Lhc construction & operation

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

2

• 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


LHC History

3

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


17.03.2010 Der LHC Beschleuniger - DPG - Bonn

The Large Hadron Collider LHC

4

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

5

Tunnel circumference 26.7 km, tunnel diameter 3.8 mDepth : ~ 70-140 m – tunnel is inclined by ~ 1.4%


6

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 !


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


Luminosity challenges

8

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


Basics of accelerator physics

9J. Wenninger LNF Spring School, May 2010

Accelerator concept

10

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


Bending

11

Magnetic rigidity

ForceLorentz force

LHC: = 2.8 km given by LEP tunnel!ρ

→ → → →


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

12

Two-in-one magnet design

B field

B

p

p

FF force

II

I

B


Focusing

13

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


Accelerator lattice

14

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!


LHC arc lattice

16

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 !


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


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?


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


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.


LHC RF system

22

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)


Visible protons !


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


RF buckets and bunches

25

∆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


Magnets & Tunnel


Superconductivity

27

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 !


The superconducting cable

28

∅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


Coils for dipoles

29

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)


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



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



Insulation vacuum for the cryogenic distribution line

Regular arc:

Vacuum

Insulation vacuum for the magnet cryostats

Beam vacuum for

Beam 1 + Beam 2


Tunnel view (1)


Tunnel view (2)


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


Magnet cooling scheme

37

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


Cryogenics


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


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


Luminosity and interaction regions


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

42

**

2

4 yx

fkNL

σπσ=

1230105.3 −−×= scmLFor k = 1:


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 !


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!


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


Limits to β*

46

The more one squeezes the beam at the IP (smaller β*) the larger it becomes in the surrounding quadrupoles (‘triplets’):


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.


Crossing angles

48

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


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


Tevatron


CDFD0

Tevatron

Tevatron I

52

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


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=


Injection and injector complex



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.


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 …


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


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


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.


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.


Proton Synchrotron

Recently celebrated its first 50 years!!


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…).


Super-Proton Synchrotron


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

…


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


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 !!


(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.


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


Spare slides



PS - bunch splitting


Injection elements

72

12 mrad

0.8 mrad

TED

TED

From the LHC Page1


Role of the TDI collimator

73

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.

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Lhc construction & operation