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OCR for page 98
5
Accelerators for
Elementary-Particle Physics
INTRODUCTION TO ACCELERATORS
The Why and How of Accelerators
Accelerators are the essential tools in most elementary-particle
physics research. They provide the high-energy particles used in
experiments; the costs of their construction and operation command
the major portion of the support budget for particle physics; and a
sizable fraction of the community of high-energy physicists is primarily
concerned with accelerator technology. Particle physics has always
been characterized by the fact that a part of this scientific community
has devoted its professional energy and ingenuity to the continuing
development of these tools of research. Accelerators thus exemplify
imaginative ideas at the frontier of technical complexity and sophisti-
cation. The spinofffrom accelerator research and development has had
applications ranging from radar to controlled thermonuclear fusion and
to high-intensity x rays for biological research.
Figure 5.1 shows how an accelerator works. A bunch of electrically
charged particles, either electrons or protons, passes through an
electric field. The particles gain energy because they are accelerated by
the electric field, hence the name accelerator. The energy gained by
each particle is given by the voltage across the electric field. Thus an
electron passing through a voltage of I volt gains an energy of I
98
OCR for page 99
ACCELERATORS FOR ELEMENTARY-PARTICLE PHYSICS 99
Bunch of
Electrons
1
Negative High
Voltage Plate
Positive F1 igh
Voltage Plate
FIGURE 5.1 Accelerators work by exerting an electric force on a charged particle. In
the example here a negative plate repels the bunch of electrons and a positive plate
attracts them. The electrons thus gain energy in moving from the negative plate to the
positive plate. By the time they reach the positive plate they are traveling so quickly that
they pass through the hole in the plate and can be used for experiments.
electron volt, abbreviated 1 eV. And an electron passing through I
million volts gains an energy of I million electron volts, abbreviated 1
MeV. In scientific notation 1 MeV = 106 eV. (Since protons have the
same electric charge as electrons, a proton passing through a million
volts also gains an energy of ] MeV.) The highest-energy accelerator in
the world is the Tevatron proton accelerator at Fermilab, which is
designed to produce an energy of I TeV, which is 106 MeV or 10'2 eV.
Accelerators are either linear or circular (Figure 5.21. In the linear
accelerator the particle is propelled by strong electromagnetic fields to
gain all of its energy in one pass through the machine. In the circular
accelerator, the particles are magnetically constrained to circulate
many times around a closed path or orbit, and the particle energy is
increased on each successive orbit by an accelerating electric field.
Until the 1960s, experiments in particle physics had been conducted
using only stationary (fixed) targets. In this case, the beam of acceler-
ated particles is extracted from the accelerator and directed at a fixed
target that may consist of a gas, a liquid, or a solid. Usually the target
material is the simplest element, hydrogen, whose nucleus is a single
proton. A wide variety of proton-proton and electron-proton experi-
ments have been performed that study the absorption or scattering of
the beam particles in the target material, the production of new
OCR for page 100
100 ELEMENTARY-PARTICLE PHYSICS
Accelerating Plates A
I I I I I r ~ 1 1 1 1 1 1 ~
. 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1
Li near Accelera tor
;\~\ ~
Circular Accelerator
~~ Accelerating
FIGURE 5.2 Very high energies cannot be obtained by using just one pair of plates, as
in Figure 5.1. There are two ways to solve this problem. In the linear accelerator, many
pairs of plates are lined up, and the particles being accelerated are given more and more
energy as they pass through each pair of plates. In a circular accelerator, only one pair
of plates is used, but the particles are made to travel in a circle, thus passing through that
pair of plates again and again. Each time they pass through the pair of plates they are
given more energy.
secondary particles during the collision, and the transformation of the
incident and target particles into new kinds of matter.
Not only are the primary reactions of the accelerated particles on
fixed targets studied, but also in many experimental situations the
secondary particles (such as pions, muons, and K mesons) are them-
selves selected and collimated to produce beams of projectiles that
interact with other targets.
As efforts were made to increase the energy in the primary interac-
tion in fixed-target experiments, it was recognized that a large fraction
of the energy of the incident particles was not available for the
interaction itself but was rather retained as the energy of motion of the
recoiling products of the collision. At relativistic energies (i.e., ener-
gies that are large compared with the rest energy of the accelerated
particles) the collision between a projectile particle and a similar
particle at rest makes available for interaction only an amount of
energy that is proportional to the square root of the energy of the
projectile. That is,
J
E = V2mEparticle.
OCR for page 101
ACCELERATORS FOR EfEMENTARY-PARTICLE PHYSICS 101
E is the usable or center-of-mass energy, Ep~jC~e is the energy of the
accelerated particle, and m is the mass (in energy units) of the target
particle. Thus as the energy of the accelerated particle increases, more
and more of it is wasted, since only E is usable. For example, if the
energy of the incident particle is increased by a factor of 100, the
energy available in the center of mass is increased by only a factor of
10. Eventually it becomes economically and technically impractical to
continue to increase the usable energy in fixed-target accelerators.
Hence for very high energies we have gone to a different and newer
accelerator concept: the particle collider.
