CHAPTER
THE TEVATRON AND BEYOND
1.1 INTRODUCTION
A strong effort is presently spent at Fermilab in developing new high field
superconducting magnets for next generation accelerators. The vanishing
electrical resistance of superconducting coils and their ability to provide magnetic
fields far beyond those of saturated iron is the main motivation for the use of
superconductor technology in all new large proton, antiproton and heavy ion
circular accelerators. Superconductivity does not only open the way to much
higher particle energies, but at the same time leads to a substantial reduction of
operating costs. Beam energies in the TeV regime are hardly accessible with
standard technology, due to the enormous power they would require. The
electrical power consumption of an accelerator cryogenic plant may easily be 1-2
orders of magnitude lower than the power needed in an equivalent warm machine
of the same energy. In this chapter, the accelerating machines at Fermilab, the
laboratory options for future acceler tors, and some highlights on
superconducting magnets are described.
1
Chapter 1 Tevatron and beyond 2
1.2 HIGH ENERGY ACCELERATORS AND FERMILAB
A discussion of the scientific motivations of the strong effort going on all
over the world to build accelerators of higher and higher energy is beyo d the
scope of this thesis. Still the main themes of today's particle physics are
mentioned here below.
1.2.1 Premise
High energy machines are mainly motivated by the need of understanding
the origin of symmetry breaking of electroweak interactions of eleme tary
particles, the origin of their masses and of the masses of the force carriers, the
reason why matter predominates over anti-matter n the universe. Besides
completing our present understanding of the Standard Model, future observations
will also hopefully lead to extend the theory and eventually reach the unification
of gravity with the other forces. New observations might also lead to understand
what is the composition of dark matter in the universe. Most of these searches are
Figure 1.1 Fermilab site.
Chapter 1 Tevatron and beyond 3
carried out by smashing particles of very high energy into each other, and by
analyzing the nature and the characteristics of the new particles produced at the
expense of the collision energy. These interactions are obtained either by blasting
high momentum particles onto a fixed target or by making them collide head-on
among themselves. In head-on colliders, in order to achieve high event rates, the
particles are bunched together and the bunches are formatted into high intensity
beams. For the deepest studies of particle structures and for the production of
more massive new particles, higher and higher energies are needed, and of course
the more complex are the accelerators. Accelerators can be divided in two types:
• Linear accelerators;
• Circular accelerators.
In a linear accelerator, charged particles travel along a straight trajectory
and go through a number of accelerating stations. An outstanding example is the
45 GeV electron/positron LINAC at SLAC, Stanford University, CA, USA.
In a circular accelerator, the beam is circulated many times in a closed orbit
along which a number of accelerating stations are present. Bending magnets and
focusing elements are distributed over the accelerator arcs to keep the particles,
during acceleration, on the same orbit and within the accelera or acceptance.
Beside Fermilab's Tevatron, that will be described in some more detail below,
LEP at CERN, Geneva, Switzerland, and HERA at DESY, Hamburg, Germany,
are examples of circular accelerators. LEP is an electron-positron collider of
maximum energy 101x101 GeV as of today. HERA is a proton-elec ron collider,
whose superconducting proton ring has an energy of 820 GeV, whereas its
electron/positron ring has an energy of 28 GeV.
1.2.2 Fermilab and the Tevatron
Fermilab was started in 1967. The first large circular accelerator operating
on site was the Main Ring with its injection stages consisting of a proton source, a
linear accelerator (LINAC) and a booster ring. The main ring, shown at the center
of Figure 1.1, had a circumference of 6.2 km. The proton b am had maximum
Chapter 1 Tevatron and beyond 4
energy of 450 GeV, and was ejected and used against fixed targets. A few years
later, the Tevatron, the first accelerator made with superconducting magnets, was
built in the same tunnel. The main ring served as last injector element to the
Tevatron. The proton beam energy doubled to 900 GeV. In 1984, the Antiproton
Source became integral part of the Fermilab accelerator complex, allowing the
Tevatron to operate as a proton-antiproton collider with a center of mass energy
of 1800 GeV. More cently, the Antiproton Recycler, to increase the intensity of
the antiproton source, and the Main Injector, to replace the main ring and increase
the intensity of the primary proton beam, were built. The latter can be seen in
Figure 1.1 in the foreground. The Recycler is presently being tested and will be
operative next year for collider Run II, while the Main Injector is operative now
in the on going Tevatron fixed target run.
