1. Operating IP8 at high luminosity in the HL-LHC era
G. Arduini, R. De Maria, N. Karastathis,
Y. Papaphilippou, D. Pellegrini, R. Tomas
Beyond the LHCb Phase-1 Upgrade, May 2017

2. HL-LHC Machine

3. LHC/ HL-LHC Plans Preparation for HL-LHC
Integrate as much luminosity compatibly with the expected lifetime of the triplet (≥300 fb-1) in ATLAS/CMS
Use LIU beams in the LHC compatibly with the expected limitations:
Pile-up and pile-up density in the experiments → limiting the bunch population
Impedance → limiting the bunch population
Heat load due to Electron cloud → limiting the number of bunches
Limit in the instantaneous luminosity imposed by the triplet (bayonet heat exchanger)

4. HL-LHC ATLAS and CMS Targets
Nominal scenario
Ultimate scenario
After LS4, proton physics days increase from standard 160 days to 200 and after LS5 to 220
Reach 3000 fb-1 and possibly 4000 fb-1 during HL-LHC lifetime:
Maximize availability, levelled luminosity per bunch, number of bunches, bunch population, brightness.
Crab-cavities to alleviate geometric reduction factor and pile-up density due to small β* and long bunches.

5. LHCb Targets LHC Phase I Phase II Max Luminosity [1034 cm-2s-1 ] 0.04
0.2
1-2
Time frame
Run II
HL-LHC
HL-LHC baseline for LHCb is Phase I:
Collisions in LHCb do not perturb beam lifetime (besides burn-off) thanks to low luminosity, extensive separation levelling, large external crossing angle
Increase of integrated luminosity obtained as beam become brighter thanks to machine upgrades with the same LHC parameter
Phase-II upgrade is under study and poses the additional challenges:
Instantaneous luminosity comparable with the one of Atlas/CMS
Machine parameters needs to be pushed to obtain higher luminosity
Long levelling duration not possible unless reducing integrated luminosity
Bayonet heat exchanger limits luminosity to 1.6 x 1034 cm-2s-1.
2017 2fb-1 at 4e32 for

6. Challenges in machine parameters
To integrate more luminosity, one needs to:
Reduce β* in IP8 below 3 m to reduce beam-size at the IP, however:
Limited by the optics flexibility and interplay with optics matching in Point 1.
Limited by the aperture in the triplet and protecting devices (e.g. TCDDM)
Increase the impact of the luminosity geometric reduction factor (e.g. also increasing the impact of polarity switches) and reduce lifetime due to beam-beam long range interactions
Enhance impact of field imperfection and chromatic aberration
Reduce crossing angle can mitigate the luminosity geometric reduction factor and aperture limits, however
Reduce luminosity lifetime further
Increase tune spread, tune difference between bunches
Reduced levelling time (not clear if desired by the experiments):
Implies larger tune spread at the beginning of the fill, when it is at the peak and is detrimental to lifetime.

7. Aperture and crossing angle
Horizontal and vertical external crossing angles (rotated during the ramp) can be considered:
Horizontal crossing aperture limited by TCDDM (Beam 2) then triplet
Vertical crossing aperture limited by triplet (less aperture due to beam screen rotation)
Crossing angle limited by orbit corrector strength to 310 µrad (with 20 µrad margin and repaired MCBY)
Protected aperture assumed at 14.6 σ (smallest aperture for worst phase advance from MKD).
Maximum crossing angle without or (*) with new TCDDM
from aperture consideration only
β* [m]
H* [µrad, σ]
H [µrad, σ]
V [µrad, σ] 1
±220, 15.5
±165, 11.6
±115, 9.9
1.4
±270, 22.5
±220, 18.3
±160, 15.4 2
±310, 30.9
±265, 26.3
±205, 22.6 3
±310, 37.5
±310,37.5
±250, 30
Crossing angles may not be feasible due to LRBB interactions.

