1. LHC Accelerator Upgrade
20/03/2000
LHC Accelerator Upgrade
J.-P. Koutchouk
CERN/AT
11/16/2018
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2. Outline Introduction Goals (phases I and II, energy upgrade)
Phase I: the consolidation
Phase II: the luminosity upgrade
How to increase further the luminosity?
Increasing the beam current
Focusing more
Luminosity issues
Technological challenges
The energy upgrade
Conclusions
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3. 1- Introduction
The present focus of the accelerator sector is obviously the baseline LHC.
Planning the upgrade is nevertheless timely as it is largely technology-driven with lead times of 5 to 15 years, depending on goals and complexity.
Yet, several choices require LHC results (machine and physics)
Hence upgrade studies aim primarily at identifying the necessary hardware to launch in time the R&D programmes and give clear goals to the action of CERN partners (US-LHC, CARE-HHH and NED)
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4. 2a- The goals tentativeplanning goal Phase I: LHC+ 2012
(or SLHC1)
2012
Triplet consolidation for high integrated luminosity; Luminosity 1 to  2;
no interference with detectors
Phase II:
SLHC
(or SLHC2)
2016
Luminosity  10:
Phase III:
LHC-D or LHC-T
energy doubler or tripler
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5. 2b- The goals: the luminosity profile
LHC baseline
SLHC
LHC+ & SLHC
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6. 3a- Phase I (LHC+) Motivations: Boundary conditions:
Facilitate reaching the ultimate luminosity of 2.31034cm-2s-1 (made difficult following increase of crossing angle, introduction of a beam screen and collimator impedance)
Improve the running efficiency at nominal performance & create margins if some of the design parameters would not be reached.
Boundary conditions:
no interference with detectors,
fastest implementation: Nb-Ti s.c. technology;
fast performance increase: no new beam dynamics
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7. 3b- Phase I (LHC+)
Solution: change the IR1 & IR5 triplets (70mm) for larger aperture ones (130mm). This gives the potential to recover the “ultimate” luminosity with a safety margin of ~50%. D1 may have to be changed. The superconducting cable is available (spare LHC dipole cable).
Status:
Feasibility studies done
Included in Proposal SLHC-PP to FP7-Infrastr coord. L. Evans, issued in May 2007.
Project coordinator appointed (R. Ostojic) to evaluate cost and manpower requirements.
Timescale: operational in 2012
Budget: remains to be found
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8. 4a- Phase II (SLHC)
Challenge: LHC baseline was pushed to “maximum” in the competition with SSC. Going beyond requires creative solutions.
Luminosity goal: ~ 101034cm-2s-1
Boundary conditions: For such a significant upgrade, the added complexity should be minimized for a fast progress of performance and the risks mitigated for a graceful degradation in case of unexpected.
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9. 4b- P.II: The luminosity formula
crossing angle
standard
Variation of β along bunch
F decreases with decreasing b*, increasing kb, Nb and ss. H is in the range {0.9, 1} for practical b*.
A constraint is the “beam-beam limit”, usually given by:
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10. Luminosity at the beam-beam limit:
4c- P.II: The strategies
Luminosity at the beam-beam limit:
Two main tracks:
Luminosity increase by increasing the beam current
Luminosity increase by lowering b*
+ work on form factors and beam-beam limit
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11. 4d- P.II: Increase of beam current
Strategy:
Increase significantly the bunch charge Nb (*4).
Respect the beam-beam limit (that would be exceeded) by a change of regime: lengthen the bunch to reach a quasi coasting beam regime by longitudinal emittance blow-up.
Reduce the number of bunches (50 ns spacing) to control e-cloud and image current heat deposition & beam stability.
Recover the lost factor of 2 by reducing beta* by a factor of 2 (25 cm)
Reduce the crossing angle by wire compensation
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12. 4e- P.II: Increase of beam current
Merits: no elements in detectors; quasi nominal insertion beam optics with small chromatic aberrations; higher average luminosity per run.
Challenges: new beam dynamics: novel beam-beam regime not experienced at this level of performance; higher peak beam current coupling to machine elements, sophisticated rectangular beam distribution; machine protection: higher bunch/beam power; higher collimator robustness required; radiation protection: protection to be re-assessment when exceeding the ultimate beam current in the LHC (INB). injectors: new beam preparation; injectors’ upgrade for full operational performance: Linac4, PS2, SPS improvement.