Particle Colliders
A simplified example of a particle collider is shown in Figure 5.3. In
a circular machine, a bunch of electrons and a bunch of positrons
circulate in opposite directions, the particle bunches being held in the
machine by a magnetic guide field. (These machines are also called
storage flings.) At two opposite places in the machine, the bunches
, Interaction Point
O~ _ Bunch of
Positrons
Bunch of Am_
Electrons ~
~ I nteract ion Point
\
FIGURE 5.3 This colliding-beam storage-ring accelerator has two bunches of particles
moving in opposite directions. The bunches collide at the two interaction points. Even
though the bunches collide, most of the particles in the bunch pass right through the
other bunch; therefore the bunches continue to rotate again and again around the orbits.
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102 ELEMENTARY-PARTICLE PHYSICS
collide head on. The usable energy is now
E
_
where Epanjc~e is the energy of a particle in either bunch. Thus all the
particle energy is usable. (This is the usual case, where both colliding
particles have the same energy. If that is not the case, as in an
electron-proton collider, then not all the particle energy is usable.)
When the bunches come together, most of the particles in one bunch
simply pass through the other bunch without actually colliding. Thus
they continue to rotate around the storage ring. The bunches may
rotate for hours or even days, making thousands or even millions of
rotations per second.
The particles are put into the storage ring by an auxiliary accelerator
called an injector. In lower-energy storage rings the particles are
usually injected with their full energy. In higher-energy storage rings,
the particles are accelerated after injection to their full energy. The
following combinations of particles are now used or will be used in
colliders:
e+- e~
P - P
P - P
e~- p
e+- P
electrons colliding with positrons
protons colliding with protons
protons colliding with antiprotons
electrons colliding with protons
positrons colliding with protons
A critical property of colliders is called luminosity, which is a
measure of the rate at which particle collisions occur. Since particle
collisions are the essence of particle experiments, the more collisions
per second, the more useful the collider. A quantity called the cross
section, S. measures the relative probability of two particles colliding.
In a collider the rate, R. of collisions per second is
R = LS,
where L is the collider luminosity. Since the cross section S has units
of centimeters squared, the units of L are
collisions
centimeters2 second
(This is abbreviated as cm~2 sol, the numerator's unit being omitted.)
Existing colliders have luminosities in the range of 1029 to 1032 cm-2
s I.
An alternative to storage rings for particle colliders is the use of
colliding beams produced by linear accelerators (Figure 5.41. The
colliding bunches of particles pass through each other just once. Much
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ACCELERATORS FOR ELEMENTARY-PARTICLE PHYSICS 103
Interaction Point
1 ,,o-- - ~ ~ ~
Bunch of Bunch of
Positrons Electrons
FIGURE 5.4 In this sketch of a linear colliding-beam accelerator the two bunches
collide only once. To make full use of that single collision the bunches have to be much
denser than in a circular collider.
denser bunches must be used to compensate for the absence of
repeated collisions. One form of such a device is currently being con-
structed that will accelerate in close succession bunches of electrons
and positrons in a single linear accelerator. In this case, the charges of
opposite sign are separated by magnets and then brought into a head-on
collision in a single pass.
Superconducting Magnets in Accelerators
In circular fixed-target accelerators or in circular colliders, the
particles are kept moving in a curved path by strong magnetic fields.
Those fields are generated by electromagnets that fill most of the
circumference of the ring. One of the practical limitations on the
achievement of higher energies with circular proton machines has been
the size of the ring and the cost of electric power to operate the
magnets. The present largest accelerators have a four-mile circumfer-
ence and consume many tens of megawatts of power.
An innovation that has led to much higher available energy for
circular proton accelerators and storage rings has been the develop-
ment of superconducting magnets. Superconducting metals, such as a
niobium-titanium (NbTi) alloy, have zero electrical resistance when
cooled to liquid helium temperature. This is a temperature just a few
degrees above absolute zero. Since the electrical resistance is zero, no
power is consumed in operating electromagnets whose coils are made
of a superconductor, although some power must be used for refriger-
ation to keep the magnets cold. This is one advantage of super-
conducting magnets.
There is also a second advantage. Superconducting magnet coils can
carry extremely high currents. These can give magnetic fields two to
four times stronger than ordinary magnets. The circumference of a
circular proton machine depends on the strength of the magnetic field
for a fixed energy. Hence the use of superconducting magnets allows a
smaller circumference to be used or, conversely, a higher energy can
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104 ELEMENTARY-PARTICLE PHYSICS
be achieved in the same circumference. This has been done during the
last several years at Fermilab, where the change from ordinary to
superconducting magnets has doubled the energy of the proton accel-
erator.
Progress in Accelerators and the Energy Frontier
Over the last 50 years there has been a continuous development of
new accelerator ideas and engineering achievements. It is remarkable
that as each set of concepts appeared to reach a dead end, a new idea,
a new technology, has evolved to continue to roll back the frontiers of
energy and luminosity. This is most strikingly illustrated in Figure 5.5,
which was first published over 20 years ago but is still a good repre-
sentation of our progress on the energy frontier. Here we have only
adjusted our definitions to represent particle colliders in terms of the
equivalent energy of the particle striking a stationary target.
