1

Commissioning challenges of the LHCCommissioning challenges of the LHC

Jörg Wenninger CERN Accelerators and Beams

Department Operations groupFebruary 2008

• LHC overview• Magnets• Luminosity• High ‘energy’ issues• Hardware & beam commissioning

2Rüdiger Schmidt Bullay Oktober 2007 2

The CERN Beschleuniger Komplex

CERN (Meyrin)

LHC

SPSControl room

ATLAS

LHCB

CMS

ALICE

7 years of construction to replace

LEP: 1989-2000

in the same 26.7 km tunnel by

LHC : 2008-2020+

3

IR6: Beam dumping system

IR4: RF + Beam

instrumentation

IR5:CMS

IR1: ATLAS

IR8: LHC-BIR2:ALICE

Injection ring 2

Injection ring 1

IR3: Momentum collimation (normal

conducting magnets)

IR7: Betatron collimation

(normal conducting magnets)

Beam dump blocks

LHC Layout8 arcs. 8 long straight sections (insertions), ~ 700 m long.beam 1 : clockwisebeam 2 : counter-clockwiseThe beams exchange their positions (inside/outside) in 4 points to ensure that both rings have the same circumference !

The main dipole magnets define the geometry of

the circle !

4

Top energy/GeV Circumference/m Linac 0.12 30PSB 1.4 157CPS 26 628 = 4 PSBSPS 450 6’911 = 11 x PSLHC 7000 26’657 = 27/7 x SPSLEIR

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 – and not more !The gain is typical for the useful range of magnets !!!

The CERN Accelerator Complex

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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 2x1032 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 100 in luminosity

LHC Magnets & Arc Layout

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The Superconductor

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Temperature [K]

Applie

d f

ield

[T

]Superconducting

state

Normal state

Bc

Tc

The 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

LHC magnets use NbTi, with a typical usable current density @ 4.2 K

2000 A/mm2 @ 6T

To reach 8-10 T, the temperature must be lowered to 1.9 K – superfluid Helium:

Very low viscosity coil penetration.

Large specific heat (2000 x conductor / unit volume).

Large heat conduction (3000 x Cu).

The superconducting cable

8

1 mm

6 m

Typical value for operation at 8T and 1.9 K: 800 A

NbTi cable

width 15 mm

A.Verweij

A.Verweij

Dipole magnets

9

Dipole length 15 m

The coils must be aligned very precisely to ensure a good field

quality (i.e. ‘pure’ dipole)

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!

Dipole field map - cross-section

10

Superconducting coil

Beam

Iron

Non-magnetic

collars

B = 8.33 Tesla I = 11800 A

11Rüdiger Schmidt 11

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

LHC arc : not just dipoles…

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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 !

~ 8000 superconducting magnets ae installed in the LHC

QF QD QFdipolemagnets

small sextupolecorrector magnets

decapolemagnets

LHC Cell - Length about 110 m (schematic layout)

sextupolemagnets

13J. Wenninger - ETHZ - December 2005 13

1232 main dipoles +

3700 multipole corrector magnets

(sextupole, octupole, decapole)

392 main quadrupoles +

2500 corrector magnets (dipole, sextupole, octupole)

Regular arc:

Magnets

14J. Wenninger - ETHZ - December 2005 14

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

15J. Wenninger - ETHZ - December 2005 15

Insulation vacuum for the cryogenic distribution line

Regular arc:

Vacuum

Insulation vacuum for the magnet cryostats

Beam vacuum for

Beam 1 + Beam 2

16J. Wenninger - ETHZ - December 2005 16

Regular arc:

Electronics

Along the arc about several thousand electronic crates (radiation tolerant) for:

quench protection, power converters for orbit correctors and instrumentation (beam, vacuum + cryogenics)

Tunnel view

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Complex interconnects

18CERN 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 dipoles

Vacuum chamber

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Beamenvel. ~ 1.8 mm @ 7 TeV

50 mm

36 mm

The beams circulate in two ultra-high vacuum chambers made of Copper that are cooled to T = 4-20 K.

A beam screen protects the bore of the magnet from image currents, synchrotron light etc from the beam.