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13. 4f- P.II: Increase of beam current
Investments: new triplet (b*= 25 cm) & D1, install the 200 MHz RF system, possibly more resistant collimators, upgrade of dumping system, extend the dynamic range of some beam instruments, upgrade radiation protection, upgrade of injectors (SPS, PS, Booster and Linac).
What if: - if beam current cannot be reached or if new beam-beam regime inefficient or if injectors not ready, return to 25 ns spacing to recover the Phase I performance (nominal  1 to 2)
Could then gain +30% luminosity with additional crab crossing.
Recover Phase II performance if new triplet is b*= 10cm type AND if larger angle crab crossing would be successful.
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14. 4g- P.II: Decrease of beta*
To improve significantly the luminosity by a b* reduction, a modification of the crossing scheme or parameters is mandatory
(Sep1 = 5)
(Sep1 = 3)
(Sep1 = 3)
PAC07
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15. 4h- P.II: Early separationD0
Full Early Separation (50 ns only if D0 not in inner detector)
First encounter D0
First encounter
Partial Early Separation (25 or 50 ns)
We need a residual crossing angle
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16. 4i- P.II: Detector geometrical constraints
We cannot put the D0 in the inner detector.
There are potential slots starting at 3.5 m and 6.8 m (ATLAS).
A “partial” early separation should be considered
Courtesy of M. Nessi, ‘Machine upgrade, ATLAS considerations’, June 2006
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17. 4j- P.II: Decrease of beta*
Merits: modification of LHC only in IR’s with no consequence for the global machine; no beam current increase beyond the agreed LHC & INB programs (collimation, machine and radiation protection); same beam dynamics mode and operations strategy; easy luminosity leveling, expected faster build-up of performance related to a lower complexity; compatible with 25 and 50 ns spacing (with reduced performance by 2), mild upgrade of injectors but benefits from an injector upgrade program.
Challenges: installation of dipoles deep inside the detectors, higher chromatic aberrations, a few encounters at a reduced beam separation, lower integrated luminosity per run.
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18. 4k- P.II: Decrease of beta*
Investments: {new triplet (b*= 10 cm), D1}, D2, one matching quad?, 4 early sep. dipoles, optional crab cavities and electron lenses, improvements in injectors.
What if: - if reduced separation not acceptable: full recovery possible but using new untested solutions: i) e-lens compensation or ii) increase separation and use crab or iii) turn to 50 ns operation with a loss by  2, that could be compensated by some current and bunch length increase (other strategy).
if chromatic aberrations too large: fast decrease with increase of beta*, reduction of l*, Q0, or achromatic collimation insertions?
If conceptual problem, turn to intensity increase with same hardware in IR’s.
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19. two draft upgrade scenarios compromises between heat load
20/03/2000
parameter
symbol
25 ns, small b*
50 ns, long
transverse emittance
e [mm]
3.75
protons per bunch
Nb [1011]
1.7
4.9
bunch spacing
Dt [ns] 25 50
beam current
I [A]
0.86
1.22
longitudinal profile
Gauss
Flat
rms bunch length
sz [cm]
7.55
11.8
beta* at IP1&5
b* [m]
0.08
0.25
full crossing angle
qc [mrad]
381
Piwinski parameter
f=qcsz/(2*sx*)
2.0
hourglass reduction
0.99
peak luminosity
L [1034 cm-2s-1]
15.5
10.7
peak events per crossing
294
403
initial lumi lifetime
tL [h]
2.2
4.5
effective luminosity
(Tturnaround=5 h)
Leff [1034 cm-2s-1]
3.6
3.5
Trun,opt [h]
4.6
6.7
e-c heat SEY=1.4(1.3)
P [W/m]
1.04 (0.59)
0.36 (0.1)
SR heat load K
PSR [W/m]
0.36
image current heat
PIC [W/m]
0.33
0.78
gas-s. 100 h (10 h) tb
Pgas [W/m]
0.06 (0.56)
0.09 (0.9)
extent luminous region
sl [cm]
3.7
5.3
comment
D0 + crab
wire comp.
two draft
upgrade
scenarios
(courtesy F. Zimmermann, Valencia 2006)
compromises
between
heat load
and # pile up
events
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20. IP1& 5 luminosity evolution for 25-ns and 50-ns spacing
20/03/2000
IP1& 5 luminosity evolution for 25-ns and 50-ns spacing
F. Zimmermann
25 ns
spacing
50 ns
spacing
average
luminosity
initial luminosity peak
may not be useful for physics
(set up & tuning?)
