lhc construction & operation
TRANSCRIPT
1
LHC : construction and operationLHC : construction and operationJörg Wenninger
CERN Beams Department / Operation groupLNF 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
Outline
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• 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. 2008 and consequences
• LHC operation
Part 1
Part 2
J. Wenninger LNF Spring School, May 2010
LHC History
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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
17.03.2010 Der LHC Beschleuniger - DPG - Bonn
The Large Hadron Collider LHC
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CMS, TotemCMS, Totem
ATLAS, LHCfATLAS, LHCf
LHCbLHCb
ALICEALICE
Lake of Geneva
Installed in the 26.7 km LEP tunnelDepth of 70-140 m
Control RoomControl Room
LHC ring
LHC ring
SPS ringSPS ring
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Tunnel circumference 26.7 km, tunnel diameter 3.8 mDepth : ~ 70-140 m – tunnel is inclined by ~ 1.4%
J. Wenninger LNF Spring School, May 2010
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IR6: Beam dumping systemIR4: RF + Beam
instrumentation
IR5:CMS
IR1: ATLAS
IR8: LHC-BIR2:ALICE
Injection ring 2Injection ring 1
IR3: Momentum collimation (normal
conducting magnets)
IR7: Betatron collimation (normal
conducting magnets)
Beam dump blocks
LHC Layout8 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 !
J. Wenninger LNF Spring School, May 2010
Beam1Beam
2
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
Luminosity challenges
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The event rate N for a physics process with cross-section σ is proprotional to the collider Luminosity L:
To maximize L:
• Many bunches (k)
• Many protons per bunch (N)
• A small beam size σ*u = (β *ε)1/2
β * : the beam envelope (optics)
ε : is the phase space volume occupied by the beam (constant along the ring).
**
2
4 yx
fkNL
σπσ=
k = number of bunches = 2808N = no. protons per bunch = 1.15×10 11
f = revolution frequency = 11.25 kHzσ*x,σ*y = beam sizes at collision point (hor./vert.) = 16 µm
σLN =
High beam “brillance” N/ε (particles per phase space
volume)
Injector chain performance !
Small envelope
Strong focusing !Optics
propertyBeam property
J. Wenninger LNF Spring School, May 2010
Basics of accelerator physics
9J. Wenninger LNF Spring School, May 2010
Accelerator concept
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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
Bending
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Magnetic rigidity
ForceLorentz force
LHC: = 2.8 km given by LEP tunnel!ρ
→ → → →
J. Wenninger LNF Spring School, May 2010
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
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Two-in-one magnet design
B field
B
p
p
FF force
II
I
B
J. Wenninger LNF Spring School, May 2010
Focusing
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N
NS
S
ByF
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
Accelerator lattice
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horizontal plane
vertical plane
Focusing in both planes is achieved by a succession of focusing and de-focusing quadrupole magnets :
The FODO structure
Alternating gradient lattice
15
s
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
LHC arc lattice
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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 !
QF QD QFdipolemagnets
small sextupolecorrector magnets
decapolemagnets
LHC Cell - Length about 110 m (schematic layout)
sextupolemagnets
One rarely talks about the multi-pole magnets, but they are essential for good machine performance !
J. Wenninger LNF Spring School, May 2010
Beam envelope
J. Wenninger LNF Spring School, May 2010 17
Betatron functions in a simple FODO cell
QF QF QF QFQD QD QD
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…
The envelope peaks in the focusing elements !
HorizontalVertical
Beam emittance and beam size
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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):
εβσ =×= EmittanceEnvelope
For unperturbed proton beams, the normalized emittance εn is conserved:
constant== εγεn γ = Lorentz factor
γγεβσ 1∝= n
The beam size shrinks with energy:
Why does the transverse emittance shrink?
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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 (γ), including the quadrupole gradients.
With constant <pt> and increasing quadrupole gradients, the transverse excursion of the particles becomes smaller and smaller !
constant=tp
LHC beam sizes
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m000'55.0 ÷=β
m18030 ÷=β
Beta-function at the LHC
ARC
Nominal LHC normalized emittance :
m5.3 µεγε ==n
Energy (GeV)
ε (nm) σ (mm)
450 7.2 1.14
3500 0.93 0.41
7000 0.47 0.29
Example LHC arc, peak β = 180 m
Acceleration
21
s
)(tE
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.
J. Wenninger LNF Spring School, May 2010
LHC RF system
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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
Visible protons !
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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 SPS(injector)
Synch. light
Flying wire LHC
Cavities in the tunnel
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RF buckets and bunches
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∆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 12.8 cm 5.8 cm
RMS energy spread 0.031% 0.02%
450 GeV 3.5 TeV
The particles oscillate back
and forth in time/energy
RF bucket
2.5 ns
J. Wenninger LNF Spring School, May 2010
Magnets & Tunnel
26J. Wenninger LNF Spring School, May 2010
Superconductivity
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Temperature [K]
Ap
plie
d f
ield
[T
]Superconducting
state
Normal state
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 NbTi @ 4.2 K
2000 A/mm2 @ 6T
To reach 8-10 T, the temperature must be lowered to 1.9 K – superfluid Helium !
