LHC : construction and operation
Jörg Wenninger
CERN Beams Department / Operations group
LNF Spring School 'Bruno Touschek' - May 2010
Part 2:
• Machine protection
• Incident and energy limits
• Commissioning and operation
J. Wenninger LNF Spring School, May 2010
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Machine protection
J. Wenninger LNF Spring School, May 2010
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The price of high fields & high luminosity…
When the LHC is operated at 7 TeV with its design luminosity & intensity,
 the LHC magnets store a huge amount of energy in their magnetic fields:
per dipole magnet
all magnets
Estored = 7 MJ
Estored = 10.4 GJ
 the 2808 LHC bunches store a large amount of kinetic energy:
Ebunch = N x E = 1.15 x 1011 x 7 TeV = 129 kJ
Ebeam = k x Ebunch = 2808 x Ebunch = 362 MJ
To ensure safe operation (i.e. without damage) we must be able to
dispose of all that energy safely !
This is the role of machine protection !
J. Wenninger LNF Spring School, May 2010
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Stored Energy
Increase with respect to existing accelerators :
• A factor 2 in magnetic field
• A factor 7 in beam energy
• A factor 200 in stored energy
J. Wenninger LNF Spring School, May 2010
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Comparison…
The energy of an A380 at
700 km/hour corresponds to
the energy stored in the LHC
magnet system :
Sufficient to heat up and
melt 15 tons of Copper!!
The energy stored in one
LHC beam corresponds
approximately to…
• 90 kg of TNT
• 10-12 litres of gasoline
• 15 kg of chocolate
It’s how easily/quickly the energy is
released that matters most !!
J. Wenninger LNF Spring School, May 2010
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Machine protection: beam
J. Wenninger LNF Spring School, May 2010
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To set the scale..
Few cm long groove of an SPS vacuum
chamber after the impact of ~1% of a
nominal LHC beam (2 MJ) during an
‘incident’:

Vacuum chamber ripped open.

3 day repair.
The same incident at the LHC implies a
shutdown of > 3 months.
>> Protection of the LHC must be much
stricter and much more reliable !
J. Wenninger LNF Spring School, May 2010
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Beam impact in a target
Simulation of a 7 TeV LHC beam impact into a 5 m long Copper target
20 bunches
180 bunches
100 bunches
380 bunches
The 7 TeV LHC beam
can drill a hole through
~35 m of Copper
Courtesy N. Tahir / GSI
J. Wenninger LNF Spring School, May 2010
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From a real 450 GeV beam…
Shoot a 450 GeV beam into a target…
6 cm
Shot
Intensity / p+
A
1.2×1012
B
2.4×1012
C
4.8×1012
D
7.2×1012
Cu plate ~20 cm
inside the ‘target’.
~0.1% nominal LHC beam
A
J. Wenninger LNF Spring School, May 2010
B
D
C
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‘Safe’ beams at the LHC…
2011
‘Un-safe beam’
‘Safe beam’
L ~ 21028 cm-2s-1
LHC 2010
J. Wenninger LNF Spring School, May 2010
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Schematic layout of beam dump system in IR6
When it is time to get rid of the beams (also in case of
emergency!), the beams are ‘kicked’ out of the ring by a
system of kicker magnets and send into a dump block !
Septum magnets
deflect the
extracted beam
vertically
Beam 1
Q5L
Ultra-high reliability
system !!
Kicker magnets
to paint (dilute)
the beam
Beam dump
block
Q4L
700 m
15 fast ‘kicker’
magnets deflect
the beam to the
outside
Q4R
 500 m
Q5R
The 3 ms gap in the beam
gives the kicker time to reach
full field.
J. Wenninger LNF Spring School, May 2010
quadrupoles
Beam 2
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The dump block
Simulation
Measurement
beam absorber
(graphite)
 The
ONLY element in the LHC that can
withstand the impact of the full beam !
 The block is made of graphite (low Z
material) to spread out the showers over a
large volume.
 It is actually necessary to paint the beam
over the surface to keep the peak energy
densities at a tolerable level !
J. Wenninger LNF Spring School, May 2010
concrete
shielding
12
Dump line in IR6
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Dump line
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Dump installation
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‘Unscheduled’ beam loss due to failures
In the event a failure or unacceptable beam lifetime, the beam must be
dumped immediately and safely into the beam dump block
Two main classes for failures (with more subtle sub-classes):
Beam loss over a single turn
during injection, beam dump.
Beam loss over multiple turns
(~millisecond to many seconds) due to
many types of failures.
Passive protection
- Failure prevention (high reliability systems).
- Intercept beam with collimators and absorber blocks.
Active Protection
- Failure detection (by beam and/or equipment monitoring)
with fast reaction time (< 1 ms).
- Fire beam dumping system.
 Because of the very high risk, the LHC machine protection system is of
unprecedented complexity and size.
 A general design philosophy was to ensure that there should always be at
least 2 different systems to protect against a given failure type.
J. Wenninger LNF Spring School, May 2010
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Failure detection example : beam loss monitors
 Ionization
–
–
–
–
chambers to detect beam losses:
N2 gas filling at 100 mbar over-pressure, voltage 1.5 kV
Sensitive volume 1.5 l
Reaction time ~ ½ turn (40 ms)
Very large dynamic range (> 106)
 There
are ~3600 chambers distributed over the ring to detect
abnormal beam losses and if necessary trigger a beam abort !
J. Wenninger LNF Spring School, May 2010
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Beam loss monitoring
J. Wenninger LNF Spring School, May 2010
Machine protection
and quench prevention:
collimation
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Operational margin of SC magnet
Applied Field [T]
The LHC is ~1000 times
more critical than
TEVATRON, HERA, RHIC
Applied field [T]
Bc critical
Bcfield
quench with fast loss
of ~106-7 protons
8.3 T / 7 TeV
Normal state
Superconducting
state
quench with fast loss
of ~few 109 protons
0.54 T / 450 GeV
1.9 K
J. Wenninger LNF Spring School, May 2010
Temperature
Temperature
[K] [K]
QUENCH
Tc critical
temperature
Tc
9K
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Quench levels