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21. 4n-P.II: Luminosity leveling
The relatively fast luminosity decay and high multiplicity call for Luminosity Leveling.
…but the issue is how to do it efficiently:
dynamic beta*: uses existing hardware; probably complex due to large number of side-effects in IR’s AND arcs.
dynamic bunch length: needs new RF; possible side effects in whole machine related to modification of peak current.
dynamic crossing angle: using the early separation hardware, no side effects identified. Even better: use crab.
EXCEPT, valid for all, a modulation of the length of the luminous region.
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22. 4p-P.II: Luminosity leveling
Multiplicity around 50
G. Sterbini
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23. 4p-P.II: Learning period
Performance rise depends on complexity. Statistical law by V. Shiltsev. Using/extending his approach yields:
complexity
luminosity
The strategy with beam current increase requires about 3 years after Phase I (4 years without).
In the ISR, a comparable beta* decrease (/7) took a few weeks at reduced current; one year for the LHC at full current?
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24. 4q- P.II: LHC technological challenges
Triplets: the key issue (challenge, lead time, cost) The most promising technology is Nb3Sn for larger field (50%) and larger temperature margin. A very recent success for this delicate technology: US-LARP TQS02a reached ~ 11T peak. The aperture barrier of 90 to 110 mm is being jumped (stress limit) and the required length (~9 m) should not cause an additional problem.
Fall-back solution: very large and long low gradient Nb-Ti triplet for larger temperature margin & ability to collect the collision debris on masks; presently considered for b*≥ 25 cm but might be pushed further (?)
Energy deposition: being included in magnet design
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25. 4r- P.II: LHC technological challenges
Early separation dipoles: feasibility study done (e-m and power deposition). It appears so far technically doable. Before further studies, an assessment of its impact on the detection of particles is needed for ATLAS (organized) and CMS (to be organized).
Wire compensation: promising results in SPS; implemented in RHIC by US-LARP and under study.
Crab crossing: under test at KEK (electrons); being considered for US-LARP R&D: accuracy challenging.
Electron lenses: implemented at Tevatron; considered for installation in RHIC and support by US-LARP: very challenging for full beam-beam compensation.
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26. 5-The energy upgrade
The progress of super-conducting magnet technologies opens the possibility to consider doubling the LHC energy with Nb3Sn (25 TeV c.m.) or perhaps tripling it with emerging technologies based on HTS superconductors.
This would be a major upgrade of the CERN accelerator complex that requires feasibility studies yet to be done.
The first and critical element is the availability of collider quality high-field superconducting magnets that can stand or are shielded from the emitted synchrotron radiation.
The phase II studies do not include the energy upgrade but should be used to prepare and e.g. foster external contributions if the physics motivation is expressed/confirmed.
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27. 11/16/2018
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28. Conclusion
The upgrade of the LHC luminosity is a natural and necessary development for such a unique facility; it is very challenging. A two-phase strategy minimizes the increments in complexity.
Two roads are considered to reach the second phase with ~10 in peak luminosity. They are both promising with different challenges and risks. One should aim, if possible, at combining both for a robust solution.
A tight collaboration with the experimentalists and sharing of risks is needed, e.g. for an early separation scheme, for the luminosity leveling options,…
The technology needed requires the R&D program proposed in the White Paper and the joint effort of the community: US-LARP, CARE and, of course, the Experimenters…
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29. Annexes Variation of the luminous region with dynamic c
Expression of the F factor
Nominal LHC parameters
Minimum crossing angle
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30. Variation of the luminous region with dynamic c
Sterbini
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31. luminosity reduction factor from crossing angle
20/03/2000
luminosity reduction factor from crossing angle
Piwinski angle
nominal LHC
Zimmermann
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32. 11/16/2018
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33. 11/16/2018
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34. Ruggiero
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