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Phil. Trans. R. Soc. A (2012) 370, 3887–3923
doi:10.1098/rsta.2011.0053
REVIEW
The engineering needed for particle physics
B Y S TEVE M YERS*
CERN, 1211 Geneva 23, Switzerland
Today’s particle accelerators and detectors are among the most complicated and
expensive scientific instruments ever built, and they exploit almost every aspect of today’s
cutting-edge engineering technologies. In many cases, accelerator needs have been the
driving force behind these new technologies, necessity being the mother of invention.
This paper gives an overview of some engineering requirements for the construction and
operation of present-day accelerators and detectors.
Keywords: engineering; Large Hadron Collider; accelerator physics
1. Particle accelerators
Rutherford was the ‘godfather’ of accelerators; he challenged future generations
of accelerator builders to invent reliable machines that could accelerate particles
to higher and higher energies. In his inaugural presidential address to the Royal
Society in London in 1928, he said ‘I have long hoped for a source of positive
particles more energetic than those emitted from natural radioactive substances’.
This was the start of a long quest for the production of high-energy beams of
particles in a very controlled way.
Particle accelerators are also used in many different applications, such as
material analysis and modification, and spectrometry, especially in environmental
science. About half of the world’s 15 000 accelerators are used as ion implanters,
for surface modification, sterilization and polymerization. The ionization arising
when charged particles are stopped in matter is often used, for example, in
radiation surgery and therapy of cancer. At hospitals, about 5000 electron
accelerators are used for this purpose. Accelerators also produce radioactive
elements that are used as tracers in medicine, biology and material science. In
material science, ion and electron accelerators are used to produce neutrons and
photons over a wide range of energies. Well-defined beams of photons are, for
example, increasingly used for lithography in order to fabricate the very small
structures required in electronics.
In this paper, an overview is given of some of the engineering requirements
for construction and operation of modern-day accelerators. The subject is so
*[email protected]
One contribution of 16 to a Discussion Meeting Issue ‘Ultra-precision engineering: from physics to
manufacturing’.
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LHC
equivalent energy of a fixed-target accelerator (E2cm/2Mp)
1017 eV
1016 eV
TeV
1015 eV
100 TeV
Sppbar S
HERA
(ep)
LEP/SLC
storage rings
10 TeV
ISR
PETRA (e+/–)
FNAL/SPS
1 TeV
100 GeV proton synchrotron
weak focusing
10 GeV electron
AG
AG
SLED
Cornell
electron linacs
synchrocyclotrons
proton linac
synchrotron
1 GeV weak focusing
100 MeV
betatron
cyclotron
sector focused
cyclotron
10 MeV
electrostatic
generator
1 MeV
rectifier generator
100 keV
1930 1940 1950
academic 11 3/6/98 slide 2
1960
1970
1980 1990 2000
Figure 1. The history of accelerators. (Online version in colour.)
enormous that it is not possible in the limited space to go into any kind of
detail. For this reason, references to books and papers [1–36] are given for
further reading.
2. History
Particle accelerators and detectors have existed and been developed for more
than 80 years. During this time, many different types of accelerators have been
built and operated, and the equivalent maximum energy available in the collisions
has increased by 12 orders of magnitude from 100 keV to 1017 eV (figure 1). This
enormous energy increase has been brought about by increases in the energy of the
primary beams and the use of colliders that use collisions of beams as opposed to
colliding a primary beam with a fixed target. The sustained exponential increase
of the beam energy for the past 80 years has been achieved through repeated
jumps from saturated technologies to emerging technologies. The accelerators
and detectors have increased enormously in complexity, size and, of course,
cost. In recent years, the type of accelerator/collider most commonly (almost
exclusively) used for particle physics is the synchrotron. In CERN (Geneva), there
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CMS
LHC
2008 (27km)
North Area
ALICE
TT40
p SPS
T12
ATLAS
CNGS
2006
TT60
AD
TT2
1999 (182m)
p
n-ToF
2001
p (proton)
ion
BOOSTER
1972 (157m)
LHC Large Hadron Collider
p (antiproton)
ISOLDE
1989
p
LINAC 2 p
neutrons
LINAC 3
ions
neutrons
neutrinos
T18
1976 (7km)
TT10
LHCb
TT41
Gran Sasso
East Area
PS
1959 (628m)
CTF3
e–
LEIR
2005 (78m)
proton/antiproton conversion
SPS Super Proton Synchrotron
neutrinos
electron
PS Proton Synchrotron
AD Antiproton Decelerator CTF3 Clic Test Facility CNGS CERN Neutrinos to Gran Sasso ISOLDE Isotope Separator Online Device
LEIR Low Energy Ion Ring
LINAC Linear Accelerator
n-ToF neutrons Time of Flight
Figure 2. Schematic layout of CERN’s accelerator complex. (Online version in colour.)
are a total of nine accelerators operating on the site (see schematic in figure 2).
The CERN Proton Synchrotron (CPS) dates back to 1959, when it was first
brought into operation, and still operates with high availability. Most recently,
the largest accelerator and detectors to be brought into operation are the Large
Hadron Collider (LHC) machine and the four large detectors (ALICE, ATLAS,
CMS and LHCb), which are distributed around the LHC circumference.
3. Basics: what is a particle accelerator?
A particle accelerator:
— provides a ‘beam’ of highly energetic particles;
— employs a ‘vacuum chamber’ inside which the particles travel;
— employs electric fields to impart energy to ‘accelerate’ the particles in their
direction of motion;
— employs transverse ‘magnetic fields’ to steer and focus the beam; and
— makes ‘collisions’ either against a fixed target, or between two beams of
particles.
(a) Beam
The ‘beam’ refers to a stream of highly energetic particles moving at speeds
very close to that of light, analogous to a beam of light. The beam usually consists
of ‘bunches’ of particles longitudinally distributed around the circumference of
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Figure 3. LHC beam pipe with vacuum pumping slots. (Online version in colour.)
the accelerator. The bunching results from the acceleration (radio-frequency,
RF) process, which uses high-frequency cavities that resonate at a harmonic
of the revolution frequency of the particles (described later). For high-energy
accelerators, the typical properties of the beams are:
— a ‘bunch’ length of a few centimetres (measured in the direction of motion
of the particles);
— cross-sectional size of bunches below the millimetre scale (horizontally and
vertically at right angles to the direction of motion) and much smaller in
the interaction regions of the detectors;
— around 1011 charged particles per bunch;
— 3000 bunches per beam (LHC); and
— ultra-relativistic particle velocity; at 7 TeV (LHC) the particles are
travelling at 0.999 999 991 times the speed of light.
(b) Vacuum chamber
This is a metal pipe (also known as the beam pipe) inside which the beam of
particles travels (figure 3). It is kept at an ultra-high vacuum (pressures less than
10−10 Torr (approx. 10−10 mbar; approx. 10−8 Pa) to minimize the amount of gas
present to avoid collisions between gas molecules and the particles in the beam.
(c) Acceleration system
High-frequency longitudinal electric fields are generated in resonant cavity
structures. These fields provide incremental acceleration to the beam of particles
each time it passes the accelerating gap of the cavity (figure 4), once per revolution
of the accelerator circumference. The acceleration becomes cumulative when the
frequency of the accelerating electric field (E) is synchronous with a harmonic
of the revolution frequency of the beam. RF cavities are located intermittently
along the beam pipe. Each time a beam passes the cavity gap, some of the energy
from the radio wave is transferred to the particles.
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E
.
.
beam
Figure 4. Acceleration principle and some cavities. (Online version in colour.)
+
v
2
F = qvB = mv
r
r
B
+
magnetic field
out towards
observer
Figure 5. Particle motion in a bending field and cross section of an LHC dipole. (Online version
in colour.)
(d) Magnetic fields
(i) Bending
Various types of electromagnets are used to serve different functions. Dipole
magnets are used to bend the path of a beam of particles that would otherwise
travel in a straight line. The higher the energy a particle has, the greater the
magnetic field needed to bend its path (illustrated in figure 5).
