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LHC status and perspectives

J. Wenninger

CERN

Beams Department

Operation group / LHC

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LuminosityL

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Recall the formula for the luminosity (head-on collision):

FfkN

FfkN

Lyx

*

2

**

2

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*

** yx

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:

(Round beams)

Injector beams0

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50 ns spacing is used as operationally since the successful vacuum

condition for electron clouds (‘beam scrubbing’) in April.

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).

Spacing N [rad]

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

Limit

Present param.

LHC beam parameters (SPS extraction)

Limits on *0

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* is constrained by aperture and beam size in the triplet quadrupoles,

space is needed for:

*

tripleto the beam envelope (12for

tertiary collimators TCT),

o a 2 margin from TCT to triplet ,

o the crossing angle –separation

of the beams at the parasitic

encounters.

~ 7 mm

Triplet TripletTCT TCT

≥14 12

Limits on *0

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Interpolating aperture measurements performed at INJECTION in

2010/11, and taking into account the 2010 experience lead to select:

Refined aperture measurements at 3.5 TeV end of August paved the

way for smaller *:

* = 1.5 m for startup 2011

* = 1 m after August

technical stop

(commissioned in less than a week)

From 2010 to 20110

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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

(m rad) 2.4-4 1.9-2.3 3.75

* (m) 3.5 1.5 1 0.55

L (cm-2s-1) 21032 3.31033 1034

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.

Luminosity 20110

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LHCb luminosity limited to ~3.5×1032 cm-2s-1 by leveling (beams collide with transverse offsets)

Peak luminosity

75 ns

50 nsincrease k

Reduce ,increase N

* 1 m3.3×1033 cm-2s-1

1380 bunches

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2*min

*min

2

**

2

44

fkNfkNL

yx

The reach in * depends on beam size in the triplets where we have the aperture limits:

*

triplet

1*min

(Approximate) luminosity scaling with energy

Scaling the 3.5 TeV performance to 7 TeV yields a luminosity of 1.2×1034 cm-2s-1 - design !

Luminosity 20110

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Integrated proton luminosity 2011 now > 4 fb-1 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

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‘Scheduled loss’ of days:

o 18 days of technical stops,

o 10 days of scrubbing run,

o ~16 days of MD.

Effective no. of days for physics : 201 – 44 = 157 days.

>> efficiency for stable beams : 29%

Lost ~ 11 days due to cryogenics issues (frequently knock on from another issue,

like electric network glitch) between July and September

Fill length0

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Luminosity lifetime is typically 16-25 hours.

Optimum fill length ~12-15 hours.

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

Projections0

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~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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High intensity issues0

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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.

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N

e-

N+1

e-

N+2

e-

Bunch N liberates an e-Bunch N+1 accelerates the e-,

multiplication at impactBunch N+2 accelerates the e-,

more multiplication…

++++++++++++++++++

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 !

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.

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Pressure decrease (normalized to intensity) as a function of effective beam time in April – very effective vacuum cleaning.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.

During intensity ramp up, additional (and progressive) cleaning occurred in the background.

J.M. Jimenez

Example of vacuum issue – IR20

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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)

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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

10-7

10-6

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Loss rate

Collisions points

Collimators

With the increasing luminosity tunnel electronics starts to suffer from SEE (Single Event Errors).

o In particular the quench protection ad cryogenics systems.

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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).

Radiation effect were expected to affect the LHC performance in above1033 cm-2s-1 – we clearly hit this problem in 2011 !

SEE failure rate evolution

22

QPSPatch

Slope: ~1 SEE dump per 60 pb-1

M. Brugger

The Number to remember : ~1 SEE dump per 60 pb-1

R2E Situation

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.

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Very fast beam loss events (~ millisecond) in super-conducting 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.

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UFO rate in 2011L

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UFO Rate ~ slowly decreasing to ~ 3-4 / hourFill number

5827 candidate UFOs in cell 12 or larger during stable beams for fills longer than 1 hour.

Techical StopTechical Stop(09. – 13.05.2011)

Techical Stop Techical Stop (04. – 08.07.2011)

Techical Stop Techical Stop (29.08. – 02.09.2011)

Dust particles

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BLM loss distribution for 3670 arc UFOs

  

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Most likely hypothesis for UFO: small ‘dust ‘ particle falling into the beam (1-100 m).

UFO loss amplitude distribution is consisted with measured dust particle distributions in the assembly halls…

UFO distribution in ring0

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The UFOs are distributed around the machine. About 7% of all UFOs are

around the injection kickers..

Mainly UFOs around injection kickers (MKI)

3.5 TeV3.5 TeV3686 candidate UFOs.

Signal RS05 > 2∙10-4 Gy/s.Red: Signal RS01 > 1∙10-2 Gy/s.

450 GeV450 GeV486 candidate UFOs.

Signal RS05 > 2∙10-4 Gy/s.

>> we are focusing on the understanding of UFO at the MKIs

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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.

Outline0

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Ion beams0

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The Pb ion beam details are still under discussion.

Most likely scenario:

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 rad).

2010 2011

Spacing (ns) 500 200

Colliding pairs (ATLAS) 131 356

* (m) 3.5 1

Luminosity (cm-2s-1) 3x1025 ~3x1026

P-ion test0

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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).

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 conditions (time constraints).

Ion run schedule0

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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…

o We should be able to reach 50-80 b-1 (2010 : 8 b-1)

p-ion test (tentative)

Outline0

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Lower beta*0

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* could be lowered further in 2012 if the collimators are set tighter around the beams:

* ~ 0.7 m looks feasible > 30% higher luminosity…. 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.

