LHC status and perspectives

1
LHC status and perspectives
J. Wenninger CERN Beams Department Operation
group / LHC
2
Outline
Proton operation 2011 High intensity issues Ions
2011 Outlook for 2012
3
Luminosity
Recall the formula for the luminosity (head-on
collision)
(Round beams)

• f is the revolution frequency (11.25 kHz), g
  E/m,
• k is the number of colliding bunch pairs,
• N is the bunch population,
• F is a geometric (loss) factor ( 1) from the
  crossing angle, F ? 0.95 in 2011,
• e is the normalized emittance, b the betatron
  (envelope) function at the IP,
• s is the beam size at IP

4
Injector beams

• 50 ns spacing is used as operationally since the
  successful vacuum condition for electron clouds
  (beam scrubbing) in April.
• Progressive increase of the bunch number ? June.
• The 50 ns beam has the highest luminosity
  potential.
• 25 ns beam was anyhow not ready in the LHC (?
  later).

LHC beam parameters (SPS extraction)
Spacing N e 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
Limit
Present param.
5
Limits on b

• b is constrained by aperture and beam size in
  the triplet quadrupoles, space is needed for

• the beam envelope (12s for tertiary collimators
  TCT),
• a 2s margin from TCT to triplet ,
• the crossing angle separation of the beams at
  the parasitic encounters.

TCT
Triplet
Triplet
TCT
14 s
12 s
6
Limits on b

• 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 b

b 1.5 m for startup 2011
b 1 m after August technical stop
(commissioned in less than a week)
7
From 2010 to 2011

• Main changes in 2011
• No change of the beam energy 3.5 TeV.
• Reduction of b ? better understanding of the
  triplet aperture.
• Faster ramp, faster squeeze.
• 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
e (mm rad) 2.4-4 1.9-2.3 3.75
b (m) 3.5 1.5 ? 1 0.55
L (cm-2s-1) 2?1032 3.3?1033 1034
8
Luminosity 2011
Peak luminosity
b 1 m
3.31033 cm-2s-1 1380 bunches
Reduce e, increase N
50 ns increase k
LHCb luminosity limited to 3.51032 cm-2s-1 by
leveling (beams collide with transverse offsets)
75 ns
9
Luminosity and energy

• The reach in b depends on beam size in the
  triplets where we have the aperture limits

(Approximate) luminosity scaling with energy
Scaling the 3.5 TeV performance to 7 TeV yields
a luminosity of 1.21034 cm-2s-1 - design !
10
Luminosity 2011
Integrated proton luminosity 2011 now gt 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
11
Efficiency

• Scheduled loss of days
• 18 days of technical stops,
• 10 days of scrubbing run,
• 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.
• gtgt efficiency for stable beams 29

12
Fill length

• Luminosity lifetime is typically 16-25 hours.
• Optimum fill length 12-15 hours.
• but beams are frequently dumped before due to HW
  issues.
• Ideally we could produce up to 120 pb-1/day
  800 pb-1/week
• our best is 520 pb-1.

Av. 6 hours
13
Projections

• 3 weeks of p operation left in 2011 (some days
  also for tests !).
• Present production
• 0.4 fb-1 / week
• gt 5 fb-1

14
Outline
Proton operation 2011 High intensity issues Ions
2011 Outlook for 2012
15
High intensity issues

• 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
• Vacuum pressure increases,
• Radiation induced failures of critical tunnel
  electronics,
• Heating of the beam screen temperature,
• Heating of injection kickers, collimators,
• Losses due to (supposed) dust particles,
• RF beam loading,
• Beam instabilities leading to emittance blow-up.
• Those effects have slowed down the pace of the
  intensity increase, and affect the machine
  availability.

16
Vacuum - electron clouds

• Since LHC switched to trains, electron cloud are
  with us.
• E-clouds induced pressure rise and beam
  instabilities (? large emittance).
• 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.
• Provided adequate conditions for operation
  (vacuum, beam stability) at 3.5 TeV, with gradual
  increase of intensity / number of bunches.

17
Vacuum cleaning with beam

• Pressure decrease (normalized to intensity) as a
  function of effective beam time in April very
  effective vacuum cleaning.
• Gain one order of magnitude/15 hours.
• 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
18
Example of vacuum issue IR2

• When ALICE insisted on flipping the polarity of
  the solenoid and spectrometer
• Strong local pressure prevented ALICE from
  switching ON for some time. Still difficult 3
  weeks after the change.
• ALICE had to be patient until vacuum was
  conditioned, plus install new solenoids to reduce
  multi-pacting from the electrons (see below)

19
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
• This issue is coming and going has also appeared
  around CMS recently.