The phase transition from the super-conducting to a normal conducting state is
called a quench.
 Quenches are initiated by an energy in the order of few milliJoules
–
–
–
–
Movement of the superconductor by several m (friction and heat dissipation).
Beam losses.
Cooling failures.
..
Example of the energy (mJ/cm3) required to
quench an LHC dipole magnet with an
instantaneous loss.
J. Wenninger LNF Spring School, May 2010

Steep energy dependence.

Models are confirmed at 0.45 TeV.
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Beam lifetime
Consider a beam with a lifetime t :
N (t )  N 0 e t / t

dN (t )
  N (t ) / t
dt
Number of protons lost per second for different lifetimes (nominal intensity):
t = 100 hours
t = 25 hours
t = 1 hour
109 p/s
4x109 p/s
Quench level ~ 106-7 p
1011 p/s
While ‘normal’ lifetimes will be in the range of 10-100 hours (in collisions most of the
protons are actually lost in the experiments !!), one has to anticipate short periods of low
lifetimes, down to a few minutes !
 To survive periods of low lifetime we must intercept the protons that are lost
with very high efficiency before they can quench a magnet : collimation!
J. Wenninger LNF Spring School, May 2010
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Collimation
A 4-stage halo cleaning (collimation) system is installed to protect the LHC
magnets from beam induced quenches.
 A cascade of more than 100 collimators is required to prevent the protons and their
debris to reach the superconducting magnet coils.
 The collimators will also play an essential role for protection by intercepting the beams.
 the collimators must reduce the energy load into the magnets due to particle
lost from the beam to a level that does not quench the magnets.
in front of the exp.
Exp.
Detectors
Courtesy C. Bracco
J. Wenninger LNF Spring School, May 2010
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Collimator settings at 7 TeV
 At the LHC collimators are essential for machine operation as soon as we
have more than a few % of the nominal beam intensity at injection !
The collimator opening corresponds
roughly to the size of Spain !
Carbon jaw
1 mm
Opening
~3-5 mm
RF contact ‘fingers’
J. Wenninger LNF Spring School, May 2010
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Collimation performance
Measurements of the collimation efficiency from beam loss maps confirms the
excellent performance : > 99.9% efficiency !
Cleaning
Momentum
Cleaning
IR2
IR5
Dump
Protection
IR8
IR1
Collimation team
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Machine protection: magnets
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LHC powering in sectors

