Wenninger1

Report
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
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
• Incident 19th Sept. 2008 and consequences
Part 2
• LHC operation
J. Wenninger LNF Spring School, May 2010
2
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
3
The Large Hadron Collider LHC
Installed in the 26.7 km LEP tunnel
Depth of 70-140 m
Lake of Geneva
CMS, Totem
Control Room
LHCb
ATLAS, LHCf
ALICE
17.03.2010
4
Der LHC
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
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
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
J. Wenninger LNF Spring School, May 2010
Injection ring 2
6
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
Tevatron pp
3x1032
cm-2
s-1
SppbarS pp
6x1030 cm-2 s-1
A factor 30
in luminosity
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
Luminosity challenges
The event rate N for a physics process with cross-section s is proprotional to
the collider Luminosity L:
N  Ls
2
L
kN f
4 s x s
*
*
y
k = number of bunches = 2808
N = no. protons per bunch = 1.15×1011
f = revolution frequency = 11.25 kHz
s*x,s*y = beam sizes at collision point (hor./vert.) = 16 mm
High beam “brillance” N/e
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)
(particles per phase space volume)
 Injector chain performance !
Optics property
 Strong focusing !
e : is the phase space volume occupied
by the beam (constant along the ring).
J. Wenninger LNF Spring School, May 2010
Small envelope
Beam property
8
Basics of accelerator physics
J. Wenninger LNF Spring School, May 2010
9
Accelerator concept
Charged particles are accelerated, guided and confined by electromagnetic fields.
- Bending:
- Focusing:
- Acceleration:
Dipole magnets
Quadrupole magnets
RF cavities
In synchrotrons, they are ramped together synchronously to match beam energy.
- Chromatic aberration:
J. Wenninger LNF Spring School, May 2010
Sextupole magnets
10
Bending
Lorentz force
→
→ → →
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
11
Bending Fields
I
II
B
B
p
B field
F force
F
p
Two-in-one magnet design
J. Wenninger LNF Spring School, May 2010
12
Focusing
N
S
F
By
y
F
x
S
N
x
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).
y
J. Wenninger LNF Spring School, May 2010
Focusing in one plane, de-focusing in the other!
13
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.
s
y
The lattice effectively constitutes a
particle trap!
x
J. Wenninger LNF Spring School, May 2010
15
LHC arc lattice
QF
dipole
m agnets
decapole
m agnets
QD
sextupole
m agnets
QF
sm all sextupole
corrector m agnets
L H C C ell - L ength about 110 m (schem atic layout)

Dipole- und Quadrupol magnets
–

Sextupole magnets
–

Provide a stable trajectory for particles with nominal momentum.
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
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…
QF
QD
QF
QD
QF
QD
QF
The envelope peaks in
the focusing elements !
Vertical
Horizontal
Betatron functions in a simple FODO cell
J. Wenninger LNF Spring School, May 2010
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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.
beam size s at any point along the accelerator
is given by (neglecting the contribution from energy
spread):
 The
s 
Envelope
 Emittance

be
For unperturbed proton beams, the normalized emittance en is conserved:
e n  e  constant
The beam size shrinks with energy:
J. Wenninger LNF Spring School, May 2010
 = Lorentz factor
s 
b en


1

18
Why does the transverse emittance shrink?
 The
acceleration is purely longitudinal, i.e the transverse momentum is not
affected:
p t  constant
 The
emittance is nothing but a measure of <pt>.
 To
maintain the focusing strength, all magnetic fields are kept proportional to E
(), 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
19
LHC beam sizes

