Transcript Document

03.10.2011
LHC performance in 2011 - PAF - La Londe
LHC status and perspectives
J. Wenninger
CERN
Beams Department
Operation group / LHC
1
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LHC performance in 2011 - PAF - La Londe
Outline
Proton operation 2011
2
Luminosity
Recall the formula for the luminosity (head-on collision):
kN f
kN f 
L
F
F
* *
*
4 x y
4  
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LHC performance in 2011 - PAF - La Londe
2
2
(Round beams)
o
f is the revolution frequency (11.25 kHz),  = E/m,
o
k is the number of colliding bunch pairs,
o
N is the bunch population,
o
F is a geometric (loss) factor (≤ 1) from the crossing angle, F  0.95 in 2011,
o
 is the normalized emittance, * the betatron (envelope) function at the IP,
o
 is the beam size at IP:
*


*
*
x y 

3
Injector beams

50 ns spacing is used as operationally since the successful vacuum
condition for electron clouds (‘beam scrubbing’) in April.
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LHC performance in 2011 - PAF - La Londe
o

Progressive increase of the bunch number  June.
The 50 ns beam has the highest luminosity potential.
o
25 ns beam was anyhow not ready in the LHC ( later).
LHC beam parameters (SPS extraction)
Limit
Present param.
Spacing
N
 [mrad]
150 ns
1.1 x 1011
1.6
75 ns
1.2 x 1011
2.0
50 ns
1.6 x 1011
1.8
50 ns
1.2-1.35 x 1011
1.3-1.5
25 ns
1.2 x 1011
2.7
4
Limits on *
LHC performance in 2011 - PAF - La Londe

* is constrained by aperture and beam size in the triplet quadrupoles,
space is needed for:
 triplet 
o
the beam envelope (12 for
tertiary collimators TCT),
o
a 2 margin from TCT to triplet ,
o
the crossing angle –separation
of the beams at the parasitic
encounters.
TCT
Triplet
Triplet

 *
TCT
≥14 
12

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~ 7 mm
5
Limits on *

Interpolating aperture measurements performed at INJECTION in
2010/11, and taking into account the 2010 experience lead to select:
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LHC performance in 2011 - PAF - La Londe
* = 1.5 m for startup 2011

Refined aperture measurements at 3.5 TeV end of August paved the
way for smaller *:
* = 1 m
after August
technical stop
(commissioned in less
than a week)
6
From 2010 to 2011
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LHC performance in 2011 - PAF - La Londe
Main changes in 2011:
o
No change of the beam energy: 3.5 TeV.
o
Reduction of *  better understanding of the triplet aperture.
o
Faster ramp, faster squeeze.
o
50 ns bunch spacing.
Parameter
2010
2011
Nominal
N ( 1011 p/bunch)
1.2
1.35
1.15
k (no. bunches)
368
1380
2808
Bunch spacing
150
50
25
2.4-4
1.9-2.3
3.75
3.5
1.5  1
0.55
21032
3.31033
1034
 (mm rad)
* (m)
L (cm-2s-1)
7
Luminosity 2011
Peak luminosity
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LHC performance in 2011 - PAF - La Londe
3.3×1033
* 1 m
cm-2s-1
Reduce ,
increase N
1380 bunches
50 ns
increase k
LHCb luminosity limited
to ~3.5×1032 cm-2s-1 by
leveling (beams collide
with transverse offsets)
75 ns
8
Luminosity and energy
 The reach in * depends on beam size in the triplets where we have
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LHC performance in 2011 - PAF - La Londe
the aperture limits:
 triplet 