Cooling channel (Helium)

Beam screen

Magnet bore

Luminosity

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Luminosity challenges

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The event rate N for a physics process with cross-section is proprotional to the collider Luminosity L:

To maximize L:

•Many bunches (k)

•Many protons per bunch (N)

•A small beam size u = ()1/2

: characterizes the beam envelope (optics),

varies along the ring, mim. at the collision points.

: is the phase space volume occupied by the beam (constant along the ring).

**

2

4 yx

fkNL

k = number of bunches = 2808N = no. protons per bunch = 1.15×1011

f = revolution frequency = 11.25 kHzx,y = beam sizes at collision point (hor./vert.) = 16 m

LN

High beam “brillance” N/

(particles per phase space

volume)

Injector chain performance

!Small envelope

Strong focusing !

Beam enveloppe

2222

Fits through the hole of a needle!

ARC ARC

ATLAS collision

point

The beam size follows the local beam optics :• In the arcs the optics follows a regular pattern.

• In the long straight sections, the optics is matched to the ‘telescope’ that provides very strong focusing at the collision point.

Collision point size (rms, defined by ‘*’):CMS & ATLAS : 16 m LHCb : 22 – 160 m ALICE : 16 m (ions) / >160 m (p)

Combining beams for collisions

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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 circulate in separate vacuum chambers in most of the ring, but they must be brought together to collide.

Over a distance of about 260 m, the beams circulate in the same vacuum chamber and they are a total of ~ 120 encounters in ATLAS, CMS, ALICE and LHCb.

Crossing angle

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IP

From the point of view of beam dynamics, colliding beams is one of the worst things one can do to a beam due to the very non-linear fields of the opposing beams (diffusion to large amplitudes, lifetime reduction…).

To avoid un-necessary direct beam encounters wherever the beams share a common vacuum:

COLLIDE WITH A CROSSING ANGLE IN ONE PLANE !

The price to pay :

A reduction of the luminosity due to the finite bunch length of 7.6 cm and the non-head on collisions L reduction of ~ 17%.

Crossing planes & angles•ATLAS Vertical 280 rad•CMS Horizontal 280 rad•LHCb Horizontal 300 rad•ALICE Vertical 400 rad

7.5 m

Bunch Pattern

25 25

The nominal LHC pattern consists of 39 groups of 72 bunches (spaced by 25 ns), with variable spacing between the groups to accommodate the rise times of the fast injection and extraction magnets (‘kickers’).

There is a long 3 s hole (5)for the LHC dump kicker (see later).

72 bunches

Stored Energy & Protection

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The price of high fields & high luminosity…

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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 Estored = 7 MJ

all magnets 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 will be a major challenge for the LHC operation crews !

Protection of the machine components from the beam is becoming a major issues for all new high power machines (LHC, SNS, …).

> 1000 x above damage limit for

accelerator components !

Stored Energy

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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 energy

Comparison…

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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 12 tons of Copper!!

The energy stored in one LHC beam corresponds

approximately to… 90 kg of TNT

Quench

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A quench corresponds to the phase transition from the super-conducting to a normal conducting state.

Quenches are initiated by an energy in the order of few mJ– Movement of the superconductor (friction and heat dissipation)

‚training‘ quenches of the magnets until coils are stable.– Beam losses.– Cooling failures.– ...

When part of a magnet quenches, the conductor becomes resistive, which can lead to excessive local energy deposition (temperature rise !!) due to the appearance of Ohmic losses. To protect the magnet:

– The quench must be detected.– The energy is the magnet circuit must be extracted.– The magnet current has to be switched off within << 1 second.

Quench : discharge of energy

3131

Magnet 1 Magnet 2

Power Converter

Magnet 154

Magnet i

Protection of the magnet after a quench:• The quench is detected by measuring the voltage increase over coil : R ~ 0 to

R > 0 !• 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 of all other magnets flows through a ‚bypass‘ diode that can stand

the current for 100-200 s.• The current of all other magnets is dischared into the dump resistors.

Discharge resistor

Dump Resistors

3232

Those large air-cooled resistors can absorb the 1 GJ stored in a dipole magnet circuit (they heat up to few hundred degrees Celsius).