Beta-function at the LHC
b  0 . 5  5 ' 000 m
b  30  180 m

ARC
Nominal LHC normalized emittance :
e n  e  3 . 5 m m
Example LHC arc, peak b = 180 m
e (nm)
s (mm)
450
7.2
1.14
3500
0.93
0.41
7000
0.47
0.29
Energy (GeV)
J. Wenninger LNF Spring School, May 2010
20
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)
J. Wenninger LNF Spring School, May 2010
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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
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Cavities in the tunnel
J. Wenninger LNF Spring School, May 2010
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RF buckets and bunches
RF Voltage
The particles
oscillate back
and forth in
time/energy
The particles are trapped in the RF voltage:
this gives the bunch structure
2.5 ns
E
time
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
Magnets & Tunnel
J. Wenninger LNF Spring School, May 2010
26
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
Bc
 Typical for NbTi @ 4.2 K
2000 A/mm2 @ 6T
 To reach 8-10 T, the temperature must be lowered
to 1.9 K – superfluid Helium !
A p p lie d fie ld [T ]
 higher fields.
N o rm a l s ta te
S u p e rc o n d u c tin g
s ta te
Tc
T e m p e ra tu re [K ]
J. Wenninger LNF Spring School, May 2010
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The superconducting cable
6 mm
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
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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
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
30
Regular arc:
Magnets
1232 main dipoles
+
392 main quadrupoles +
2500 corrector magnets
(dipole, sextupole, octupole)
J. Wenninger LNF Spring School, May 2010
3700 multipole
corrector magnets
(sextupole,
octupole,
decapole)
J. Wenninger - ETHZ - December 2005
31 31
Regular arc:
Connection via
service module and
jumper
Supply and recovery of
helium with 26 km long
cryogenic distribution line
J. Wenninger LNF Spring School, May 2010
Cryogenics
Static bath of superfluid
helium at 1.9 K in cooling
loops of 110 m length
J. Wenninger - ETHZ - December 2005
32 32
Regular arc:
Beam vacuum for
Beam 1 + Beam 2
Insulation vacuum for the
cryogenic distribution line
J. Wenninger LNF Spring School, May 2010
Vacuum
Insulation vacuum for the
magnet cryostats
J. Wenninger - ETHZ - December 2005
33 33
Tunnel view (1)
J. Wenninger LNF Spring School, May 2010
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Tunnel view (2)
J. Wenninger LNF Spring School, May 2010
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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
36
Magnet cooling scheme
10000
SOLID
P [kP a]
1000
 lin e
HeII
100
CRITICAL POINT
HeI
Pressurized H e II
GAS
10
S aturated H e II
1
1
10
T [K]

