the hitchhikers’ guide to beam-beam based optimization of
TRANSCRIPT
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Beam-Beam Based Optimization
of the LHC Performance:
A view towards Run-III
S. Fartoukh, N. Karastathis, Y. Papaphilippou
with many thanks to
LCR3 Working Group
Beam-Beam & Luminosity Studies WG
07.05.2019
The Hitchhikers’ Guide to
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Large Hadron Collider
• LHC: p-p, I-I collider
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• 2 rings (... to revolve them all)
• 8 arcs (... to bind them)
• The beams share the same region at
the 4 experimental Insertion regions
(IR) and cross at a single point in
space namely the Interaction Point (IP)
(... to smash them all)
• 2 High-Luminosity
(...and the dark matter find, man)
• 2 Low-Luminosity
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Our clients’ needs
• The detectors at the experimental insertions are looking for the particle debris generated by the p-p collisions.
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An ATLAS event display from a H WW* eνμν event [1]
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Luminosity
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• The instantaneous rate of event production at an IP is linearly correlated to the
delivered luminosity as 𝒅𝑵
𝒅𝒕= 𝝈𝒑𝒑 ⋅ 𝑳
where 𝜎𝑝𝑝 is the cross-section of proton-proton interactions is a characteristic of each
process and depends on the center-of-mass energy 𝑠.
• The luminosity, is defined as the product
of the incident particle (A) flux and the
target (B) area density:
𝐿 = Φ𝐴 ⋅ 𝜌𝐵
• In our case is overlap in space and time of
the colliding bunches [2] normalized by the
the beam parameters
𝐿 = 2𝑐𝑁𝑏1𝑁𝑏2𝑛𝑏𝑓𝑟𝑒𝑣 cos2𝜙
2න
−∞
+∞
න
−∞
+∞
න
−∞
+∞
න
−∞
+∞
𝜌 1 𝑥 𝜌 1 𝑦 𝜌 1 𝑧 + 𝑐𝑡 𝜌 2 𝑥 𝜌 2 𝑦 𝜌 2 𝑧 − 𝑐𝑡 𝑑𝑥𝑑𝑦𝑑𝑧𝑑𝑡
Beam 1Beam 2
𝜎𝑥(1) 𝜎𝑥
(2)𝜎𝑧(2)
IP
φ
Particle Physics Break
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Luminosity (II)
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𝑳 =𝑵𝒃𝟏𝑵𝒃𝟐𝒏𝒃𝒇𝒓𝒆𝒗
𝟒𝝅𝝈𝒙𝝈𝒚⋅ 𝑾 ⋅ 𝒆
𝑩𝟐
𝑨 ⋅ 𝑺
• So I could increase my delivered luminosity by simply increasing the injected
bunch intensity, but what can I do if the injection parameters are fixed?
𝑊 = 𝑒1
4𝜎𝑥2⋅𝑑
2
, 𝐴 =sin2
𝜙
2
𝜎𝑥2 ⋅
cos2𝜙
2
𝜎𝑧2 ,
𝐵 =𝑑⋅𝑠𝑖𝑛
𝜙
2
2𝜎𝑥2 , 𝑆 =
1
1+𝜎𝑧𝜎𝑥⋅𝜙
2
2
Beam 1 Beam 2IP
φ’<φ
φ
Beam 1 Beam 2IP
IPBeam 1
Beam 2
For Gaussian profiles
Some tools:
y2 [4] | C [5] | available too
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Simulation of an LHC Fill
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μ =𝐿𝜎𝑖𝑛𝑒𝑙𝑛𝑏𝑓𝑟𝑒𝑣
Beam Parameters
We could also work in reverse:
Modify the machine parameters to keep
the luminosity constant “Luminosity Leveling” [3]
Leveling by offsetting the beams is used continuously in the LHCb/ALICE
experiments and in the ATLAS/CMS when needed.
It is an easy and efficient method of keeping the luminosity under control.
Turnaround time [h]
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Beam-Beam simulations
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[6]
• A close collaboration of ABP - OP group resulted in a continuous optimization of
the LHC, based on beam-beam DA simulations.
• For example, we saw that improving the working point, we improve DA thus
lifetime!
• Without the WP optimization we would have to increase the crossing-angle to reduced the
effect of the beam-beam (𝜉𝑏𝑏 ∝ 1/𝑑𝑏𝑏2 ) Luminosity
• Even if the DA is ok-ish, optimizing it we could spend additional margin on reducing the
crossing angle event further, at the same initial bunch intensity Luminosity
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Beam-Beam simulations (II)
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[6]
• Why optimize only the initial crossing
angle?
• At the regime of the optimized WP the
correlation of the crossing angle and the
bunch intensity in terms of DA is almost
linear for the targeted amplitudes (5,6σ)
• Αs the intensity decays, the margin of DA
increases in the case the half-crossing
angle is fixed.
• A further reduction of the crossing angle
within the fill could slightly buy us some
additional luminosity
• Limiting ourselves in maintaining throughout the fill the initial iso-DA line
would keep the lifetime always above a certain threshold.