8. Vertical vs Horizontal Crossing
Vertical crossing:
Crossing µrad → -160 µrad
Separation mm → -0.5 mm [2→7 TeV]
Crossing plane 0 → 90° [2→7 TeV]
V Angle offset -40 µrad → 0 [2→7 TeV]
β* m → 1.4 m [2→7 TeV]
Horizontal crossing:
Crossing µrad → -270 µrad
Separation mm → -1 mm [2→7 TeV]
Crossing plane 0
V Angle offset -40 µrad → 0 [2→7 TeV]
β* m → 1.4 m [2→7 TeV]
TCDDM replaced
Still better trade-off with H crossing at constant aperture, despite complex gymnastic during the ramp. Rotation of the beam screen would reduce the gap, but clear advantage for V crossing.

9. Vertical vs Horizontal Crossing
Vertical crossing:
Crossing µrad → -160 µrad
Separation mm → -0.5 mm [2→7 TeV]
Crossing plane 0 → 90° [2→7 TeV]
V Angle offset -40 µrad → 0 [2→7 TeV]
β* m → 1.4 m [2→7 TeV]
Horizontal crossing:
Crossing µrad → -220 µrad
Separation mm → -1 mm [2→7 TeV]
Crossing plane 0
V Angle offset -40 µrad → 0 [2→7 TeV]
β* m → 1.4 m [2→7 TeV]
TCDDM not replaced
Still better trade-off with H crossing at constant aperture, despite complex gymnastic during the ramp. Rotation of the beam screen would reduce the gap, but clear advantage for V crossing.

10. Peak and Integrated luminosity
Estimates for 2.5 µm/γ, 2524 colliding bunches, ±250 µrad based on scaling from nominal scenario.
Large impact of spectrometer polarity on luminosity.
Needs to push β* and reduce levelling to maximize integrated luminisity

11. Luminosity evolution β* [m] 2 Angle [µrad] ±250 Polarity -
Virtual Lumi
[1034 cm-2s-1]
1.0
Pushed parameters not confirmed.
Short to no levelling.
For illustration based on scaling.

12. Luminosity evolution β* [m] 2 Angle [µrad] ±250 Polarity +
Virtual Lumi
[1034 cm-2s-1]
1.54
Pushed parameters not confirmed.
Short to no levelling.
For illustration based on scaling.

13. Luminosity evolution β* [m] 1.4 Angle [µrad] ±250 Polarity -
Virtual Lumi
[1034 cm-2s-1]
1.25
Pushed parameters not confirmed.
Short to no levelling.
For illustration based on scaling.

14. Luminosity evolution β* [m] 1.4 Angle [µrad] ±250 Polarity +
Virtual Lumi
[1034 cm-2s-1]
2.14
Pushed parameters not confirmed.
Short to no levelling.
For illustration based on scaling.

15. Comparison of LHCb Settings
Hz/cm2 (HO), 250 urad
IP1/5 Beta* [m]
Working point exists with enough margin allowing to run in the nominal scenarios.
Both the beam separation and the crossing angle of LHCb, have a visible impact on DA therefore lifetime.
More pushed LHCb settings require larger crossing in Atlas and CMS increasing the pileup density and potentially a shortening of the levelling time.
Studies with reduced β* and in other points of the fill are pending.

16. Conclusion
Beginning of the levelling is the most critical point during the fill during which high luminosity from LHCb would further constrain the operating conditions.
Aperture considerations limit the minimum beta* to ~1.4 m and a half crossing angle of ~250 um and this requires HW modifications (TCDDM) not in baseline.
Preliminary investigations indicate that the beam-beam effects are not negligible even with well optimized settings.
Further simulations and machine studies to test these configurations are necessary.
Vertical crossing does not give clear advantages.
There is a limit in the triplet heat load at a luminosity of 1.6 x 1034 cm-2s-1 at 7 TeV.
New TAS-TAN shielding for increased radiation to be studied.