Chapter 1
LHeC: The Option of a TeV energy scale LeptonHadron Collider using the LHC Infrastructure
This chapter describes the conceptual design proposal for a future Lepton-Hadron
collider of unprecedented luminosity and energy, which is based on the existing
LHC infrastructure.
1.1 Introduction
The Large Hadron electron Collider (LHeC) project provides the unique
possibility to explore lepton-proton collisions in the TeV Center of Mass (CM)
range, for the study of new phenomena in the partonic structure of protons and
nuclei, precision Higgs physics and the search for physics beyond the Standard
Model of particle physics [1,2]. The LHeC may become the first electron-ion
collider ever built. The LHeC is designed to use one of the hadron beams of the
LHC in a synchronous operation mode. It therefore represents an important
opportunity for a further exploitation of the LHC existing infrastructure and its
massive investment already taken and to come. Achieving CM collision energies
in the TeV range with a 7 TeV energy proton beam demands the lepton beam
energy to significantly exceed the electron beam energy of HERA (27.6 GeV)
[3], the first ep collider built. The presented conceptual design of the LHeC [1] is
based on a lepton beam energy of 60 GeV, with an option considered of much
higher energy (140 GeV).
Following a first study of the LHeC [4], a more comprehensive design study
was initiated by the CERN Scientific Program Committee (SPC) in autumn 2007,
followed by the mandate from the CERN directorate and the European
Committee for Future Accelerators (ECFA) to provide a conceptual design study
on the physics, the accelerator and a detector for the LHeC. This mandate was
soon after also supported by the Nuclear Physics European Collaboration
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Committee (NuPECC). An international LHeC study group was formed, today
comprising about 200 physicists from 75 institutes, which for working out the
“Conceptual Design Report” (CDR) [1] conducted under the auspices of CERN,
ECFA and NuPECC four workshops, see [5], in the time between September
2008 (held at Divonne, Switzerland) and June 2012 (Chavannes-de-Bogis,
Switzerland). Prior to its publication in July 2012, the CDR was peer reviewed
by more than twenty experts on the various LHeC topics who had been invited by
the CERN directorate for scrutinizing the design.
The LHeC has been designed for exploitation in parallel with the HL-LHC
operation over a time scale of approximately a decade. Synchronous pp and ep
operation provides the possibility for collecting a total integrated luminosity of
the order of 100 fb-1 with peak ep luminosities of the order of L = 1033 cm-2 s-1.
The luminosity prospects are thus exceeding the HERA achievements by about
two orders of magnitude. The total electrical power consumption of the electron
facility was limited to 100 MW. The CDR describes in some detail two options
for the LHeC: a so-called Ring-Ring (R-R) and a Linac-Ring (L-R) option, which
are conceptually different in the realization of the electron beam, and are both
sketched here. The electron beam energy was set to 60 GeV, for both R-R and LR, a value between the beam energies of LEPI and LEPII and not too demanding
in either of the two configurations. This value may be altered in a further design.
P X4 6
P Z4 5
Point 5
Point 4
P M4 5
P X5 6
P M5 4
TX4 6
UJ 4 7
UJ 4 6
R A4 7
UX4 5
UJ 4 4
R A4 3
UJ 5 3
RE 48
RE 52
UP 5 3
P M5 6
UL 54
RR 53
UJ 5 7
UXC5 5
US 4 5
UW 4 5
TD 62
TU5 6
UJ 5 6 1
CMS
P M6 5
Fig.
R A6 3
UJ 63
UA6 3
UJ 6 4
UX6 5
the additional lepton
UJ 6 6
UL6 4
R A6 7
RE 32
UW 6 5
US 6 5
UL6 6
UA6 7
UJ 6 7
P M3 2
UJ 6 8
TD 6 8
TZ3 2
Schematic
layout of the LHC with
TX6 4
UJ 3 3
UJ 3 2
1.1.
