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