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, 28-31 May 2017
HL-LHC Machine
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)
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.
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.
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 beambeam 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.
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.
Vertical vs Horizontal Crossing
Vertical crossing:
Crossing
-170 µrad → -160 µrad
Separation
-3.5 mm → -0.5 mm [2→7 TeV]
Crossing plane 0 → 90° [2→7 TeV]
V Angle offset -40 µrad → 0 [2→7 TeV]
β*
10 m → 1.4 m [2→7 TeV]
Horizontal crossing:
Crossing
-170 µrad → -270 µrad
Separation
-3.5 mm → -1 mm [2→7 TeV]
Crossing plane 0
V Angle offset -40 µrad → 0 [2→7 TeV]
β*
10 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.
Vertical vs Horizontal Crossing
Vertical crossing:
Crossing
-170 µrad → -160 µrad
Separation
-3.5 mm → -0.5 mm [2→7 TeV]
Crossing plane 0 → 90° [2→7 TeV]
V Angle offset -40 µrad → 0 [2→7 TeV]
β*
10 m → 1.4 m [2→7 TeV]
Horizontal crossing:
Crossing
-170 µrad → -220 µrad
Separation
-3.5 mm → -1 mm [2→7 TeV]
Crossing plane 0
V Angle offset -40 µrad → 0 [2→7 TeV]
β*
10 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.
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
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.
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.
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.
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.
Comparison of LHCb Settings
IP1/5 Beta* [m]
LHCb@1e34 Hz/cm2 (HO), 250 urad
•
•
•
•
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.
15
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.