The Large Hadron Collider

LHC Operation II: pushing the limitsUFOsSEUse-cloud25 ns operationBeam-beam effects

[R. Alemany][CERN BE/OP]

[Engineer In Charge of LHC]Lectures at NIKHEF (21.03.2013)

High intensity beam issues

Pushing the limits does not come for free.

The 2011 and 2012 operation encountered issues related to the increased storage intensity >2 x 1014 p+:

• Vacuum pressure increase• Heating of machine elements (BSRT mirrors,

injection kickers, collimators) by the beam induced high order modes

• Losses due to dust particles falling into the beam (UFO)

• Single Event Upsets (SEU)• Beam losses due to tails• Beam instabilities blowing up the beamsThe last two issues dumped 35 physics fills in 2012

UFOs (Unidentified Flying Objects)

• UFO rate = f(beam intensity) up to few hundred of nominal (1.5· 1011p) bunches

• UFO rate ~ kte for higher beam intensities

• UFOs can give losses over the dump threshold and dump the beams.

Unforeseen sudden losses appearing around the ring on

the ms time scale interaction of macro-particles, of sizes estimated to be 1-100 µm, with the proton beams.

UFOs in LHC during 2011 & 2012

Tobias Baer, Evian Workshop December, 19th 2012

• The UFO rate (h-1) decreases with operation time cleaning effect

• 2011 ~ 10 UFO/h 2 UFO/h• After a major machine intervention, like the Xmas shut

downs, the UFO rate increases• In 2012 initially about 2.5xUFO rate of 2011

• 25 ns scrubbing (2012) showed 10xUFO rate of 2012

UFOs: extrapolation to 7 TeV

Additionally (not considered): UFOs around IRs until cell 11, at collimators/movable devices and Ufinos in experiments.

Tobias Baer, Evian Workshop December, 19th 2012

21

91

Single Event Upset (SEU)SEU is caused by a very high energy deposition in a small

volume of the electronics chip data is lost or device destroyed

Radiation sources at LHC: 20 MeV hadrons and thermal neutrons

0.00E+00

5.00E+07

1.00E+08

1.50E+08

2.00E+08

2.50E+08

3.00E+08

0.00E+00

1.00E+03

2.00E+03

3.00E+03

4.00E+03

5.00E+03

6.00E+03Point 1 - cumulativeUJ14UJ16

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M. Calviani R2E Review 21 Nov 2011

Coming from:IP1,5&8 luminosityIR3&7 collimator lossesDS leakage from luminosity, collimator losses and beam gasARC beam gas

Single Event Upset (SEU)• 2011 237 events detected. 22% of STABLE BEAMS

were dumped by SEU. Cryogenics and Quench Protection Systems most affected.

EquipmentDump 2012 >LS1 Expectations

QPS 31 5Power Converter 14 3

Cryo 4 1EN/EL 1 0

Vacuum 4 2Collimation 1 0

Other 5

10-20 Expected

Dumps

50

Crucial mitigation measurements: equipment relocation outside high radiation areas, use radiation hard electronics, shielding

2012

e-cloudvacuum chamber wall

p+ bunch

e-

e-cloud @injection energy ionization of gas molecules by p+

e-cloud @higher energy photoelectrons from synchrotron light (44 eV photons = critical energy for

photoemission yield from cooper(beam screen))

e-cloud mechanism at injection

e- from ionization:• ~eV slow motion still inside the beam screen when the next proton

bunch passes• accelerated to ~100 – 1000 eV by the Coulomb field of the next bunch• before arrival of the next bunch, strike the wall, yielding one or more

secondary electrons.

e-cloud mechanism at injection

If δ >1 re-generative process and the ambient electron density will grow exponentially.

Beam screen (copper) δ ~1.1 to ~1.7

secondary electron yield δ=emitted e-/incident e-

e-cloud mechanism at injection

Re-generative process

e-cloud: effects on collider operation

Transverse mode-coupling instability (TMCI), coupled-bunch instabilities, head-tail motion within the proton bunch, tune spread, beam loss and incoherent emittance growth• Beam unstable right

after the injection (beams dumped due to losses)

• Probably triggered by e-cloud in the main dipoles

• Observed vertical motion in the trailing bunches

• Beam stable with high chromaticity settings Q’=15 (while normally 2) Courtesy of W. Hofle, D. Valuch

Injection tests with 48bunches trains (26.08.2011)

e-cloud: effects on collider operatione-cloud desorbs gases from the walls of the beam screen

• Pour beam lifetime• Important emittance growth• Preassure bumps instabilities

G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

e-cloud: effects on collider operationEnergetic electrons heat the surfaces that they impact heat load could exceed installed refrigeration capacity for 25 ns bunch spacing.