To limit the stored energy within one
electrical circuit, the LHC is powered
by sectors.
 The main dipole circuits are split into
8 sectors to bring down the stored
energy to ~1 GJ/sector.
 Each main sector (~2.9 km) includes
154 dipole magnets (powered by a
single power converter) and 47
quadrupoles.
5
4
6
DC Power feed
3
LHC
DC Power
27 km Circumference
7
 This also facilitates the
commissioning that can be done
sector by sector !
Powering Sector
8
2
Sector
1
J. Wenninger LNF Spring School, May 2010
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Powering from room temperature source…
6 kA power converter
Water cooled 13 kA Copper cables
! Not superconducting !
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…to the cryostat
Feedboxes (‘DFB’) : transition from Copper cable to super-conductor
Cooled Cu cables
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Quench detection

When part of a magnet quenches, the conductor becomes resistive, which can
lead to excessive local energy deposition (tT rise !!) due to Ohmic losses.
 To protect the magnet:
– The quench must be detected: this is done by monitoring the voltage that appears
over the coil (R > 0).
– The energy release is distributed over the entire magnet by force-quenching the
coils using quench heaters (such that the entire magnet quenches !).
– The magnet current is switched off within << 1 second.
18/04/10
Example of the voltage signals
over 5 quenching dipole magnets
(beam induced at injection).
Threshold of quench
protection system (QPS)
J. Wenninger LNF Spring School, May 2010
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Quench - discharge of the energy
Power Converter
Discharge resistor
Magnet 1
Magnet 2
Magnet 154
Magnet i
Protection of the magnet after a quench:
• The quench is detected by measuring the voltage increase over coil.
• The energy is distributed in the magnet by force-quenching using quench heaters.
• The current in the quenched magnet decays in < 200 ms.
• The current flows through the bypass diode (triggered by the voltage increase over the
magnet).
• The current of all other magnets is dischared into the dump resistors.
J. Wenninger LNF Spring School, May 2010
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Dump resistors
Those large air-cooled resistors can absorb the 1 GJ stored in the
dipole magnets (they heat up to few hundred degrees Celsius).
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Machine protection philosophy
Stored
magnetic
energy
Stored
beam
energy
Power Permit
→ Authorises power on
→ Cuts power off in
case of fault
Beam Permit
→ Authorises beam operation
→ Requests a beam dump
in case of problems
BLMs
Cryo
RF
access
Experiments
Power permit
Beam permit
Software interlocks
vacuum
Power Converters
Collimators
QPS
J. Wenninger LNF Spring School, May 2010
Warm
Magnets
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Beam interlock system
Over 20’000 signals enter the interlock system of the LHC that will
send the beam into the dump block if any input signals a fault !
Timing
LHC
LHC
LHC
Devices
Devices
Devices
Safe
Mach.
Param.
Software
Interlocks
Movable
Devices
SEQ
CCC
Operator
Buttons
Safe
Beam
Flag
BCM
Beam Loss
Experimental
Magnets
Experiments
Transverse
Feedback
Collimator
Positions
Beam
Aperture
Kickers
Environmental
parameters
Collimation
System
BTV
screens
FBCM
Lifetime
Mirrors
BTV
Beam
Dumping
System
Beam Interlock System
Injection BIS
PIC essential
+ auxiliary
circuits
WIC
RF
System
BLM
BPM in
IR6
Access
System
Vacuum
System
Timing System
(Post Mortem)
Magnets
QPS
(several
1000)
FMCM
Power
Converters
~1500
Monitors
aperture
limits
(some 100)
Power
Converters
AUG
UPS
J. Wenninger LNF Spring School, May 2010
Cryo
OK
Monitors
in arcs
(several
1000)
Doors
EIS
Vacuum
valves
Access
Safety
Blocks
RF
Stoppers
Isn’t it a miracle that it works !
34
Incident of September 19th 2008
& Consequences
J. Wenninger LNF Spring School, May 2010
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Event sequence on Sept. 19th
Introduction: on September 10th when the first beam made it around the
LHC, not all magnets had not been fully commissioned for 5 TeV.
A few magnets were missing their last commissioning steps.
The last steps were finished the week after Sept. 10th.

Last commissioning step of the dipole circuit in sector 34 : ramp to 5.5 TeV.

At ~5.1 TeV an electrical fault developed in the dipole bus bar (the bus bar
is the cable carrying the current that connects all magnet of a circuit).
Later traced to an anomalous resistance of 200 nW (should be 0.3 nW).

