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Proton Intensity Evolution
Estimates for LHC
R. Assmann, CERN/BE
19/3/2009
LMC
Acknowledgements to:
Chiara Bracco, Elias Metral (CERN) and Thomas Weiler (Uni
Karlsruhe) for simulation data.
Werner Herr for collaboration on beam-beam related parameters.
Bernd Dehning for input on beam loss monitors.
Mike Lamont for getting me going on this work.
Massimiliano Ferro-Luzzi and Roger Bailey for discussions.
John Jowett for optics and layout work.
Collimation Study Group and SLAC/LARP for many years of
studies from many different persons and Commissioning Meeting
for feedback.
“Cassandra has always been misunderstood
and misinterpreted as a madwoman or crazy
doomsday prophetess.” L. Fitton
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Recent Reference
PhD report available for
download from web site LHC
collimation project:
http://www.cern.ch/lhccollimationproject/PhD/bracco-phdthesis-2009.pdf
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Nothing New on Limits, Except More Detail…
All the connections and
expected limitations announced
since many years.
For example, see SPC report:
Phase II part of our 2003
collimation plan and effort put
into place (White Paper) to find
solution.
Phase II prepared to maximum
in the LHC tunnel (equipped
slots).
R. Assmann, CERN
LHC Proton Intensity Limit
• Impossible to predict the future precisely. Especially as LHC enters into
new territory with intensities above 0.5% of its nominal design value.
• However, baseline assumptions have been agreed for the design of the
LHC, taking into account experience with previous projects (ISR, SppS,
Tevatron, HERA, …). All checked and supported by external experts.
• Simulations predict performance limitation from beam losses, based on
clear physics process (“single-diffractive scattering”) and limitation in offmomentum phase space coverage in LHC collimation.
• Here, take baseline assumptions and assume simulations results are
correct. Add some evolution to these values. Calculate performance.
• Concentrate on collimation efficiency (assume impedance less severe, as
predicted – or solved with transverse feedback).
• All is ongoing work…
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Looks very ambitious and successful,
doesn’t it?
better
Result: Achievement Factor Beyond World
Record in Stored Energy
Beat world record (mature HERA/Tevatron)
in first year LHC by factor 10-20!
worse
Later you might be disappointed
by this performance!
LHC is bigger, has much higher complexity, has magnets with lower quench limits, has demanding beam-beam & beam loss issues, has restricted operational flexibility from protection, …
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Collimation: Ideal Cleaning Inefficiency
versus Re(Tune Shift)
R. Assmann, T. Weiler, E. Metral
worse
Ideal Performance
Phase I
Phase II
Review on
April 2/3!
In the following:
Concentrate on Phase I
better
Phase II
worse
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Input: Ideal Cleaning Efficiency
Two cases considered:
More difficult to stop 7
TeV protons  no
black hole available for
sucking them up!
1) Tight: Collimators always at tightest possible settings (6/7 s).
Best performance but increasingly tight tolerances. Ramp and
squeeze with closed collimators.
2) Intermediate: Intermediate settings with good protection and
relaxed tolerances. Reduced but still good cleaning.
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better
worse
… as Inefficiency (Leakage Rate) …
Simulation results (points) fitted (lines) to represent energy dependence.
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better
worse
Impact of Imperfections on Inefficiency
(Leakage Rate) – 7 TeV
40% intensity ideal reach
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PhD C. Bracco
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Impact of Alignment Errors on Inefficiency
(Leakage Rate)
Year 1
better
Year 2
Year 3
Predicted inefficiency over 20 different seeds of magnet
alignment errors  Always worse than ideal (as expected).
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PhD C. Bracco
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Why Do We Believe Strongly in Limitation?
• Because it is related to clear and well-known physics processes:
– Primary collimators intercept protons and ions, as they should.
– Small fraction of protons receive energy loss but small transverse kick (singlediffractive scattering), ions dissociate, …
– Subsequent collimators in the straight insertion (no strong dipoles) cannot
intercept these off-momentum particles (would require strong dipoles).
– Affected particles are swept out by first dipoles after the LSS. Main bends act
as spectrometer and off-momentum halo dump  quench.
• Off-momentum particles generated by collimators MUST get lost at
the dispersion suppressor (if we believe in physics and LHC optics).
• No hope that this is not real (e.g. LEP2 was protected against this – not
included for the LHC design and too late to be added when I got involved).
• Predicted for p, ions of different species (with different programs).
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halo
Downstream of IR7 b-cleaning
Halo Loss Map
Losses of off-momentum protons from
single-diffractive scattering in TCP
cryo-collimators
Upgrade Scenario
transversely shifted by 3 cm
NEW concept
without new magnets
and civil engineering
halo
-3 m shifted in s
+3 m shifted in s
better
worse
Input: Imperfection Factor
Imperfections always make cleaning efficiency worse. Imperfection factor describes worsening
of inefficiency!
Warning: Only simulated in detail for 7 TeV. Assumed to be independent of energy.
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worse
better
Input: Quench Limit
Takes expected magnet quench limit and some rough dilution into account.
Warning: Transient quench limit seems at least factor 2-6 lower than expected from first
beam quenches. Ignored here. However, not much hope to win in the quench limit.
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worse
Input Bernd Dehning and BLM team
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Input: BLM Threshold
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From FLUKA results
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Input: Dilution Factor
Losses are diluted (lowered) by the showers! Calculated in detail by FLUKA. This factor takes
this detailed dilution into account. Makes proton and FLUKA results coherent.