For ultra-relativistic particles, the maximum energy for a beam in a
circular accelerator is simply proportional to the product of the maximum
dipole field strength (B) and the average radius of the particle trajectory
(r). Consequently, to produce the highest-energy beams requires very largecircumference accelerators as well as the maximum magnetic field that can be
produced (see below).
(ii) Focusing
Magnets having magnetic fields that vary linearly with displacement from
the magnetic centre (quadrupoles) are employed to focus the beams of particles
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y
N
By=
∂By
.x
∂x
S
S
x
N
Figure 6. Quadrupolar field and cross section of an LHC quadrupole. (Online version in colour.)
(figure 6), in analogy with the use of optical focusing lenses used for focusing a
beam of light.
(iii) DC powering
The coils of the electromagnets are fed by high-power, high-precision,
controllable DC power converters, which convert AC power into DC with very
low residual noise in the form of either ripple or long-term drift.
(e) Collisions
Counter-rotating beams are magnetically steered so that they collide at discrete
locations around the circumference of the collider.
Particle detectors are placed around these collision points to record the
new particles that are created by the high-energy collisions. Figure 7 shows
schematics of the ATLAS and CMS detectors at the LHC, indicating the
principal components.
4. Engineering requirements
In order to put into context the engineering requirements associated with particle
physics, it is useful to consider the various phases in the life cycle of a particle
collider complex. The life cycle phases can be listed as follows:
—
—
—
—
—
—
—
—
conception;
conceptual design;
technical development, prototyping, costing and proposal to build;
construction and procurement;
installation;
hardware commissioning;
beam commissioning; and
operation for physics data-taking.
(a) Conception
In this phase, the particle physics results obtained from previous experiments
steer the choice of the particle beam conditions needed for the next step forward.
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muon detectors
(a)
electromagnetic calorimeters
solenoid
ATLAS
Review. Engineering for particle physics
detector characteristics
width: 44 m
diameter: 22 m
weight: 7000 t
forward calorimeters
CERN AC-ATLAS V1997
end cap toroid
barrel toroid
inner detector
hadronic calorimeters
shielding
(b)
crystal calorimeter
silicon tracker
forward hadron
calorimeter
magnet
yoke
superconducting
solenoid magnet
hadron
calorimeter
muon
chambers
one of the 15
detector sectors
Figure 7. Exploded diagrams of the (a) ATLAS and (b) CMS detectors. (Online version in colour.)
To a large extent, the definition of a new project in particle physics involves
specifying the type of particles, the beam energy and the interaction rate. During
the past decades, the procedure followed has been to search for new results using
hadron beams, and, in the event of discovering new phenomena, to make precision
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measurements of the new particles by using a lepton collider. The most recent
examples of this are the discovery (1984) of the W and Z particles in the SppbarS
(Spp̄S) collider, followed by the precise measurement of the properties of these
particles in the CERN Large Electron–Positron (LEP) collider (1989–2000).
The types of particle to be accelerated have a very strong influence on
the design of the machine. For high-energy circular electron–positron colliders,
the important design consideration is the influence of synchrotron radiation.
The particle energy lost per turn scales with the fourth power of the relative
energy (g) of the beam (g = m/m0 , where m0 is the rest mass of the particle).
This lost energy must be replaced by the acceleration system. Consequently,
for such high-energy circular electron–positron colliders, the RF system is
the most critical, and the most costly. One of the key parameters in the
design of the RF system is the electrical power consumption. Consequently,
the LEP room-temperature cavities were designed to have maximum shunt
impedance so as to minimize the electrical power consumption. The upgrade
of LEP to higher energies (LEP2) used superconducting cavities in order to
minimize the power consumption as well as to allow much higher accelerating
gradient fields.
Even with the advent of superconducting cavities, the very strong dependence
of energy lost per turn on the beam energy imposes a severe limitation on the
maximum beam energy achievable in a circular lepton collider. For this reason,
recent studies for future electron–positron colliders concentrate on linear colliders,
in order to reduce the energy losses due to synchrotron radiation.
For protons, since the rest mass is about 2000 times that of an electron, the
energy loss per turn is much less of an issue, and consequently protons can be
accelerated to very high absolute energies. Hence, for proton colliders, in order
to increase the discovery potential, the goal is to achieve the maximum energy
in collisions. As shown previously, the maximum energy depends linearly on the
product of the bending radius of the trajectory of the tunnel and the maximum
bending field strength that can be achieved. The achievable bending field in roomtemperature magnets is limited by the saturation of the magnetic material to
around 2 T. Hence, for recent high-energy colliders such as the Relativistic Heavy
Ion Collider (RHIC), the Tevatron and the LHC, superconducting magnets have
been developed and have allowed operational fields of 5–8.5 T.
For both hadron and lepton colliders, the interaction rate is primarily
determined by the particle density at the collision points, i.e. the total number
of particles divided by the beam cross section. The beam size (cross section) for
hadron colliders is determined by the beam coming from the injectors, whereas
for lepton colliders, the properties of the synchrotron radiation determine the
beam size. In summary, the most stringent limitations to be considered for these
machines are the energy (and energy density) stored in the beam for hadrons
(protons) and the radiated power for leptons (electrons).
(b) Conceptual design
Following the identification of the type of particles to be accelerated, the
requested energy and the hoped-for event rate (luminosity), the conceptual
design phase begins. The outcome of this phase should ensure the feasibility of
the project, and produce a coherent set of parameters. The design should also
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1000
900
800
700
600
500
400
300
Jura
P4
170 m
altitude (m)
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molasse
limestone
0
1
P8
plain
2
3
4
1.4% slope of
5
6
SPS
moraine
LEP tunnel
7
8
9
10
11
distance (km)
Figure 8. The inclination of the LHC tunnel. (Courtesy of J. Osborne.) (Online version in colour.)
identify some of the most critical components and initiate more detailed technical
studies on these items. It is often necessary at this stage to identify the need for
demonstrators of the critical components. For example, in the case of the LEP
collider, demonstrators of the room-temperature RF cavities, the magnets and
the vacuum chamber were built. In order to design and build these components,
extensive use of existing electro-mechanical computer codes was necessary, as well
as developing the technical expertise in the CERN workshops. The experience
gained in building these demonstrators proved invaluable for the future success
of the project by identifying the most critical technical areas and allowing very
rough cost estimates.
In order to plan the civil engineering, extensive samples of the ground (rock)
conditions are required in the vicinity of the proposed tunnel and the surface
buildings. For the LEP/LHC tunnel, this analysis resulted in a repositioning of
the tunnel, as well as inclining the plane of the tunnel (by 1.4%) so as to follow
the approximate plane of the rock (figure 8). This obviated the need for extremely
deep vertical pits at the highest points of the trajectory of the tunnel. In addition,
these results reduce the risk as seen by the civil engineering contractors and
therefore produce lower bids.
In order to answer some of the more fundamental beam dynamics questions,
machine studies are often performed on existing accelerator facilities.
The conceptual design report should identify the most high-risk technical
components, as well as giving a first indication of the possible capital cost, running
costs and electrical power consumption.
(c) Technical development, prototyping, costing and proposal to build
Following the conceptual design phase, the detailed technical development
begins. This work involves first of all refining the technical details of all major
systems, including civil engineering, survey and geodesy, magnets, acceleration
system, beam instrumentation, vacuum system, cryogenic system, power supplies,
injection and extraction systems, and collimators. By definition, this implies
expertise in many branches of engineering and technology. For the most critical
components, demonstrators and prototypes often need to be built to confirm and
endorse their design and performance. For components that are needed in large
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numbers, engineering industrialization is a key process to reduce the total cost.
Some issues are specific to accelerator design:
— the precision of the circumference of the accelerator tunnel;
— configuration management to allow installation in the confined area of the
tunnel;
— the impact of ionizing radiation on electronic components; and
— the mechanical properties of materials when subjected to impacts by highenergy beams.
At this stage, it is of the utmost importance to take into account the multiple
interdependences of systems on other systems. In addition, owing to the fact
that a beam will only continue to circulate when every single component reacts
correctly within very tight tolerances, the mean time between failures must be
minimized for the systems. Sometimes this implies additional redundancy, with
concomitant cost increase.