 *

*
min

1

(Approximate) luminosity scaling with energy
kN 2 f
kN 2 f 

2
L




*
*
4 x* *y 4  min
  min
Scaling the 3.5 TeV performance to 7 TeV yields
a luminosity of 1.2×1034 cm-2s-1 - design !
9
Luminosity 2011
Integrated proton luminosity 2011 now > 4 fb-1
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LHC performance in 2011 - PAF - La Londe
But we do not transform all our gain in peak luminosity into gain in
integrated luminosity – radiation effects on electronics ( later)
~0.4 fb-1/week
0.2 fb-1/week
10
Efficiency
 ‘Scheduled loss’ of days:
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LHC performance in 2011 - PAF - La Londe
o 18 days of technical stops,
o 10 days of scrubbing run,
o ~16 days of MD.
Lost ~ 11 days due to
cryogenics issues (frequently
knock on from another issue,
like electric network glitch)
between July and September
 Effective no. of days for physics : 201 – 44 = 157 days.
>> efficiency for stable beams : 29%
11
Fill length
 Luminosity lifetime is typically 16-25 hours.
 Optimum fill length ~12-15 hours.
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LHC performance in 2011 - PAF - La Londe
o but beams are frequently dumped before due to HW issues.
 Ideally we could produce up to ~ 120 pb-1/day ~ 800 pb-1/week
o our best is 520 pb-1.
Av. ~6 hours
12
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LHC performance in 2011 - PAF - La Londe
Projections
 ~3 weeks of p
operation left in 2011
(some days also for
tests !).
 Present production:
~0.4 fb-1 / week
> 5 fb-1
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LHC performance in 2011 - PAF - La Londe
Outline
High intensity issues
14
High intensity issues
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LHC performance in 2011 - PAF - La Londe
With 50 ns operation and with stored intensity of ~1.8x1014 protons
(1380 bunches) a number of issues related to high intensity have started
to surface:
o
Vacuum pressure increases,
o
Radiation induced failures of critical tunnel electronics,
o
Heating of the beam screen temperature,
o
Heating of injection kickers, collimators,
o
Losses due to (supposed) dust particles,
o
RF beam loading,
o
Beam instabilities leading to emittance blow-up.
Those effects have slowed down the pace of the intensity increase, and
affect the machine availability.
15
Vacuum - electron clouds
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LHC performance in 2011 - PAF - La Londe

Since LHC switched to trains, electron cloud are with us.
o
E-clouds induced pressure rise and beam instabilities ( large emittance).
o
Can be conditioned away… by e-clouds !
Bunch N+2 accelerates the e-,
more multiplication…
Bunch N+1 accelerates the e-,
multiplication at impact
Bunch N liberates an e-
e-
++++++
N+2
e-
++++++
N+1
++++++
N
e-

In April high intensity beams of 50 ns (up to 1080b) were used at injection to
condition the vacuum chamber over a ~10 day period.
o
Provided adequate conditions for operation (vacuum, beam stability) at 3.5
TeV, with gradual increase of intensity / number of bunches.
16
Vacuum cleaning with beam
 Pressure decrease (normalized to intensity) as a function of effective
beam time in April – very effective vacuum cleaning.
LHC performance in 2011 - PAF - La Londe
o Gain one order of magnitude/15 hours.
o Common vacuum chamber
regions around experiments
are most critical due to
the overlap of the beams.
J.M. Jimenez
 During intensity ramp up, additional (and progressive) cleaning
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occurred in the background.
17
Example of vacuum issue – IR2
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LHC performance in 2011 - PAF - La Londe

When ALICE insisted on flipping the polarity of the solenoid and spectrometer:
o Strong local pressure – prevented ALICE from switching ON for some time.
Still difficult 3 weeks after the change.
o ALICE had to be patient until vacuum was conditioned, plus install new
solenoids to reduce multi-pacting from the electrons (see below)
18
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LHC performance in 2011 - PAF - La Londe
Vacuum spikes

Vacuum conditions in IR2 and IR8 have been periodically so poor that they lead
to beam dumps from beam losses.

Causes of the vacuum spikes are not well understood, could be linked to gas at
the warm-cold transitions of the stand-alone cryostats, e-clouds…
o
This issue is coming and going… has also appeared around CMS recently.
Example of vacuum issue in IR8
10-6
10-7
19
Radiation induced problems
 With the increasing luminosity tunnel electronics starts to suffer from
SEE (Single Event Errors).
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LHC performance in 2011 - PAF - La Londe
o In particular the quench protection ad cryogenics systems.
Collisions points
Collimators
Loss rate
S
20
Mitigation of radiation effects
Radiation effect were expected to affect the LHC performance in
above1033 cm-2s-1 – we clearly hit this problem in 2011 !
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LHC performance in 2011 - PAF - La Londe
 As the peak luminosity increased, we have suffered from increasing
rates of radiation induced failures (SEE) leading to premature beam
dumps – dominated by ‘luminosity radiation’ in IR1 and IR5.
SEE failure are now the dominant cause of beam dumps !
 Some mitigation of radiation effects in 2011:
o Relocation of equipment away from the tunnel (cryo and interlock PLC)
during the technical stops.
o Improvement of codes (FPGAs, PLCs) to cope with errors and avoid reset
involving tunnel access (Quench protection system - QPS, cryogenics).
21
SEE failure rate evolution
M. Brugger
QPS
Patch
The Number to remember : ~1 SEE dump per 60 pb-1
22
R2E Situation
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LHC performance in 2011 - PAF - La Londe

2011 operation:
o Identified the most critical equipment – dominated by QPS, cryo.
o Mitigation through better firmware (QPS), special reset procedures (cryo),
signal filtering (RF) etc. SEEs rates from cryo have been reduced a lot.
o Presently the fill length is basically defined by R2E effects !