ELEMENTARY-PARTICLE PHYSICS AND THE VARIETY OF
ACCELERATORS
In the last section we described how accelerators work. We now turn
to the reasons for the variety of accelerators used in elementary-
particle physics: fixed-target accelerators and particle colliders, proton
accelerators and electron accelerators, low-energy accelerators and
high-energy accelerators. This variety exists to serve the many dif-
ferent purposes of elementary-particle physics experiments. We will
outline these purposes and give some illustrations.
Study of the Properties of Known Particles
Often we know that a particle exists, but we know little about its
properties. An example in present-day research is the B meson, which
contains a b or bottom quark and has a mass of about 5 GeV. The B
meson can decay in many different ways through the weak interaction,
and we would like to know much more about these different modes of
decay. The cleanest way to study those decay modes at present is to
produce a single B meson and a single anti-B meson (B) in an
electron-positron collision using an electron-positron collider. Since
the total mass to be created is about 10 GeV, an electron-positron
collider that has its maximum luminosity at about 10 GeV is best. Such
a collider is the CESR facility at Cornell University. Lower-energy
electron-positron colliders do not have enough energy to create the BB
OCR for page 105
ACCELERATORS FOR ELEMENTARY-PARTICLE PHYSICS
1000 TeV
100 TeV
I O TeV
I TeV
co
cr
As 100 GeV
m
`,: 10 GeV
o
I GeV
v
100 MeV
10 MeV
I MeV
105
o //
i//
/
Proton Storage Ring _;
( Equiv. Energy)
/
/
Proton Synchrotron
Weak Focusing
\ AG
/
/
Electron Synchrotron f
Weok Focusing /f'/ ~ ~~
q Proton Li nac
~Sector- Focused
- /~r
i/~
///
PI
~ _% ~ —
Betatron ~
~0
into
Electron Linac
_ Synchrocyclotron
/~
n
\ E lectrostati c
Generotor
Rectifier
Generotor
100 KeV I l l l l l l
1 930 1940 1950 1960 1970 1980 1990
FIGURE 5.5 The maximum energy achievable by an accelerator has increased
exponentially with time over the last 50 years. This exponential increase has been
maintained by a succession of new inventions in accelerator technology. The highest
energies have been achieved by storage rings, the latest invention in accelerator
technology. In this figure the energy of storage rings is denoted by the equivalent energy
that a fixed-target accelerator would have to possess to give the same useful energy.
OCR for page 106
106 ELEMENTARY-PARTIC~E PHYSICS
pair, while higher-energy colliders have less luminosity at the required
energy.
On the other hand, to measure the lifetime of the B meson rather than
its decay modes, the meson should have high velocity. Then it is best
to produce it at higher energy, and the PETRA and PEP electron-
positron colliders have that higher energy. Thus the first measurements
of the lifetime of the B meson were made by experiments at PEP. The
recently discovered Z° particle is another example. The discovery of
the Z° was made at the CERN proton-antiproton collider because that
was the only existing collider or accelerator with enough energy to
create the 93-GeV mass of this particle. But electron-positron colli-
sions should provide the cleanest and easiest way to create Z° particles
in great numbers so that their properties can be studied in great detail.
Indeed, studying the physics of the Z° is the first purpose of two
electron-positron colliders now under construction, the Stanford Lin-
ear Collider (SLC) and LEP at CERN (see the section below on
Accelerators We Are Using or Building). Existing electron-positron
colliders do not have enough energy to create Z° particles.
The study of the decays of K mesons provides another example. The
puzzling phenomenon of CP violation is observed only in such decays.
To study these decays in detail we need a large number of K mesons,
which are best produced in fixed-target proton accelerators. Thus the
Tevatron, the Alternating Gradient Synchrotron (AGS), and the SPS
machines are all used to produce K beams for various studies of
K-meson decays.
Study of the Known Forces
Three of the four known forces, the electromagnetic force, the weak
force, and the strong force, can be studied using accelerators. But the
most suitable accelerator depends on the force to be studied and how
it is to be studied. An old but still interesting example is the discovery
that the total cross section (that is, the total rate) for the interaction of
protons with protons through the strong force increases as the energy
increases. The increase is not large, but it is a clear increase. This is
called the rising total cross-section effect, and we do not understand
why it occurs. To make progress on this problem we need more data on
proton-proton interactions at yet higher energy. These data can only
come from a higher-energy proton-proton collider.
Further studies of the weak force at higher energy require a different
facility. The weak interaction can only be studied in a collision if the
OCR for page 107
ACCELERATORS FOR EfEMENTARY-PARTICLE PHYSICS 107
strong force is not present; otherwise the strong force masks the weak
force. Therefore one of the particles in the collision must be a lepton,
because leptons do not feel the strong force. The classic way to study
the weak interaction has been to collide neutrinos with protons or with
neutrons in a fixed-target experiment. The neutrinos must come from a
secondary neutrino beam produced at a proton accelerator.