Several stages progressively raise the beam energy. The accelerating steps
of the proton beam at Fermilab include (see also Figure 1.2):
• Cockcroft-Walton electrostatic accelerator;
• LINAC;
• Booster;
• Main Injector;
• Tevatron.
For the collider mode of the Tevatron operation, the Main Injector also
feeds an antiproton source. The source c mprises an external target where
antiprotons are generated, a collecting and focusing channel debuncher ring,
where single shot antiprotons are collected, an accumulator ring fed by the
debuncher, and a recycler ring, where the "old" protons are rescued at the nd of a
Tevatron collider run.
The Cockcroft-Walton provides the first stage of acceleration. In this
device, electrons are added to hydrogen atoms. The resulting negative ions, each
consisting of two electrons and one proton, are attracted by a positive volt ge and
accelerated to an energy of 750 keV. After leaving the Cockcroft-Walton, the
negative hydrogen ions enter a linear accelerator called the LINAC. The LINAC
consists of five tanks containing sets of drift tubes. An oscillating electric field is
Chapter 1 Tevatron and beyond 5
applied to the tubes. The particles travel through the drift tubes in phase with the
electric field, shielded by the tubes when the electric field would slow them down,
and emerging in the gaps in between the tubes when the field is accelerating. In a
recent upgrade the LINAC energy was increased to 400 MeV. After exiting the
LINAC, the ions are stripped of their electrons by a carbon foil, resulting in a
proton beam that is injected into the Booster synchrotron ring. The Booster
accelerates the protons to an energy of 8 GeV, and, via pulsed operation, it
organizes the high frequency sequence of LINAC pulses into a smaller number of
bunches for injection into the Main Injector. The Main Injector is the most
important improvement for Run II. It accelerates alternatively protons and
antiprotons, up to 150 GeV for injection in the Tevatron. Alternatively it sends a
120 GeV beam to the antiproton production target. The final stage of acceleration
is provided by the Tevatron, a superconducting synchrotron of 1 km in diameter,
with bending dipole magnets reaching a 4 T magnetic field. In collider mode,
protons and antiprotons are injected separately into the Tevatron, and circulate in
the same beam pipe. The acceleration in the Tevatron is provided by a set of RF
superconducting cavities. An energy of 900 GeV was reached by the Tevatron
beams during Run I. In the future Run II, thanks primarily to an improved cooling
system, 1 TeV per beam will possibly be reached.
The luminosity in the Tevatron collider is proportional to the beam currents,
the antiproton current being the critical element. A recycler collects the
antiprotons survived at the end of the physics run and makes them available for
next stores. This recycler ring is made of permanent magnets, and it is located in
the same tunnel as the Main Injector. The accelerating chain is quite complex
since many machines are used in series. All of them have to be synchronized and
must work to specification in order to obtain the optimum beam configuration.
Collisions of the beam bunches must occur at the center of the particle detectors
surrounding the beam pipe at specific azimuths around the Tevatron ring. The two
main detectors operating at the Tevatron Collider are CDF (Collider Detector at
Fermilab) and D0. These detectors discovered the Top Quark in 1995.
Chapter 1 Tevatron and beyond 6
The Tevatron is the highest energy accelerator in the world. Its magnet ring
is based on a FODO (focusing-drift-defocusing-drift) cell magnet sequence,
where magnets with separated functions are used. For beam bending,
superconducting dipole magnets with NbTi technology are used, while
superconducting quadrupole magnets provide focusing. This machine will set the
energy frontier in the particle physics until approximately 2005, when the new
proton-proton Large Hadron Collider (LHC) at CERN will become operative.
New much larger and very challenging accelerators are being studied right now in
order to extend the research in particle physics beyond the present energy limits.