If it does not work…

3333P.Pugnat

During magnet testing the 7 MJ stored in one magnet were released into one spot of the coil (inter-turn short)

Operational margin of SC magnet

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Temperature [K]

App

lied

field

[T]

Superconductingstate

Normal state

Bc

Tc

9 K

Applied Field [T] Bc critical field

1.9 K

quench with fast loss of ~1010 p ~ 0.01% of total int.

Quench with fast loss of ~106-7 p

~0.01-0.1 ppm of the total int.8.3 T / 7 TeV

0.54 T / 450 GeV

QUENCH

Tc critical temperatur

e

Temperature [K]

The LHC is ~1000 times more critical than TEVATRON,

HERA, RHIC

‘Cohabitation’

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Even to reach a luminosity of 1033 cm-2 s-1, i.e. 10% of the design, requires unprecedented amounts of stored beam energy.

The stored energy will be circulating a few cms from extremely sensitive super-conducting magnets, which can quench following a fast loss of less than 1 part per million of the beam:

>>> requires a very large collimation system (> 100 collimators).

LHC commissioning has to be much more rigorous than what was done for previous machines – no shortcuts and dirty ‘tricks’ can be used to go ahead as soon as the intensities exceed a few % of the design.

>>> Predictions on the commissioning duration are particularly tricky!

Collimation

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Primary collimator

Secondary collimators Absorbers

Protectiondevices

Tertiarycollimators

Tripletmagnets

Experiment

Beam

Primaryhalo particle Secondary halo

Tertiary halo

+ hadronic showers

hadronic showers

A multi-stage halo cleaning (collimation) system with over 100 collimators has been designed to protect the sensitive LHC magnets from beam induced quenches :

Halo particles are first scattered by the primary collimator (closest to the beam).The scattered particles (forming the secondary halo) are absorbed by the secondary collimators, or scattered to form the tertiary halo.

Primary and secondary collimators are made of Carbon to survive severe beam impacts !

The collimators must be very precisely aligned (< 0.1 mm) to guarantee efficiency above 99.9% at nominal intensities.

the collimators will have a strong influence on detector backgrounds !!

Collimation @ 7 TeV

3737

1 mm

Opening ~3-5 mm

The collimator opening corresponds roughly to the size of Spain !

For colliders like HERA, TEVATRON, RHIC, LEP collimators are/were used to reduce backgrounds in the experiments ! But the machines can/could actually operate without collimators !

At the LHC collimators are essential for machine operation as soon as we have more than a few % of the nominal beam intensity !

Beam Dumping System IR6

3838

Q5R

Q4R

Q4L

Q5L

Beam 2

Beam 1

Beam dump block

Kicker magnets to

paint (dilute) the beam

about 700 m

about 500 m

15 fast ‘kicker’ magnets

deflect the beam to the

outside

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

quadrupoles

Dump Block…

39 39

Approx. 8 m

concrete shielding

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 hadronic 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 !

Unscheduled beam loss & protection

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Beam loss over multiple turns due to many types of failures.

Passive protection - Failure prevention (high reliability systems).- Intercept beam with collimators and absorber

blocks.

Active protection systems have no time to react !

Active Protection- Failure detection (by beam and/or equipment

monitoring) with fast reaction time (< 1 ms).- Fire beam dumping system

Beam loss over a single turn during injection, beam dump or any other fast ‘kick’.

In the event a failure or unacceptable beam lifetime, the beam must be dumped immediately and safely into the beam dump

blockTwo main classes for failures (with more subtle sub-classes):

The LHC machine protection (interlock) system is designed for high reliability with a design target of < 1

failure in 1000 years.

LHC Commissioning :

the road to first beam

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Injector status : beam at the gate to the LHC

42L.R. Evans – EDMS Document No. 521217

SPC

2

TI 8 commissioning

TI 8 commissioning / V.Mertens / TCC, 29.10.2004

First shot on BTVI87751 on 23 October 2004, 13:39

TV screen at end transfer line

Beam image taken less than 50 m away from the

LHC tunnel in IR8 (LHCb) !

