The Large Hadron Collider LHC Operation II: pushing the limits UFOs SEUs e-cloud 25 ns operation...
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Transcript of The Large Hadron Collider LHC Operation II: pushing the limits UFOs SEUs e-cloud 25 ns operation...
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
HE
H fl
ue
nce
(cm
-2)
AT
LA
S l
um
i (p
b-1
)
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.
Tota
l bea
m in
tens
ityH
eat
load
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
Sec
onda
ry
elec
tron
yi
eld
e-cloud: scrubbing runs
2012
from 1.55 down to 1.45
B1 2748b
B2 2748b
Sec
onda
ry
elec
tron
yie
ld
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)