10-6
Example of vacuum issue in IR8
10-7
20
Radiation induced problems

• With the increasing luminosity tunnel electronics
  starts to suffer from SEE (Single Event Errors).
• In particular the quench protection ad cryogenics
  systems.

Collisions points
Collimators
Loss rate
S
21
Mitigation of radiation effects

• 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
• Relocation of equipment away from the tunnel
  (cryo and interlock PLC) during the technical
  stops.
• 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 !
22
SEE failure rate evolution
M. Brugger
QPSPatch
Slope 1 SEE dump per 60 pb-1
The Number to remember 1 SEE dump per 60 pb-1
23
R2E Situation

• 2011 operation
• Identified the most critical equipment
  dominated by QPS, cryo.
• Mitigation through better firmware (QPS), special
  reset procedures (cryo), signal filtering (RF)
  etc. SEEs rates from cryo have been reduced a
  lot.
• Presently the fill length is basically defined by
  R2E effects !
• 2011/12 Christmas Break (and Technical Stops)
• Relocation of most critical elements.
• Additional shielding of most critical areas (UJs
  in Pt1).
• 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.

24
UFO status

• 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
• 18 beams dumps in 2010,
• 11 beam dumps in 2011, last beam dump mid-July
  2011.
• 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
• Extrapolation to 7 TeV predicts 100 dumps /
  year.
• Due to lower quench thresholds and larger
  deposited energy density.

25
UFO rate in 2011
5827 candidate UFOs in cell 12 or larger during
stable beams for fills longer than 1 hour.
Techical Stop (09. 13.05.2011)
Techical Stop (04. 08.07.2011)
Fill number
UFO Rate slowly decreasing to 3-4 / hour
26
Dust particles
 
 

• 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

27
UFO distribution in ring
3.5 TeV3686 candidate UFOs. Signal RS05 gt 210-4
Gy/s. Red Signal RS01 gt 110-2 Gy/s.
450 GeV486 candidate UFOs. Signal RS05 gt 210-4
Gy/s.
The UFOs are distributed around the machine.
About 7 of all UFOs are around the injection
kickers. .
Mainly UFOs around injection kickers (MKI)
gtgt we are focusing on the understanding of UFO at
the MKIs
28
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.

29
Outline
Proton operation 2011 High intensity issues Ions
2011 Outlook for 2012
30
Ion beams

• The Pb ion beam details are still under
  discussion.
• Most likely scenario
• Bunch spacing of 200 ns (nominal 100 ns also
  possible).
• 358 bunches (24 per injection), 350 colliding
  pairs per experiment.
• Intensity 1.2x108 ions / bunch.
• b 1 m in ATLAS, CMS and ALICE (if aperture OK
  !).
• ATLAS CMS standard crossing angles (120 mrad).

2010 2011
Spacing (ns) 500 200
Colliding pairs (ATLAS) 131 356
b (m) 3.5 1
Luminosity (cm-2s-1) 3x1025 3x1026
31
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 10s Hz).
• The beams slip one wrt the other locations of
  (parasitic) collisions move longitudinally.
  Possible source of poor lifetime.
• 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
• Ramp and cogging at 3.5 TeV.
• Test of very low intensity collisions but
  unlikely to provide stable beams conditions (time
  constraints).

32
Ion run schedule

• Success oriented planning given that the p-ion
  test is taking place more or less at the same
  time.
• Duration of p-ion not well defined
• We should be able to reach 50-80 mb-1 (2010 8
  mb-1)

p-ion test (tentative)
33
Outline
Proton operation 2011 High intensity issues Ions
2011 Outlook for 2012
34
Lower beta

• b could be lowered further in 2012 if the
  collimators are set tighter around the beams
• b 0.7 m looks feasible gt 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.

Settings in sigma (e 3.5 mm)
Collimator family 2011 Tight
TCP-IR7 (primary) 5.7 4.0
TCSG-IR7 (secondary) 8.5 6.0
TCLA-IR7 (absorbers) 17.7 8.0
TCT IP1/5 (tertiary) 11.8 9.3
TCSG-IR6 (secondary IR6) 9.3 7.0
TCDQ-IR6 (dump protection) 9.8 7.5
2.5 sigma extra margin in the triplet for b
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
35
The diode story

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

36
The diode
Diode is used as current bypass in case of a
quench
Half moon contact
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
37
High current quench simulation
Comsol output for the final temperature after a 6
kA quench with Rc,moon40 mW (adiabatic
conditions)
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
38
Diode studies

• Cold test in SM18 on several diodes (2011).
• Warm test of diodes (ongoing).
• Tests in SM18 on magnetdiode (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.