The LHC injectors are ready after a long battle to achieve nominal beam brightness.

Beam was sent down both ~3 km long transfer lines to the LHC.

Filling exercise from SPS to LHC

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Extraction of a high intensity beam from the SPS to the LHC transfer lines was tested towards both rings in the fall of 2007.

Ready to go …

LHC Machine Commissioning

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Commissioning of the LHC equipment (‘Hardware commissioning’) has started in 2005 and is now in full progress. This phase includes:

Testing of ~8000 magnets (most of them superconducting). 27 km of cryogenic distribution line (QRL). 4 vacuum systems, each 27 km long. > 1600 magnet circuits with their power converters (60 A to 13000 kA). Protection systems for magnets and power converters. Checkout of beam monitoring devices. Etc…

Hardware commissioning sequence

45

The present LHC commissioning phase is designated as ‘Hardware Commissioning’. The most ‘visible’ job is the commissioning of the magnets and their powering systems.

When all magnets of one of the eight LHC sectors installed and interconnected:

Vacuum system is pumped to nominal pressure. Magnets are cooling down to 1.9 (4.5) Kelvin. The Cool down proceeds in steps,

interleaved with electrical tests (HV insulation, ground faults…). Connection of the power converter to the magnets. Commissioning of the power converter + interlock system + magnet

protection system (low current). Commissioning of magnet powering + magnet protection system (high

current). Powering of all magnets in a sector to nominal current.

This commissioning sequence is run individually on each of the 8 LHC sectors (arcs).

Circuit by circuit

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Each electrical circuit (magnet group + power converter) has to pass a number of tests before it is released as ‘commissioned’.

For the large circuits, there are more than 10 intermediate steps. Each step is validated by system experts.

Commissioning status

47

Installation and interconnections finished for magnets.

Cryogenics :

- Sector 78 (IR8IR7) was cooled down to 1.9 K in June/July 2007.

- Sector 45 is presently at 1.9 K.

- Sectors 56 and 78 : cool down in progress, expected at 1.9 K mid-March. Magnet powering :

- Commissioning of sector 78 in June/July 2007:

>> All circuits with individual magnets commissioned (except triplets).

>> The main magnets were commissioned to 1 TeV: limited by electrical non-conformities that were repaired during a warm up in September-October.

- Commissioning of sector 45 in progress (until mid-February).

Other systems (RF, beam injection and extraction, beam instrumentation, collimation, interlocks, etc) are essentially on schedule for first beam in 2008.

Cool-down

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From 300K to 80K pre-cooling with 1200 tons of liquid N2 (60 trucks).

LHC He inventory: 100 tons.

The duration of the first cool-down of one arc/sector is ~6 weeks

(nominal ~ 2 weeks). So far this was too optimistic… The cool down of sector 45 took ~8-9

weeks, delayed by heat isolation problems in certain cells etc The stability of the cryogenics so far proves to be ‘delicate’.

Xmas break

01/10/07

1.9 K

High field

quenches

Approaching 7 TeV

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Main dipole magnet high field ramps for sector 45 started some days ago.

The first ‘re-training’ quenches occurred around 5.6 TeV. After 3 ramps/training quenches, the magnets have reached 6 TeV. 1 TeV/1500 A to go…

31/01/2008

Currentdischarge into

dump resistors

Quench at 10270

A > 6 TeV

Injection level

Cryo Recovery

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1.9 K

After a high current/energy quench, the temperature in the affected magnet(s) increases above 20 K.

Recovery is ‘slow’ – few hours.

>> Progress is slow !

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K. Foraz TS-IC-PL 12 23 34 45 56 67 78 81

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20

303132333435

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20

303132333435

Power limitation

It is still possible to have a beam test through 78

around 21st May… Assumes very fast cool-down of

each sector (6 weeks)

Inner triplet interconnect

Flushing

Cool-down

Warm up

Powering Tests

CV maintenance

LHC Schedule, status Week 03LHC Schedule, status Week 03

Focus for 2008:Get beam in at 450 GeV asap !!The HWC will be ‘paused’ to make room for a beam test, presently expected around JUNE!

