Beam Losses and Background Loads on Collider Detectors Due to Beam-Gas Interactions in the LHC Page: 3 of 4
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sections of these processes are calculated using the
Glauber model with inelastic corrections. The
contribution of electromagnetic elastic (Coulomb)
scattering is calculated using the screened Rutherford
cross section. Atomic and nuclear screening is taken into
The calculated differential cross sections of proton-
carbon elastic scattering at 7 TeV are shown in Fig. 1. At
angles 0 > 7 grad, responsible for beam loss in LHC, the
role of Coulomb scattering is diminishing.
1 Nuclear incoherent
to _ 0=7 Arad
Figure 1: Differential cross
elastic scattering at 7 TeV.
q' in (GeV/C)2
sections of proton-carbon
MODELING BEAM-GAS INTERACTIONS
As with beam-gas inelastic collisions, the rate of
Coulomb scattering and nuclear elastic beam-gas
interactions is proportional to the beam intensity and
residual gas pressure in the beam pipe. Longitudinally it
follows the pressure maps . The points of beam
interactions with residual gas nuclei are sampled from the
data shown in Fig. 2.
10 - H
0 5000 10000 15000 20000 25000
P1, P2, IRS, IRS +/-- 270Gm
For the nominal beam current of 0.6 A, one gets
1.154x108 nuclear elastic and Coulomb interactions per
second in the ring for each of the two beams. Multi-turn
modeling of these interactions was performed separately
for Beam 1 and Beam 2 with the STUCT code . All
the apertures were included in the model. Fig. 2 shows
calculated beam loss distribution around the ring for
Beam2. The first maximum at -6500 m corresponds to
IP7, second at -13182 m to tertiary collimators at IP5,
third at -20000 m to IP3, and fourth to the tertiary
collimators at IPL.
0 5000 10000 15000 20000 25000
Path length, m
Figure 3: Beam loss distribution along the LHC ring for
Beam 2 elastically interacted with residual gas.
The Beam 2 loss rates (MHz) are 35.78 (IP7), 3.07
(TCT.R5), 7.19 (TCT.L1) and 2.08 (rest of the ring).
Similarly, the loss rates (MHz) for Beam 1 are 33.81
(IP7), 7.05 (TCT.L5), 4.68 (TCT.R1) and 2.58 (rest of the
ring). With the mandatory tertiary collimators in IPI and
IP5, the products of elastic beam-gas interactions in the
ring are locally intercepted by these TCTs. With the arcs
being the main source of this component, the role of the
limiting aperture of the betatron cleaning system in IP7 is
quite obvious: for Beam 1, and a of the ring contribute
to loss rate in IPI and IP5, correspondingly, while for
Beam 2, the situation for IPI and IP5 is reversed.
Fig. 4 shows a distribution of the number of turns an
elastically scattered proton of Beam 1 or Beam 2 makes
before being intercepted by the IPI and IP5 tertiary
collimators. Most of the scattered protons are intercepted
by TCTs on the first turn, while some of them make 2, 3
and more turns before being lost on the tertiary
10 -- H
- - c
1 50 100 150 200 250 300
Figure 2: Density of residual gas molecules in the LHC
ring (top) and interaction regions (bottom).
0 1 2
4 5 6~
Figure 4: Contribution of elastically scattered protons to
the loss rates on TCTs as a function of a number of turns.
- U -
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Drozhdin, A.I.; Mokhov, N.V.; Striganov, S.I. & /Fermilab. Beam Losses and Background Loads on Collider Detectors Due to Beam-Gas Interactions in the LHC, article, April 1, 2009; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc935212/m1/3/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.