The quality of the work done at this phase has a crucial impact on the
authorization or rejection of the project and the ultimate performance of the
machine.
(d) Construction and procurement
Following the project approval, the procurement and construction follow,
controlled by the project management.
Figure 9 shows the implementation of the LEP/LHC tunnel; the detailed
civil engineering numbers are given in table 1. Tunnelling was done by boring
machines, which were guided by the alignment/geodesy experts. The final result
was that the 27 km tunnel circumference was accurate to 2 cm, as later measured
by the beam itself.
Figure 10 shows a typical example engineering drawing and a final photograph
of the CMS tunnel at point 5.
Calls for tender are issued to European companies for all the major systems.
Contracts are given to the lowest bidders that have the technical capabilities
to complete the work within the given time scale. During construction, CERN
engineers regularly visit the factories to ensure the quality of the products and
to transfer technical expertise. This technology transfer is highly appreciated
by the contracting companies. Careful budget control is exercised throughout
this process, which finishes with the reception and technical acceptance of the
final products.
(e) Installation
The installation of an accelerator involves a wide range of disciplines,
particularly project planning, integration, heavy handling, transport, alignment,
survey, mechanical engineering, electrical engineering, cryogenics, vacuum,
quality assurance, safety, etc. Owing to the restricted space in the tunnel and
the high level of coactivity, very precise and rigorous planning is essential. Safety
is also an extremely important issue at this stage owing to many hazards existing
simultaneously, e.g. transport of heavy objects, pressurized vessels, high voltage
and oxygen deficiency.
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(a)
point 5
point 4
point 6
point 3.3
CMS
point 3.2
N
point 7
point 2
point 8
SPS
point 1.8 point 1
ALICE
existing structures
LHC project structures
LHC-b
ATLAS
(b)
point 4
point 5
point 3.3
point 3.2
point 6
N
point 7
point 2
point 8
point 1.8
point 1
existing buildings
LHC project buildings
Figure 9. Civil engineering implementation of the LEP/LHC tunnel, showing (a) underground
works and (b) surface buildings of LHC project. (Courtesy of J. Osborne.) (Online version
in colour.)
(f ) Cool down and hardware commissioning
Following the installation of all of the accelerator equipment, the cool down
begins. Owing to the large dimensional contractions with temperature (approx.
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up to 55 m of moraine overburden
PM54
USC
PX56
20 m minimum
rock cover
UXC
UP554
35 m
UP55
58 m
Figure 10. CMS Cavern (courtesy of J. Osborne). (Online version in colour.)
Table 1. Civil engineering numbers for the LHC tunnel project. Note that the LHC numbers are
in addition to those needed for LEP.
number of shafts
number of underground caverns
tunnel lengths, all diameters (km)
number of buildings
surface area of buildings (103 m2 )
excavated volumes (103 m3 )
volume of concrete underground (103 m3 )
volume of concrete on surface (103 m3 )
LEP
LHC
19
37
32.6
70
59
1100
230
85
6
32
6.5
30
28
420
125
42
80 m in total for the LHC ring), cool down can provoke continuity problems with
electrical, vacuum, cryogenics and beam connections. All possible measurements
are made during the cool down phase. Nevertheless, some non-conformities may
be provoked that are not evident from the measurements or simply cannot be
measured during this phase. When the machine is cold (1.9 K), the hardware
commissioning begins. This is a preparatory phase, which is essential before beam
commissioning is started. It is crucial to test in a very comprehensive way all
systems that can be tested without the beam. All systems are put through their
operational cycle to ensure that they behave in the specified way and with the
correct time constants. Electrical quality assurance is critical for the magnets and
the power converters, and all other systems are carefully monitored and tested
(RF, vacuum, cryogenics, etc.).
The overall system is tested with dry runs, which use the control systems,
synchronization system and the applications software.
(g) Beam commissioning
The ultimate test for the hardware installed in an accelerator is the circulation
of a particle beam. There are systems that need the beam before they can
be commissioned (e.g. beam instrumentation), and there are possible polarity
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inversions for any single element, of which there are tens of thousands. In addition,
it is crucial that the connections to the equipment locations in the ring are correct;
errors can arise due to naming conventions, the problem of having two beams
travelling in opposite directions, colour codes, etc. (For example, is longitudinal
left as seen from the inside of the ring? Is transverse left as seen for beam 1 or
2?) Initial beam commissioning (examples are specific to the LHC) involves:
— beam extraction from the injector (Super Proton Synchrotron, SPS), beam
transfer through the injection beam line (3 km long, filled with accelerator
equipment) and injection into the LHC;
— ‘threading’ the injected beam (octant by octant) through the 27 km of the
LHC. This is done by applications software that measures and corrects
the beam trajectory on successive injections. When the beam has passed
through the whole circumference, it is a joyous occasion, as this is one of
the first indications that the whole ensemble of millions of components is
behaving properly (‘the beam does not lie’);
— following the successful completion of one turn, the closed orbit is ‘closed’
and the beam may circulate for tens or even hundreds of turns. This is
another joyous occasion, as this indicates that there are neither serious
obstructions in the beam pipe aperture nor any polarity errors;
— with many turns, some rudimentary measurements of the beam parameters
can be made, but at this stage, ‘capture’ of the beam by the RF system
allows the beam to circulate for very long times (hours); and
— with captured circulating beam, one starts testing the applications
software, and the rest of the beam instrumentation.
(h) Operation for physics
The operational mode of an accelerator is by definition one of the most exciting
and challenging. In this mode, all the subsystems and components must be
controlled in perfect harmony so that the beam can be injected, accumulated,
accelerated and brought into collision. The system’s ‘glue’, which holds the
operations together, is the applications software, which makes extensive use of
the control systems, synchronization and timing systems, as well as the local
dedicated (‘hardware’) controls for each of the major components. Beams will
only survive the energy ramp if all systems are ‘ramped’ in perfect synchronism.
All magnetic fields must be ramped in harmony so that the ‘optics’ seen by
the beam remains under control. The RF system must be in synchronism
with the magnetic field increase, otherwise an energy mismatch occurs. Secondorder corrections are carried out by dedicated feedback systems, which measure
small deviations from the required conditions and apply corrections. These
systems are, of course, totally dependent on the beam instrumentation system
providing reliable data throughout the ramp. In addition, all safety systems
must be fully validated, since the ‘ramp’ produces significant increases in
the stored energy of the beam as well as the magnetic stored energy. The
details of this mode of operation could only be covered in many hours of
intensive lectures and involve understanding of nonlinear beam dynamics as
well as knowledge of the accelerator system and the individual components in
the system.
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5. System-by-system description of uses in accelerators
Owing to the enormous complexity in such a large number of high-technology
systems, it is impossible here to give detailed technical descriptions or
details. On each of the subjects presented here, there are yearly specialized
conferences involving hundreds of people, e.g. magnets, vacuum, accelerators,
power converter, cryogenics, etc.
(a) Survey, geodesy
Particle accelerators impose very tight tolerances in absolute and relative
accuracy for the positioning of their installed components. These tolerances result
from beam dynamics and mechanical (geometrical) issues, both of which influence
the aperture available for the circulating beam. The task of the surveyors is to
measure and align the position, orientation, shape and size of all major accelerator
components as well as the particle detectors to accuracies not needed in any other
domain. In order to position a component, a reference system (frame) must be
defined. A coordinate system that defines the position of the object is attached
to this frame. The reference system at CERN has evolved with the increase of
the size of its installations. Initially, the smaller dimensions allowed the surface
of the Earth to be considered as a plane without introducing significant errors.
However, with the advent of the much larger accelerators of the 1970s, the
surface of the Earth could no longer be considered as a plane. A new reference
surface was adopted using a sphere and the average sea level throughout the
continents. A new coordinate H , the altitude, was defined as the distance
measured with respect to this surface.
