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1Drawing by
Sergio Cittolin
Der LHC Beschleuniger: Herausforderungen auf
dem Weg zu Teilchenkollisionen
J. Wenninger CERN
Outline17
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Introduction
Installation and preparation for beam
Incident in sector 34, repair and consequences
LHC beam operation
Conclusions
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The Large Hadron Collider LHC
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CMS, TotemCMS, Totem
ATLAS, LHCfATLAS, LHCf
LHCbLHCb
ALICEALICE
Lake of Geneva
Installed in 26.7 km LEP tunnelDepth of 70-140 m
Control RoomControl Room
LHC ringLHC ring
SPS ringSPS ring
ATLAS17
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CMS17
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LHCb17
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ALICE17
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TOTEM and LHCf17
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Total X-section, elastic and diffractive scattering.
Forward production of neutral particles (cosmic ray shower modeling)
Collider luminosity17
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FfkNLyx
b
4
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“Thus, to achieve high luminosity, all one has to do is make (lots of) high population bunches of low emittance to collide at high frequency at locations where the beam optics provides as low values of the amplitude functions as possible.” PDG 2005, chapter 25
Parameters:– Number of particles per bunch – Number of bunches per beam kb
– Beam sizes at the collision point
– Revolution frequency f– Crossing angle factor F ~ 1
Collision rate is proportional to luminosity
Interaction RegionBeam quality (emittance)
Intensity
LHC challenges17
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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
SppS pp 6x1030 cm-2 s-1
Very high field magnets and very high beam intensities:Operating the LHC is a great challenge.There is a significant risk to the equipment and experiments.
A factor 2 in field
A factor 4 in size
A factor 30 in luminosity
LHC dipole magnet17
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1232 dipole magnets. B field 8.3 T (11.8 kA) @ 1.9 K
(super-fluid Helium) 2 magnets-in-one design : two beam
tubes with an opening of 56 mm.
Operating challenges:o Dynamic field changes at injection.o Very low quench levels (~ mJ/cm3)
Stored energy17
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Increase with respect to existing accelerators :• A factor 2 in magnetic field• A factor 7 in beam energy• A factor 200 in stored beam energy
Damage threshold
Collimation17
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13beambeam
1.2 m
To operate at nominal performance the LHC requires a large and complex collimation system
o Previous colliders used collimators mostly for experimental background conditions.
Ensure ‘cohabitation’ of:o 360 MJ of stored beam energy, o super-conducting magnets with quench
limits of few mJ/cm3
Almost 100 collimators and absorbers.
Alignment tolerances < 0.1 mm to ensure that over 99.99% of the protons are intercepted.
Primary and secondary collimators are made of Carbon to survive large beam loss.
Outline17
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Installation17
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Transport in the tunnel with an optically guided vehicle.
Approximately 1600 magnet assemblies transported over up to 20 km at 3 km/hour.
First dipole lowered March 2005.Magnet installation until spring 2007Interconnection work finished end 2007
3 km long arc cryostat17
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LHC cool-down 200817
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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]
All sectors at nominal
temperature
First beam around the LHC
LHC Hardware Commissioning17
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Commissioning of the magnets & circuits (power converter, quench protection, interlocks..) follows predefined test steps. 1’700 circuits, 10’000 magnets
Commissioning time ~5 months
11’122 test steps(2008)
April September
September 10th - control (show) room17
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For 3 days all went perfectly well with beam…
Outline17
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Incident of Sept. 19th 200817
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The final circuit commissioning was performed in the week following the startup with beam.
During the last commissioning step of the last main dipole circuit an electrical fault developed at ~5.2 TeV (8.7 kA) in the dipole bus bar (cable) at the interconnection between a quadrupole and a dipole magnet.
Later correlated to quench due to a local R ~220 n – nominal 0.35 n
An electrical arc developed and punctured the helium enclosure.Around 400 MJ from a total of 600 MJ stored in the circuit were
dissipated in the cold-mass and in electrical arcs. Large amounts of Helium were released into the insulating vacuum.
The pressure wave due to Helium flow was the cause of most of the damage (collateral damage).
Magnet Interconnection17
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Dipole busbar
Melted by arcMelted by arc
Collateral damage17
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Quadrupole-dipole interconnection
Quadrupole support
Main damage area covers ~ 700 metres. 39 out of 154 main dipoles, 14 out of 47 main quadrupoles
from the sector had to be moved to the surface for repair (16) or replacement (37).
Sooth clad beam vacuum chamber
Bus-bar joint17
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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.
Joint quality17
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bus U-profile bus
wedgeSolder No solder
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.
During repair work in the damaged sector, inspection of the joints revealed systematic voids caused by the welding procedure.
