The Large Hadron ColliderContents:

1. The machineII. The beam

III. The interaction regionsIV. First LHC beam

[R. Alemany][CERN AB/OP]

[Engineer In Charge of LHC]Lectures at NIKHEF (12.12.2008)

III. The interaction regionsContents:I. The straight sectionsII. Betatron and momentum cleaning

insertionsIII. The experiments:

II. High luminosity insertions (ATLAS & CMS)III. Low luminosity insertions (ALICE & LHCb)

IV. SqueezeV. Colliding with a crossing angleVI. Luminosity optimization

III.I. The straight sections

SPS (~7 km)LHC (27 km)

IR ARC

Sector

DS MS

IR

IT

ITIP

III.I. The straight sections• A straight section is composed

of:1. Matching section (MS)2. Inner triplets (IT) ( there is

an experiment)3. IR: collimators, RF, dump

system, experiments

III. The interaction regionsContents:I. The straight sectionsII. Betatron and momentum cleaning

insertionsIII. The experiments:

II. High luminosity insertions (ATLAS & CMS)III. Low luminosity insertions (ALICE & LHCb)

IV. Luminosity optimization

III.II Momentum and betatron cleaning insertions (IR3, IR7)

Particles with large momentum offset are scattered by the primary collimators in IR3.

Particles with large H, V or H&V betatron amplitudes are scattered by the primary collimators in IR7.

In both cases the scattered particles are absorbed by secondary collimators.

Typical quadrupole strength 30-35 T/m

Note: IR3 & IR7 have special DS (arc quadrupoles in series + trim quadrupoles) because of lack of space to place the power converters.

IR3

IR7

Q4 Q5 Q6 Q7D3 D4

(collimators are not shown)

(collimators are not shown)

Warm magnets224 mm

III.II Momentum and betatron cleaning insertions (IR3, IR7)

14506(9)

Settings @7TeV and *=0.55 mBeam size () = 300 µm (@arc)Beam size () = 17 µm (@IR1, IR5)

7 8.5

III. The interaction regionsContents:I. The straight sectionsII. Betatron and momentum cleaning

insertionsIII. The experiments:

II. High luminosity insertions (ATLAS & CMS)III. Low luminosity insertions (ALICE & LHCb)

IV. SqueezeV. Colliding with a crossing angleVI. Luminosity optimization

III.III The experiments: High luminosity

insertions• The high luminosity insertions are IR1 (ATLAS) and

IR5 (CMS)• They are identical in terms of hardware and optics• The optics design is guided by two main

requirements:• Large dynamic range of * values while

keeping the total phase advance over the IR constant:• * = 18 m for injection• * = 0.55 m for collisions

• When changing from injection to collision optics, the quadrupole magnets must change smoothly with * to have under control the beam size, the beam separation and the chromaticity

III.III The experiments: High luminosity

insertions• The hardware constraints:

• The beams share the same beam pipe and the same low beta triplet quadrupoles, so the optics solution must have the same triplet gradients. The maximum gradients are constraint

• The overall beam size must be small enough to fit into the tight aperture of the LHC at this location

• Optics:

* The beams at pre-collision are displaced from the ideal orbit to increase the mechanical aperture of the low beta triplet quadrupoles ** phase advance for the whole insertion region (Q13.R – Q13.L)

Optics * (m) µx/2π**

µy/2π**

Qx Qy

Injection

18 2.618 2.644 64.28 59.31

Collision

0.55 2.633 2.649 64.31 59.32

Q2

Q1

Q3

III.III. The experiments: High luminosity

insertions

IP1 TAS

* Q1 Q2 Q3 D1(1.38 T) TA

N* D2 Q4

(3.8 T)Q5 Q6 Q7

4.5

K 1.9 KWarm

Separation/ Recombination

Matching QuadrupolesInner Triplet

1.9 K

ATLASR1

* Protect Inner Triplet (TAS) and D2 (TAN) from particles coming from the IP

4.5

K

4.5

K188 mm

Tertiary collimators

6.45 kA 10.63 kA

23.8518.95

22.517.6

29.024.0

To provide sufficient aperture for the XangleThe mechanical aperture of the inner triplets limits the maximum * @IPs and the maximum Xangle limit peak lumi

slide 12

III.III. The experiments:High luminosity

insertionsATLAS

five-storey building

CMS

III.III. The experiments: Low luminosity

insertionsLHCb

Q7Q6Q5Q4D2

MKI

Q1 Q2 Q3

ALICE

IP8

LHCb

MSI

TDI

TCD

DD1

Beam 1

Beam 2

Beam 2

Beam 1

LHCb experimentCenter of the exp cavern

III.III. The experiments: Low luminosity

insertionsALICE

LHCb

III. The interaction regionsContents:I. The straight sectionsII. Betatron and momentum cleaning

insertionsIII. The experiments:

II. High luminosity insertions (ATLAS & CMS)III. Low luminosity insertions (ALICE & LHCb)

IV. SqueezeV. Colliding with a crossing angleVI. Luminosity optimization

III.IV. Squeeze• Squeeze: change quadrupole currents (magnet

strength) in a way that the beta function at the interaction point is very small to increase luminosity

• Magnets: matching quadrupolesRB

RQD/RQF

III.IV. Squeeze

• So even though we squeeze our 100,000 million protons per bunch down to 16 microns (1/5 the width of a human hair) at the interaction point. We get only around 20 collisions per crossing with nominal beam currents.

• The bunches cross (every 25 ns) so often we end up with around 600 million collisions per second - at the start of a fill with nominal current.

• Most protons miss each other and carry on around the ring. The beams are kept circulating for hours 10 hours

Squeeze the beam size down as much as possible at the collision point to increase the chances of a collision

III.IV. Squeeze

IR2

IR1

IR3 IR4

IR5

IR6 IR7 IR8

IR1

Injection

Beta function at top energy and after squeeze

III.IV. Squeeze

ATLAS=CMS

Q1 Q3 D2 Q5

Q2 D1 Q4 Q6

Q7

2MB

Q8

2MB

Q9

2MB

Q10

2MB

Q11IT MS DS

III. The interaction regionsContents:I. The straight sectionsII. Betatron and momentum cleaning

insertionsIII. The experiments:

II. High luminosity insertions (ATLAS & CMS)III. Low luminosity insertions (ALICE & LHCb)

IV. SqueezeV. Colliding with a crossing angleVI. Luminosity optimization

III.IV Colliding with a XangleWhy? to minimize beam-beam interaction effects

Vertical Xangle (160 µrad @ injection, 142.5 µrad @collis)

Horizon Xangle (160 µrad @ injection, 142.5 µrad @collis)

III. The interaction regionsContents:I. The straight sectionsII. Betatron and momentum cleaning

insertionsIII. The experiments:

II. High luminosity insertions (ATLAS & CMS)III. Low luminosity insertions (ALICE & LHCb)

IV. SqueezeV. Colliding with a crossing angleVI. Luminosity optimization

22

21

22

21

21

2 yyxx

brevNfNNL

III.IV Luminosity optimization• Luminosity formulae:

AB

yyxx

brev eWFNfNN

L2

22

21

22

21

21

2

Ni = number of protons/bunchNb = number of bunchesfrev = revolution frequencyix = beam size along x for beam iiy = beam size along y for beam iAssume Gaussian distributions for the beam distribution functions and equal bunch length.

)(2)(2

221

212

xx

dd

eW

W is a pure beam offset contribution. If the offset is in the horizontal plane beam 1 is displaced by d1 and beam 2 is displaced by d2 with respect to their reference orbits, thus W takes the form:

2tan21

1

222

21

2

xx

s

F

F is a pure crossing angle (Φ) contribution, which for a crossing angle in the horizontal plane (XS, with S the direction of movement) takes the form:

FLHC = 0.836

Φ

ρ1(x,y,s,-s0) ρ2(x,y,s,-s0)

s

x

III.IV Luminosity optimization

2

2

22

21

2

2cos

2sin2

sxx

A

22

21

12 2sin

xx

ddB

exp(B2/A) is a term that appears when beams collide with a crossing angle and an offset at the same time. For a crossing angle and an offset in the x direction:

Luminosity monitors in the machine BRAN detectors:

Luminosity scans:1: Get the beams into collision (the first days of beam commissioning); 2: Optimize luminosity every fill3: Calibrate luminosity based on machine parameters dedicated runsEach of these is of course applicable at each of the four LHC interaction

points.

III.IV Luminosity optimization• Method orthogonal separation scans

Example from LEP

x

y

III.IV Luminosity optimization

• Luminosity ≠ cte over a physics run. It decays due to degradation of intensities and emittance.

• The main cause of lumi decay are the collisions themselves, but there are other contributions like beam-gas scattering, beam-beam interactions

~ 15 hours (lumi lifetime)

• 600 million collisions/sec = 20 coll/crossx2808x11000Hz

• Raw data rate is 1015 bytes/sec• equivalent to >1 million CD-roms/sec

• Only 0.00025% recorded for analysis• experimental “trigger” rejects the rest