As the circumference of the accelerators increased to the size of LEP (27 km),
it was necessary to consider an ellipsoid of revolution as reference surface of the
Earth. In addition, account had to be taken of the gravitational effects of the
nearby Jura Mountains and Lake Geneva. An equipotential surface of gravity,
called the geoid, to which the force of gravity is perpendicular everywhere, has
been defined by means of zenithal camera and gravimetric measurements. The
measurements taken with survey instruments are therefore linked to this geoid.
Presently, the CERN reference system is a local ellipsoid that fits the Earth
in the CERN area. The deviation of vertical between this ellipsoid and the local
‘plumb line’ vertical has been calculated and taken into account in the new geoid.
(i) Geodetic network
The absolute positioning of accelerator components, known in an XYZ
coordinate system, is ensured by means of a geodetic network. The first level
of this geodetic network is a surface network that comprises monuments solidly
anchored to the Earth, forming a very well-defined basic framework from which
the links to national and international reference systems can be established. The
determination of the coordinates of these monuments is done by very accurate
triangulation, trilateration, levelling measurements and more recently by Global
Positioning System (GPS) measurements. The accuracy of these network points
has to be in the range of one millimetre.
This surface network, once measured, is transferred to the underground
network of the accelerator.
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power (kW at 4.5 K)
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180
160
140
120
100
80
60
40
20
0
1960
LHC
ATLAS, CMS
LEP2+
LEP2
ALEPH, DELPHI,
LEP Low-Beta
OMEGA, BEBC
ISR Low-Beta
1965
1970
1975
1980 1985
year
1990
1995
2000
2005
Figure 11. Evolution of CERN installed cryogenic power (courtesy of L. Tavian). (Online version
in colour.)
The underground network comprises tripods regularly spaced along the
accelerator tunnel. The topographical traverse linking one access shaft to the
adjacent one is realized by gyro-theodolites, theodolites and very accurate
electronic distance-measuring devices. Offset distances with respect to a stretched
nylon wire are also frequently measured in order to improve the ‘smoothness’ of
the network.
(b) Cryogenics
With the advent of superconducting magnets and RF cavities, cryogenics has
become a key technology for particle accelerators. Figure 11 shows the evolution
of the cryogenics installed to cool CERN’s facilities.
In these applications, the superconductor must operate at a fraction of its
critical temperature in order to preserve current-carrying capability at high field
(magnets) or to limit AC losses (RF cavities), thus imposing the use of helium
in the case of low-temperature superconductors. Additional important benefits
of operating the accelerator beam pipes at low temperature are the achievement
of high vacuum through cryogenic pumping of all residual gas species except
helium, and the reduction of wall resistance, which controls image-current losses
and transverse impedance.
Figure 12 indicates the geographical distribution of the CERN cryogenic plants
(figure 12a) and the schematic outlay of the systems for the LHC machine
and detectors. Two examples of large cryogenic refrigerators are shown in the
photographs of figure 13.
(c) Magnets
Magnets are at the core of both circular and linear accelerators. The main
function of a magnet is to guide the charged-particle beam by virtue of the Lorentz
force, given by the following simple expression:
F = q · v × B,
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P5
P6
P4
8×18 kW at 4.5 K
1800 SC magnets
24 km and 20 kW at 1.8 K
37 000 t at 1.9 K
130 t He inventory
P3
P2
P7
P8
1.8
P1
cryogenic plant
Figure 12. Cryogenics footprint and schematic of plants. (Online version in colour.)
Figure 13. Large cryogenic helium refrigerators (33 kW at 50–75 K, 23 kW at 4.6–20 K; 41 g s−1
liquefaction) (courtesy of L. Tavian). (Online version in colour.)
where q is the electrical charge of the particle, v its velocity and B the magnetic
field induction. The trajectory of a particle in the field hence depends on the
particle velocity and on the space distribution of the field. The simplest case is
that of a uniform magnetic field with a single component and velocity v normal
to it, in which case the particle trajectory is a circle. A uniform field has thus
a pure bending effect on a charged particle, and the magnet that generates it is
generally referred to as a dipole. Equating the Lorentz force to the centripetal
force (figure 5), we obtain the following relation between the strength of the field
B, the radius of the circumference r and the particle momentum p:
Br [T m] = 3.3356
p [GeV c −1 ]
,
Z
where Z is the charge number of the particle (in multiples of the electron charge),
and the momentum is expressed in practical units of GeV c −1 . The product Br
is known as the magnetic rigidity and provides the link between dipole strength
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3903
and length based on the desired momentum of a charged particle in a circular
accelerator. Note how the formula shows clearly the trade-off between the bending
magnetic field B and the size of the machine (related to r).
Modern accelerators employ the following types of magnets for the given
applications:
— dipoles, to bend the particles in a circular trajectory;
— quadrupoles (magnetic field proportional to distance from the centre of
the magnetic axis), to focus the particle beams;
— sextupoles (field proportional to the square of the distance from the
magnetic axis), to compensate the chromatic aberrations introduced by
the quadrupoles;
— octupoles (field proportional to the cube of the distance from the magnetic
axis), to generate particle stability by introducing a frequency dependence
on particle amplitude (Landau damping); and
— in addition, ‘skew’ magnets that subject the particles to a ‘crossed’
horizontal and vertical field, an example being the use of skew quadrupoles
to reduce the coupling of particle motion between the horizontal and
vertical planes.
The requirements of magnets to be operated in an accelerator are many and
varied. A list of the most important operational requirements is given below:
—
—
—
—
—
—
—
—
—
—
—
physical constraints (space, transport, weight, etc.);
magnetic field strength;
‘good field’ region (may depend on working point);
field quality at the different working conditions;
physical aperture;
power supply constraints;
cooling by water or by cryogenic system;
radiation resistance of the coils;
ability to align and re-align during complete lifetime;
high reliability; and
protection against the uncontrolled release of the magnetic stored energy.
The requirement on field quality is crucial to the successful operation with
beam. The magnetic field of any magnet may be represented by a Taylor series
expansion. Higher-order terms in this expansion can drive nonlinear motion of the
particles and ultimately cause beam loss. In a superconducting magnet, this is
one of the most crucial requirements, and the nonlinear magnetic terms usually
define the ‘dynamic aperture’ of the machine. The dynamic aperture denotes
the largest transverse amplitude that particles can have and still be on stable
trajectories. Amplitudes larger than the dynamic aperture result in particle
losses. The dynamic aperture is derived by tracking ‘particles’ in a computer
simulation for millions of turns in full six-dimensional phase space, with the
particles subjected to the measured higher-order field components in the magnets.
As an example of the impact of nonlinearities, figure 14 shows a one-dimensional
phase space plot of particle motion close to a fifth-order nonlinear resonance.
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(a)
(b)
2
1
x¢ 0
–1
–2
–2
–1
0
x
1
2
–2
–1
0
x
1
2
Figure 14. Plots of phase space structure near fifth order (a) without and (b) with a nonlinear
element (courtesy of W. Herr and E. Forest). (Online version in colour.)
The left plot is for a completely linear machine and the right plot results from
the addition of a single nonlinear element (a sextupole in this case), showing
clearly the distortion of the phase space caused by the nonlinearity as well as the
chaotic motion between the resonant islands.
(i) Room-temperature magnets
‘Normal conducting’ (alternatively ‘resistive’, ‘warm’ or ‘conventional’) are
electromagnets in which the magnetic field is generated by conductors such as
copper or aluminium, which oppose an electrical resistance to the flow of current.
The magnetic field induction provided in the physical aperture of these magnets
is limited to 1.7–2.0 T, owing to magnetic saturation. In these conditions, the
yoke provides a closure of the magnetic path with small use of magneto-motive
force, and its pole profile determines the magnetic field quality.
Figure 15 shows numerous images of room-temperature magnets used in
recently built accelerators; their more important characteristics are listed
in table 2.