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G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

e-cloud: scrubbing runs

2011

from 2.2 down to 1.52

B1 2100b

B2 1020b

1. Inject a high current beam to induce e-cloud many gas molecules trapped inside the beam pipe metal released.

2. Then pump

G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

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e-cloud: scrubbing runs

2012

from 1.55 down to 1.45

B1 2748b

B2 2748b

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G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

e-cloud observations at 4 TeV•After Scrubbing Run machine studies with 25ns beams at 4TeV were possible. Main observations:• The heat-load strongly increases during the ramp since the EC

is enhanced by the photoelectrons due to synchrotron radiation This violent transient on the heat load in the arcs limits the number of bunches which can be accelerated

• Despite the larger number of electrons, at high energy the beam becomes less affected by EC the beam quality achievable at collisions is determined by the EC effects at 450GeV

25 ns operation (from 2015)• 25 ns operation is a request from the

experiments less pile-up less computational resources needed cleaner event reconstruction

• But it is a challenge for the machine. First suspect?

e-cloud

Beam-beam interactions at LHC• Two counter rotating beams made of a large

number of p+ bunches interact at the IPs.• When these two density of charge particles come

close together electromagnetic interaction beam-beam interaction• Head-on (HO) unavoidable if we want to do

physics• Long-range (LR) pseudo-unavoidable we

need a crossing angle to avoid more than one HO

Crossing angle

Courtesy of M. Schaumann

Beam-beam interactions at LHC• Each beam represents an electromagnetic potential to the

other beam:• Acts like a non-linear electromagnetic lens at the location of

the interaction (adding additional very non-linear multipoles in the IP)

• Localized, periodic beam force

LR: non-linear force amplitud dependent tune shift

Courtesy of W. Herr

HO: linear force quadrupole likeamplitud independent tune shift

Courtesy of W. Herr

Long range interactions tune spread

• Number of LR interaction depends on spacing and length of common part• In LHC 15 LR interactions (for 25 ns) on each side of the IP 4x2x15 =120!• Effects depend on separation

(for large enough d)

1. Large effects for largest amplitudes where non-linearities are strong

2. The size of the effect depends on d for small d problems

3. The tune spread is very asymmetric since all the non-linear part of the beam-beam force curve is scanned.

Courtesy of W. Herr

Long-range interactions closed orbit effects

For d >> σ Maclaurin series

Amplitude independent kick dipolequadrupole

sextupoleoctupole

Pacman bunches• Orbit can be corrected, but only global

corrections are possible• Pacman bunches will always be

overcorrected they’ll no have the optimum position

The difference in orbit kick before and after the IP is cancelled for bunches in the core of the train, but for Pacman bunches not!

Long range effects in ATLAS IP1

vertical

Effect arising from missing LR interactions in the vertical plane of IP1

Different history of LR encounters for head and trail bunches responsible for the asymmetry

Courtesy of M. Schaumann

Courtesy of M. Schaumann

Horizontal effect in ATLAS arises from LR in horizontal plane in CMS! And propagates to IP1

LR effects when reducing the crossing angle

Beam losses = f(number HO)

HO IP1,5,8

HO IP8

Beam losses = f(number HO)

HO IP1,5,8

HO IP8

Lead ion beam production

Small sliver of solid isotopically pure 208Pb is placed in a ceramic crucible that sits in an "oven"

The metal is heated to around 800°C and ionized to become plasma. Ions are then extracted from the plasma and accelerated.

The accelerator chain consumes about 2 mg of lead every hour – a tiny amount, but 10 g costs some SwFr 12,000

II. LHC Operational cycle:Injection

1

2

3

4

5

7

8

6

SPS

LIN

AC

3

CPSLEIR

Top energy Circumference(m) LINAC3 4.2 MeV/u ~10LEIR 72 MeV/u 78CPS 4.2 GeV/u 628 = 4 PSBSPS 157 GeV/u 6911 = 11 x PSLHC 2760/u 26657=27/7xSPS

B2 Dump

B1 Dump

Pb54+

Pb82+

Strip foil

Strip foil

Ion source Pb29+ (2.5 keV/u)