An electrical arc developed which punctured the helium enclosure.
Secondary arcs developed along the arc.
Around 400 MJ were dissipated in the cold-mass and in electrical arcs.

Large amounts of Helium were released into the insulating vacuum.
In total 6 tons of He were released.
J. Wenninger LNF Spring School, May 2010
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Pressure wave
QV
QV
Q
SV
D
D
D
Q
Cold-mass
Vacuum vessel

Line E
Cold support post
Warm Jack

Compensator/Bellows
Vacuum barrier
QV
D
D
D
PT
Q
SV
D
D
D
Q
QV
D
D
D
QV
Q
Pressure wave propagates in both directions along the magnets
inside the insulating vacuum enclosure.
Rapid pressure rise :
– Self actuating relief valves could not handle the pressure.
designed for 2 kg He/s, incident ~ 20 kg/s.
– Large forces exerted on the vacuum barriers (every 2 cells).
designed for a pressure of 1.5 bar, incident ~ 10 bar.
– Several quadrupoles displaced by up to ~50 cm.
– Connections to the cryogenic line damaged in some places.
– Beam vacuum to atmospheric pressure.
J. Wenninger LNF Spring School, May 2010
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One of ~1700 bus-bar connections
Dipole busbar
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Incident location
Dipole bus bar
J. Wenninger LNF Spring School, May 2010
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Collateral damage : displacements
Quadrupole-dipole
interconnection
Quadrupole support
Main damage area ~ 700 metres.

39 out of 154 dipoles,
 14 out of 47 quadrupole short straight
sections (SSS)
from the sector had to be moved to the
surface for repair (16) or replacement
(37).
J. Wenninger LNF Spring School, May 2010
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Collateral damage : beam vacuum
The beam vacuum was affected over entire 2.7 km length of the arc.
Beam Screen (BS) : The red color is
characteristic of a clean copper
Clean Copper surface.
surface
J. Wenninger LNF Spring School, May 2010
Contamination
with multiBS
with some contamination
by
super-isolation
(MLI
multi layer
layer magnet
insulation
insulation)
debris.
BS with soot contamination. The
grey color varies depending on the
Contamination with sooth.
thickness of the soot, from grey to
dark.
 60% of the chambers
 20% of the chambers
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Quench - discharge of the energy
The bus-bar must carry the
current for some minutes,
through interconnections
Magnet 1
Power Converter
Discharge resistor
Magnet 2
Magnet 154
Magnet i

In case of a quench, the individual magnet is protected (quench protection and diode).

Resistances are switched into the circuit: the energy is dissipated in the resistances (current
decay time constant of 100 s).
>> the bus-bar must carry the current until the energy is extracted !
J. Wenninger LNF Spring School, May 2010
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Bus-bar joint

24’000 bus-bar joints in the LHC main circuits.

10’000 joints are at the interconnection between magnets.
They are welded in the tunnel.
Nominal joint resistance:
• 1.9 K
0.3 nΩ
• 300K
~10 μΩ
For the LHC to operate safely at a certain energy, there is a
limit to maximum value of the joint resistance.
J. Wenninger LNF Spring School, May 2010
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Joint quality

The copper stabilizes the bus bar in the event of a cable quench (=bypass for
the current while the energy is extracted from the circuit).
Protection system in place in 2008 not sufficiently sensitive.

A copper bus bar with reduced continuity coupled to a superconducting cable
badly soldered to the stabilizer can lead to a serious incident.
Solder
No solder
wedge
bus
U-profile
bus
X-ray of joint

J. Wenninger LNF Spring School, May 2010
During repair work in the damaged
sector, inspection of the joints revealed
systematic voids caused by the welding
procedure.
44
Normal interconnect, normal operation


