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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

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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

2010-201117

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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 !