He II: super-fluid
Very low viscosity
o Very high thermal conductivity
o
Courtesy S. Claudet
J. Wenninger LNF Spring School, May 2010
37
Cryogenics
Pt5
Pt4
Pt6
8 x 18kW @ 4.5 K
1’800 SC magnets
Cry
plan&
t Dis
tion
24 okm
20 trib
kWu@
1.8 K Pt7
Present Version
36’000 t @ 1.9K
Pt3
130 t He inventory
Pt2
Pt8
Pt1.8
Courtesy S. Claudet
Pt1
Cr yogenicplant
Grid power ~32 MW
J. Wenninger LNF Spring School, May 2010
38
Cool down
Cool-down
to 1.9 K is
~4 weeks/sector
Firsttime
cool-down
ofnowadays
LHC sectors
[sector = 1/8 LHC]
300
T e m p e r a tu re [K ]
250
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
J. Wenninger LNF Spring School, May 2010
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Vacuum chamber
 The
50 mm
beams circulate in two ultra-high
vacuum chambers, P ~ 10-10 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.
36 mm
Beam screen
Magnet bore
Cooling channel (Helium)
Beam envel ( 4 s)
~ 1.8 mm @ 7 TeV
J. Wenninger LNF Spring School, May 2010
40
Luminosity and interaction regions
J. Wenninger LNF Spring School, May 2010
41
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 !!
2
L
kN f
4 s x s
*
*
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
For k = 1:
 k : the number of bunches – k=2808
30
L  3 . 5  10 cm
- 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
J. Wenninger LNF Spring School, May 2010
2
s
1
42
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
‘Beam-beam’ interaction
 When a particle of one beam encounters the
Y
Force
Quadrupole
lens
Quadrupole Lense
Y
Force
Beam(-beam)
lens
Beam - Beam Lense
J. Wenninger LNF Spring School, May 2010
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!
44
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 :
J. Wenninger LNF Spring School, May 2010
45 mm
45
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
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
Combining the beams for collisions
q uad ru po le
Q4
q uad ru po le
Q5
q uad ru po le
Q4
sep aratio n
in ner qu ad rup o le
d ipo le (w arm ) triplet
reco m bin atio n
d ipo le
b eam II
b eam
d istance
194 m m
in ner qu ad rup o le sep aratio n
triplet
d ipo le
reco m bin atio n
d ipo le
q uad ru po le
Q5
A TLA S
or C M S
b eam I
collision point
24 m
200 m
E xam ple for an LH C insertion w ith A TLA S or C M S
 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
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
7.5 m
J. Wenninger LNF Spring School, May 2010
Crossing planes & angles
• ATLAS Vertical
280 mrad
• CMS
Horizontal 280 mrad
• LHCb
Horizontal 300 mrad
• ALICE
Vertical
400 mrad
48
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 !
J. Wenninger LNF Spring School, May 2010
~ 7 mm
49
Tevatron
J. Wenninger LNF Spring School, May 2010
50
Tevatron
CDF
D0
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 ~ 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
J. Wenninger LNF Spring School, May 2010
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:
2
L
fTevatron  4 fLHC
kN f
4 s x s
*
*
y
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
53
Injection and injector complex
J. Wenninger LNF Spring School, May 2010
54
Beam 2
4
Beam 1
5
LHC
6
7
3
TI8
2
protons
SPS
TI2
Booster
8
1
LINACS
CPS
Ions
LEIR
Top energy/GeV
Linac
0.05
PSB
1.4
CPS
26
SPS
450
LHC
7000
Circumference/m
30
157
628 = 4 PSB
6’911 = 11 x PS
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
Principle of injector cycling
The beams are handed from one accel. to the next or used for its own customers !
B field
SPS
ramp
SPS top energy,
prepare for
transfer …
Beam transfer
SPS waits at
injection to be
filled by PS
SPS
B field
time
PS
B
time
PS Booster
J. Wenninger LNF Spring School, May 2010
time
56
Principle of injection (and extraction)
Circulating
beam
Kicker B-field
Injected beam
Injected
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
Linac2
Radio-frequency
quadrupole (RFQ)
Delivered beam current:
Beam energy:
Repetition rate:
Radio-frequency system:
J. Wenninger LNF Spring School, May 2010
Alvarez’s drift-tube
~150mA
90 keV (source) → 750 keV (RFQ) → 50 MeV
1 Hz
202 MHz
58
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.
J. Wenninger LNF Spring School, May 2010
59
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
J. Wenninger LNF Spring School, May 2010
60
Proton Synchrotron
Recently celebrated its first 50 years!!
J. Wenninger LNF Spring School, May 2010
61
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…).
P S e je c tio n :
32 0 ns b eam gap
7 2 b u n ch e s
7 2 b u n ch e s
on h=84
in 1 tu rn
Q u a d ru p le s p littin g
at 25 G eV
A c ce le ra tio n
1 8 b u n ch e s
to 2 5 G e V
on h=21
T rip le s p littin g
a t 1 .4 G e V
6 b u n ch e s
2+4 bunches
on h=7
in 2 b a tch e s
J. Wenninger LNF Spring School, May 2010
E m p ty
b u ck et
P S in je ctio n :
62
Super-Proton Synchrotron
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63
SPS-to-LHC transfer lines
Courtesy of J. Uythoven
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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.
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Collision point symmetry
= collision point
CMS
Symmetry
axis
 ATLAS, ALICE and CMS are positioned
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
Alice
Atlas
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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,
OR
in ALICE,
OR
in LHCb,
LHC
LHCb
but never in all experiments at the same
time !!
Alice
Atlas
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(Some) LHC filling patterns
Schema
Nominal bunch
distance (ns)
No. bunches
Comment
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
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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 ms long particle free gap for the beam dump kicker.
J. Wenninger LNF Spring School, May 2010
68
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
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Spare slides
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PS - bunch splitting
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Injection elements
TED
12 mrad
0.8 mrad
TED
From the LHC Page1
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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.
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