• Studying the evolution of beam parameters and correlate with the simulated DA
one could provide some time instances along an operational fill that the crossing
angle would be safe to be reduced by a certain amount. Anti-leveling
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LHC 2017
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+100pb/day
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Looking a bit further: HL-LHC
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[7]
• The HL-LHC is
planned to be
operated in
leveling mode for
most of the fill
length.
• The leveling
method considered
is adapting the β*
at the ATLAS/CMS
experiments
(ALICE/LHCb
remain in offset
leveling).
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LHC 2018 – One step further
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• So in 2018 several development of controls and improving our understanding of
the machine based on our simulations
• The evolution of the
crossing angle as a
function of the bunch
intensity was
automatized.
• Steps in further
squeezing the β* have
been implemented at the
end of each fill.
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Run-III – LIU Commissioning
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[8]
• LIU will slowly ramp up the injected bunch intensity throughout the Run-III.
• Certain limitations restrict the
injected bunch intensity into the
LHC and the maximum luminosity
accepted by the two high
luminosity experiments.
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So what is Run-III?
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Fact #1: Run-III is a preparatory HL-LHC run period.
Fact #2: LIU is completing its commissioning, will continuously
push higher intensity into the LHC.
Fact #3: The experiments are still there!
• Back to DA simulations:
After optimizing the working point define the function of 𝝈𝒃𝒃𝒏𝒐𝒓𝒎 𝑵𝒃
Force at least 9.43σ 1.17e11 ppb, as in the present LHC
We need to prepare the
mechanics towards HL-
LHC, but not to
compromise the
integrated performance
• Since HL-LHC is based around β* leveling expand 2018 to a full leveling.
• We already have the evolution of the crossing angle as a function of the bunch intensity
implemented. Could we combine the two to gain an additional knob?
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Run-III – Adapting β* & X
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These intensities
are not reachable
• Based on the DA simulations and the scaling law acquired for the normalized crossing angle define scaling
law of 𝐍 𝜷∗ under the constraint of 𝐋 = 𝟐 × 𝟏𝟎𝟑𝟖 𝑯𝒛/𝒎𝟐.
• Keep in mind that there are some additional unknowns:
• How well do we preserve emittance from injection to collisions? Take the extremes 1.8μm & 2.5μm
• What is the impact of non-simulated diffusive mechanisms (e-cloud)? Leave additional DA margin & e-
cloud can be mitigated by smaller number of bunches Study for 2736b & 2484b
• The commissioned optics should
be such that they accommodate
the large dynamic range of
initial β*
• Leave additional margin in the start of the leveling when the bunch intensity is high.
• Overall, one can define the
𝝈𝒃𝒃 𝑵𝒃 𝜷∗ ȁ𝑳=𝟐×𝟏𝟎𝟑𝟒𝑯𝒛/𝒄𝒎𝟐 and
therefore the evolution of the crossing
angle.
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Run-III – Evolution along the fill
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• However, the linear fits are only valid at the optimal WP setting Test the evolution of the WP for the
various commissioned optics under the constraint of the leveled luminosity.
• (.313, .318) looks
optimal in all
configurations.
nb = 2736
nb = 2484• The results suggest a
DA above the 5σ target
for most of the path
• Fixing the WP one can
scan the β* as a
function of the
intensity to find the
leveling path 𝑁𝑏 𝛽 ȁ𝜑(𝛽)for a certain luminosity
target.
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Run-III – Fill Profile
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[9, 10]
9.7h @ 𝟏. 𝟑𝟒 ⋅ 𝟏𝟎𝟏𝟏𝐩𝐩𝐛
13.4h
1.2𝟔𝐟𝐛−𝟏
52.6 evt
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Run-III – Our Crystal Ball
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(**) Telescopic Optics & CRATS
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• The ATS optics offers the flexibility to reduced the β* while controlling the chromatic aberrations
induced the usual β* squeezing at an IP stops at a certain value (𝛽𝑝𝑟𝑒∗ ) and then the magnets in the
neighboring IRs are used to further reduce the β* at the target IP
• A characteristic of the telescopic squeeze is the tele-index defined as 𝑟 =𝛽𝑝𝑟𝑒∗
𝛽𝑡𝑎𝑟𝑔𝑒𝑡∗
[9,10,11]
(**) = in layman’s terms
• Interestingly enough, by
applying a tele-index, the
variation of the β-beating
wave in the arcs,
modifies the effectiveness
of the octupoles by
enhancing the provided
detuning by ∝1
4(𝑟2 +
1
𝑟2)
550A (with safety
factor of 2)
• This affects the coherent
stability, mainly during the
RAMP, allowing for
additional Landau damping
some telescope is
needed in the RAMP
already successful MDs
during 2018
• In addition the FP experiments prefer the
squeezing of the β* during collisions in
telescopic mode as the transfer matric is
constant.