2011/12 Christmas Break (and Technical Stops):
o Relocation of most critical elements.
o Additional shielding of most critical areas (UJ’s in Pt1).
o It seems we cannot do much for QPS equipment located in the dispersion
suppressors areas – source of a large fraction of dumps – for LS1.

Next long-shutdown:
Relocation & Shielding for all critical areas.
23
UFO status
 Very fast beam loss events (~ millisecond) in super-conducting
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LHC performance in 2011 - PAF - La Londe
regions of the LHC were THE surprise of 2010 – nicknamed UFOs
(Unidentified Falling Object).
 Beam dumps triggered by UFO events:
o 18 beams dumps in 2010,
o 11 beam dumps in 2011, last beam dump mid-July 2011.
o All but one dump at 3.5 TeV.
 Things are ‘calming down’ at 3.5 TeV, but the situation is worrying
for future 7 TeV operation:
o Extrapolation to 7 TeV predicts ~ 100 dumps / year.
o Due to lower quench thresholds and larger deposited energy density.
24
UFO rate in 2011
5827 candidate UFOs in cell 12 or larger during
stable beams for fills longer than 1 hour.
Techical Stop
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LHC performance in 2011 - PAF - La Londe
(09. – 13.05.2011)
Techical Stop
(04. – 08.07.2011)
Techical Stop
(29.08. – 02.09.2011)
UFO Rate ~ slowly decreasing to ~ 3-4 / hour
Fill number
25
Dust particles
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LHC performance in 2011 - PAF - La Londe
BLM loss distribution for
3670 arc UFOs
 Most likely hypothesis for UFO: small ‘dust ‘ particle falling into the
beam (1-100 mm).
 UFO loss amplitude distribution is consisted with measured dust
particle distributions in the assembly halls…
26
UFO distribution in ring
3.5 TeV
LHC performance in 2011 - PAF - La Londe
3686 candidate UFOs.
Signal RS05 > 2∙10-4 Gy/s.
Red: Signal RS01 > 1∙10-2 Gy/s.
The UFOs are distributed around the
machine. About 7% of all UFOs are
around the injection kickers.
450 GeV
486 candidate UFOs.
Signal RS05 > 2∙10-4 Gy/s.
Mainly UFOs around injection kickers
(MKI)
.
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>> we are focusing on the understanding of UFO at the MKIs
27
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LHC performance in 2011 - PAF - La Londe
Studies of injection kicker UFOs
 Detailed FLUKA model
of the injection region to
reproduce UFO losses
and help localizing the
source(s).
 Spare MKI that was
removed from the LHC
last year will be opened
for dust analysis.
28
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LHC performance in 2011 - PAF - La Londe
Outline
Ions 2011
29
Ion beams
 The Pb ion beam details are still under discussion.
 Most likely scenario:
LHC performance in 2011 - PAF - La Londe
o Bunch spacing of 200 ns (nominal = 100 ns – also possible).
o 358 bunches (24 per injection), ~350 colliding pairs per experiment.
o Intensity ~1.2x108 ions / bunch.
o * = 1 m in ATLAS, CMS and ALICE (if aperture OK !).
o ATLAS & CMS standard crossing angles (120 mrad).
2010
2011
Spacing (ns)
500
200
Colliding pairs (ATLAS)
131
356
* (m)
3.5
1
3x1025
~3x1026
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Luminosity (cm-2s-1)
30
P-ion test
 Issue with mixed p-ion mode is the difference in revolution
frequencies at injection (4.5 kHz). At 3.5 TeV the frequencies can be
forced to be equal (differences very small, few 10’s Hz).
LHC performance in 2011 - PAF - La Londe
o The beams slip one wrt the other: locations of (parasitic) collisions move
longitudinally. Possible source of poor lifetime.
o At 3.5 TeV, the frequencies are locked together and the beams must be
cogged longitudinally to collide at the right place.
 An injection test of protons into ring 1 and Pb into ring 2 is foreseen
during the next MD (before the ion run).
 Then – depending on the ‘smoothness’ – tests of:
o Ramp and cogging at 3.5 TeV.
o Test of very low intensity collisions – but unlikely to provide stable beams
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conditions (time constraints).
31
Ion run schedule
 Success oriented planning given that the p-ion test is taking place
more or less at the same time.
o Duration of p-ion not well defined…
LHC performance in 2011 - PAF - La Londe
o We should be able to reach 50-80 mb-1 (2010 : 8 mb-1)
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p-ion test (tentative)
32
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LHC performance in 2011 - PAF - La Londe
Outline
Outlook for 2012
33
Lower beta*

