divergence (phase space conservation !)
 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

47. Combining the beams for collisions
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

48. COLLIDE WITH A CROSSING ANGLE IN ONE PLANE !
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 mrad
CMS Horizontal mrad
LHCb Horizontal mrad
ALICE Vertical mrad
7.5 m
J. Wenninger LNF Spring School, May 2010

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 mm
J. Wenninger LNF Spring School, May 2010

50. Tevatron
J. Wenninger LNF Spring School, May 2010

51. Tevatron
CDF D0

52. Tevatron I Tricky to operate !! E E
The Tevatron is presently the ‘energy frontier’ collider in operation at FNAL, with a beam energy of 980 GeV and a size of ~ ¼ LHC (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
J. Wenninger LNF Spring School, May 2010

53. Tevatron II Compare LHC and Tevatron:
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:
fTevatron  4 fLHC Tevatron gets a factor 4 ‘for free’ due to ring size !!
kLHC  100 kTevatron LLHC  30 LTevatron
N2/(sx sy) ~ equal
Luminosity gain of LHC comes basically from the number of bunches (k) !!
J. Wenninger LNF Spring School, May 2010

54. Injection and injector complex
J. Wenninger LNF Spring School, May 2010

55. LHC Beam 2 5 4 6 Beam 1 7 3 2 SPS 8 Booster 1 LINACS CPS LEIR
Top energy/GeV Circumference/m
Linac
PSB
CPS = 4 PSB
SPS ’911 = 11 x PS
LHC ’657 = 27/7 x SPS
LEIR
CPS
SPS
Booster
LINACS
LHC 3 4 5 6 7 8 1 2
Ions
protons
Beam 1
Beam 2
TI8
TI2
Note the energy gain/machine of 10 to 20.
The gain is typical for the useful range of magnets.

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
transfer …
SPS ramp
Beam transfer
SPS waits at injection to be filled by PS
SPS
B field
time PS B
time
PS Booster
time

57. Principle of injection (and extraction)
time
Injected beam
Circulating beam
Kicker B-field
Septum magnet
Kicker magnet
Injected beam
Circulating beam B
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 !

58. Linac2 Delivered beam current: ~150mA
Radio-frequency quadrupole (RFQ)
Alvarez’s drift-tube
Delivered beam current: ~150mA
Beam energy: 90 keV (source) → 750 keV (RFQ) → 50 MeV
Repetition rate: 1 Hz
Radio-frequency system: 202 MHz

59. PS Booster
Constructed in the 70ies to increase the intensity into the PS
Made of four stacked rings
Acceleration to Ekin=1.4 GeV
Intensities > 1013 protons per ring.

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.
x 3

61. Proton Synchrotron
Recently celebrated its first 50 years!!

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…).

63. Super-Proton Synchrotron

64. SPS-to-LHC transfer lines
Courtesy of J. Uythoven

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.
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.
2.5 ns
25 ns
= bunch position
= filled position …
J. Wenninger LNF Spring School, May 2010

66. Collision point symmetry
LHC
CMS
Atlas
Alice
LHCb displaced by c x 37.5 ns = m
LHCb
Symmetry
axis
ATLAS, ALICE and CMS are positioned on the LEP symmetry axis (8 fold sym.)
LHCb is displaced from the symmetry axis by m <<-->> 37.5 ns.
For filling patterns with many bunches this is not an issue, but it becomes a bit tricky with few bunches.
J. Wenninger LNF Spring School, May 2010

67. Filling pattern example: 1x1
LHC
CMS
Atlas
Alice
LHCb
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, OR
in ALICE,
in LHCb,
but never in all experiments at the same time !!
J. Wenninger LNF Spring School, May 2010

68. (Some) LHC filling patterns
Schema
Nominal bunch distance (ns)
No. bunches
Comment
43x43
2025 43
No crossing angle required
156x156
525
156
25 ns 25
2808
Nominal p filling
50 ns 50
1404
run target
Ion nominal
100
592
Nominal ion filling
Ion early
1350 62
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 ms long particle free gap for the beam dump kicker.
J. Wenninger LNF Spring School, May 2010

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 t5 t3
b=bunch, e=empty t2 t1
J. Wenninger LNF Spring School, May 2010

70. Spare slides
J. Wenninger LNF Spring School, May 2010

71. PS - bunch splitting

72. Injection elements
12 mrad
0.8 mrad
TED
From the LHC Page1

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.