Transcript Beam Loss and Collimation at the LHC R. Assmann, CERN/AB 15/11/2007 for the Collimation Team GSI Beschleunigerpalaver RWA, GSI 11/07

Beam Loss and Collimation
at the LHC
R. Assmann, CERN/AB
15/11/2007
for the Collimation Team
GSI Beschleunigerpalaver
RWA, GSI 11/07
What is the LHC Beam?
Protons/ions stored in circular
accelerator.
Top view
Particles travel with light velocity in a
27 km long vacuum tube.
Revolution frequency is 11 kHz.
p
Ideally fully stable without any losses.
Two beams with opposite travel
directions and well defined collision
points.
7.6 cm
0.2 mm
25 ns
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25 ns
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1) Introduction: The LHC Challenge
The Large Hadron Collider:
Circular particle physics collider with 27 km circumference.
Two colliding 7 TeV beams with each 3 × 1014 protons.
Super-conducting magnets for bending and focusing.
Start of beam commissioning: May 2008.
LHC nominal parameters
Particle physics reach defined from:
1) Center of mass energy

14 TeV
super-conducting dipoles
Number of bunches:
Bunch population:
Bunch spacing:
2808
1.15e11
25 ns
Top energy:
Proton energy:
Transv. beam size:
Bunch length:
Stored beam energy:
7 TeV
~ 0.2 mm
8.4 cm
360 MJ
Injection:
2) Luminosity
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1034 cm-2 s-1
Proton energy:
Transv. Beam size:
Bunch length:
450 GeV
~ 1 mm
18.6 cm
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The LHC SC Magnets
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LHC Luminosity
• Luminosity can be expressed as a function of transverse energy density
re in the beams at the collimators:
d = demagnification (bcoll/b*)
Np = protons per bunch
frev = revolution freq.
Eb = beam energy
• Various parameters fixed by design, for example:
– Tunnel fixes revolution frequency.
– Beam-beam limit fixes maximum bunch intensity.
– Machine layout and magnets fix possible demagnification.
– Physics goal fixes beam energy.
• Luminosity is increased via transverse energy density!
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pp, ep, and ppbar collider history
Higgs +
SUSY + ???
~ 80 kg TNT
2008
1992
Collimation
Machine Protection
SC magnets
1971
1987
1981
The “new Livingston plot“ of proton colliders: Advancing in unknown territory!
A lot of beam comes with a lot of garbage (up to 1 MW halo loss, tails, backgrd, ...)
 Collimation. Machine Protection.
Proton Losses
• LHC: Ideally no power lost (protons stored with infinite lifetime).
• Collimators are the LHC defense against unavoidable losses:
– Irregular fast losses and failures: Passive protection.
– Slow losses: Cleaning and absorption of losses in super-conducting
environment.
– Radiation: Managed by collimators.
– Particle physics background: Minimized.
• Specified 7 TeV peak beam losses (maximum allowed loss):
– Slow:
0.1% of beam per s for 10 s
0.5 MW
– Transient:
5 × 10-5 of beam in ~10 turns (~1 ms)
20 MW
– Accidental:
up to 1 MJ in 200 ns into 0.2 mm2
5 TW
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The LHC Collimators…
• Collimators must intercept any
losses of protons such that the rest
of the machine is protected („the
sunglasses of the LHC“):
> 99.9% efficiency!
Top view
• To this purpose collimators insert
diluting and absorbing materials into
the vacuum pipe.
• Material is movable and can be
placed as close as 0.25 mm to the
circulating beam!
• Nominal distance at 7 TeV:
≥ 1 mm.
• Presently building/installing phase 1!
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Preventing Quenches
• Shock beam impact: 2 MJ/mm2 in 200 ns
(0.5 kg TNT)
• Maximum beam loss at 7 TeV: 1% of beam over 10 s
500 000 W
• Quench limit of
SC LHC magnet:
8.5 W/m
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Machine Protection
• There are a number of LHC failure scenarios which lead to beam loss.
• No discussion of machine protection details here. However, comments on
collimator role in machine protection. R. Schmidt is Project Leader for MP.
• Slow failures:
– First losses after >10-50 turns appear at collimators as closest aperture
restrictions.
– Beam loss monitors detect abnormally high losses and dump the beam within
1-2 turns.
• Fast failures (dump and injection kicker related):
– Sensitive equipment must be passively protected by collimators.
• In all cases, the exposed collimators must survive the beam impact:
up to 2 MJ in 200 ns (0.5 kg TNT)
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2) LHC Collimation Basics
Beam axis
Beam propagation
Impact
parameter
Core
Collimator
Particle
Unavoidable losses
Primary
halo (p)
CFC
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CFC
W/Cu
Tertiary halo
p
Superconducting
magnets
Absorber
e
Absorber
e p
Shower
Secondary
collimator
Secondary
p halo
p Shower
p
Primary
collimator
Impact
parameter
≤ 1 mm
Multi-Stage Cleaning
SC magnets
and particle
physics exp.