• The only possible solution of
combining telescope in the RAMP and
collision in telescopic mode, within the
limits of matchability would be to start
colliding from an anti-telescope
i.e. 𝑟 =1
1.8
S. Fartoukh
Beam
-Beam
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(**) Run-III – Flat Optics & Triplet Lifetime
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[10, 12]
• So far we were considering colliding in
ATLAS/CMS with 𝛽𝑋𝐼𝑃1 = 𝛽𝑌
𝐼𝑃1 = 𝛽𝑋𝐼𝑃5 =
𝛽𝑌𝐼𝑃5 (round optics)
Slightly better performance (hindered by the leveled
luminosity threshold), but…
• Flat Optics assumes asymmetric β*
aspect ratio 𝑟∗ =𝛽𝑋∗
𝛽ȁȁ∗ ≠ 1, with 𝛽𝑋
∗𝛽ȁȁ∗ =
𝛽𝑟𝑜𝑢𝑛𝑑∗
• It is an alternative to HL-LHC
operation without crab cavities as
increasing the β* aspect ratio rapidly
mitigates the geometric luminosity
loss.
Improvement on the irradiation of the triplet
magnets
Reducing the crossing angle has also a beneficial
effect
the parametric variation of the crossing angle
with the β* keeps the crossing angle as low as
possible for as long as possible!
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Some things yet to solve...
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• There is work yet to be done both from our side (optics production and further tests) but also from the side of the experiments.
• Some on-going discussions include
[10, 13, 14]
ATLAS, V CMS, HThe variation of the crossing angle and β*
significantly shrinks the longitudinal
luminous region, thus increasing the pileup
density.
2018 BCMS 2017 8b4e
The bunch-by-bunch luminosity variation
along the train might impact the leveling
(on luminosity? on pileup? Maximum or
average?)
5x48 bpi, 2736 collisions in ATLAS/CMS, 2250/2376 in Alice/LHCb
Baseline: Pure BCMS
Backup: Mixed BCMS with 8b+4e (56b) inserts
What is the impact on using mixed filling
schemes?
G. Iadarola
I. Efthymiopoulos
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Summary
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• The estimate of the performance of a collider is given by its luminosity, which depends both on the
beam and machine parameters.
• For fixed injected beam parameters, modifying the crossing-angle, the β* and the parallel
separation changes the delivered luminosity at the IPs luminosity constant (leveling)
• Beam-beam DA simulations were used extensively to identify the optimal path to modifying the
machine parameters along the fill, and after it was proved safe, the technique was pushed into
nominal operation during Run-II.
• In the future, during Run-III the LIU will commission the upgrades and will gradually increase the
delivered bunch intensity LHC must adapt, and prepare for the HL-LHC era without hindering
the its integrated performance.
• Building upon the knowledge and tools developed already, Run-III operation could be:
• Injection : the same
• Combined Ramp and Anti-Telescopic Squeeze
• Collide & Squeeze : in telescopic mode β* && xing in IP1/5, offset in IP2/8
• A plan for the full cycle of the Run-III is in place and the optics are being finalized. Of course the
final decision on the operational settings and LHC deliverables falls on the management.
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Some references
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1. ATLAS Collaboration, “Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC”,
Phys. Lett. B 716 (2012) 1-29
2. B. Muratori, “Luminosity and luminous region calculations for the LHC”, LHC-PROJECT- NOTE-301
3. B. Muratori, T. Pieloni, “Luminosity leveling techniques for the LHC”, Proceedings of the ICFA Mini-Workshop on Beam–Beam Effects in
Hadron Colliders, CERN, Geneva, Switzerland, 18–22 March 2013 CERN–2014–004 (CERN, Geneva, 2014)
4. “beamCal” : https://github.com/nkarast/beamCal
5. ”BeamParameterEvo”: https://github.com/nkarast/BeamParameterEvo
6. N. Karastathis, K. Fuchsberger, M. Hostettler, Y. Papaphilippou, and D. Pellegrini, “Crossing angle anti-levelling at the LHC in 2017”,
Journal of Physics: Conference Series 1067 022004 (2018)
7. E. Metral et al, “Update of the HL-LHC operational scenarios for proton operation”, CERN-ACC-NOTE-2018-0002
8. G. Rumolo et al, “What to expect from the Injectors during Run-III”, Proceedings of 9th Evian LHC Operations Workshop, 30 Jan– 1 Feb.
2019
9. N. Karastathis, S. Fartoukh et al., “Report from the LHC Run-III Configuration Working Group”, Proceedings of 9th Evian LHC Operations
Workshop, 30 Jan– 1 Feb. 2019
10. S. Fartoukh, N. Karastathis et al., LHC Machine Committee, 6 Mar 2019, 803742 (GPN ONLY)
11. S. Fartoukh, “Achromatic telescopic squeezing scheme and applications to the LHC and its luminosity upgrade”, Phys. Rev. ST. Accel.
Beams 16, 111002, 19 Nov 2013
12. S. Fartoukh, N. Karastathis, M. Solfaroli Camillocci, R. Tomas Garcia, “About flat telescopic optics for the future operation of the LHC”,
CERN ATS Reports, CERN-ACC-2018-0018
13. N. Karastathis, S. Fartoukh, et al, LHC Program Coordination Meeting, 25 Mar 2019, 806576
14. LHC Program Coordination Meeting, 20 May 2019, 820221
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Tune along the fill (additional)
07/06/2019 Document reference 24