* could be lowered further in 2012 if the collimators are set tighter
around the beams:
* ~ 0.7 m looks feasible > 30% higher luminosity….
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LHC performance in 2011 - PAF - La Londe

Tight settings were tested in 2011 (MDs) and operation should be
possible – but more delicate since a significant beam halo is then
touching the primary collimators.
Collimator family
TCP-IR7 (primary)
TCSG-IR7 (secondary)
TCLA-IR7 (absorbers)
TCT IP1/5 (tertiary)
TCSG-IR6 (secondary IR6)
TCDQ-IR6 (dump protection)
2011
5.7
8.5
17.7
11.8
9.3
9.8
Tight
4.0
6.0
8.0
9.3
7.0
7.5
Settings in sigma
( = 3.5 mm)
2.5 sigma
extra margin
in the triplet
for *
At some point it is only worth pushing the peak
luminosity if we can improve the situation of the SEEs.
Or fills will become shorter and shorter…
34
The diode story

Quench propagation tests have been performed during the technical
stops. Very positive outcome:
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LHC performance in 2011 - PAF - La Londe
o
Quenches did not reach the joint at 3.5 TeV. Operation at 3.5 TeV is
safer than had been estimated.
But there were some ‘worrying’ side results on the DIODEs:



Resistances of the diode leads are up to 15 times larger than measured
during cold tests in SM18, and seem to strongly increase with the current.
The observed spread in the resistance of 12 diodes leads is very large
(factor 20), much larger resistances are likely to be present in some of the
other 4000 diode leads of the machine.
The results are irreproducible, and correct simulation is presently not
possible due to the large number of unknowns.
35
The diode
‘Half moon’ contact
Diode is used as current
bypass in case of a quench
Lower diode
busbar
Main busbars
Upper diode busbar
(partially flexible)
Upper
heat sink
‘Half moon’ contact
towards diode
Lower
heat sink
Diode box,
Helium contents : 5 l
High current quench simulation
Comsol output for the final temperature after a 6 kA quench with Rc,moon=40 mW
(adiabatic conditions)
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LHC performance in 2011 - PAF - La Londe
A. Verweij
90 K
180 K
95 K
If the anomalous resistance is located at the half-moon
connection, there is a risk of melting down at 7 TeV
37
Diode studies
 Cold test in SM18 on several diodes (2011).
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LHC performance in 2011 - PAF - La Londe
 Warm test of diodes (ongoing).
 Tests in SM18 on magnet+diode (2012).
 Proposal to warm up a short section and remove a diode
from the machine.

The CSCM project for splice measurements (that is described
next) could probe the resistances of all the installed diodes.
38
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LHC performance in 2011 - PAF - La Londe
Energy reach and CSCM

Energy reach is so far limited to 3.5 TeV due to risk of bad (= high
resistance) splices in the busbar connections of the main circuits.

The CSCM project (Copper Stabilizer Continuity Measurements)
aims to develop a method for measuring the resistance of all busbar
connections with limited risk and to determine the maximum safe
energy sector by sector.
Conditions similar to a quench, but no stored energy in the magnets –
thermal run away can be stopped ‘easily’.
o Measurements are performed at 20 K when the magnets are not
superconducting – current bypasses magnets via the diodes.
o Requires modifications to power converters and protection systems.
o
39
CSCM current cycle
T=20 K (DT to be defined)
H. Thiesen TE/EPC
2011performance
H. Thiesen – 16 AugustLHC
– TE-TM
in 2011 - PAF - La Londe
03.10.2011
I
Trip by mQPS
tplateau
Iplateau
4-6 kA
Fast ramp down
if V>Vthr
Fast
dI/dt
500 A/s
Power
Abort
if V>Vthr
dV/dt to open
the diodes
500 A
t1
PC in voltage mode
60 s
t2
t
PC in current mode
40
2011performance
H. Thiesen – 16 AugustLHC
– TE-TM
in 2011 - PAF - La Londe
03.10.2011
CSCM measurements
 All
main circuits (RB, RQD, RQF)
 All
interconnection splices
 All
current lead-busbar connections at the DFB
 All
bypass diode paths
A. Siemko TE/MPE
41
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LHC performance in 2011 - PAF - La Londe
CSCM and energy status

Reviews of the CSCM will take place this week to analyze the risks
and readiness of the project.