W/Cu
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“Phase 1”
System Design
Momentum
Collimation
Betatron
Collimation
“Final” system:
Layount is 100%
frozen!
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C. Bracco
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A Virtual Visit to IR7
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LHC Collimator Gaps
Collimator settings:
5 - 6 s (primary)
6 - 9 s (secondary)
s ~ 1 mm (injection)
s ~ 0.2 mm (top)
Small gaps lead to:
1. Surface flatness
tolerance (40 mm).
2. Impedance
increase.
3. Mechanical
precision demands
(10 mm).
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Required Efficiency
Quench threshold
Allowed
intensity
(7.6 ×106 p/m/s @ 7 TeV)
Illustration of LHC dipole in tunnel
N
max
p
   Rq  Ldil /c
Cleaning inefficiency
=
Number of escaping p (>10s)
Number of impacting p (6s)
Beam lifetime
(e.g. 0.2 h minimum)
Dilution
length
(~10 m)
Collimation performance can limit the intensity and therefore
LHC luminosity.
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Intensity Versus Cleaning Efficiency
For a 0.2 h
minimum beam
lifetime during
the cycle.
99.998 % per m efficiency
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The LHC Phase 1 Collimation
•
Low Z materials closest to the beam:
– Survival of materials with direct beam impact
– Improved cleaning efficiency
– High transparency: 95% of energy leaves jaw
•
Distributing losses over ~250 m long dedicated cleaning insertions:
– Average load ≤ 2.5 kW per m for a 500 kW loss.
– No risk of quenches in normal-conducting magnets.
– Hot spots protected by passive absorbers outside of vacuum.
•
Capturing residual energy flux by high Z absorbers:
– Preventing losses into super-conducting region after collimator insertions.
– Protecting expensive magnets against damage.
•
No shielding of collimators:
– As a result radiation spread more equally in tunnel.
– Lower peak doses.
– Fast and remote handling possible for low weight collimators.
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3) Collimator Hardware
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Hardware: Water Cooled Jaw
Up to 500 kW impacting on
a jaw (7 kW absorbed in jaw)…
Advanced material: Fiber-reinforced graphite (CFC)
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The LHC “TCSG” Collimator
1.2 m
3 mm beam passage with RF contacts for
guiding image currents
Designed for maximum robustness:
Advanced CC jaws with water cooling!
Other types: Mostly with different jaw
materials. Some very different with 2
beams!
360 MJ proton beam
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Robustness Test with Beam
C-C jaw
TED Dump
Microphone
~ Tevatron beam
Graphite
450 GeV
3 1013 p
2 MJ
0.7 x 1.2 mm2
Fiber-reinforced
graphite (CFC)
C jaw
~ ½ kg TNT
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Operational Control
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Using Sensors to Monitor LHC Jaw Positions
Side view at one end
CFC
CFC
Vacuum tank
Temperature sensors
Microphone
Movement
for spare
surface
mechanism
(1 motor,
2 switches,
1 LVDT)
Reference
Reference
Motor
Motor
Sliding table
Gap opening (LVDT)
Resolver
Resolver
Gap position (LVDT)
+ switches for IN, OUT, ANTI-COLLISION
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Collimator Controls
S. Redaelli et al
Collimator Beam-Based Alignment
Successful test of LHC collimator control architecture with SPS beam (low, middle, top level)
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Position Measurement and Reproducibility
LVDT Calibration Repeatability test (TT40)
36 repetitions
1.01
1.005
20 µm
1
Normalized position [mm]
0.995
0.99
0.985
0.98
0.975
~ 25 µm mechanical play
0.97
0.965
0.96
0.955
0.95
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
distance [mm]
•
Measured during test in TT40 (Oct. 31st) in remote!!!!
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R. Losito et al
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Compatibility with LHC UHV
J-P. BOJON, J.M. JIMENEZ,
D. LE NGOC, B. VERSOLATTO
Conclusion:
Graphite-based jaws are compatible with the LHC vacuum.
The outgassing rates of the C jaws will be optimized by material and heat
treatment under vacuum, an in-situ bake-out and a proper shape design.
No indication that graphite dust may be a problem for the LHC.
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Other collimator features
• In-situ spare concept by moving the whole tank
(move to fresh surface if we scratch the surface
with beam)
• Direct measurements of jaw positions and
absolute gap (we always know where the jaws
are)
• Precision referencing system during production
• Measurements of jaw temperature
• Radiation impact optimization: Electrical and
water quick plug-ins, quick release flanges,
ceramic insulation of cables, ...