J. Wenninger LNF Spring School, May 2010
The superconducting cable
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∅1 mm∅6 µm
Typical value for operation at 8T and 1.9 K: 800 A
Rutherford cable
width 15 mm
A.Verweij
A.Verweij
J. Wenninger LNF Spring School, May 2010
Coils for dipoles
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Dipole length 15 mI = 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
30Rüdiger Schmidt 30
Beam tube
Superconducting coil
Non-magnetic collars
Ferromagnetic iron
Steel cylinder for Helium
Insulation vacuum
Supports
Vacuum tank
Weight (magnet + cryostat) ~ 30 tons, length 15 m J. Wenninger LNF Spring School, May 2010
31J. Wenninger - ETHZ - December 2005 31
1232 main dipoles +
3700 multipole corrector magnets
(sextupole, octupole, decapole)
392 main quadrupoles +
2500 corrector magnets (dipole, sextupole, octupole)
Regular arc:
Magnets
J. Wenninger LNF Spring School, May 2010
32J. Wenninger - ETHZ - December 2005 32
Regular arc:
Cryogenics
Supply and recovery of helium with 26 km long cryogenic distribution line
Static bath of superfluid helium at 1.9 K in cooling loops of 110 m length
Connection via service module and jumper
J. Wenninger LNF Spring School, May 2010
33J. Wenninger - ETHZ - December 2005 33
Insulation vacuum for the cryogenic distribution line
Regular arc:
Vacuum
Insulation vacuum for the magnet cryostats
Beam vacuum for
Beam 1 + Beam 2
J. Wenninger LNF Spring School, May 2010
Tunnel view (1)
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Tunnel view (2)
35J. Wenninger LNF Spring School, May 2010
Complex interconnects
36CERN visit McEwen
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 Afor the main quadrupole magnets
J. Wenninger LNF Spring School, May 2010
Magnet cooling scheme
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1
10
100
1000
10000
1 10
T [K]
P [k
Pa]
SOLID
HeII HeI
CRITICAL POINT
GAS
λ line
Saturated He II
Pressurized He II
1
10
100
1000
10000
1 10
T [K]
P [k
Pa]
SOLID
HeII HeI
CRITICAL POINT
GAS
λ line
Saturated He II
Pressurized He II
Courtesy S. Claudet
He II: super-fluido Very low viscosityo Very high thermal conductivity
J. Wenninger LNF Spring School, May 2010
Cryogenics
J. Wenninger LNF Spring School, May 2010 38
Pt 3
Pt 4
Pt 5
Pt 6
Pt 7
Pt 8
Pt 1
Pt 2
Pt 1.8
Cryoplant DistributionPresent Version
Cryogenic plant
8 x 18kW @ 4.5 K
1’800 SC magnets
24 km & 20 kW @ 1.8 K
36’000 t @ 1.9K
130 t He inventory
Courtesy S. ClaudetGrid power ~32 MW
Cool down
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First cool-down of LHC sectors
0
50
100
150
200
250
300
12-Nov-2007
10-Dec-2007
07-Jan-2008
04-Feb-2008
03-Mar-2008
31-Mar-2008
28-Apr-2008
26-May-2008
23-Jun-2008
21-Jul-2008
18-Aug-2008
15-Sep-2008
Tem
pera
ture
[K]
ARC56_MAGS_TTAVG.POSST ARC78_MAGS_TTAVG.POSST ARC81_MAGS_TTAVG.POSST ARC23_MAGS_TTAVG.POSSTARC67_MAGS_TTAVG.POSST ARC34_MAGS_TTAVG.POSST ARC12_MAGS_TTAVG.POSST ARC45_MAGS_TTAVG.POSST
Cool-down time to 1.9 K is nowadays ~4 weeks/sector[sector = 1/8 LHC]
Vacuum chamber
40
Beam envel (± 4 σ) ~ 1.8 mm @ 7 TeV
50 mm
36 mm
The 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.
Cooling channel (Helium)
Beam screen
Magnet bore
J. Wenninger LNF Spring School, May 2010
Luminosity and interaction regions
41J. Wenninger LNF Spring School, May 2010
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=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- Injectors (more beam)- 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
Luminosity
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**
2
4 yx
fkNL
σπσ=
1230105.3 −−×= scmLFor k = 1:
J. Wenninger LNF Spring School, May 2010
Collective (in-)stability
43
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
Q u a d r u p o l e L e n s e
B e a m - B e a m L e n s e
F o r c e
F o r c e
Y
Y
‘Beam-beam’ interaction
44
Quadrupole lens
Beam(-beam) lens
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!