However, as we discussed in the last section, fixed-target experi-
ments are more limited in their maximum energy than are collider
experiments. Thus the highest-energy weak-force studies will have to
be done using an electron-proton collider. No such collider exists, but
the knowledge and technology needed to build such a facility do exist.
The DESY laboratory in Germany is now building such a collider,
called HERA.
Tests of New Ideas and Theories
It is rare that a new idea or theory can be tested with experimental
data that already exist. More commonly it is necessary to carry out
new experiments to test the new ideas or theory. Such experimental
tests often stretch the capabilities of the accelerator being used. For
example, the principle of lepton conservation states that the decay
muon ~ electron + photon
cannot occur. This decay has been looked for but has not been found
to a precision of about 1 part in 10"'. To test some theories that say that
this decay should in fact occur at a level of I part in 10~2 the
experimenter needs a great number of muons. The best source for such
muons is the secondary muon beam from a high-intensity proton
accelerator. High intensity, not high energy, is important. Therefore
experimenters use a relatively low-energy but high-intensity proton
accelerator such as the 800-MeV LAMPF machine at Los Alamos.
Other tests of new ideas and theories require high energies. For
example, in Chapter 4 the technicolor theory was mentioned; this
theory predicts new particles in the mass range of I TeV. No existing
collider can produce particles with such a large mass. Therefore a
higher-energy proton-proton or proton-antiproton collider is needed.
The proposed Superconducting Super Collider (SSC), discussed below
in the section on The Superconducting Super Collider, A Very-High-
Energy Proton-Proton Collider, would have sufficient energy to pro-
duce these massive new particles.
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108 ELEMENTAR Y-PARTICLE PHYSICS
The Search for New Particles and the Mass Scale
The need for higher-energy colliders to search for new particles is so
fundamental to our goals that we will discuss this in more detail. There
are two questions involved in the search for a new particle. How much
energy is needed? How much intensity or luminosity is needed?
The answer to the energy question depends on the type of collider
used to produce the particle. In electron-positron colliders, when the
electron and positron annihilate, they can give all their energy to the
production of the particle. If a single particle is to be produced, then
the total energy of the collider need only be equal to the mass of the
single particle.
Proton-proton colliders or proton-antiproton colliders require more
total energy than the mass of the particle that is to be produced. This
is because the production process actually occurs through the collision
of a single quark or gluon in one proton with a single quark or gluon in
the other proton (or antiproton). On the average a single quark or gluon
in a proton only carries about 1/6 of the total energy. Therefore the
total collision energy needed to produce a particle of a certain mass is
about 6 times that mass. This is a rough rule, because the second
question how much luminosity is required is also important. If the
production process for ~ new particle is rare, then a high luminosity is
required.
The range of masses that can be produced at a collider should
overlap the mass range or mass scale of the theory that is to be tested.
To achieve this mass range both high energy and high luminosity are
necessary. To reach the mass scales of the theories discussed in
Chapter 4, colliders should have the following general properties:
10-TeV minimum total energy
proton-proton or proton-antiproton
electron-positron
1032 cm~2 so minimum
luminosity
1-TeV minimum total energy
1032 cm~2 s~ ~ minimum
luminosity
As will be described in the section below on The Superconducting
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ACCELERATORS FOR ELEMENTARY-PARTICLE PHYSICS 121
range of several TeV:
· What is the origin of mass, and what sets the masses of the
different elementary particles? Is the Higgs hypothesis correct, and can
the Higgs particles be found? If the Higgs hypothesis is wrong, what
replaces it?
· Are there more quark or lepton generations? Why do these particles
form generations?
· Are the quarks and leptons truly elementary?
· Are new theoretical ideas like technicolor or supersymmetry correct?
Can the strong and electroweak interactions be unified?
-- Are there undiscovered fundamental forces?
The mass range needed to study these problems is illustrated in
Figure 5.12. This mass range cannot be reached with fixed-target
accelerators; it requires a hadron-hadron collider. As mentioned ear-
lier, when hadrons collide, the full energy of the hadrons is not
available for conversion into mass, even in a colliding-beam accelera-
tor. This is because the hadron-hadron collision really consists of a
quark-quark, quark-gluon, or gluon-gluon collision; and these constit-
uents only carry a fraction of the total energy of the hadron. The rough
rule is that 1/6 of the total energy is available, on the average, for
conversion into large masses. We emphasize that this is an average.
There is a large probability that 1/10 to 1/20 of the energy can be
converted into large masses and a small probability that 1/3 can be
converted.