Towards beam…

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“As usual” the latest schedule is VERY aggressive: Assumes parallel commissioning of almost the entire machine, while the initial schedules assumed parallel commissioning of only 1/4 LHC in parallel.

There is a strong push for beam tests @ 450 GeV, even before the magnets are commissioning to nominal field:

>>> Get beam in asap at injection.

A beam test of a few weeks at 450 GeV of 1 or 2 sectors or of the entire machine will be performed, possibly as soon as June.

Planning is delicate due to interference with the experiments installation and hardware commissioning.

Various scenarios are been evaluated, including beam tests in certain sectors in parallel with hardware commissioning in others !!

LHC machine cycle (commissioning)

5353

0

2000

4000

6000

8000

10000

12000

-4000 -2000 0 2000 4000

time from start of injection (s)

dip

ole

cu

rre

nt (A

)

energy ramp

preparation and access

beam dump

injection phase

collisions

collisions

450 GeV

7 TeV

start of the ramp Squeeze

Step by step

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Beam commissioning will start with single low intensity bunches:• Get the beams around and establish an orbit.

• Capture the beams in the RF systems.

• Commissioning all beam instrumentation.

• Understand /check the behaviour of the SC magnets at injection (field drifts due to ‘super-conducting eddy currents’).

• Etc…

Then there will more or less parallel branches to:• Establish operation with both beams together.

• Increase intensity & number of bunches.

• Ramp, ‘squeeze’ (focus more at IP).

• Collide …

If all goes very well… 2 months !

Please take this as a LOWER limit…

Operation phases

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Parameter Phase A Phase B Phase C Nominal

k / no. bunches 43-156 936 2808 2808

Bunch spacing (ns) 2021-566 75 25 25

N (1011 protons) 0.4-0.9 0.4-0.9 0.5 1.15

Crossing angle (rad)

0 250 280 280

*/*nom) 2 1 1

* (m, IR1&5) 32 22 16 16

L (cm-2s-1) 6x1030-1032 1032-1033 (1-2)x1033 1034

Beam commissioning will proceed in phases with increased complexity: Number of bunches and bunch intensity. Crossing angle (start without crossing angle !). Less focusing at the collision point.

It will most likely take YEARS to reach design luminosity !!!

Summary

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We are getting there at last !!!!

• After many years of delay the LHC is taking shape in the tunnel.

• The first sector is approaching 7 TeV.

• The entire machine could be at 1.9 K around June.

Beam is knocking at the door :

• The injectors are ready.

• There is a strong push/pressure to inject beam as soon as possible and perform some beam commissioning at 450 GeV – we may have beam in at least one sector in June 2008.

• Although in the best of all Worlds we could achieve collisions after 2 months, the road to 7 TeV collisions is long.... Be patient !!

For more details on the LHC, have a look at the 2nd Hadron Collider Summer School site : http://indico.cern.ch/conferenceDisplay.py?confId=6238

‘RF fingers’ problems

57

RF bellows are used to maintain electrical contact between adjacent pieces of vacuum chamber (essential for beam stability).

The bellows must cope with the thermal expansion of ~ 4 cm between 1.9 K and room temperature (when the magnets are cooled down/warmed up).

Bellows are installed at every interconnection (1700 in total).

At room temperature

At operating temperature

59

Damaged bellows

60

RF fingers that bend into the beam are a classic problem for accelerators. RHIC suffered a lot from it…

So no excuses for not being careful in design and manufacturing ! And yet it happened ! X-ray imaging of some bellows revealed bend fingers in

the sector that was tested in July and warmed up since then for repair.

Cause & solution for RF fingers

61

The fingers bend due to a combination of a wrong ‘finger angle’ PLUS a gap between the magnet apertures larger than nominal (still inside specification).

Only few interconnects are affected. Complete survey of one sector was performed using X-ray techniques. Repair is not difficult…once the bad fingers are found.

>> This problem will stay around until every sector is warmed up again in 200X…

‘Solution’: A small (ping-pong size) ball equipped

with a 40MHz transmitter is blown through the beam vacuum pipe with compressed air.

Beam position monitors are used to follow the ball as it rolls inside the vacuum chamber until it stops or exits on the other end..