• RF contacts to avoid trapped modes or additional
impedance
C. Rathjen, AT/VAC
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Collimator Deliveries
Production deadline
for initial installation
120
# collimators
100
Initial 7 TeV installation
80
60
40
20
0
z
Mr
06
Jun
06
p
Se
06
z0
De
6
z
Mr
07
Ju n
07
p
Se
07
z0
De
7
z
Mr
08
Time
Industry: 87% of production for 7 TeV initial ring installation has been completed (66/76).
All collimators for first run should be at CERN by end of the year.
Total production should be completed in April.
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4) Tunnel Installations
(vertical and skew shown)
Water
Connections
Vacuum pumping
Modules
Collimator
Tank (water cooled)
Quick connection
flanges
A. Bertarelli
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BLM
Beam 2
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Tunnel Preparations IR7
Cable routing from top (radiation)
Water
connection
Cable trays
Pumping domes
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Series of collimator plug-in supports
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Collimator Installation
Quick plug-in support (10 min installation)
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Installed Collimator on Plug-In
Collimator
Upper plug-in
Lower plug-in
Base support
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Remote Train
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Remote Survey
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4) Collimation Performance
Simulations:
5 million halo protons
200 turns
realistic interactions in all collimator-like objects
LHC aperture model
 Multi-turn loss predictions
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Efficiency in Capturing Losses
Local inefficiency [1/m]
Beam1, 7 TeV
Efficiency
99.998 % per m
TCDQ
Betatron cleaning
Ideal performance
Quench limit
(nominal I, =0.2h)
Beam2, 7 TeV
Efficiency
99.998 % per m
TCDQ
Betatron cleaning
Ideal performance
Quench limit
(nominal I, =0.2h)
99.998 % needed
Local inefficiency: #p lost in 1 m over total #p lost = leakage rate
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99.995 %
predicted
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Problem: Beam loss tails?
Observation of BLM signal tails:
BLM team:
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Up to 10-20 seconds in length
Many measurements  Beam related true signal!
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Collimation for Ions
Different physics! Two-stage b cleaning not working! Limitation to ~50% of nominal
ion intensity.
Power load [W/m]
G. Bellodi et al
 Loss predictions used for allocation of additional BLM’s for ions!
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K. Tsoulou et al
Energy Deposition (FLUKA)
FLUKA team
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CERN Mechanical Simulations
Displacement analysis – Nominal conditions (100 kW) – Load Case 2
10s Transient (500 kW) – Loss rate 4x1011 p/s (Beam Lifetime 12min)
Initial loss 8e10p/s Max.
deflect. ~20mm
Transient loss 4e11p/s
during 10s
Max deflect. -108mm
Back to 8e10p/s situation!
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A. Bertarelli & A. Dallochio
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Local Activation
•
Losses at collimators generate local heating and activation.
•
Local heating: On average 2.5 kW/m.
•
Activation: Up to 20 mSv/h on contact (better not touch it).
•
Fast handling implemented. Remote handling being developed.
Residual dose rates
One week of cooling
S. Roesler et al
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Kurchatov Collaboration Studies of CFC
Material Used in LHC Collimators
A. Ryazanov
 Working on understanding radiation damage to LHC collimators from 1016 impacting
protons of 7 TeV per year. Also with BNL/LARP…
… in addition shock wave models…
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Impedance Problem
• Several reviews of LHC collimator-induced impedance (originally not
thought to be a problem).
• Surprise in 2003: LHC impedance driven by collimators, even metallic
collimators.
• LHC will have an impedance that depends on the collimator settings!
• Strong effort to understand implications…
Third look at impedance in Feb 03
revealed a problem:
F. Ruggiero
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Transverse Impedance [MΩ/m]
First Impedance Estimates 2003
Typical collimator half gap
104
103
102
LHC impedance without collimators
10
1
10-1
0
2
4
6
Half Gap [mm]
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10
F. Ruggiero, L. Vos
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Impedance and Chromaticity
E. Metral
et al
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2006 Collimator Impedance Measurement
Improved controls in 2006:
• Possibility of automatic
scan in collimator
position.
• Much more accurate
and complete data set in
2006 than in 2004!