Collider Goals
These physics goals, searching for answers to fundamental questions
and exploring new physics in the several-Ted mass range, require a
hadron-hadron collider of very high energy and large luminosity. Our
knowledge and experience in accelerator technology enables us to set
the practical goals for the collider of a maximum energy of 40 TeV and
a maximum luminosity of 1033 cm~2 S-~. To achieve this luminosity a
proton-proton collider is favored. The richness and range of the
particle physics that can be done at such a facility dictates that there be
multiple interaction regions for particle detectors. Six or more inter-
action regions are desirable. Summarizing, the practical goals for the
Superconducting Super Collider are as follows:
Maximum total energy
Maximum luminosity
Number of interaction regions
40 TeV
1033 Cm-2 S-]
6 or more
OCR for page 122
22 ELEMENTAR Y-PARTICLE PHYSICS
4
3
-
J
~ 2
In
_
Cl)
In
~ L
1
o
Domain I
of SSC~
Posi ble
t Quark
Lepton
I nterna I
0 Structure
Domain of |
Tevatron
\; I*
`lWz' ~ I In -
TOP ? New Quarks
and Leptons
Other Ws
~ Z's
Technicolor
Higgs Super
Boson Symmetry
Pointlike Weak
Breakdown
) ~
FIGURE S. 12 The mass scale at which physicists believe that a number of fundamental
new phenomena may appear. The SSC would extend this scale beyond the mass of a few
tenths of a TeV that can be probed by facilities now under construction to the regime of
2 to 3 TeV and above.
Design Studies
Since 1982 the U.S. elementary-particle physics community has
been developing a plan for the construction of a high-luminosity
proton-proton collider in the energy range of 40 TeV. The work began
in the summer of 1982 at a meeting in Snowmass, Colorado (see
Proceedings of the 1982 Division of Plasma and Fluids Summer Study
on Elementary Particle Physics and Future Facilities, June 28-July 16,
1982, Snowmass, Colorado, R. Donaldson, R. Gustafson, and F. Paige,
OCR for page 123
ACCELERATORS FOR El EMENTARY-PARTICLE PHYSICS 123
eds.~. The result of this and other studies was that a recommenda-
tion for the construction of such a facility was made to the U.S.
Department of Energy by the 1983 High Energy Physics Advisory Panel
of the Department of Energy (HEPAP) Subpanel on New Facilities. This
collider has been named the Superconducting Super Collider (SSC)
because it requires the use of superconducting magnets to keep its size
and its operating power costs within reasonable bounds. Although the
basic technology for the machine is at hand, the scale is unprecedented.
Therefore an intensive series of design studies has been carried out.
In April 1983 an informal one-week workshop (see Report of the 20
TeV Hadron Collider Technical workshop, Newman Laboratory,
Cornell University, Ithaca, New York) was held at Cornell to study the
design problems and to make initial estimates of feasibility, time scale,
and costs. This was followed by meetings and workshops on hadron
collider detectors, on the physics that can be done at the SSC, on
accelerator issues related to the SSC, and on cryogenic issues related
to superconducting magnets for accelerators. During this period a
subpanel of HEPAP was set up to provide advice on the content and
implementation of a preliminary research and development (R&D)
effort. The most intensive design work at present is the National SSC
Reference Designs Study (see SSC Reference Designs Study Group
Report, May 1984), which was conducted from February through May
1984. This study addressed three areas:
· Technical feasibility: the designs of 40-TeV total-energy proton-
proton colliders were explored using three of several possible
superconducting magnet styles as study models.
· Economic feasibility: the likely cost range was estimated using
preliminary engineering designs for the three magnet styles and the
other hardware and conventional facilities required to construct and
operate technically feasible colliders.
· Required R&D: the R&D needed to verify design calculations and
technical assumptions was identified.
It was not intended' however, that the Reference Designs Study Group
Report be either a design proposal or a site preference study. Some of
the material in this section is based on this study.
Superconducting Magnets
The feasibility of constructing the SSC has been substantially
enhanced by the recent success in accelerating protons to high energy
OCR for page 124
124 ELEMENTARY-PARTICLE PHYSICS
in a superconducting accelerator, the Tevatron at Fermilab. This
machine uses about 1000 superconducting magnets in a circumference
of about 4 miles. It now operates at 800 GeV for physics experiments,
and it has also operated in the beam-storage mode preliminary to its use
as a proton-antiproton collider. Although the full operating energy of
1000 GeV and full beam intensity are yet to be attained, the perform-
ance of the Fermilab machine is a definitive verification of the
practicality of using superconducting technology for obtaining beams
of very high energy. The Tevatron has opened the door to a new era.
In the course of its construction a great deal has been learned. Great
strides have been made in the development of superconducting,
niobium-titanium cables, so that high-quality materials are now avail-
able in large quantities at reasonable cost. The technique for constrain-
ing the superconducting cable in the magnet with the required high
precision has been well demonstrated, and protection systems have
been developed to cope with the inevitable magnet quenches. (A
superconducting magnet quenches when a part of its coil becomes too
warm to maintain zero electrical resistance.) During the same period,
the industrial capability for producing refrigeration equipment has
grown rapidly, and large machines with much higher reliability are now
available. The ability to transport large volumes of helium liquid over
long distances has been demonstrated. Automatic control over the
refrigeration and cryogenic systems has been remarkably successful,
and the capability for beam location and control has been demon-
strated.