P Z6 5
UJ 62
RE 58
RE 62
RE 42
RE 38
P X6 4
UP 62
UL5 6
US C5 5
UA4 3
PZ 33
UD 62
RR 57
UA4 7
UL4 6
UL4 4
R Z3 3
Point 6
UJ 5 6
UJ 4 3
P o in t 3 . 3
RF
R Z5 4
P o in t 3 . 2
UD6 8
UP 68
N
RE 68
beam bypasses (marked
in blue) around the
RE 28
ATLAS
Point 7
UJ 2 7
Point 2
UA2 7
R A2 7
P GC2
UL 26
experiments
P M2 5
UP 2 5
UW 2 5
UJ 7 6
eP X2 4
Point 8
UL 24
SPS
UX2 5
ALICE
CMS
for
the
RR7 3
TZ7 6
US 2 5
UJ 2 6
and
RE 72
P M7 6
p
P o in t 1 . 8
R A2 3
RE 22
RH2 3
TT 40
Point 1
UJ 23
UJ 2 4
RE 78
PM 18
P M1 5
P X1 6
RE 18
UJ 2 2
ATLAS
P X1 5
UW 8 5
P X8 4
PGC 8
TI 8
US 1 5
UJ 1 8
UL1 4
R R 1 7 UJ 1 7 UL1 6
UJ 1 4
RT 18
UJ 1 3
TX8 4
UJ 86
RA 87
UX1 5
US A1 5
UJ 8 7
RE 88
RH 87
UJ 12
UJ 1 6
UJ 88
R A8 3
UJ 8 4
RE 12
RR1 3
RF
PMI 2
R T1 2
e+, e- injector
an
interaction at Point 2.
UJ 83
UA8 7
UL 86
TI 2
and
UJ 8 2
US 85
TJ 8
P X1 4
LHeC
UA 83
P Z8 5
UL 84
LS S 4
RR7 7
RE 82
PM 85
UA2 3
Ring-Ring option of the
UX 85
LHCB
LEP
LHC
LHeC, RF
ep
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1.2 Ring-Ring Option
The Ring-Ring configuration features the installation of a new electron
synchrotron storage ring inside the LHC tunnel, on top of the existing LHC ring.
The R-R option is technically rather straightforward (in between the technologies
of the LEP-I and LEP-II projects). It yet is challenging as it requires integration
work inside a tunnel with an already operational accelerator. Around the existing
experiments for the HL-LHC accelerator configuration, bypasses have to be built
and connected, in which the RF for the electron beam can be housed. Fig. 1.1
illustrates the schematic layout of the LHC with the additional lepton beam
bypasses around the ATLAS and CMS experiments [1]. Each bypass has a total
length of ca. 1.2km and provides a maximum beam separation of about 20m with
respect to the proton beam Interaction Points (IP). Fig. 1.2 shows the schematic
bypass layout around the CMS experiment in the LHC Interaction Region 5. The
ATLAS bypass concept exploits the existing survey gallery in IP1 in order to
minimize the size of the bypass extension around the large ATLAS cavern. It
thus consists of two separate parts left and right to the IR1 of about 500m each,
see Fig 1.3.
Fig. 1.2. Schematic layout of the CMS bypass in IP5: Top view. The bypass has about 2 times
600m length. The RF is installed in a parallel tunnel of 120m length, at a distance of about 8m to
the bypass tunnel.
Fig. 1.4 illustrates a typical cross section of the LHC tunnel with the LHC
equipment and the space required for the additional components for the electron
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beam. The position of the electron synchrotron magnets is shifted inwards with
respect to the LHC magnets in order to compensate for the path length increase
due to the bypasses and to generate a circumference for the LHeC lepton beam
equal to that for the proton beams of the LHC machine.
Fig. 1.3. Schematic layout of the LHeC bypass (blue) of ATLAS (grey) in IR1. The bypass has
two parts and the two 60m long parallel tunnels are designed to house part of the RF for the
electron beam. Each side is accessed with a separate shaft.
As an example of space limitation for the installation of a new electron storage
ring, Fig. 1.5 shows the existing LHC installations in IR 4. However, in spite of
such limitations in the CDR [1], a lattice beam optics could be designed and
principally shown to work exploiting asymmetric FODO cells. The interference
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with the existing LHC installations is large but as discussed in [1] not
insurmountable. The main challenge for the R-R solution would be the time and
risk it took for installation.