(ii) Superconducting magnets
The prime difference between superconducting and normal conducting magnets
is in the way the magnetic field is generated. While in normal conducting
magnets the field is dominated by the magnetization of the iron yoke, in their
superconducting counterpart the field is generated by a suitable distribution of
current, properly arranged around the beam aperture. Superconducting magnet
technology relies heavily on the ability to produce technical superconducting
materials in the form of high-current cables. Figure 16 shows an artist’s view of
the LHC superconducting magnet (figure 16a) as well as a mechanical engineering
cross section (figure 16b).
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Elettra
ALBA
ANKA
SPring-8
SOLEIL
Diamond
SLS
CLS
Figure 15. Photographs of conventional magnets in many accelerator projects (courtesy of D.
Tommasini). (Online version in colour.)
Table 2. Important characteristics of the conventional magnets used in many accelerator projects.
bending radius (m)
number of magnets
dipole field (T)
gradient (T m−1 )
gap (mm)
current (A)
Elettra
ALS
ESRF
ANKA
ASP
ALBA
SOLEIL
SPring-8
SLS
Diamond
5.5
24
1.21
2.86
70
1420
∞
36
1.35
5.19
50
924
23.37
64
0.86
0
54
700?
5.56
16
1.5
0
41
660
∞
28
1.3
3.35
42
695
7.05
32
1.42
5.65
36
530
5.36
32
1.71
0
37
538
39.27
88
0.68
0
64
1090
5.73
36
1.4
0
41
557
7.16
48
1.4
0
46.6
1337
Figure 17 shows a coil cross section of the LHC dipole, the field distributions
in the ‘two-in-one’ magnet, as well as a pictorial view of the field lines.
Figure 18 shows the superconducting cable used in the LHC magnets
(Rutherford cable). In total, over 7000 km of this cable was needed in the
construction of the LHC magnets.
Superconducting magnet technology has been used in the construction
of the largest particle accelerators. Table 3 shows the main characteristics
of the four large-scale hadron accelerators built and operated using
superconducting magnets.
(d) Stability and margins, quench and protection
We have already remarked that an accelerator magnet superconductor must
be operated below its critical surface. The transition of a superconductor to a
‘normal’ conducting state is called a ‘quench’ and can be provoked by any sudden
temperature increase caused by internal mechanical energy release or by impact
by some of the particles in the high-energy beam. During the initial stages of
the quench, the resistance of the superconducting cable increases and generates
resistive heating (i 2 R), resulting in thermal runaway.
In spite of taking all reasonable precautions by good design, an irreversible
transition to the normal conducting state cannot be completely excluded and the
magnet must be actively protected against burn-out by its own stored magnetic
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(a)
superconducting coils
beam pipe
heat exchanger pipe
helium-II vessel
spool piece
bus bars
superconducting bus bar
iron yoke
non-magnetic collars
vacuum vessel
quadrupole
bus bars
radiation screen
thermal shield
protection
diode
(b)
auxiliary
bus bar tube
instrumentation
feed throughs
15 m long
LHC cryodipole
alignment target
main quadrupole bus bars
heat exchanger pipe
superinsulation
superconducting coils
beam pipe
vacuum vessel
beam screen
auxiliary bus bars
shrinking cylinder/He-I vessel
thermal shield (55–75 K)
non-magnetic collars
iron yoke (cold mass, 1.9 K)
dipole bus bars
support post
Figure 16. (a) Dipole magnet artist’s impression and (b) LHC dipole standard cross section. (Online
version in colour.)
energy. Superconducting magnets in general, and more specifically the highly
compact accelerator magnets, tend to have large stored magnetic energy density.
The protection system measures the resistance in the magnet coils and, in
the event of detecting an irreversible resistive transition, triggers a system to
discharge the magnet stored energy into a safe place. Such incidences are, of
course, to be avoided, as they must be preceded by an abort of the circulating
beam, reducing the operational efficiency. In addition, quenches may reduce the
useful life of the magnet.
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|B101| (T)
0
10
20
60
2.652–2.8
2.505–2.652
2.357–2.505
2.210–2.357
2.063–2.210
1.915–2.063
1.768–1.915
1.621–1.768
1.473–1.621
1.326–1.473
1.178–1.326
1.031–1.178
0.884–1.031
0.736–0.884
0.589–0.736
0.442–0.589
0.294–0.442
0.147–0.294
0–0.147
Figure 17. Coil distribution and field lines. (Online version in colour.)
Figure 18. The superconducting cable for the LHC magnets; a total of 7000 km of cable was used
(courtesy of L. Rossi). (Online version in colour.)
Table 3. Characteristics of the four major superconducting hadron accelerators (courtesy of
L. Bottura).
maximum collision energy (GeV)
injection energy (GeV)
ring length (km)
dipole field (T)
aperture (mm)
operating temperature (K)
first beam
Tevatron
HERA
RHIC
LHC
980
151
6.3
4.3
76
4.2
Jul/1983
920
45
6.3
5.0
75
4.5
Apr/1991
100/n
12
3.8
3.5
80
4.3–4.6
Jun/2000
7000
450
26.7
8.3
56
1.9
Sep/2008
6. Power converters
The powering sources of particle accelerator magnets are mainly DC power
converters and, in most cases, the feedback control system regulates the magnet
current. The peak and the r.m.s. ratings of the current and voltage to power the
magnet are crucial parameters, and the voltage and current ripple are critical for
beam stability and lifetime.
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D
Y
D
Y
Figure 19. Six-pulse thyristor rectifier (courtesy of F. Bordry and J.-P. Burnet).
Three main families of power converter are used for particle accelerators:
thyristor-controlled rectifier, switch-mode power converter and discharged
power converter.
The thyristor-controlled rectifier was the main type used from the 1970s
up to the 1990s. Many different configurations are implemented with thyristor
devices; the simplest one is the six-pulse thyristor rectifier with freewheeling diode
(figure 19).
(a) Switch-mode power converter with high-frequency transformer
One of the main interests in using the switch-mode power converter with a
high-frequency transformer is to reduce the volume of the power converters.
Ferrite cores are widely produced and available at a competitive price. A classical
topology is shown in figure 20.
In the case of the CERN LHC, where the power converters must be installed
underground with limited space, the switch-mode power converter was chosen
for power converter up to 200 kW. In this case, many sub-converters are placed
in parallel to reach the required power level. The superconducting magnets
require high current (many kiloamps) and low voltage (less than 20 V); for this
application, the solution is to drive many high-frequency transformers in series
with one inverter.
(b) Switch-mode power converter with 50 Hz transformer
If the volume of the power converter is not an issue, 50 Hz transformers can
be used, as they are produced by industry for other applications at affordable
prices. The most classical topology used for particle accelerators can be seen in
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Y
Y
D
+
D
Y
Y
D
–
Figure 20. Switch-mode power converter with high-frequency transformer (courtesy of F. Bordry
and J.-P. Burnet).
figure 21. For power converters above 10 kW, the classical switching frequency is
in the range of 1–20 kHz.
(c) High accuracy in power converters for particle accelerators
The power converters that drive the magnets in particle accelerators can be
considered as controlled current sources with current feedback based on reference
values as a function of time.
The accuracy of the feedback control is determined primarily by the current
measurement transducer and, in the case of digitally controlled power converters,
the control algorithm and the analogue-to-digital converter (ADC) employed in
the feedback loop.
(d) Digital power converter control
The advantages of digital control are numerous: increased stability
and reproducibility, less susceptibility to noise and thermal effects, easy
implementation of different control methods as well as easy loop parametrization.
On the negative side, the use of digital control increases system complexity and
introduces new sources of error such as those related to ADC measurement.
When maximum accuracy is required, the most effective present strategy is to
control the power supply voltage source by the output current of the converter.
In this case, the output current of the converter is read by an ADC connected
to a current transducer and then compared with a digital reference. The error is
fed into a digital regulator and the result sent to a digital-to-analogue converter
(DAC) that provides an analogue signal to control the voltage source.
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S. Myers
main switch
bouncer
Figure 21. Switch-mode power converter with 50 Hz transformer (courtesy of F. Bordry and J.P. Burnet).
This solution is implemented in the control of the LHC power converters.