Preliminary Collider Designs and Considerations
The design studies, particularly the National SSC Reference Designs
Study, have shown that a conservative extension of existing or
near-term technology can lead to the successful achievement of an
SSC. Several design options exist' and the selection of a particular
design to optimize the cost is one of the most important considerations.
The final cost will depend on the results of the R&D program that will
be carried out before initiating construction. One of the principal
factors determining the detailed design of the collider is the strength of
the magnetic guide field. The options cover a broad range of magnetic-
field values. The Reference Designs Study has considered the three
superconducting, niobium-titanium magnet designs (a), (b), and (c)
listed next. Other work has considered the design (d). As shown in
Figure S.13(a), the diameter of the collider decreases as the magnetic
field increases.
OCR for page 125
A CCELERA TORS FOR E! EMENTAR Y-PARTIC~E PH YSICS 125
(b) /
, 1 Col l l der and I n ~ actors
, ~J, to Scole
(a)
Lo
~ 20
at
10
o
Nigh Ene: Enlorgement
~ Booster tog/ of
\> 1 TeV By In jectors
}/ -
-
Totol Energy - 40 TeV
0 2 4 6 8 10 . Ll noc
MAGNETIC FIELD (Teslo) O to 0.001 TeV
~ Low Energy Booster to
0.07 TeV
-
Interoc:\
colons —~D:
FIGURE 5.13 (a) The diameter of a proton-proton or proton-antiproton collider
depends on the total energy desired and the magnetic field used. (b) Schematic layout of
the SSC indicating the injector complex and the main ring, where protons are
accelerated to 20 TeV in counterrotating bunches that collide at six points around the
circumference. The total collision energy is 40 TeV.
(a) A high-field magnet design has a 6.5-tesla field, with both beam
tubes and both coil sets side by side in a common iron yoke contained
in a single cryostat. This approach is referred to as the 2-in-1 design.
Intrinsic to this approach is magnetic coupling, limiting the extent to
which the field strengths in the two apertures may differ. This results in
an SSC main ring about 18 miles in diameter.
(b) A medium-field dipole magnet has a 5-tesla field, with each beam
tube and coil in its own cryostat. Each cryostat has only enough iron
to shield one coil from the magnetic field of the other. This is referred
to as the 1-in-1 no-iron design. This results in an SSC main ring about
22 miles in diameter.
(c) A low-field magnet has a 3-tesla field, with each beam tube and
each coil set in separate iron yokes, one above the other, in a single
cryostat. In this design, although the iron is driven well into saturation,
the field is determined primarily by the iron pole faces, and the
magnetic fields of the two rings are not strongly coupled. This magnet
is referred to as the superferric design. This results in an SSC main ring
about 32 miles in diameter.
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126 ELEMENTARY-PARTICLE PHYSICS
(d) The designs listed above use a niobium-titanium superconductor,
with which we have a great deal of experience. Very high magnetic
fields, 8 teslas or more, can be achieved with a niobium-tin supercon-
ductor, but there is little experience at present with such magnets.
a
The choice among these systems is complex. The medium-field
technology and to some extent the high-field technology have already
been proven in the Tevatron and in the Brookhaven CBA design.
Although such magnets could simply be copied and manufactured in
quantity, without cost-saving design and production changes the over-
all cost of the installation would be great. The accelerator tunnel in this
case would have a moderate length.
The low-field design is expected to be reliable because of the low
values of the forces and low field strengths in the superconductor. It
also has the advantage that since the iron profile largely determines the
field accuracy, it should be less sensitive to the placement of conduc-
tors. It has the disadvantage of requiring a larger tunnel perimeter.
The use of very high field magnets would minimize the tunnel length.
However, suitable superconducting cable has not yet been produced in
quantity, and the appropriate technology has not yet been developed.
This type of magnet might therefore require much more R&D than
lower-field designs.
No matter what magnetic-field strength is chosen, the cost of the
collider can be reduced if magnets with a smaller aperture can be
developed and used. This requires R&D both in magnet design and in
the accelerator physics of the collider. Finally, advantage must be
taken of the increased scale of production. New fabrication methods
suited to mass production will have to be developed.
The SSC facility is shown schematically in Figure 5.13(b). The
collider itself sets the size of the site. The injector complex would lie
against one portion of the collider ring. The six interaction regions
would be distributed around the ring.
An important question is the site required for such a machine. A
large number of factors must be taken into account in the site selection.
These include ring diameter; environmental considerations; availability
of water' power, and roads; and proximity to airports, villages, and
cities. Stating first with the technical considerations, it is clear that the
number of suitable sites will be strongly dependent on the radius of the
machine. From the point of view of beam dynamics, gentle deviations
from flatness of the ring might be tolerated. One might be able to take
advantage of this in order to locate the interaction halls and service
buildings near the surface; and it may permit the use of contour-
following, cut-and-cover techniques for the machine closure instead of
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ACCELERATORS FOR ELEMENTAR Y-PARTICLE PHYSICS 127
the more expensive mode of tunneling. A cursory search for suitable
sites has suggested that several can be found that would be suitable for
even the largest of the rings. one of 100-mile perimeter or more.