Fig. 1.4. Typical cross section of the LHC tunnel with the LHC equipment and the space required
for the additional components for the electron synchrotron (red box), adjusted to ensure equal
circumferences of the e and the p beams.
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Fig. 1.5. View on the LHC installation in Interaction Region 4, illustrating the tight space
restrictions for the installation of an additional electron storage ring at certain places inside the
LHC tunnel.
The interaction region for the LHeC, running synchronously with the LHC, has
the novel feature of accommodating three beams: the colliding proton and lepton
beams and the non-colliding second proton beam of the LHC. A schematic view
of the interaction region layout for the R-R solution is shown in Fig.1.6. The lowbeta electron beam quadrupoles are placed close to the detector and are shifted in
the horizontal plane to generate an additional dipole deflection field for
separating the lepton beam from the proton beam left and right from the IP,
making them effectively act as combined function magnets. They are followed by
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additional dipole separation magnets for the electron beam (light blue colored
elements in Fig. 1.6) and then by low-beta quadrupole magnets for the proton
beam (in red).
0.3
Electron final
focusing quads
0.2
x [m]
0.1
Proton final
focusing half
quadrupole
Electron beam
0
Proton beam
-0.1
SR Absorber
Electron separator
dipole
-0.2
-0.3
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
z [m]
Fig. 1.6. Schematic layout of the LHeC interaction region in the R-R machine configuration with
maximum coverage of the polar angle acceptance by the detector placed between z=±6m.
Fig. 1.7 shows the schematic cross section of the first quadrupole magnet next
to the ep IP that is acting on the proton beam. It features a low field aperture
(right-hand side) for the lower energy electron beam, a high field region (lefthand side) for the colliding high-energy proton beam and two field free apertures
for the non-colliding proton beam (top and bottom of the magnet cross section in
Fig. 1.7). Table [1] summarizes the optics parameters for the electron beam while
Table [2] lists the main beam parameters and performance projections for the
LHeC R-R configuration indicating that a peak luminosity of about L = 1033 cm-2
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s-1 is reachable. Note that this projection is based on the so-called “ultimate”
proton beam parameters of the LHC.
Fig. 1.7. Cross-section of the first low-beta superconducting half quadrupole in the proton lattice.
The Lepton beam will pass on the right hand side of the mirror plate in a low field region.
The Ring-Ring option of the LHeC requires also the construction of a new
electron beam injector complex. The LHeC CDR [1] considers a compact and
efficient 10 GeV injector based on the principle of a recirculating Linac, taking
advantage of the studies for ELFE at CERN [6]. Such a low injection energy
requires dipole magnets with a well reproducible, low magnetic field, for which
prototypes had been successfully built both by BINP Novosibirsk and CERN [1].
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Table 1: Optics parameters of the LHeC e-ring lattice.
Electron beam energy
60 GeV
Phase advance per half cell, H/V
180o/120 o
Cell length
106.881 m
Dipole fill factor
0.75
Damping partition Jx / Jy / Je
1.5 / 1 / 1.5
Coupling constant κ
0.5
Horizontal emittance (no coupling)
3.96 nm
Horizontal emittance (κ = 0.5)
2.97 nm
Vertical emittance (κ = 0.5)
1.49 nm
Table 2: LHeC R-R performance estimate and main parameters
Parameter
Electron beam energy
Value
60 GeV
Proton beam energy
7 TeV
e+, e- intensity per bunch
2×1010
p intensity per bunch
#bunches
ep Luminosity (HL-layout)
Total wall plug power
Transverse lepton emittances  x,y (conservative estimate)
1.7 x 1011
2808
0.8×1033 cm-2s-1
100 MW
5.0nm, 2.5 nm
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1.3 Linac-Ring Option
The Linac-Ring option requires a new linear accelerator for the electron beam
that intersects in one location with the existing LHC machine. Several options
have been considered for the linear accelerator (pulsed, re-circulating and Energy
Recovery Linac configurations). These provide a range of energy and luminosity
combinations. The baseline option for the LHeC CDR is a recirculating 60 GeV
Energy Recovery Linac (ERL) which allows for high luminosity operation, is
based on a modest civil engineering effort and directly compares with the LHeC
Ring-Ring option. A pulsed linac option provides still an interesting option for
maximizing the energy reach of the LHeC (at the cost of a reduced peak
luminosity performance) as could be demanded by findings at the LHC. Table 3
summarizes key parameters for both options. The 60 GeV ERL version is
capable of reaching a luminosity as high as the R-R option. First considerations
have been made as to possibly reach a level of 1034 cm-2 s-1 which would enhance
the potential of the LHeC for precision Higgs measurements [2]. It is notable, as
was recently pointed out [7], that essentially the same machine in a 4-pass regime
and going to 80 GeV has an interesting application as a cost effective photonphoton collider for the study of the newly observed particle at 125 GeV.