Everything is at 1.9 Kelvin.
Current passes through the superconducting cable.
Magnet
Magnet
For 7 TeV : I = 11’800 A
Helium bath
copper bus bar 280 mm2
current
copper bus bar 280 mm2
Interconnection joint (soldered)
superconducting cable
with about 12 mm2 copper
This illustration does not represent the real geometry
J. Wenninger LNF Spring School, May 2010
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Normal interconnect, quench
 Quench
in adjacent magnet or in the bus-bar.
 Temperature
Magnet
 The
increase above ~ 9 K.
superconductor becomes resistive.
Magnet
 During
the energy discharge the current passes for few minutes
through the copper bus-bar.
copper bus bar 280 mm2
copper bus bar 280 mm2
superconducting cable
interconnection
J. Wenninger LNF Spring School, May 2010
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Non-conform interconnect, normal operation
 Interruption
Magnet
of copper stabiliser of the bus-bar.
 Superconducting
 Current
cable at 1.9 K
Magnet
passes through superconductor.
copper bus bar 280 mm2
copper bus bar 280 mm2
superconducting cable
interconnection
J. Wenninger LNF Spring School, May 2010
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Non-conform interconnect, quench.
 Interruption
of copper stabiliser.
 Superconducting
Magnet
cable temperature increase to above ~9 K and
cable becomes resistive.
Magnet
 Current
cannot pass through copper and is forced to pass
through superconductor during discharge.
copper bus bar 280 mm2
copper bus bar 280 mm2
superconducting cable
interconnection
J. Wenninger LNF Spring School, May 2010
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Non-conform interconnect, quench.

Magnet
The superconducting cable heats up because of the
combination of high current and resistive cable.
copper bus bar 280 mm2
Magnet
copper bus bar 280 mm2
superconducting cable
interconnection
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Non-conform interconnect, quench.
 Superconducting
Magnet
cable melts and breaks if the length of the
superconductor not in contact with the bus bar exceeds a critical
value and the current is high.
 Circuit
Magnet
is interrupted and an electrical arc is formed.
copper bus bar 280 mm2
copper bus bar 280 mm2
superconducting cable
interconnection
Depending on ‘type’ of non-conformity, problems appear :
 at
different current levels.
 under
different conditions (magnet or bus bar quench etc).
J. Wenninger LNF Spring School, May 2010
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September 19th hypothesis
 Anomalous
resistance at the joint heats up and finally quenches
the joint – but quench remains very local.
Magnet
 Current
sidesteps into Copper that eventually melts because the
electrical contact is not good enough.
copper bus bar 280 mm2
Magnet
copper bus bar 280 mm2
superconducting cable
quench at the interconnection
>> A new protection system will be installed this year to
anticipate such incidents in the future:
new Quench Protection System (‘nQPS’)
J. Wenninger LNF Spring School, May 2010
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LHC repair and consolidation
14 quadrupole
magnets replaced
New longitudinal restraining
system for 50 quadrupoles
39 dipole magnets
replaced
Almost 900 new helium
pressure release ports
Collateral damage mitigation
J. Wenninger LNF Spring School, May 2010
204 electrical interconnections repaired
Over 4km of vacuum
beam tube cleaned
6500 new detectors and
250km cables for new Quench
Protection System to protect
busbar joints: nQPS
52
A glimpse in the future: the consolidated connection

In the next long (> 1 year) shutdown all the
connections will be re-done.
 Latest design:
o Better electrical contact
o Mechanical clamping
Mechanical clamping
(not present in Tevatron
and Hera …)
Courtesy F. Bertinelli
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LHC energy target - way down
When
All main magnets commissioned for 7TeV
operation before installation
Detraining found when hardware
commissioning sectors in 2008
o 5 TeV poses no problem
7 TeV
Why
2002-2007 Design
12 kA
5 TeV
9 kA
Summer 2008 Detraining
o Difficult to exceed 6 TeV
Machine wide investigations following
S34 incident showed problem with joints
Commissioning of new Quench
Protection System (nQPS)
3.5 TeV
6 kA
1.18 TeV
2 kA
Late 2008
Spring 2009
Nov. 2009
Joints
nQPS
450 GeV
54
J. Wenninger LNF Spring School, May 2010
LHC energy target - way up
When
Train magnets
o 6.5 TeV is within reach
o 7 TeV will take time
7 TeV
6 TeV
Repair joints
Complete pressure relief system
Commission nQPS system
3.5 TeV
2014 ? Training
2013
Joint repair
2011
2010
1.18 TeV
What
nQPS
2009
450 GeV
55
J. Wenninger LNF Spring School, May 2010
Beam operation
J. Wenninger LNF Spring School, May 2010
56
Goals for 2010-2011
2009
Repair of Sector 34
No Beam
2010
1.18 nQPS
TeV 6kA
B
3.5 TeV
Isafe < I < 0.2 Inom
2011
3.5 TeV
~ 0.2 Inom
Ions
β* ~ 2 m
Ions
β* ~ 2 m
Beam
Beam
Ambitious goal for the run: collect 1 fm-1 of data/exp at 3.5 TeV/beam.
To achieve this goal the LHC must operate in 2011 with
Mininum β* in various IP’s
L ~ 21032 Hz/cm2 ~ Tevatron Luminosity
which requires ~700 bunches of 1011 p/bunch
(stored energy of ~ 30 MJ – 10% of nominal)
Implications:
Strict and clean machine setup.
Machine protection systems at near nominal performance.
J. Wenninger LNF Spring School, May 2010
β*inj
β*min
IP1 / IP5
11 m
2m
IP2/IP8
10 m
2m
IP5-TOTEM
11 m
90 m
57
LHC schedule