Measurement of the entire ring will take ~2-3 months: a full campaign
would delay the startup with beam to May 2012.

For the moment the most likely scenario is a measurement of 1-2
sectors as tests in shutdown 2011-2012. The remaining sectors
would be measured after the end of the 2012 run.

The decision about the energy at the startup of 2012 may be taken
before the CSCM would provide data, either 3.5 or 4 TeV.
o
Overhead for beam setup of 4 TeV is negligible if we startup with that
energy. Some additional hardware commissioning needed (order of 1
week).
42
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LHC performance in 2011 - PAF - La Londe
25 ns beams

25 ns beam was injected in batches of 12 (nominal 278).

Severe electron cloud instability issues – 48 bunches are heavily
unstable.
o
Required beam scrubbing time ~ 5-10 x longer than for 50 ns.

This week a tests with collisions at 3.5 TeV of short trains (12 or 24
bunches) is foreseen.

Injector performance:
Bunch
spacing
N/bunch
Emittance
H&V [mm]
50
1.6 x 1011
2.0
50
1.3 x 1011
1.5
25
1.2 x 1011
2.7
43
50 versus 25

Assuming similar emittance blow-up in the LHC (twice as many
bunches with 25 ns !):
N/bunch
 @ 450 GeV
[mm]
 @ 3.5 TeV
[mm]
Relative
Luminosity
50
1.6 x 1011
2.0
2.6 ?
1.22
50
1.3 x 1011
1.5
2.1
1
25
1.2 x 1011
2.7
3.4 ?
1.05
LHC performance in 2011 - PAF - La Londe
Bunch
spacing

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
We expect 25 ns to be more difficult:
o
Larger emittance : injection.
o
Smaller spacing : e-cloud ( longer scrubbing run), vacuum.
o
Long range beam-beam (twice as many encounters).
o
Larger stored energy (UFO amplitude and rate?).
But it would of course half the no. events per crossing….
44
2012 run
 LHC
o
3 weeks for startup with beam (to first moderate intensity).
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LHC performance in 2011 - PAF - La Londe
 End
1
startup with beam : 7th March 2012.
of the run ~ mid-November.
month Pb-Pb or p-Pb run ?
 No
schedule available to date.
 Integrated
luminosity projection:
o
50 ns beam,
o
same performance of 0.5 fb-1 / week,
o
assuming 20 effective weeks of high intensity
>> ~ 10 fb-1 integrated L
or even more if we
increase L or efficiency !
45
Summary
 The
peak performance in 2011 exceeded our most optimistic
expectations – we are now in routine ~3x1033 cm-2s-1 regime.
of the beams is smooth, yet we have trouble to achieve long
fills and highest integrated performance.
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LHC performance in 2011 - PAF - La Londe
 Operation
o
Limited 0.4-0.5 fb-1 / week
 The
main issue are SEEs that now trigger the majority of the beam
dumps. The Quench Protection System is in the first line…
energy discussion for 2012 is open (3.5 or 4 TeV) – wait for
Chamonix Workshop in January.
 The
performance for 2012 is ~ 10 fb-1 assuming similar
performance in integrated luminosity.
 Extrapolated
o
The machine favors 50 ns over 25 ns, but a common request from the
experiments could change the balance towards 25 ns.
Thank you for your attention !
46
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LHC performance in 2011 - PAF - La Londe
Spares
47
* limits
 The focusing at the IP is defined by * which relates to the beam size 
 2 = * 
 * is limited by the aperture of the triplet quadrupoles around the collision
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48 performance in 2011 - PAF - La Londe
LHC
point and by the retraction margins between collimators.
Smaller size  at the IP implies:
 Larger divergence (phase space conservation !)
 Faster beam size growth in the space from IP to first quadrupole !
33 mm
 = 2.8 mm
* = 11 m
90 mm
1.5 m
Squeeze
Separation and crossing: example of ATLAS
Horizontal plane: the beams are combined and then separated
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LHC performance in 2011 - PAF - La Londe
194 mm
ATLAS IP
~ 260 m
Common vacuum chamber
Vertical plane: the beams are deflected to produce a crossing angle at the IP
to avoid undesired encounters in the region of the common vac. chamber.
a (mrad)
ATLAS
-120 / ver.
ALICE
80 / ver.
CMS
120 / hor
LHCb
-250 /hor
2011 !
~ 7 mm
a
Not to scale !
49
1380 bunches with 50 ns spacing
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LHC performance in 2011 - PAF - La Londe
Beam 1
Beam abort
gap
LHC circumference
50
Ghost bunches
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LHC performance in 2011 - PAF - La Londe