39
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.
• Measurements are performed at 20 K when the
  magnets are not superconducting current
  bypasses magnets via the diodes.
• Requires modifications to power converters and
  protection systems.

40
CSCM current cycle
T20 K (DT to be defined)
H. Thiesen TE/EPC
I
Trip by mQPS
tplateau
Iplateau
4-6 kA
Fast ramp down if VgtVthr
Fast Power Abort if VgtVthr
dI/dt
500 A/s
dV/dt to open the diodes
500 A
H. Thiesen 16 August 2011 TE-TM
t
t1
t2
60 s
PC in voltage mode
PC in current mode
41
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
H. Thiesen 16 August 2011 TE-TM
42
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.
• Overhead for beam setup of 4 TeV is negligible if
  we startup with that energy. Some additional
  hardware commissioning needed (order of 1 week).

43
25 ns beams

• 25 ns beam was injected in batches of 12 (nominal
  278).
• Severe electron cloud instability issues 48
  bunches are heavily unstable.
• 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 HV mm
50 1.6 x 1011 2.0
50 1.3 x 1011 1.5
25 1.2 x 1011 2.7
44
50 versus 25

• Assuming similar emittance blow-up in the LHC
  (twice as many bunches with 25 ns !)

Bunch spacing N/bunch e _at_ 450 GeV mm e _at_ 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

• We expect 25 ns to be more difficult
• Larger emittance injection.
• Smaller spacing e-cloud (? longer scrubbing
  run), vacuum.
• Long range beam-beam (twice as many encounters).
• Larger stored energy (UFO amplitude and rate?).
• But it would of course half the no. events per
  crossing.

45
2012 run

• LHC startup with beam 7th March 2012.
• 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
• 50 ns beam,
• same performance of 0.5 fb-1 / week,
• assuming 20 effective weeks of high intensity
• gtgt 10 fb-1 integrated L

or even more if we increase L or efficiency !
46
Summary

• 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.
• 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.
• 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 !

47
Spares
48
b limits

• The focusing at the IP is defined by b which
  relates to the beam size s
• b is limited by the aperture of the triplet
  quadrupoles around the collision point and by the
  retraction margins between collimators.

s2 b e
Smaller size s at the IP implies ? Larger
divergence (phase space conservation !) ?
Faster beam size growth in the space from IP to
first quadrupole !
33 mm
e 2.8 mm
b 11 m
1.5 m
Squeeze
90 mm
49
Separation and crossing example of ATLAS
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.
a (mrad)
ATLAS -120 / ver.
ALICE 80 / ver.
CMS 120 / hor
LHCb -250 /hor
a
2011 !
50
1380 bunches with 50 ns spacing
Beam 1
Beam abort gap
LHC circumference
51
Ghost bunches

• Parasitic bunches are usually present between the
  main bunches (and essentially unavoidable),
  spaced by
• 2.5 ns LHC RF system
• 5 ns SPS RF system
• 25 ns PS RF system
• Amplitude per-mill could be used for
  main-parasitic collisions in ALICE !

52
Blow up from e-clouds
Example of bunch by bunch transverse sizes with
804 bunches / beam
With strong electron cloud activity
and after some time of vac. chamber scrubbing !
LHC 830 meeting
53
Solenoids (around ATLAS) as cure for clouds
Unfortunately solenoids only work in field-free
regions
54
Quench Protection System SEU counter
100 SEUs reached 20th September
11 beam dumps
55
Radiation Levels Extrapolation
M. Calviani
Shielding
Shieldingalreadyin Place!

• 2011 assuming 4fb-1 for ATLAS/CMS, 1fb-1 for
  LHCb and 11015 p loss/beam/P7
• 2012 assuming 10fb-1 for ATLAS/CMS, 1 fb-1 for
  LHCb and 11015 p loss/beam/P7
• Nominal assuming 50fb-1 1.5x en. scal.
  (ATLAS/CMS), 2 fb-1 1.5x en. scal. (LHCb) and
  11016 p loss/beam/P7
• NB missing effect of an eventual beam-gas
  induced radiation increase!

56
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.
• A copper bus bar with reduced continuity coupled
  to a superconducting cable badly soldered to the
  stabilizer can lead to a serious incident.

X-ray of joint

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

57
LHC Dipoles Beam Screens
Beam screen as seen by the beam
Slots (3 surface coverage)
Beam Screen cooling pipes
Cold Bore (2K)
58
Beam screen temperatures

• 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
• vacuum pressure increase / electron clouds,
• RF heating from EM fields.

• 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.
• Increased bunch length blow-up.

59
IT beam-screen temperatures

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

Injection /ramp
3.5 TeV stable beams
dump
BS T (K)
25 K
17 K
Courtesy S. Claudet