Proton run until November.

Lead ion setup and run
November and Decenber.
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2010 commissioning milestones
27th Feb
First injection
28th Feb
Both beams circulating
5th March
Canonical two beam operation
8th March
Collimation setup at 450 GeV
12th March
Ramp to 1.18 TeV
15th - 18th March Technical stop – dipoles magnets good for 3.5 TeV
19th March
Ramp to 3.5 TeV
30th March
First 3.5 TeV collisions in all experiments (media event)
4th - 5th April
19h-long physics fill with b* 10/11 m - L ~ 1027 cm-2s-1
7th April
b*squeeze to 2 m in IP1 and IP5
24th April
30h-long physics fill with b* of 2 m - L ~ 1.31028 cm-2s-1
J. Wenninger LNF Spring School, May 2010
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LHC machine cycle
collisions
collisions
beam
dump
energy
ramp
7 TeV
start of the
ramp
Squeeze
3.5 TeV
injection phase
preparation
and access
J. Wenninger LNF Spring School, May 2010
450 GeV
60
LHC cycle

Ramp-down/pre-cycle: 1 ½ - 2 hours
o

Depending on initial conditions, the magnets must either be ramped down (3.5 TeV
 injection) or cycled (from access).
Injection: > 15 minutes
Probe beam: low intensity bunch (≤ 1010 p) that must be first injected when a ring is
empty (safety !). Used to verify that beam parameters are OK for higher intensity.
o Nominal bunch sequence follows when beam parameters have been adjusted.
o

Ramp: 40 minutes
Machine energy is increase at constant optics (b-function) from 450 to 3.5 TeV. For
the moment the ramp rate is limited to ~1 GeV/s.
o Ramp rate to nominal value ~5 GeV/s in June >> ramp will become a lot faster!
o

Squeeze: ~30 minutes
Injection b* is larger (10 m in IR1/5, 11 m in IR2/8) because more aperture is
needed to accommodate the larger beam emittance (triplet magnets).
o In the squeeze b* is reduced to its nominal physics value (presently 2 m in all IRs).
This implies gradient changes of most quadrupoles in the straight sections and first
part of the arc.
o
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Beam squeeze
11 m to 2 m
Duration will soon be
reduced to ~ 20 minutes
10 m to 2 m
30 min
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How equal are the IPs?

The b-function can be measured by exciting (shaking) the beams.

After correction of optics errors, the residual error is in a band of around +- 20%
(specification).
>> b* may vary from IP to IP by up to 20% !
Example of optics
errors along the ring
for b* 2 m (beam1)
Horizontal
Vertical
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LHC cycle : physics

Stable beams: 10’s of hours and more…
Period with quiet running conditions for the experiments (with HV ON, etc).
o Beam tuning activity reduced to the minimum (to keep good lifetimes…).
o Collimators in fixed positions (they move during ramp and squeeze).
o Note however that abrupt beam loss can occur in any phase due to powering
problems, cooling etc etc – even in stable beams.
 Experiments are protected by the collimators (shadowing), the LHC machine
protection system and by their own protection system (BCM : Beam Condition
Monitors).
o
[1030 cm-2s-1]
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Beam overlap

Because the beams circulate most of the time in different vacuum chambers, they also see
slightly different magnetic fields.
>> no guarantee that they collide at the IP ! Small beam sizes !!

To optimize the beam overlap (hor. and vert. planes) the beams are scanned across one
another until the peak luminosity if found (typical +- 2 sigma).
>> settings are quite reproducible – more experience needed.
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Beam operation at 3.5 TeV

Luminosity (head on case, no crossing angle) :
kN 2 f
L
4 x* *y

The beam size s depends on b /b* and on beam emittance en:
en
  b

*
*
Everywhere in the ring the beam
size scales with 1/ ~ 1/E.
Aperture margins are reduced wrt 7 TeV !