Warning: FLUKA results only available for 7 TeV and the ideal machine. Dilution factor
assumed to be independent. Can be different.
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Putting it together: Performance Model
• The various important input parameters have been put together into a
preliminary performance model.
• All is preliminary work.
• However, should give some good idea about what we are looking at and
what are the main parameters expected to limit the LHC performance.
• Such an approach takes into account the agreed assumptions, the
technical results and the simulations of achievable performance.
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Result: Intensity Limit vs Loss Rate 5 TeV
better
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Result: Intensity Limit vs Loss Rate 7 TeV
better
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Remarks Beam Loss Rate
• The LHC beams will have most of the time > 20h beam lifetime!
• Original assumption for stored LHC beams:
Min. intensity lifetime = 20 h (after 20 min about 1% of beam lost).
• However, every accelerator experiences regular reductions of beam
lifetime due to various reasons:
– Machine changes in operational cycle: Snapback, ramp, squeeze
– Crossing of high-order resonances during operational cycle.
– Operator actions during empirical tuning (tune, orbit, chromaticity, coupling,
…) with some small coupling of parts of beam to instabilities…
• A very short drop in beam lifetime is sufficient to have a quench and to
end the fill. Collimation must protect against these loss spikes.
• Collimator design assumption changed to:
Min. intensity lifetime = 0.2 h (after 10s about 1% of beam lost).
• Based on real world experience (SppS, HERA, Tevatron, RHIC, ISR, …).
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Examples for 0.001/s Loss Rate
• It is really the loss rate that matters above a few ms. So what counts is
the ratio of loss amount over loss duration (short loss spikes are very
dangerous). We get the peak loss rate 0.001/s from:
– 1% of beam lost in 10 s.
– 0.1% of beam lost in 1 s.
– 0.01% of beam lost in 100 ms.
– 0.001% of beam lost in 10 ms.
• Stick with the official loss rate 0.001/s from now on, adding some
evolution.
• Assume 0.002/s is achieved in the first year of LHC operation at 5 TeV,
as shown in following slides.
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Result: Intensity Limit vs Energy
LHC could store lot’s of intensity at 1 TeV  Shows effort put on improvements!
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Result: Limit Stored Energy vs Beam Energy
x 300
LHC could store lot’s of energy at 1 TeV  Shows effort put on improvements!
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Input: Beam-Beam Related (W. Herr)
Beta*
Crossing Angle (LR BB)
Limit bunch intensity
(head-on BB)
Limit on bunch spacing (LR BB)
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Result: Intensity Limit vs Energy
R. Assmann and W. Herr
beam-beam limited
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Result: Limit Stored Energy
vs Beam Energy
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Result: Peak Instantaneous Luminosity
R. Assmann and W. Herr
beam loss limited
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Evolution versus Time
• All LHC systems are supposed to work much better than comparable
systems in HERA and Tevatron in the slides before. They have been
designed to do so.
• However, there are no miracles (usually) and systems will not start up
with their final performance. Issues must be understood and solved one
by one (a 0.1% beam tail of the LHC corresponds to full Tevatron/HERA
beam).
• Some time evolution was added to the different parameters to reflect the
experience that critical issues are usually improved with time.
• Also include an upgrade scenario (Scenario 1): Collimation upgrade
completed in 2013/14 shutdown. Triplet phase I upgrade.
• Assume 5 TeV  6 TeV  7 TeV. Just my guess, can be changed…
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Inputs I
Beta*
Ideal inefficiency
Peak loss rate
Limit bunch intensity
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Inputs II
BLM threshold
Crossing angle
Dilution factor (FLUKA)
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Imperfection factor
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A Look at Tevatron Efficiency vs Time
D. Still
~ factor 2 improvement per year
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Result: Intensity versus Time
(Scenario 1)
Collimation limited
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Beam-beam limited
PRELIMINARY
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Result: Stored Energy versus Time
(Scenario 1)
Collimation limited
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Beam-beam limited
PRELIMINARY
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Result: Peak Luminosity versus Time
(Scenario 1)
Collimation limited
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Beam-beam limited
PRELIMINARY
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Scenario 2
• As before, but early collimation upgrade completed in 2011/12.
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Result: Intensity versus Time
(Scenario 2)
Collimation limited
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Beam-beam limited
PRELIMINARY
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Result: Stored Energy versus Time
(Scenario 2)
Collimation limited
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Beam-beam limited
PRELIMINARY
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Result: Peak Luminosity versus Time
(Scenario 2)
Collimation limited
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Beam-beam limited
PRELIMINARY
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Conclusion
• Nothing new on expected beam loss limitations for LHC.
• Collected baseline LHC assumptions (originating from real-world collider
experience: Tevatron, SppS, RHIC, HERA, LEP, SLC, PEP-2, ISR).
• Put together available performance simulations around collimation and
beam loss (optimistic approach). Other high intensity effects assumed OK (electromagnetic noise, heating from image currents, instabilities, R2E, …).
• Used info as input parameters to model intensity reach of the LHC.
• Introduced some evolution in input parameters. BB limits from W. Herr.
• Obtain performance estimates versus time based on technical arguments.
• Will not claim that this is the truth but this is the best estimate that I can do
and it is not in contradiction with simulations.
• If different input parameters are agreed we can evaluate the effect on
performance! Also allows analyzing LHC performance once we have data!
• All preliminary: M. Ferro-Luzzi is coordinating a strategy note.
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From Peak to Integrated Luminosity
LEP Example
Can look into a LEP model which can
be applied to LHC.
Note: LHC much more complex and
sensitive than LEP!
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