Parasitic bunches are usually present between the main bunches
(and essentially unavoidable), spaced by:
o
2.5 ns :
LHC RF system
o
5 ns :
SPS RF system
o
25 ns :
PS RF system
Amplitude ~ per-mill – could be used for main-parasitic collisions in
ALICE !
ghost bunches
36 bunch train
51
Blow up from e-clouds
Example of bunch by bunch transverse sizes with 804 bunches / beam
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LHC performance in 2011 - PAF - La Londe
With strong electron cloud activity…
… and after some time of vac. chamber scrubbing !
52
LHC 8:30 meeting
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LHC performance in 2011 - PAF - La Londe
Solenoids (around ATLAS) as cure for clouds…
Unfortunately solenoids only work in
field-free regions…
53
Quench Protection System SEU counter
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LHC performance in 2011 - PAF - La Londe
100 SEUs reached 20th September
11 beam dumps
54
Radiation Levels: Extrapolation
M. Calviani
HEH/cm2/year
UJ14
UJ16
RR17
UJ56
RR53
UJ76
RR73
RR77
UX85B
US85
2011 up to Aug11
1.1E+08
7.4E+07
3.9E+06
1.7E+07
5.2E+06
2.7E+06
4.8E+06
7.0E+06
1.3E+08
2.5E+07
2011
1.7E+08
1.1E+08
6.0E+06
2.8E+07
8.4E+06
4.4E+06
7.7E+06
1.1E+07
1.8E+08
3.4E+07
2012
4.2E+08
2.9E+08
1.5E+07
6.9E+07
2.1E+07
4.4E+06
7.7E+06
1.1E+07
1.8E+08
3.4E+07
nominal
3.2E+09
Shielding
2.1E+09
1.1E+08
5.2E+08
1.6E+08
Shielding
4.4E+07
already
7.7E+07 in Place!
1.1E+08
5.3E+08
1.0E+08
 2011: assuming 4fb-1 for ATLAS/CMS, 1fb-1 for LHCb and 1*1015 p loss/beam/P7
 2012 : assuming 10fb-1 for ATLAS/CMS, 1 fb-1 for LHCb and 1*1015 p loss/beam/P7
 Nominal: assuming 50fb-1 +1.5x en. scal. (ATLAS/CMS), 2 fb-1 + 1.5x en. scal. (LHCb) and
1*1016 p loss/beam/P7
 NB: missing effect of an eventual beam-gas induced radiation increase!
55
Joint quality

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.
03.10.2011
LHC performance in 2011 - PAF - La Londe

A copper bus bar with reduced continuity coupled to a superconducting
cable badly soldered to the stabilizer can lead to a serious incident.
Solder
No solder
wedge
bus
U-profile
bus
X-ray of joint

During repair work in the damaged
sector, inspection of the joints revealed
systematic voids caused by the welding
procedure.
56
LHC performance in 2011 - PAF - La Londe
LHC Dipoles & Beam Screens
Beam screen as seen
by the beam
03.10.2011
Slots (3% surface coverage)
Beam Screen cooling pipes
Cold Bore (2K)
57
Beam screen temperatures
beam screen (BS) shields the magnet cold bore from the beams.
 Gases are trapped on the cold bore (colder than BS).
 In the presence of beam, the beam screen maybe be heated by
03.10.2011
LHC performance in 2011 - PAF - La Londe
 The
vacuum pressure increase / electron clouds,
o RF heating from EM fields.
o
 In
some cases (triplets) out-gassing from
the BS has been observed – very careful T
control during technical (or cryo) stops.
 Part of the effect could be correlated to
(too) short bunches at 3.5.
o Increased bunch length blow-up.
Beam screen as
seen by the beam
Cold Bore (2K)
58
IT beam-screen temperatures
 Triplet
BS temperature tricky to stabilise at injection and in the ramp.
Fine tuning/manual intervention by cryo operators.
o Effect no fully understood.
LHC performance in 2011 - PAF - La Londe
03.10.2011
o
Injection
/ramp
3.5 TeV stable
beams
dump
BS T (K)
25 K
17 K
Courtesy S. Claudet
59