In this case, the control challenge is more serious owing to the eight-sector
powering strategy used for the main dipole and quadrupole circuits. Hence, not
only does the control of each converter have to be extremely accurate, but also
the generation of the different current references and their synchronization along
the 27 km circumference of the LHC. For this purpose, each power converter
in the LHC has dedicated control electronics, which is actually an embedded
microcontroller-based computer capable of performing full local state control,
reference function generation and measurement acquisition as well as running
a digital current regulation loop. Reference functions are synchronized using a
timing network. Each digital controller is connected to a field bus (WorldFIP)
and the timing network is used to synchronize the cycles of all segments of the
field bus. The digital controller disciplines a phase-locked loop to align its clock
to the start of each WorldFIP cycle, guaranteeing synchronism of the references
along the machine.
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(e) Current measurement in particle accelerators
The requirements for beam current measurement have driven progress in
transducer technology, culminating with the invention of the direct-current
current transformer (DCCT) at CERN in the late 1960s. The idea was to
build a magnetic beam current transformer (BCT) with frequency response
extended down to DC to measure beam current in the Intersecting Storage Rings
(ISR) accelerator. Although the new transducer was not initially intended for
power supply regulation applications, its advantages compared with previous DC
instrument transformers soon became obvious.
The last decade has seen important progress in DCCT technology with the
development and deployment of the DCCTs for the main dipole and quadrupole
power supplies of the LHC. Short-term stability of the order of two parts per
million (ppm), yearly drifts better than 15 ppm and linearity better than 2 ppm
have been achieved.
7. Ultra-high vacuum
In particle accelerators, the beam pipes must be at ultra-high vacuum (UHV)
in order to reduce the beam–gas interactions, i.e. the scattering of beam
particles resulting from collisions with the molecules of the residual gas. These
interactions reduce the beam lifetime (nuclear scattering) and the luminosity
(multiple Coulomb scattering). They can also cause intensity limitations provoked
by pressure instabilities and by electron-induced instabilities (for positive
beams only).
Beam–gas scattering can also increase the background in the detectors and
the radiation dose rates in the accelerator tunnels. The latter leads to material
activation, increased radiation dose rates to intervention crews, premature
degradation of tunnel infrastructure such as cables and electronics, and finally
higher probability of electronic single event upsets (SEUs) produced by neutrons.
SEUs are of great concern for the electronics in the tunnel as well as in the
service galleries.
The design of an accelerator vacuum system must observe severe additional
constraints, which must be taken into account at the design stage. Among these
constraints, the ‘impedance’ seen by the beam must be minimized in order to
preserve beam stability, the generation of radio-frequency higher-order modes
(HOMs) must be minimized so as to avoid local heating by the beam, and
the beam aperture in the magnets must be maintained sufficient to allow good
beam lifetime.
For accelerators operating at cryogenic temperatures, the heat load induced by
scattered beam particles and synchrotron radiation can also be an issue for the
cryo-magnets, since local heat loads can lead to magnet quenches.
(a) Synchrotron radiation
When a particle beam traverses a perpendicular magnetic field, it radiates
photons and loses energy by synchrotron radiation. The interaction of the
photons with the vacuum chamber wall stimulates molecular gas desorption and
dissipates heat.
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In high-energy electron accelerators (e.g. LEP, Diamond and SOLEIL), the
heat load due to synchrotron radiation reaches several tens of kW m−1 . This
requires careful engineering design to keep the beam pipe at a reasonable
temperature. In the LHC, the heat load of 0.2 W m−1 is evacuated by the beam
screen’s cooling circuit maintained between 5 and 20 K, while the cold bore is
operating at 1.9 K.
(b) Electron cloud
Operation with high-intensity, closely spaced, bunches can lead to the
formation of a cloud of electrons. This effect has been seen in and posed a
limitation to performance of the following machines: ISR, PSR, KEK-B, PEP-II,
SPS and recently the LHC. The generated electron cloud affects the beam
properties and contributes to the vacuum dynamics. When an electron is in
the vicinity of the beam potential, it experiences the electromagnetic forces
of the beam. Photoelectrons can be produced, which ionize the residual gas
and allow the multiplication of secondary electrons. In the LHC, for example,
the proton bunch intensity is large enough to give a kick of approximately
100 eV to the stray electrons. On interaction with the inner surface of the
vacuum pipe, these electrons stimulate gas desorption and produce secondary
electrons. The secondary electrons are further accelerated to approximately
100 eV by the following bunch, 25 ns apart, leading to an avalanche in the
production of electrons.
The secondary electron yield (SEY) is the key parameter that defines the
vacuum level in a beam tube. It is defined as the ratio of the number of produced
electrons to the number of incident electrons. Typical SEY values for metallic
surfaces are approximately 2.
(c) Vacuum pumping
The pumping scheme is often decided at the early stages of the design, since
it affects the overall engineering of the accelerator.
In particle accelerators, discrete pumping is the most commonly used solution
and is often achieved by ion pumps combined with sublimation or non-evaporable
getter (NEG) cartridge pumps, cryogenic and turbo-molecular pumps.
Ion pumps are widely used since they are very reliable and provide a
high pumping speed. Ion pumps also have the advantage of pumping all gas
species and, once baked-out at 250–300◦ C, the ultimate pressure is in the low
10−10 Pa range.
Sublimation pumps are often used to speed up the pump down to the UHV
pressure range or as a complement to ion pumps at very low pressures. Similarly
to NEG pumps, the major limitation of sublimation pumps is related to the
pumping speed of noble gases and methane.
8. Beam instabilities
(a) Impedance and collective effects
As the beam intensity increases, the beam can no longer be considered as a
collection of non-interacting single particles; in addition to the ‘single-particle
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(a)
3913
(b)
Figure 22. SPS powering system: (a) 200 MHz tetrode amplifiers and (b) 800 MHz klystrons.
(Online version in colour.)
phenomena’, ‘collective effects’ become significant. At low intensity, a beam of
charged particles moves around an accelerator under the Lorentz force produced
by the ‘external’ electromagnetic fields (from the guiding and focusing magnets,
RF cavities, etc.). However, the charged particles also interact with themselves
(leading to space-charge effects) and with their environment, inducing charges
and currents in the surrounding structures, which create electromagnetic fields
called wake fields. Furthermore, the charged particles can also interact with other
charged particles present in the accelerator (leading to two-stream effects, and
in particular to the electron cloud effect mentioned previously) and with the
counter-rotating beam in a collider (leading to beam–beam effects). As the beam
intensity increases, all these ‘perturbations’ should be properly quantified. The
motion of the charged particles will eventually still be governed by the Lorentz
force but using the total electromagnetic field, which is the sum of the external
and perturbation fields. These perturbations can lead to both incoherent (i.e. of a
single particle) and coherent (i.e. of the centre of mass) effects, in the longitudinal
and transverse planes, leading to beam quality degradation or even partial or total
beam losses. Fortunately, stabilizing mechanisms exist, such as Landau damping,
electronic feedback systems and linear coupling between the transverse planes.
9. Radio frequency
The components needed to accelerate beams in an accelerator are:
— a master RF signal generator, controlled to have the correct frequency,
phase and amplitude for acceleration;
— an RF amplifier chain that amplifies the frequency signal to high power;
and
— a power coupler that feeds the high power into the RF cavity, where the
desired large electromagnetic RF field is generated and which by the cavity
design gives the required acceleration to the beam.
Figure 22 shows two of the powering systems used in the CERN SPS.
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Figure 23. (a) Radio-frequency quadrupole (RFQ) and (b) drift-tube linac (DTL). (Online version
in colour.)
Table 4. Cavities in the PS complex.
cavity
count
harmonic
number
frequency
range (MHz)
peak
voltage (kV)
PSB
C02
C04
C16
1 per ring
1 per ring
1 per ring
1
2
8–24
0.6–1.8
1.2–3.9
6–17
8
8
6
PS
C10
C20
C40
C80
C200
10 + 1
1+1
1+1
2+1
4+2
7–21
28, 42
84
168, 169
420–433
2.7–10
13 or 20
40
80
200
1–20
15
3–350
350
30
There are many different types of cavities for different types of accelerators.