1.3 NEXT GENERATION MACHINES
In a few years (possibly in 2005), the LHC proton-proton collider at CERN
will operate in the same circular tunnel in which LEP is running now. For a given
accelerator energy the two parameters that can be adjusted, the radius of the
Figure 1.2 Accelerators at Fermilab.
Chapter 1 Tevatron and beyond 7
machine and the field of its magnets, are not independent of each other. The
higher the field in the magnets, the smaller is the machine. With a circumference
of 27 km and an 8.4 T bending magnetic field, the LHC proton beams will reach a
maximum energy of 7 TeV each. Since the LHC collides protons on protons,
special "2 in 1" magnets are employed, which accommodate the two separate
beams circulating in opposite directions. Being the machine approximately
circular, bending radius, bending field, and beam energy are related by the simple
relationship:
(1.1)
where:
q is the particle charge [units of electron charge],
Bm is the bending field of the magnets [T],
r is the radius of the circular accelerator [m].
A fraction of the LHC magnets are being built in the US, and Fermilab is
the most important center for the US LHC project. Superconducting NbTi
technology was chosen for the LHC magnets, as was done for the Tevatron first
and for HERA next, with maximum dipole fields of 4 T and 6 T respectively. The
nominal operating field of LHC is 8.4 T. Because of their higher field, the use of
superconducting magnets allows for reduction of tunneling costs. However as the
field increases, better superconductor properties are required, raising costs again.
Superconducting NbTi is a ductile alloy which is ideal for manufacturing
composite strands, for making cables out of them, and eventually wind magnet
coils. Nevertheless, with an upper critical field (see Chapter 2) of about 11.5 T at
4.2 K, the LHC NbTi coils would have to be pushed near their critical current
limits to operate at 8.4 T. A safe operation point was achieved at LHC by
lowering the magnet operating temperature to 1.9 K (superfluid helium). At this
temperature the NbTi upper critical field rises to 14 T. This choice moved the
technological effort more onto the cryogenic system than on the superconducting
material R&D.
,3.0 rqBE mGeV =
Chapter 1 Tevatron and beyond 8
( ),BvEeF
rrrr
×+=
More cost-effective solutions are presently being studied for a post-LHC
Very Large Hadron Collider (VLHC) [1, 2]. At the Snowmass_96 Summer Study
on New Directions for High Energy Physics (HEP), a goal was set of a 50 TeV x
50 TeV proton-proton collider with a 3 TeV injector. Fermilab could possibly be
the site for the VLHC. Figure 1.2 shows how the new machines could be
integrated at Fermilab [3].
At present two main options for the VLHC bending magnets are being
pursued, a low field and a high field one. The low field version would be a ring of
600 km in circumference with 2 T transmission line magnets, while the high field
version would employ 12 T dipole magnets in a ring of 100 km in circumference.
The main advantage of a high field choice would be the enhancement of
luminosity, thanks to synchrotron radiation beam damping. This phenomenon
becomes important at around 10-12 T. At higher bending the machine luminosity
is limited by other effects, while the cooling system is unnecessarily overloaded.
The choice between the low and high field options is also determined by the
overall construction costs, which are a balance between magnet production and
tunneling costs. At present the low field option appears to be of lower cost.
Whereas for the low field magnets NbTi can be used, in the case of the high field
option other kinds of superconductors have to be considered. Multifilamentary
Nb3Sn is one of the most promising materials. Strand and cable R&D is actively
pursued by Fermilab within the High Field Magnet Project (HFM), using different
Nb3Sn technologies and as a benchmark, the strand design developed for the
International Thermonuclear Experimental Reactor (ITER).
1.4 SUPERCONDUCTING MAGNETS
Keeping the charged particles confined around a circular orbit requires both
bending and focusing forces generated by electromagnetic fields. The Lorentz
force is given by:
(1.2)
Chapter 1 Tevatron and beyond 9
where :
E is the electric field,
e is the electron charge,
v is the particle velocity, and
B is the magnetic field.
