Lhc construction & operation

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 schemes

J. Wenninger LNF Spring School, May 2010 1

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 operation


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


The Large Hadron Collider LHC
Installed in the 26.7 km LEP tunnel
Depth of 70-140 m Lake of Geneva

g
C rin
LH
CMS, Totem

LHCb
Control Room
SP
S r
ing

ATLAS, LHCf

ALICE

4
17.03.2010 Der LHC

Tunnel circumference 26.7 km, tunnel diameter 3.8 m
Depth : ~ 70-140 m – tunnel is inclined by ~ 1.4%

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: Beam
positions (inside/outside) in 4
points to ensure that both rings IR4: RF + Beam dumping system
have 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 2

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 !


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).


Basics of accelerator physics


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


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!


Bending Fields
I
II
B

B field
p
B F force
F p
Two-in-one magnet design

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!


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

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

x


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 !

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 cell


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 γ

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 !


LHC beam sizes
 Beta-function at the LHC

β = 0.5 ÷ 5'000 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 0.29


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

21
J. Wenninger LNF Spring School, May 2010

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)


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)


Cavities in the tunnel


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%

Magnets & Tunnel


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]

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.Verweij


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)


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
Rüdiger Schmidt 30

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

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
line

Regular arc:
Beam vacuum for
Beam 1 + Beam Vacuum
2

Insulation vacuum for the
Insulation vacuum for magnet cryostats
the cryogenic
distribution line


Tunnel view (1)


Tunnel view (2)


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

CERN visit McEwen 36

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. Claudet


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 MW

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.POSST


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 TeV

Luminosity and interaction regions


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 quadrupoles


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 !


‘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 se


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 µm


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 !


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 CMS
beam
distance
194 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.


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 µrad


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


Tevatron


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

52

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) !!

53

Injection and injector complex


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.


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 SPS
B field
time

PS

B
time

PS Booster

time

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 !


Linac2

Radio-frequency Alvarez’s drift-tube
quadrupole (RFQ)

Delivered beam current: ~150mA
Beam energy: 90 keV (source) → 750 keV (RFQ) → 50 MeV
Repetition rate: 1 Hz
Radio-frequency system: 202 MHz

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.


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.

x3


Proton Synchrotron

Recently celebrated its first 50 years!!


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 batches

62

Super-Proton Synchrotron


SPS-to-LHC transfer lines

Courtesy of J. Uythoven


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.


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
c


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

Atlas


(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.


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

69

Spare slides


PS - bunch splitting


Injection elements
12 mrad
TED

0.8 mrad

TED
From the LHC Page1


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.

Lhc construction & operation

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