The quench levels at 3.5 TeV are a factor ~10 higher wrt 7 TeV and the
stored beam energy is lower: advantage in the early days.
J. Wenninger LNF Spring School, May 2010
Peak luminosity at 3.5 TeV

A consequence of the small beam size at the IP (small b*) is a large beam
size in the triplet magnets.

The physical aperture in the triplet quadrupoles defines the minimum of b*.
o
Limits b* to ~2 m at 3.5 TeV (instead of 0.5 m at 7 TeV)
Beam size * increase by:
Lower limit on b*
(with crossing angle)

Factor 2 (‘naturally’ larger size)

Factor ~2 (b* limit)
Luminosity loss wrt 7 TeV of:

Present operating conditions
(no crossing angle) @ 2 m
J. Wenninger LNF Spring School, May 2010
Factor ~22 per plane (*)
>> overall factor ~8
for same beam intensity
Intensity increase plan
Stage
N (protons)
k
Stored E (kJ)
Peak L (Hz cm-2)
3 fat pilots
1.00E+10
3
17
1.34E+28
4 bunches
2.00E+10
4
44.8
7.63E+28
4 bunches
5.00E+10
4
112.0
4.77E+29
8 bunches
5.00E+10
8
224.0
9.54E+29
4x4 bunches
5.00E+10
16
448.0
1.91E+30
8x4 bunches
5.00E+10
32
896.0
3.81E+30
43x43
5.00E+10
43
1204.0
5.13E+30
8 trains of 6 b
8.00E+10
48
2150.4
1.33E+31
b* = 2 m
50 ns trains
8.00E+10
96
4300.8
2.67E+31
nominal e
Present phase
Next step
(coming days)
Beams become
rather dangerous
Assumption:

To gain experience with the machine protection system, 2 weeks of running time
(~10 fills, 40 h of physics) must be integrated before proceeding to the next step.

Some steps require additional (machine protection) commissioning.
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Performance estimate
If all goes well, we
could reach
L  1032 cm-2s-1
by November!
4 coll pairs
5e10 p/bch
Weak bosons (W, Z)
4 coll pairs
2e10 p/bch
23.4 - ...
b * 2m
1.1e10 p/bch
charm, D’s, J/Psi
30.3 - 19.4
b * 10m 1.1e10 p/bch
Apr 23
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November 7
69
Integrated luminosity projections
If maintain the high availability
100 pb-1 could be integrated
until November
Apr 23
Apr 23
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LHC operation today

So far LHC operation is amazingly stable and efficient (we are still in
commissioning !!).
But we are working with ‘tiny’ beams (on the nominal LHC scale).



In physics at the moment:
b* = 2 m at IPs
*  45 m
N  1.81010 p/bunch
k = 2 bunches
 1 colliding pair / IP
L  1.31028 cm-2s-1
The next step in intensity is most likely:
N  41010 p/bunch
k = 2 bunches
 1 colliding pair / IP
L  6.81028 cm-2s-1
We have been able to collide bunches with nominal population (N = 1011) at
450 GeV: this is a good omen for 3.5 TeV (cross fingers…)
o
We may be able to push luminosity faster than anticipated (L  N2)
J. Wenninger LNF Spring School, May 2010
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To observe the LHC
http://op-webtools.web.cern.ch/op-webtools/vistar/vistars.php?usr=LHC1
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Outlook

The LHC beam commissioning has been progressing smoothly and rapidly so
far – even we are sometimes surprised !!
o

But the LHC remains a very complex machine, so far we operate under rather
simple conditions.
The problem of the LHC: to reach ‚interesting‘ luminosities we must store very
dangerous beams at a very early stage of the commissioning.
o
We have little operational experience.
o
We are still consolidating beam control software, debugging systems...
o
We must be sure that they are no unexpected flaws in the machine protection
system (or totally unexpected situations).
>> for this reason we increase the intensity/L in rather modest steps.

Assuming there are no surprises, and once we have passed the main hurdle of
the initial machine protection commissioning (soon !), progress should be
faster:
>> so far the target of 1031 – 1032 cm-2s-1 by November is within reach !
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From darkness…
…to light
Sept 2008
April 2010
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