The electric term in equation 1.2 must be used for acceleration, while the
magnetic term that does not generate work can only be used for bending. At high
energy, where v=c, a magnetic field of barely 1 T generates the same Lorentz
force as an electric field of 3x108 V/m. Although they do not increase the particle
energy, magnetic field are thus very effective in bending the trajectory. Magnetic
dipole fields perpendicular to the plane of the particle trajectory are used to bend
the beams. Quadrupole fields around the beam axis focus the particles, and
longitudinal electric fields are used to accelerate them.
A noticeable difference to be taken into account in comparing a
conventional and a superconducting magnet is that in the former the field is
present almost only in the iron sector, while in the latter the field surrounds the
entire space around it. This configuration significantly constraints the choice of
the structural materials.
The focusing lattice most frequently used in a circular accelerator is a series
of identical cells, each containing a focusing (F) and defocusing (D) quadrupole
magnets separated by drift (O) spaces (FODO lattice). In between the focusing
cells are positioned the dipole bending magnets. This structure is called separated
function, to distinguish it from systems with integrated functions, where the
bending magnets have radial dependent bending field that is also capable of
performing the required focusing. Using magnets with separated functions allows
greater design and operation flexibility.
The challenging requirements in superconducting magnet design are [4]:
• Field strength. The general rule is the higher the field strength, the better. Not
only bending, but also focusing and defocusing is more efficient at higher
fields;
Chapter 1 Tevatron and beyond 10
• Field quality. Since the beam has to circulate many times around the same
orbit, small imperfections in the field decrease the beam lifetime;
• Magnet bore size. The cost of the magn t increases dramatically with the bore
size. However, from the point of view of beam acceptance, the larger the bore
size, the better it is. At high energy, the beam size can be small but induced
fields misalignments and other factors may force to make the acceptance
much larger then beam size;
• AC-DC behavior. To keep the particle in orbit during acceleration, the
magnets have to be ramped. However at maximum beam energy and in
collider mode operation, the field must be very stable for many hours;
• Radiation hardness. The magnet has to survive in a high radiation area for the
entire expected life of the machine;
• Reliability. The malfunctioning of a single magnet can cause the loss of the
entire beam. With more than one thousand magnets in the ring, this clearly
imposes strict reliability requirements on each of them.
• Cost. Because of the large number of magnets, both their production and their
maintenance cost should be kept as low as possible.
Despite the anticipated strong saving in operating cost, the introduction of
superconducting magnets generated other problems like:
• Persistent eddy currents. Eddy currents in the superconducting filaments are
induced during the magnet current ramp. Because of the vanishing resistance
of the material, they do not decay and generate dipolar and higher multipolar
fields;
• Quench behavior. If one of the critical parameters in the superconductor is
exceeded the magnet quenches to the normal resistance state. The machine
must be protected from possible damage, and must be able to recover quickly
from quenches;
• Cryogenics. An accurate study on the cryogenic plant and transport lines is
needed in order to avoid high costs for refrigeration.
Chapter 1 Tevatron and beyond 11
In a bending magnet of the
high energy machines under
consideration, the saggita at he
magnet exit is negligible with
respect to the magnet length.
Therefore the key elements of the
magnet design are coil cross
section and conductor distribution
over it. Given bore size and
magnetic field, conductor volume
and field quality should be
optimized by a careful design of
these parameters. Presently the
most successful coil design is
based on the so called cos(θ) conductor distribution. This solution produces the
desired magnetic field with the smallest amount of superconductor. As already
mentioned, field quality is also very important. This parameter directly affects
beam optics and beam stability. Important sources of field errors are
misalignments of the conductor and of the iron yoke on magnet cross section, iron
saturation, coil deformation under Lorentz forces, and most of all the
superconductor magnetization. Superconductor magnetization is reduced
primarily by reducing the superconducting filament diameter. This is one of the
challenging goals in superconductor development.
1.5 OTHER APPLICATIONS OF SUPERCONDUCTIVITY
The realization of high field magnets for high energy physics is one of the
most fascinating and difficult applications of superconductivity, but there are
many other important fields in which superconductors can possibly be used. This
is especially true, after the discovery in 1986 of high temperature
Figure 1.3 Superconductor distribution in a cos(θ)
design dipole magnet.