Fig. 1.8 shows the schematic layout of the 60 GeV ERL option and Fig. 1.9
shows a 3D civil engineering illustration for the underground installation. It
features two 1km long Super Conducting 10 GeV linacs and a total of 6 return
arcs with a radius of curvature of ca. 1km that are installed in two half circular
tunnel sections. The ERL infrastructure requires a total of ca. 9km underground
tunnel length (a size comparable to the SPS installation at CERN or HERA at
DESY) and features a total bending arc length of ca. 20km when accounting for
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Table 3: Key parameters for two options for the L-R version of the LHeC.
LINAC Parameters for the Linac-Ring Option
Operation mode
CW
Pulsed
Beam Energy [GeV]
60
140
Peak Luminosity [cm-2s-1]
1033
4 1031
Cavity gradient [MV/m]
20
32
RF Power Loss [W/cavity]
13-37
11
W per W (1.8K to RT)
700
700
Cavity Q0
2.5 1010
2.5 1010
Power loss/GeV at RT
0.51-1.44
0.24
RF length [km]
2
7.9
Total length (including return
9
7.9
Beam current [mA]
6.4
0.27
Repetition rate
-
10 Hz
Pulse length
-
5ms
arcs) [km]
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Fig. 1.8. Schematic layout of the 60 GeV ERL option.
Fig. 1.9. Civil engineering illustration for the underground installation of the ERL Linac-Ring
option with the existing LHC and SPS (white ring near the Atlas covern) installations.
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the 6 separate return arcs. The total number of accelerator components is about
6000 which is an order of magnitude less than the CLIC500 project requires.
While the schematic layout of the Linac-Ring interaction region of the LHeC
looks on first glance similar to the one of the Ring-Ring option it requires slightly
different IR features. Most notably, the L-R option requires additional dipole
magnets in the Interaction Region (designed to be integrated in the central
detector space together with the detector solenoid [1]) to bring the electron beam
to collision with the proton beam after the last accelerating passage in the ERL.
Moreover, higher gradient requirements arise for the Super Conducting half
quadrupole low-beta magnets of the proton beam, which most likely require the
use of Nb3Sn magnet technology.
1.4 LHeC Planning and Timeline
The LHeC study assumes that the LHC hardware will reach the end of its lifetime
by the end of the HL-LHC upgrade phase. The current planning (as of January
2013) for the HL-LHC upgrade assumes a full implementation of the HL-LHC
upgrade by the end of 2023. Assuming a total exploitation time of ca. 10 years
for both the HL-LHC and the LHeC project, this implies that the LHeC project
should start operation approximately parallel to the HL-LHC exploitation phase
[8]. Fig. 1.10 shows the resulting planning schedule for the LHeC project which
lead to launching dedicated R&D work on key technical systems by 2012. The
goal for the LHeC proposal had been set by CERN at the 2012 LHeC Workshop
to develop and drive R&D activities for key technical components and concepts
such that a final decision on the LHeC project implementation can be taken by
the time the LHC reaches its full energy and large statistics at 13 TeV CM energy
will be analyzed. In parallel the detector concept is being further developed.
