568
M. Lamont et al.
9.8.7 Luminosity levelling
For colliders limited by pile-up, the integrated luminosity can be significantly
improved by implementing a luminosity levelling scheme that initially reduces the
peak luminosity and levels it at a constant value that yields an acceptable pileup throughout a physics fill [100, 101]. In case different detectors with different
levels of acceptable pile-up are installed in a collider (e.g. at the LHC), it also
allows satisfying the requirements of the low pile-up detectors without impairing
the performance of the high luminosity experiments.
The effective luminosity lifetime increases when levelling the luminosity due
to the slower burn off rate and, for certain means of levelling (e.g. offset, crossing
angle), weaker head-on beam-beam effects. The luminosity evolution over a fill with
luminosity levelling consists of a first part, where the levelling is used to reduce the
instantaneous luminosity to a certain target value, possibly followed by a second
part where the luminosity is left to decay without levelling (Fig. 9.20).
If the beam emittance is constant and beam losses are dominated by burn off
while levelling, the beam current decays linearly and the levelling time is directly
proportional to the initial beam current:
T lvl =
1 −
L lvl
L peak
n b N i
n IP σ tot L lvl
(9.83)
where n b is the number of bunches with initial intensity N i , L peak is the theoretical
peak luminosity, L lvl is the levelled luminosity, σ tot is the cross-section for the
interaction of beam particles, and n IP is the number of IPs.
In the following, the machine parameters commonly used to control the luminosity for levelling are discussed.
Fig. 9.20 Luminosity
evolution in a burn-off
dominated scenario with and
without levelling. In the
levelled case, the peak
luminosity is initially levelled
down by a factor of 0.4. The
dotted line shows the
“virtual” luminosity which is
reduced to the target through
levelling. The levelling time
is T lvl = 15 h
0
5
10
15
20
25
30
0
2
4
6
8
10
Time in collisions [h]
Luminosity [10
34
cm
−2
s
−1
]
leveling
no leveling
M. Lamont et al.
9.8.7 Luminosity levelling
For colliders limited by pile-up, the integrated luminosity can be significantly
improved by implementing a luminosity levelling scheme that initially reduces the
peak luminosity and levels it at a constant value that yields an acceptable pileup throughout a physics fill [100, 101]. In case different detectors with different
levels of acceptable pile-up are installed in a collider (e.g. at the LHC), it also
allows satisfying the requirements of the low pile-up detectors without impairing
the performance of the high luminosity experiments.
The effective luminosity lifetime increases when levelling the luminosity due
to the slower burn off rate and, for certain means of levelling (e.g. offset, crossing
angle), weaker head-on beam-beam effects. The luminosity evolution over a fill with
luminosity levelling consists of a first part, where the levelling is used to reduce the
instantaneous luminosity to a certain target value, possibly followed by a second
part where the luminosity is left to decay without levelling (Fig. 9.20).
If the beam emittance is constant and beam losses are dominated by burn off
while levelling, the beam current decays linearly and the levelling time is directly
proportional to the initial beam current:
T lvl =
1 −
L lvl
L peak
n b N i
n IP σ tot L lvl
(9.83)
where n b is the number of bunches with initial intensity N i , L peak is the theoretical
peak luminosity, L lvl is the levelled luminosity, σ tot is the cross-section for the
interaction of beam particles, and n IP is the number of IPs.
In the following, the machine parameters commonly used to control the luminosity for levelling are discussed.
Fig. 9.20 Luminosity
evolution in a burn-off
dominated scenario with and
without levelling. In the
levelled case, the peak
luminosity is initially levelled
down by a factor of 0.4. The
dotted line shows the
“virtual” luminosity which is
reduced to the target through
levelling. The levelling time
is T lvl = 15 h
0
5
10
15
20
25
30
0
2
4
6
8
10
Time in collisions [h]
Luminosity [10
34
cm
−2
s
−1
]
leveling
no leveling