Chapter 1 Tevatron and beyond 12
superconductors, where Tc is greater than 77 K (liquid nitrogen temperature).
Applications and present R&D projects include [5]:
• Magnetic resonance imaging;
• Energy storage;
• Controlled thermonuclear fusion;
• Magnetohydrodynamic power generation;
• DC motors and AC machines;
• Magnetic levitation.
1.5.1 Magnetic resonance imaging
Superconducting magnets are used routinely in many hospitals in magnetic
resonance imaging (MRI) applications. In this technique, a magnetic field is used
to align the spins of hydrogen atoms (mostly contained in H2O molecules), and an
electromagnetic pulse is then given to excite spin orbit transitions. When the pulse
is over, the spins go back to the original state emitting a characteristic
electromagnetic wave. The wave can be detected by direction-sensitive sensors
and used to create two-dimensional pictures of the human body. MRI is widely
used to diagnose tumors, especially in sensitive parts of the body like rain,
where intrusive techniques are risky.
1.5.2 Energy storage
Storage superconducting magnets have been proposed as a large reservoir
of energy (Superconducting Magnetic Energy Storage SMES) in order to balance
the daily variations in the electricity demand. When the available electric power
grid exceeds the demand, the SMES can adsorb and store energy. This
electromagnetic energy can be dumped back into the network to satisfy increased
demand during the peak hours. A SMES was built within the "S ar Wars" US
defense program. This kind of application has not been transferred yet to the
Chapter 1 Tevatron and beyond 13
civilian field due to the complexity and costs of keeping such large magnets at
cryogenic temperatures.
1.5.3 Controlled thermonuclear fusion
Magnetic confinement of hot
plasma may be the most promising
way to achieve commercial
production of power from controlled
thermonuclear fusion. Given the
extreme confinement fields required,
superconducting coils are the only
possible choice. The Joule loss of
conventional copper magnets would
be 100 times greater than the power
required for refrigeration. The most advanced project in this field is ITER, a
tokamak fusion reactor, shown in Figure 1.4. The magnetic field to confine and
stabilize the high temperature plasma is generated by two types of coil systems:
toroidal coils and poloidal coils. Fusion reactions take place when the plasma is
sufficiently hot and dense, and contained long enough for the nuclei to start fusing
together in an energy positive process. This international experiment is supported
by Europe, Japan and Russia. USA decided to withdraw from the collaboration in
1998.
1.5.4 Magnetohydrodynamic power generation
Magnetohydrodynamic generation is a technique for the direct conversion
of thermal to electrical energy. The principle of this method is based on the
induction of an EMF by means of a transverse magnetic field in a hot gas flow,
and subsequent extraction of a DC power. The purpose of using superconducting
magnets is the same as for the fusion react rs, the energy produced has to exceed
the power input. This technology will possibly become of commercial interest
Figure 1.4 ITER design.
Chapter 1 Tevatron and beyond 14
before thermonuclear fusion, contributing significantly to fossil fuel energy
saving.
1.5.5 DC motors and AC machines
The superconducting technology applied to electric motors would allow to
reduce sizes and to reach higher efficiency. The advantages are most evident in
big motors, especially those for marine propulsion. The hardest problem is to
build a rotating cryostat, and to transmit the torque between zones at room and at
low temperature. The same problem has to be solved as for AC generators. In this
case, the design of a cryostat rotating at 50-60Hz with a high centrifugal
acceleration of some 5000g rises many technical problems. In the last years,
several prototypes of DC motors have been built, but none of them is working
reliably. A prototype for a 300 MVA superconducting generator is now under
construction by Westinghouse Electric Corporation in USA.
1.5.6 Magnetic levitation
One of the most fascinating applications of superconductivity is magnetic
levitation. The idea of designing vehicles floating on magnetic fields is not new,
but the introduction of superconducting magnets made it approachable, by means
of high magnetic f elds, low weights, and low energy consumption. Magnetic
levitation applied to train transport is especially pushed in Japan. Some prototypes
have already been built, and a record speed of 577 km/h has been reached. The
advantages of these vehicles are high speeds, no contact with the ground, no
moving parts, and no noise.
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