R. Steinhagen et al
E. Metral et al
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Summary: The Staged LHC Path
Energy density
at collimators
Stored energy
in beams
Number of
collimators
(nominal 7 TeV)
State-of-the-art in SC
colliders (TEVATRON,
1 MJ/mm2
2 MJ
Phase 1 LHC
Collimation
400 MJ/mm2
150 MJ *
88
Nominal LHC
1 GJ/mm2
360 MJ
122
Ultimate & upgrade
scenarios
~4 GJ/mm2
~1.5 GJ
≤ 138
Limit (avoid
damage/quench)
~50 kJ/mm2
~10-30 mJ/cm3
HERA, …)
* Limited by cleaning efficiency (primary) and impedance (secondary)
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5) Beyond Phase 1
• The LHC phase 1 system is the best system we could get within the
available 4-5 years.
• Phase 1 is quite advanced and powerful already and should allow to go a
factor 100 beyond HERA and TEVATRON.
• Phase 2 R&D for advanced secondary collimators starts early to address
expected collimation limitations of phase 1.
• Phase 2 collimation project was approved and funded (CERN white
paper). Starts Jan 2008. Should aim at complementary design compared
to SLAC.
• Collaborations within Europe through FP7 and with US through LARP are
crucial components in our plans and address several possible problems.
• We also revisit other collimation solutions, like cryogenic collimators,
crystals, magnetic collimators, non-linear schemes.
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LHC Phase 2 Cleaning & Protection
Beam axis
Beam propagation
Impact
parameter
Core
Collimator
Particle
CFC&
CFC
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CFC Phase 2
material
1.
Phase 2 materials for system improvement.
2.
Crystals AP under study (surface effects,
dilution, absorption of extracted halo).
Shower
p
e
Absorber
e p
Shower
Hybrid Collimator TCSM
Impact
parameter
≤ 1 mm
Primary
Primary
collimator
collimator
Crystal
Secondary
p halo
p
Phase 1 ColliPhase 1 Collimator TCSG
mator TCSG
Primary
halo (p)
W/Cu
Tertiary halo
p
Superconducting
magnets
Absorber
Unavoidable losses
SC magnets
and particle
physics exp.
W/Cu
 Low electrical resistivity, good absorption, flatness, cooling, radiation, …
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 September workshop provided important input and support…
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Draft Work Packages
White Paper (WP), Europe (FP7), US (LARP)
WP1 (FP7)
–
Management and communication
WP2 (WP, FP7, LARP)
–
Collimation modeling and studies
WP3 (WP, FP7, LARP)
–
Material & high power target modeling and tests
WP4 (WP, FP7, LARP)
–
Collimator prototyping & testing for warm regions
Task 1
–
Scrapers/primary collimators with crystal feature
Task 2
–
Phase 2 secondary collimators
WP5 (FP7)
–
Collimator prototyping & testing for cryogenic regions
WP6 (FP7)
–
Crystal implementation & engineering
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SLAC Collimator Design and Prototyping:
Rotatable LHC Collimator for Upgrade
Strong SLAC commitment and effort:
Design with 2 rotatable Cu jaws
Theoretical studies, mechanical design,
prototyping.
New full time mechanical engineer hired.
Looking for SLAC post-doc on LHC collimation!
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First prototype with helical cooling circuit
(SLAC workshop)
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Working Together to Develop
Solutions…
• Many if not most new accelerators are loss-limited in one way or another!
• Collimation has become a core requirement for success. The LHC
upgrade program is or will be just one example.
• Collimation is so challenging in modern accelerators that it warrants a full
collaborative approach to extend the present technological limits.
• Collaborations exist or are under discussion with presently 17 partners:
Alicante University, Austrian Research Center, BNL, EPFL, FNAL, GSI,
IHEP, INFN, JINR Dubna, John Adams Institute, Kurchatov Institute, Milano
University, Plansee company, Protvino, PSI, SLAC, Turin Polytechnic
• The importance and intellectual challenge is reflected by the strong
support from the international community.
• Operational and design challenges impose fascinating technological and
physics R&D.
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6) Conclusion
•
LHC advances the accelerator field into a new regime of high power beams
with unprecedented stored energy (and destructive potential).
•
The understanding of beam halo and collimation of losses at the 10-5 level will be
crucial for its success (high luminosity)!
•
LHC collimation will be a challenge and a learning experience!
•
Collimation is a surprisingly wide field: Accelerator physics, nuclear physics,
material science, precision engineering, production technology, radiation physics.
•
A staged collimation approach is being implemented for the LHC, relying on the
available expertise in-house and in other labs.
•
The collaboration and exchange with other labs is very important to design and
build the best possible system (achieve our design goals)!
•
Bid for support from European Community (FP7). We hope to have GSI as major
partner in the domain of understanding and controlling beam losses.
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The Collimation Team…
Collimation team:
About 60 CERN technicians,
engineers and physicists… in
various groups and
departments.
+ many friends in connected
areas (BLM’s, MP, …)
+ collaborators in various
laboratories (SLAC, FNAL,
BNL, Kurchatov, …)
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