X-ray of joint
LHC repair and consolidation17
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14 quadrupole magnets replaced
39 dipole magnets replaced
204 electrical inter-connections repaired
Over 4km of vacuum beam tube cleaned
New longitudinal restraining system for 50 quadrupoles
Almost 900 new helium pressure release ports
6500 new detectors and 250km cables for new Quench Protection System to protect from busbar quenches
Collateral damage mitigation
LHC target energy: the way down17
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2002-20077 TeV
Summer 20085 TeV
Spring 20093.5 TeV
Nov. 2009
450 GeV
Detraining
nQPS2 kA
6 kA
9 kA
When Why
12 kA
Late 2008 Joints
1.18 TeV
Design All main magnets commissioned for
7TeV operation before installation
Detraining found when hardware commissioning sectors in 2008
– 5 TeV poses no problem– Difficult to exceed 6 TeV
Machine wide investigations following S34 incident showed problem with joints
Commissioning of new Quench Protection System(nQPS)
LHC target energy: the way up17
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Train magnets– 6.5 TeV is in reach– 7 TeV will take time
Repair joints Complete pressure relief system
Commission nQPS system
2014 ?
2010
Training
Stabilizers
nQPS
When What
7 TeV
3.5 TeV
1.18 TeV
450 GeV
2011
2013
2009
6 TeV
Outline17
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Reserve slides
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20th November 2009
14 months to repair, consolidate and re-commissioning all elements. Great relief on November 20th when both beams circulated again !!!
2009 beam operation milestones17
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20th Nov Day 0 Both beams circulating after 6 hours23rd Nov Day 3 First pilot collisions at 450 GeV29th Nov Day 9 Beams ramped to 1.18 TeV6th Dec Day 16 Stable collisions @ 450 GeV for the experiments8th Dec Day 18 Both beams ramped to 1.18 TeV – first collisions
Many LHC systems were commissioned at forced pace – aim to check as much as possible.
Overall uptime ~60% - very good at this stage. Our most optimistic plan became true !! A touch of modesty…
o The stored energy did not exceed 30 kJ – 0.01% of nominal.
Protons visible by eye17
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At the LHC momentum and magnetic fields are sufficiently strong for the protons to emit visible light that can be used to image the beams in real-time.
The energy loss per turn is 7 keV at 7 TeV.
Excellent performance of the beam instrumentation has largely contributed to the fast progress.
Synch. light
Flying wire LHC
Flying wire SPS(injector)
Noise on the beam17
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The beam is frequently self-excited, driven by noise ‘humps’ visible on the vertical beam oscillation spectrum.Amplitude of ~nm to m.Source is still unknown.Cause of emittance (beam size) blow up.
Beam optics17
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The magnetic model is in very good shape. At 1.2 TeV the optics errors are within spec without any correction.
Work on 450 optics corrections are ongoing
Relative beam size error
(
Specification
Initial optics error (‘beta-beating’) at 450 GeV & 1.2 TeV
Cleaning efficiency measurement 17
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Measurement
noise
Peak leakage to supercond. magnets
Loss at primary collimator
Full collimation setup at injection in 2009. Beam cleaning efficiencies ≥ 99.98% ~ as designed
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CLE
AN
ING
CollimationCollimation
Beam dump17
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Extraction kickers
Dilution kickers
Extraction septum magnets
Dump block
Complex beam dumping system commissioned.Beam swept over dump surface (power load)
Collisions at 450 GeV17
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Collisions were delivered to the experiments for a few days to collect data at 450 GeV for detector studies.
~1.5 million events were collected by LHC experiments.
Beam1 currentBeam2 current
24 hours
1.2 TeV Collisions17
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With very clean beam conditions, the experiments could record first collisions at 1.2 TeV.
2010-2011 run17
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Ambitious goal : collect 1 fm-1 of data/exp at 3.5 TeV/beam.
To achieve such a goal the LHC must operate in 2011 with
L ~ 21032 Hz/cm2 ~ Tevatron Luminosity
which requires ~700 bunches of 108 p/bunch(stored energy of ~ 30 MJ – 10% of nominal)
Implications:o Strict and clean machine setup.o Machine protection systems at near nominal performance.
~2-4 weeks of commissioning timeCareful and step-wise increase of intensity, starting with just 4 bunches
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Jan-Feb 2010: commissioning of LHC circuits for 3.5 TeV operation.
Beam operation 2010:o Start-up with beam.o Consolidation at 450 GeV (optics…).o Ramp to 3.5 TeV.o Low intensity collisions at 3.5 TeV.o Interaction spot size squeezing.o Low intensity collisions at 3.5 TeV squeezed.o Stepwise (factor 2-4) increase of intensity to
1-2 MJ/ beamo Switch from individual bunches to bunch train
operation (b separation 50 ns).o …o Lead ion run
End February
End MarchApril
Summer
Today
November
A possible scenario17
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mid-April
November
Evolution in this regime of stored energy will depend on actual
experience – difficult to predict
Summary17
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The incident 9 days after startup in 2008 revealed quality issues of the bus-bar joints.
14 months of repair and re-commissioning.
New diagnostics for online monitoring and protection of all joints.
The LHC beam energy will be limited to 3.5 TeV in 2010/2011. Long shutdown in 2012 to prepare LHC for 7 TeV / beam (repair of all joints).
Very successful beam commissioning in 2009 to 1.2 TeV.
Very fast commissioning pace.
Now preparing for 18 months run at 3.5 TeV.
Aiming for luminosities up to ~1032 cm-2s-1
The beam challenges are ahead of us !