Schedule and Cost
Research and development will be needed before beginning the
construction of the collider. About 2 years will be required before a
working design can be established. It will be necessary to learn how to
mass produce low-cost' high-quality magnets and how to handle.
mount' and survey the magnets into position with a high degree of
precision. It is likely that a full-scale prototype of a relatively long
tunnel section and guide field will be constructed in order to test the
practicality and integration of the system. It may even be necessary to
work on the design of more than one of these systems in parallel in
order to determine the minimum-cost system. Such R&D activity is
essential to carry out the design.
The scale of this project far exceeds any of our existing high-energy
physics facilities. It is obvious that an administrative organization will
be required that is responsible to a broadly based national representa-
tion of the elementary-particle physics community. The federal funding
agencies must indicate that they are receptive to a proposal to build
such a machine. International cooperation with respect to building
some of the detectors or other costs should be explored.
The Reference Designs Study has considered the construction
schedule, as follows: .'In this study, we have assumed a six-year
construction period, which would lead to completion in early 1994 if
construction were to begin in PY 1988. The optimum duration of the
construction period should itself be an object of study. . . It will depend
on many factors, such as the detailed scope of the facility that is
ultimately proposed, the technical means devised for its construction.
and the spending pattern needed. Finding ways for minimizing the
delay between start of construction and first use for physics research
must be given great emphasis."
The same study has estimated the construction costs of the SSC.
These costs, based on the three magnet technologies (a), (b), and (c)
listed above, range from $2.70 billion to $3.05 billion in fiscal year 1984
dollars. (The costs of research equipment, preconstruction R&D, and
possible site acquisition are not included.) Quoting the study, ''The
contingencies are intended to be sufficiently conservative that these
totals represent our best estimate today for an upper bound on the SSC
cost. With intense R&D and effective planning, lower costs could re-
sult.
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128 ELEMENTARY-PARTICLE PHYSICS
RESEARCH AND DEVELOPMENT FOR VERY-HIGH-ENERGY
LINEAR COLLIDERS
Physics Motivation
In the section above on Elementary-Particle Physics and the Variety
of Accelerators we saw that hadron-hadron and electron-positron
colliders largely complement each other in the physics that they
explore. As mentioned earlier, there is a rule of thumb that an
electron-positron collision has the same available energy as a proton-
proton collision when the actual energy of the electron plus positron is
about 1/6 of the actual energy of the two protons. Thus to reach the
same available energy as the planned 40-TeV proton-proton collider,
an electron-positron collider would require a total energy in the
several-Ted range. This is beyond the reach of the known technology
of circular electron-positron colliders; thus a new electron-positron
collider technology such as the linear collider is needed.
incidentally, although most of the thought and work on linear
colliders is for electron-positron machines, the concept may also be
applicable to electron-proton colliders.
Present Technology and Concepts
As described above in the section on Accelerators We Are Using
and Building, the first application of linear collider principles is now
being made in the construction at SLAC of the Stanford Linear
Collider, a facility with a maximum total energy of 100 to 140 GeV.
Starting from this machine, we now consider what R&D is needed in
order to build a much larger TeV machine. In linear accelerators and
colliders, the critical parameter is the accelerating gradient, i.e., the
energy gained per meter of length. In the SLC, it will be about 20 GeV
per kilometer.
A 2-TeV collider based on the present SLAC accelerating structure
would consist of two conventional linear accelerators each 50 kilome-
ters in length. With 12 electron-positron bunches per pulse, there could
be a magnetic switchyard that would feed the bunches to 6 parallel
interaction regions, each with a luminosity of the order of 1032 cm~2
s~'. Using the electrical efficiency of today's pulsed radio-frequency
power sources, the total power consumed would be approximately 300
MW. These numbers are quite large. It is desirable to reduce the length
and hence the construction cost of such a machine, and also its power
consumption. Research and development work aimed toward these
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ACCELERATORS FOR ELEMENTARY-PARTICLE PHYSICS
129
goals is now beginning. Of course experience with the operation of
SLC will also stimulate progress toward these goals.
One of the directions for improving the technology of linear colliders
is to reduce the wave length (increase the operating frequency) of the
accelerator structure. A reduction to 5 cm (SLAC uses 10 cm) doubles
the accelerating gradient and doubles the electrical efficiency. Re-
search and development is required to produce high-power klystrons at
this higher frequency, and several ideas exist for ultra-relativist~c or
laser-driven klystrons that could provide not only the requisite power
but also much higher electrical efficiency. Alternative accelerator
structures that promise much higher accelerating gradients will also be
explored. (At these shorter wavelengths there is increased energy
spread in the accelerated beam, and further development in chromatic
corrections of the final focusing systems is required to handle this
energy spread.)
The repetitive nature of linear accelerators naturally suggests auto-
mated production techniques to reduce construction costs. Also, ener-
gy-recovery schemes, perhaps using superconducting microwave ac-
celerator units, need to be explored to increase overall electrical
efficiency further. As these technologies advance, the design of a linear
collider facility can be optimized, and the construction and operating
costs can be reduced.