Both the R-R and the L-R LHeC options have been demonstrated to be
technically feasible [1]. The Linac-Ring option has the key advantage that its
civil engineering and installation work can be performed mostly decoupled from
the LHC operation. This strategic advantage of the Linac-Ring option has lead to
the decision to concentrate the technical R&D work for the LHeC during the
years 2013 to 2015 on the Linac-Ring option. As this involves the most
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demanding parts (cavity and IR magnet in particular) it remains possible to
consider all the ep collision options eventuall, by 2016 in the light of then
expected conclusive first high energy LHC results. The total number of and the
main specifications for the normal conducting magnets in the arcs and SC RF
cavities are comparable for both LHeC options.
Fig. 1.10. LHeC project planning schedule [1] as considered for the current (1/2013) LHC
shutdown schedule and with the goal of maximizing the synchronous ep/pp operation time during
the HL-LHC phase.
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Chapter Title for Chapter 1 Headings
1.5 Key Technical Systems.
The R&D effort for key technical systems will focus on the following systems
for the coming 2 to 3 years:
 Superconducting RF technology for the development of cavities with
high Q0 for CW operation.
 Superconducting magnet technology for the development of Nb3Sn
magnet technology for quadrupole designs with mirror cross sections
with apertures for high as well as low magnetic field configurations.
 Optimization for the design of normal conducting magnets suited for
the return arcs of the Energy Recovery options with multiple magnet
systems per arc.
 Design of an LHeC Energy Recovery Linac test facility for studying
and testing the various technical components and building up
operational experience for ERL facilities in the future.
 Full optics and layout integration of the LHeC into the HL-LHC
project.
 Civil engineering studies for the Linac-Ring option.
 Design of the vacuum system for the experimental insertion of the
LHeC
 Detector development.
1.6 Summary.
The high energy, intense hadron beams of the LHC provide a unique opportunity
for complementing the pp and AA physics programme of the LHC, and possibly
that of a new pure lepton collider, with deep inelastic ep and eA scattering
experiments at the energy frontier. The LHeC design [1], briefly characterized
here, has been made for synchronous operation with the LHC, which is
considered to have an operation lifetime until the early thirties with an interesting
maximum luminosity programme. Unless future LHC results lead to new
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dramatic insight, the chosen energy of 60 GeV and the high luminosity, likely in
excess of a hundred times that of the first ep collider HERA, promise to be a
most reliable basis for an innovative physics program. The most likely realization
of the LHeC is then with a 9km circumference Energy Recovery Linac
configuration using multiple passes through two 1km superconducting linear
accelerators, for which a test facility is under design. For the next about three
years, an important design and prototype phase has begun in order to facilitate a
decision on the LHeC when the high energy data of the LHC will eventually help
clarifying the strategic future of particle physics and of CERN. In comparison to
super hadron collider machines of a new generation or linear lepton collider
concepts, the LHeC, while providing fascinating new accelerator challenges, is of
a less resource demanding nature. It reaches its ultimate performance owing to
the uniqueness of the LHC hadron beams for which most substantial investments
have been taken over many years. Not to use these for the LHeC “at some point
during the LHC lifetime appeared to be a waste” [9].
References:
1.
2.
3.
4.
5.
6.
7.
8.
9.
LHeC Study Group, A Large Hadron Electron Collider at CERN, J.Phys.G:
Nucl.Part.Phys 39(2012)075001 [arXiv:1206.2913(2012)].
LHeC Study Group, On the Relation of the LHeC and LHC, [arXiv:1211.5102(2012)].
HERA, Report by F. Willeke, this book.
J.Dainton, M.Klein, P.Newman, E.Perez and F.Willeke, Deep Inelastic Scattering at
the LHC, JINST 1(2006)P10001 [hep-ex/0603016].
For a comprehensive documentation on the LHeC see : http://cern.ch/lhec
K.Aulenbacher, B.Aune, J.Aysto, J.Baldy, H.Burkardt et al., ELFE at CDR,
Conceptual Design Report, CERN, Geneva (1999).
SAPHIRE reference
S.Myers, Talk at EPS2011, Grenoble, France.
G.Altarelli, LHeC Workshop, Divonne 2008, see [5].