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…

Collimator family 2011 TightTCP-IR7 (primary) 5.7 4.0TCSG-IR7 (secondary) 8.5 6.0TCLA-IR7 (absorbers) 17.7 8.0TCT IP1/5 (tertiary) 11.8 9.3TCSG-IR6 (secondary IR6) 9.3 7.0TCDQ-IR6 (dump protection) 9.8 7.5

2.5 sigma extra margin in the triplet

for *

Settings in sigma ( = 3.5 m)

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Quench propagation tests have been performed during the technical stops. Very positive outcome:

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.

Lower heat sink

Upper heat sink

The diode

Lower diode busbar

Diode box, Helium contents : 5 l

Upper diode busbar(partially flexible)

‘Half moon’ contact

towards diode

‘Half moon’ contact

Main busbars

Diode is used as current bypass in case of a quench

Comsol output for the final temperature after a 6 kA quench with Rc,moon=40 (adiabatic conditions)

95 K

90 K

180 K

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If the anomalous resistance is located at the half-moon connection, there is a risk of melting down at 7 TeV

A. Verweij

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Cold test in SM18 on several diodes (2011). 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.

Energy reach and CSCM0

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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.o 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.

tplateau

dI/dt

Iplateau

I

t

Fast

Power

Abort

if V>Vthr

dV/dt to open

the diodes

Fast ramp down

if V>Vthr

t1 t2

T=20 K (DT to be defined)

500 A

4-6 kA

60 s

PC in voltage mode PC in current mode

Trip by mQPS

500 A/s

H.

Th

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6 A

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H. Thiesen TE/EPC

CSCM measurements All main circuits (RB, RQD, RQF)

All interconnection splices

All current lead-busbar connections at the DFB

All bypass diode paths

H.

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A. Siemko TE/MPE

CSCM and energy status0

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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).

25 ns beams0

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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/bunchEmittance H&V [m]

50 1.6 x 1011 2.0

50 1.3 x 1011 1.5

25 1.2 x 1011 2.7

50 versus 250

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Bunch spacing

N/bunch @ 450 GeV

[m] @ 3.5 TeV

[m]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

Assuming similar emittance blow-up in the LHC (twice as many

bunches with 25 ns !):

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….

2012 run0

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LHC startup with beam : 7th March 2012.o 3 weeks for startup with beam (to first moderate intensity).

End of the run ~ mid-November.

1 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 !

Summary0

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The peak performance in 2011 exceeded our most optimistic expectations – we are now in routine ~3x1033 cm-2s-1 regime.

Operation of the beams is smooth, yet we have trouble to achieve long fills and highest integrated performance.

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…

The energy discussion for 2012 is open (3.5 or 4 TeV) – wait for Chamonix Workshop in January.

Extrapolated performance for 2012 is ~ 10 fb-1 assuming similar performance in integrated luminosity.

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 !

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Spares

* limits4

8

The focusing at the IP is defined by * which relates to the beam size

* is limited by the aperture of the triplet quadrupoles around the collision 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 !

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2 =

* = 11 m 1.5 mSqueeze

= 2.8 m

90 m

33 m

194 mm ATLAS IP

~ 260 m

Common vacuum chamber

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Horizontal plane: the beams are combined and then separated

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.

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~ 7 mm

Not to scale !

(rad)

ATLAS -120 / ver.

ALICE 80 / ver.

CMS 120 / hor

LHCb -250 /hor

2011 !

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Beam 1

LHC circumference

Beam abort gap

1380 bunches with 50 ns spacing

Ghost bunches0

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36 bunch train

ghost bunches

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 !

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52LHC 8:30 meeting

With strong electron cloud activity…

… and after some time of vac. chamber scrubbing !

Example of bunch by bunch transverse sizes with 804 bunches / beam

Solenoids (around ATLAS) as cure for clouds…0

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Unfortunately solenoids only work in field-free regions…

100 SEUs reached 20th September

Quench Protection System SEU counter

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11 beam dumps

Radiation Levels: Extrapolation

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HEH/cm2/year 2011 up to Aug11 2011 2012 nominalUJ14 1.1E+08 1.7E+08 4.2E+08 3.2E+09UJ16 7.4E+07 1.1E+08 2.9E+08 2.1E+09RR17 3.9E+06 6.0E+06 1.5E+07 1.1E+08UJ56 1.7E+07 2.8E+07 6.9E+07 5.2E+08RR53 5.2E+06 8.4E+06 2.1E+07 1.6E+08UJ76 2.7E+06 4.4E+06 4.4E+06 4.4E+07RR73 4.8E+06 7.7E+06 7.7E+06 7.7E+07RR77 7.0E+06 1.1E+07 1.1E+07 1.1E+08UX85B 1.3E+08 1.8E+08 1.8E+08 5.3E+08US85 2.5E+07 3.4E+07 3.4E+07 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!

M. Calviani

Shieldingalready

in Place!

Shielding

Joint quality0

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bus U-profile bus

wedge

Solder No solder

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. A copper bus bar with reduced continuity coupled to a superconducting cable

badly soldered to the stabilizer can lead to a serious incident.

During repair work in the damaged sector, inspection of the joints revealed systematic voids caused by the welding procedure.

X-ray of joint

LHC Dipoles & Beam Screens

Cold Bore (2K)Beam Screen cooling pipes

Slots (3% surface coverage)

Beam screen as seen by the beam

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The 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

o vacuum pressure increase / electron clouds,o RF heating from EM fields. Beam screen as

seen by the beam

Cold Bore (2K)

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.

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IT beam-screen temperatures

17 K

25 K

Courtesy S. Claudet

BS T (K)

Injection/ramp

3.5 TeV stable beams dump

Triplet BS temperature tricky to stabilise at injection and in the ramp.o Fine tuning/manual intervention by cryo operators.o Effect no fully understood.

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