In the lower energy range where linear accelerators are used, the two common
structures are radio-frequency quadrupoles (RFQs) and drift-tube linacs (DTLs;
figure 23). The RFQ both focuses and accelerates the beam.
In lower energy synchrotrons, which cover a large energy range (typically a
factor of 10–30 between injection and top energy), the cavity frequency must be
increased during acceleration so as to synchronize with the increasing revolution
frequency of the beam. Table 4 shows the range of cavities and their parameters
for the Proton Synchrotron Booster (PSB) and the Proton Synchrotron (PS)
itself. In the lower energy PSB, three different types of cavities cover a frequency
range from 0.6 to 17 MHz. The PSB accelerates protons from 50 MeV to 1.4 GeV,
which increases the proton velocity and consequently the revolution frequency by
about a factor of 3 in 500 ms. In such cases, needing variable-frequency cavities,
one uses ferrite-loaded cavities with magnetic tuning. In the higher energy PS,
there are two variable-frequency systems covering the range 2.7–20 MHz as well
as three fixed-frequency systems at 40, 80 and 200 MHz.
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Figure 24. Photograph of one of the travelling wave cavities of the SPS. (Online version in colour.)
As an example of a travelling wave structure, figure 24 shows an open view
of an SPS 200 MHz travelling wave cavity. The phase advance per cell is p/2,
corresponding to a regular gap distance of 375 mm. These cavities produce an
accelerating voltage of 12 MV with a total power of just under 4 MW.
In modern RF systems, the behaviour of the beam and the acceleration system
is constantly monitored and controlled by a large number of feedback and feedforward loops, usually referred to as the low-level RF (LLRF) system. Additional
control loops maintain the correct tune of the resonance frequency, correct
vacuum and temperature conditions and interlocks for safety and protection.
Recent synchrotrons and colliders have used fixed-frequency superconducting
(SC) cavities so as to allow higher acceleration gradients. At superconducting
temperatures, the RF surface resistance is not exactly zero; however, it can still
allow quality (Q) factors of the order of 1010 , which makes RF superconductivity
extremely attractive. It should be noted, however, that even with such low power
losses in the SC cavities, cooling at 2 K is very inefficient. As a rule of thumb,
1 W lost at 2 K requires refrigeration power of about 1 kW. A novel technique for
the fabrication of SC cavities was developed for the LEP collider at CERN and is
now used for LHC. The cavities are fabricated from sheet metal copper—a well
understood process—and the copper cavities are then sputtered on the inside
with a thin layer of niobium. The advantages of this technique are the good
thermal conductivity properties of copper and the reduced raw materials costs
associated with the small quantities of niobium. The maximum accelerating fields
(approx. 10 MV m−1 ) achieved with this technique are, however, below what has
been obtained with bulk niobium cavities.
Figure 25a shows the installed 400 MHz SC cavity system in the LHC
tunnel and figure 25b shows the waveguide and power distribution plant in the
underground cavern.
10. Transverse beam feedback
The beams in accelerators are inherently unstable. Many mechanisms are used in
order to maintain the beam and its quality for many hours of circulation in the
vacuum chamber. A crucial feedback system for the LHC is depicted in figure 26a.
The transverse oscillations that appear at the onset of instability are measured
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(a)
(b)
Figure 25. LHC point 4: (a) 400 MHz SC cavities and (b) LHC RF power plant in the underground
cavern. (Online version in colour.)
(a)
Tbeam
(b)
BPM
BPM
BPM
signal
processing
and
correction
calculation
power
amplifier
kicker
Tsignal
BPM beam position monitor
ideal equilibrium orbit
beam trajectory
Figure 26. (a) Schematic of the transverse damping system and (b) photograph of damper kickers
in tunnel. (Online version in colour.)
in one location of the circumference by a suitable beam position monitor (BPM).
This signal is processed electronically and a corrective ‘kick’ signal is generated,
amplified and applied to the beam by a transverse electromagnetic field (‘kicker’).
Figure 26b shows the system as installed in the LHC tunnel.
11. Beam diagnostics and instrumentation
Galileo said ‘measure what is measurable and make measurable what is not’.
Beam instrumentation and diagnostics combine the disciplines of accelerator
physics with mechanical, electronic and software engineering. The aim of the beam
instrumentation physicist or engineer is to provide the diagnostic equipment for
the observation of particle beams with the precision required to operate and
improve the accelerators and their associated transfer lines.
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(a) Beam position measurement
(i) Pick-ups
The measurement of beam position relies on processing the information
from pick-up electrodes located in the beam pipe. Four pick-up families are
commonly employed:
— Electrostatic. This consists of two electrodes insulated from and located
on opposite sides of the vacuum chamber.
— Electromagnetic. The most common type of electromagnetic pick-up is the
strip line coupler. These consist of two strip electrodes located on opposite
sides of the vacuum chamber. The particularity of this pick-up is that the
beam-induced signal is only produced at one end of each strip and depends
on the direction of the beam.
— Resistive/inductive. Including wall current and inductive pick-ups, these
monitors make direct use of the image current flowing on the wall of the
vacuum chamber.
— Magnetic. Such monitors are usually exploited because of their relative
insensitivity to stray particles. The two electrodes are replaced by two
loops orthogonal to the plane of measurement, which couple to the
magnetic field of the beam.
In order to extract an intensity-independent position from all of these monitors,
a normalized difference signal needs to be obtained. Several different methods are
used for this normalization.
(b) Beam current and intensity measurement
The measurement of beam current or bunch intensity is crucial in any
accelerator. This is usually done by means of a BCT. In order for the transformer
to interact with the magnetic field of the beam, it has to be placed over a ceramic
gap in the vacuum chamber. To keep the impedance seen by the beam as low as
possible, an RF bypass (either a thin metallic coating or external capacitors on
the ceramic) is required for the high-frequency wall current components.
(c) Diagnostics of transverse beam motion
The stability of a particle beam is dependent on many parameters related to the
‘optics’ of the machine. For example, nonlinear resonances occur when the number
of betatron oscillations per turn (‘tune’) is an integer fraction. Consequently, it
is of the utmost importance to measure quantities such as the betatron tune,
chromaticity (tune dependence on particle energy) and transverse coupling, so
that corrective action can be taken to correct the optics functions by way of the
power converters driving the magnetic fields.
(d) Emittance measurement
In an accelerator, the ‘emittance’ is the invariant beam cross section dependent
on the azimuthal position around the ring. The ultimate luminosity of any collider
is inversely proportional to the transverse emittance of the colliding beams.
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Emittance measurements are therefore of particular importance in such machines.
The emittance is measured in many and varied ways.
(i) Secondary emission grids
Secondary emission (SEM) grids consist of ribbons or wires that are placed in
the beam. As the beam intercepts the grid, SEM produces a current in each strip
that is proportional to the beam intensity at that location. By measuring this
current for all strips, a beam profile is obtained. SEM grids are the most widely
used means to measure the density profile of beams in transfer lines.
(ii) Scintillator and optical transition radiation screens
Scintillator screens have been used for nearly a century; the modern version
consists of a doped alumina screen that is intercepted by the beam. In its simplest
form, a graduated screen is observed using a TV camera.
Optical transition radiation (OTR) screens are a less expensive alternative
to scintillator screens. OTR radiation is generated when a charged-particle beam
transits the interface of two media with different dielectric constants (e.g. vacuum
to metal or vice versa). The radiation produced is emitted in two cones and
imaging can again be performed using simple optics followed by a charge-coupled
device (CCD) camera.
(iii) Wire scanners
Wire scanners come in two different types rotating and linear. Rotating wire
scanners are operated at speeds of up to 20 m s−1 and consist of a thin wire (some
tens of micrometres in diameter) mounted on a fork that is attached to a rotating
motor, while linear scanners use motors that push/pull the wire across the beam.
There are two ways of obtaining a beam profile with wire scanners: by measuring
the SEM current as a function of wire position (similar to SEM grid acquisition)
or by measuring the flux of secondary particles created as the beam interacts
with the wire.