RESEARCH ON ADVANCED CONCEPTS FOR
ACCELERATORS AND COLLIDERS
To conclude this chapter we discuss some advanced ideas for
accelerators and colliders. We do not know if any of these ideas can be
reduced to practice. But if we are to move substantially beyond the
energy range of present accelerator technologies, we must find new
ways to accelerate particles. This section describes some of the ideas
now being explored.
Linear Accelerators and Colliders
Calculations and research are being carried out in the United States
and abroad on a variety of new and advanced concepts for obtaining
higher accelerating gradients, which is the energy gain per unit of
accelerator length. There is reason to believe that accelerator struc-
tures can be built to handle up to 200 GeV per kilometer, ten times the
currently available gradients. What is needed is a suitable high-
efficiency, high-power source of short-wavelength electromagnetic
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130 ELEMENTARY-PARTICLE PHYSICS
.
radiation that can provide a relatively large amount of energy per unit
length. We list some of the possibilities:
1. Very-high-power, very-short-pulse-length, high-frequency klys-
trons suitable for this purpose may be developed.
2. A special case of a source of short-wavelength radiation is the
wake field of a high-energy beam passing through a cavity system. This
idea is being pursued theoretically and shows considerable promise and
a special simplicity since the wake-field source cavity can be combined
with a beam-accelerating cavity within a single structure.
3. In the two-beam accelerator concept, a high-power, low-energy
electron beam travels parallel to the desired high-energy particle beam.
Using a principle such as that of the free-electron laser, the high-
power, low-energy beam radiates its power to the high-energy beam,
thus providing the acceleration.
4. A more radical approach is to use the very short wavelength
obtainable from a laser. In this case one cannot consider accelerating
structures of conventional design; the dimensions are far too small. It
appears possible, however, to use a suitable optical grating in place of
a conventional cavity. The most extreme case would be obtained if the
periodic grating were replaced by a periodic plasma, possibly formed
over a grating surface. In this case gradients as high as I TeV per
kilometer could theoretically be attained. Such high and obviously
desirable gradients can only exist in or near a plasma and not in or near
any solid conductor or dielectric.
S. A particularly interesting solution occurs when a plasma is
exposed to two laser beams of suitably close frequency. The beat
frequency between the two lasers can be matched to the natural plasma
frequency, and a strong periodic and moving charge modulation can be
induced. Large electrostatic fields are generated by this modulation,
and these could be used to accelerate suitably injected beams. Accel-
erating fields as high as 2 TeV per kilometer have been discussed, but
there remains great uncertainty about the stability, energy efficiency,
and suitability of such a mechanism to the construction of a high-
energy linear collider.
Many such ideas have been suggested. Some of them may not work.
Others may work but not have application for high-energy physics. It
is clear, however, that without some such idea, no great further step in
energy will be possible. On the other hand, with gradients of the order
of I TeV per kilometer theoretically possible, an accelerator of 100
TeV is not unthinkable. It is thus important to the future of the field
that these ideas are followed up.
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ACCELERATORS FOR ELEMENTARY-PARTICLE PHYSICS 131
Ultrahigh-Energy Circular Colliders
We have a great deal of knowledge and experience with the
technology to be used to build a 40-TeV proton-proton circular
collider. The primary limitation of that technology is that we do not
know how to increase substantially the magnetic field that guides the
particles in a circle. and hence we do not know how to decrease sub-
stantially the circumference of the collider. Some size and cost
reduction can be obtained in guide-field magnets by the use of new
superconducting materials such as niobium-tin. While such develop-
ments are important, they do not promise a radical saving or access to
much higher energies. Mechanical forces will limit the usable magnetic
fields no matter what conductors become available.
Even if we could substantially decrease the circumference of a
proton-proton collider, we would then reach a second limitation: the
protons would begin to lose large amounts of energy via synchroton
radiation, as occurs at much lower energies in circular electron-pos-
itron colliders. Indeed, no ideas have yet been proposed to enable an
increase of the energy of a circular collider beyond the 100-TeV range.
The Need for Advanced Research on Accelerators and Colliders
Thus new accelerator ideas need to be developed and explored. In
the past, new ideas have indeed occurred, resulting in the enormous
increases in accelerator energy that have been achieved in the past 50
years. However, the present scale of R&D in accelerator technology is
small and certainly not commensurate with its importance. Part of the
problem is the reluctance of individuals to commit themselves to tasks
whose possible fruition seems quite distant. Another problem is the
lack of suitably trained multidisciplinary experts. A third may be traced
to the mechanisms for supporting accelerator physics. Encouragement
to universities to expand training in accelerator physics is needed.
Possibly, too, it would be desirable to have a funding mechanism that
would allow laboratories to pursue such work with an assurance that
such funding was truly an addition to that for more immediate goals.
There is a strong and natural tendency for internal priorities to cut back
on such long-range activities.
Despite these reservations, it is encouraging to note that there are
still many people working on new ideas and that advanced accelerator
workshops and schools take place regularly. We can hope and expect
to see significant new activity in the coming decade.
Representative terms from entire chapter:
storage ring