J. Wenninger LNF Spring School, May 2010
From arc to collision point
45
Fits through the hole of a needle!
ARC cells ARC cells
CMS collision
point
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
J. Wenninger LNF Spring School, May 2010
Limits to β*
46
The more one squeezes the beam at the IP (smaller β*) the larger it becomes in the surrounding quadrupoles (‘triplets’):
J. Wenninger LNF Spring School, May 2010
Small size
Huge 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 !
Combining the beams for collisions
47
200 m
inner quadrupoletriplet
separationdipole (warm)
recombinationdipole
quadrupoleQ4
quadrupoleQ5
ATLAS or CMS
inner quadrupoletriplet
separationdipole
recombinationdipole
quadrupoleQ4
quadrupoleQ5
collision point
beam I
Example for an LHC insertion with ATLAS or CMS
24 m
beamdistance194 mm
beam II
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
Crossing angles
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IP
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%
Crossing planes & angles•ATLAS Vertical 280 µrad•CMS Horizontal 280 µrad•LHCb Horizontal 300 µrad•ALICE Vertical 400 µrad
7.5 m
J. Wenninger LNF Spring School, May 2010
Separation and crossing : example of ATLAS
49
Horizontal plane: the beams are combined and then separated
Vertical plane: the beams are deflected to produce a crossing angle at the IP
~ 7 mm
194 mm ATLAS IP
Not to scale !
~ 260 m
Common vacuum chamber
J. Wenninger LNF Spring School, May 2010
Tevatron
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CDFD0
Tevatron
Tevatron I
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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
Tevatron II
53
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/(σx σy) ~ equal
Luminosity gain of LHC comes basically from the number of bunches (k) !!
**
2
4 yx
fkNL
σπσ=
J. Wenninger LNF Spring School, May 2010
Injection and injector complex
54J. Wenninger LNF Spring School, May 2010
J. Wenninger LNF Spring School, May 2010 55
Top energy/GeV Circumference/m Linac 0.05 30PSB 1.4 157CPS 26 628 = 4 PSBSPS 450 6’911 = 11 x PSLHC 7000 26’657 = 27/7 x SPS
LEIR
CPS
SPS
Booster
LINACS
LHC
3
45
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.
J. Wenninger LNF Spring School, May 2010
Principle of injector cycling
56
PS Booster
PS
SPS
time
time
time
B field
B field
B
The beams are handed from one accel. to the next or used for its own customers !
Beam transfer
SPS waits at injection to be
filled by PS
SPS ramp
SPS top energy, prepare for transfer …
J. Wenninger LNF Spring School, May 2010
Principle of injection (and extraction)
57
Septum magnet
Kicker magnet
Kicker magnet
Injected beam
Circulating beam
B
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 !
Kicker B-field
time
Injected beam
Circulating beam
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Linac2
Delivered beam current: ~150mABeam energy: 90 keV (source) 750 keV (RFQ) → → 50 MeV Repetition rate: 1 HzRadio-frequency system: 202 MHz
Radio-frequency quadrupole (RFQ)
Alvarez’s drift-tube
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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.
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Filling the PS with LHC beams
x 3
Rings 2,3 & 4 are filled with 2 bunches per ring. The 6 bunches are transferred to the PS.
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Proton Synchrotron
Recently celebrated its first 50 years!!
J. Wenninger LNF Spring School, May 2010
Bunch Splitting at the PS
62
Triple splittingat 1.4 GeV
Quadruple splitting at 25 GeV
PS injection:2+4 bunches in 2 batches
Empty
bucket
Accelerationto 25 GeV
PS ejection:72 bunches
in 1 turn
320 ns beam gap
6 buncheson h=7
18 buncheson h=21
72 buncheson h=84
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…).
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Super-Proton Synchrotron
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Courtesy of J. Uythoven
SPS-to-LHC transfer lines
Collision schemes
65
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
Collision point symmetry
66
LHCLHC
CMS
Atlas
Alice
LHCb
displa
ced b
y
c x 3
7.5 ns
= 11
.25 m
LHCb
= collision point
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.
For filling patterns with many bunches this is not an issue, but it becomes a bit tricky with few bunches.
Symmetry axis
J. Wenninger LNF Spring School, May 2010
Filling pattern example: 1x1
67
LHCLHC
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,
OR
in LHCb,
but never in all experiments at the same time !!
J. Wenninger LNF Spring School, May 2010
(Some) LHC filling patterns
68
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 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.
J. Wenninger LNF Spring School, May 2010
Nominal filling pattern
69
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’).
τ1
τ2
τ 3
τ 5
72 bunches
b=bunch, e=empty
J. Wenninger LNF Spring School, May 2010
Spare slides
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PS - bunch splitting
J. Wenninger LNF Spring School, May 2010
Injection elements
72
12 mrad
0.8 mrad
TED
TED
From the LHC Page1
J. Wenninger LNF Spring School, May 2010
Role of the TDI collimator
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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.