(iv) Residual gas and luminescence monitors
Residual gas monitors are used in many high-energy accelerators in order to
reconstruct transverse beam distributions. The signal results from the collection
of either the ions or the electrons produced by the beam ionizing the small amount
of residual gas in the vacuum chamber. These ions or electrons are accelerated
using a bias voltage of several kilovolts and collected on a microchannel
plate (MCP). The avalanche of electrons produced by the MCP then hits a
phosphor screen, giving an image of the beam profile that can be monitored
using a CCD camera.
(v) Synchrotron radiation monitors
Synchrotron radiation monitors are limited to highly relativistic particles and
offer a completely non-destructive and continuous measurement of the twodimensional density distribution of the beam. These monitors make use of the
light produced when highly relativistic particles are deflected by a magnetic field.
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The most common way of measuring the beam size with synchrotron
radiation is to directly image the extracted light using traditional optics and
a camera.
(e) Beam loss monitoring
Beam loss monitors (BLMs) have three main uses in particle accelerators:
damage prevention, diagnostics and machine optimization. The most common
type of BLM is the ionization chamber, owing to their robustness and large
dynamic range. The chamber provides the medium with which the secondary
particles created by the beam loss can interact (typically a gas such as nitrogen
or argon).
The BLM systems are crucial for the detection of particles ‘lost’ from the beam.
They form part of the machine protection system, which detects particle losses
above a certain threshold and triggers a beam abort so as to avoid a quench of
the magnets.
12. Injection and extraction techniques
Transfer of beam between accelerators or onto external dumps, targets and
measurement devices requires injection, extraction and beam transfer lines.
Injection is the final component of the transfer of beam between one accelerator
and another, either from a linear to a circular accelerator or between circular
accelerators. Extraction is the removal of beam from an accelerator, either for
the transfer to another accelerator or to a target, dump or measurement system.
Injection and extraction systems need to be designed to transfer beam with
minimum beam loss, to achieve the desired beam parameters and often with
minimum dilution of the beam emittance.
Single-turn injection and extraction methods are rather straightforward for
both lepton and hadron machines. They generally involve a septum (or series of
septa) to deflect the beam into or out of the accelerator aperture, a kicker to
deflect the beam onto or away from the closed orbit, and a closed orbit bump
to reduce the required kick strength. For these methods, the beam losses can be
very low, and the emittance dilution associated with the injection or extraction
can be very small.
13. Injection- and extraction-related hardware: kickers and septa
Although kickers and magnetic septa are also dipole magnets following the same
principles as ordinary bending elements, they have very distinct features, and
must often fulfil conflicting design requirements. Kickers and septa are typically
purpose-built single elements or only produced in small series. Many of the
elements are installed under vacuum, with the resulting implications.
The rise time of kickers must be very short (ranging from several microseconds
down to several nanoseconds), while the requirements on the reproducibility of the
flat-top amplitude and the ripple are still quite stringent (at least considering
the pulse-type excitation). Redundancy can also be a design criterion: if one
of the devices fails, those remaining should be sufficient, in combination with
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special protection elements, to handle the beam safely. There is only one single
action on the beam and the timing needs to be precisely set up; online corrections
are not possible. Applications are often safety critical (beam abort systems) and
imminent failures must be recognized in real time.
(a) Electrostatic and magnetic septa
A septum, either electrostatic or magnetic, constitutes the separation between
an area of ideally zero field, which is traversed by the circulating beam, from
an area with high field in which the injected or extracted beam experiences
a deflection. The septum should be as thin as possible to keep the strength
requirements for the associated kicker at a reasonable level, and to reduce particle
losses and irradiation of the septum and the surroundings. Since the beam passes
only once through the high-field area of a septum, its field homogeneity is not
as critical as for normal bending magnets. However, the stray field into the
‘field-free’ region must be minimized, since the circulating beam experiences
it at every passage.
(b) Electrostatic septa
Electrostatic septa can be made very thin since normally they do not carry
currents. They consist of either a series of wires (several 100 to over 1000,
made, for instance, out of tungsten–rhenium) or a set of foils (for instance, of
molybdenum), which are precision-aligned on a support frame.
14. Collimators
Collimators are special accelerator devices that place scattering or absorbing
blocks of materials around the beam. They can be fixed or movable with respect
to the beam. The collimator jaws are the blocks of material that are placed close
to the beam. The jaw material is characterized by its nuclear properties, thermal
conductivity, electrical resistivity, mechanical properties (surface roughness and
flatness) and vacuum properties (residual outgassing rates). A collimation system
is an ensemble of collimators that is integrated into the accelerator layout to
intercept stray particles and to protect the accelerator.
(a) Requirements for modern collimators
Modern collimators must support the operation of accelerators with high power
loss and beams that are often beyond the destruction limits of available materials.
At the same time, collimators must be placed very close to the beam, and
allowable tolerances have reached the micrometre regime. Collimators thus are
cooled, high-power devices, which are highly radioactive, must be good absorbers,
extremely robust and work as precision tools.
(b) High power loads
State-of-the-art accelerators have advanced into the regime of high beam
brightness. This regime is characterized by a high beam power that is compressed
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(a)
(b)
LHC
100
ISR
10
1
PEP-II
0.1 SNS
0.01
1
KEKB
energy density (MJ mm–2)
stored energy (MJ)
1000
LHC
(2010)
SPS HERA-p
ILC
Tevatron
RHIC
LEP-2
CLIC
100
1000
10
beam momentum (GeV c–1)
10 000
ILC CLIC
1000
LHC
(2010)
100
10
KEKB
1
0.1
SPS
RHIC
HERA-p
Tevatron
SNS
PEP-II
ISR
1
10
100
1000
beam momentum (GeV c–1)
0.01
10 000
LHC
LEP-2
10 000
Figure 27. (a) The stored energy and (b) the density of stored energy versus the beam
momentum for various accelerators. Filled symbols refer to proton, while open symbols indicate
electron/positron accelerators. ILC and CLIC are design studies. (Courtesy of R. Assmann.)
(Online version in colour.)
into a small transverse beam size. The beam power can be characterized by
considering the energy that is stored in one beam with Np charged particles:
Estored = pceNp .
(14.1)
Here, c is the light velocity. We consider particles with charge q = e and
relativistic momentum p. The stored energy in the beams is compared in figure 27
for several accelerator facilities. It is seen that modern accelerators operate at or
are designed for beam momenta between a few GeV c −1 to a few TeV c −1 . The
stored beam energies are in the range 10 kJ to 500 MJ. Losses and power loads
must be distinguished for different types of accelerators.
(c) Destructive beam densities
The transverse beam size sz at collimator locations has decreased over the
years, either due to lower normalized emittances from injectors or due to the
operation at high beam energies. This increases the energy density in the beam:
rE =
Estored
.
psx sy
(14.2)
The stored energy densities are compared in figure 27 for several facilities; the
beam size is taken at typical collimator locations. It is noted that this parameter
is directly proportional to relevant performance parameters such as luminosity
and therefore is usually maximized.
The stored energy densities range from 10 kJ mm−2 to 4 GJ mm−2 . Damage
limits depend on the type and length of material that is hit by the beam. Typical
values for metals are around 50 kJ mm−2 . Very robust materials such as fibrereinforced carbon can survive an impacting proton energy density of around
5 MJ mm−2 for 1 m long blocks. In many cases, collimators can only survive
fractions of the collimated beam. A full beam impact must be avoided and
collimators must be designed for maximum robustness, non-catastrophic failure
in case of beam impact and in situ handling of damaged surfaces.
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15. Accelerator systems that were not covered in this review
Owing to limitations in space there are several crucial accelerator systems that
have not been covered in this review. They include:
—
—
—
—
—
beam dump system;
machine protection;
magnetic measurements and mapping;
SEU and radiation to electronics; and
large-scale simulation.
For further reading on these subjects, please consult the ‘general’ references.
I have profited enormously with material and advice from numerous colleagues at CERN without
which this paper would not have been possible. I sincerely thank them.
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