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Chapter V. Synchrotrons
V.1 Science Motivation
Although the cyclotrons and betatrons described in Chapters II and IV were able to apply a
modest accelerating voltage many thousands of times, their size grew with their output energy.
Their magnets had pole pieces that saturated at about 2 T and the only way to double the top
energy of the particle, for example, was to make the circular path twice as big. If the poles were
twice as big, the volume of steel in the magnet yoke would be eight times larger. Lawrence’s
largest cyclotron already had as much steel in it as a naval frigate and it looked as if a whole
fleet would have to be sacrificed to make a significant leap in energy. It seemed to be the end of
the road. But then it was realized that only the outer rim of the magnet was needed to channel
particles at the top energy and, if particles could be persuaded to use this rim at all energies,
there was a hope that the huge cyclotron might be replaced by a slender necklace of magnets
around this rim. To make this work, particles would have to be injected into the machine in
pulses; each pulse would then be accelerated and the magnetic field of the necklace increased to
keep in step with the rising energy. Physicists had to wait until World War II was over before
they had the time to invent and fully develop this idea, but when they did they called it the
synchrotron. The name comes from the need to synchronies the rise in field — and, as it turns
out, also the accelerating frequency — to match the rising energy of the bunch of particles as
they are accelerated.
The synchrotron has proved a trustworthy companion in the quest for higher energies.
The ongoing development of the synchrotron principle from machines of a few hundred MeV
to monsters of several TeV was made possible by two important discoveries, that of
alternating-gradient focusing and the use of colliding beams. The world’s two most recent big
synchrotrons, both at the European laboratory CERN near Geneva, are LEP for electrons and
the LHC project for protons. These are many kilometers in girth, but recent years have also
seen a diversification of powerful lower energy machines for the production of synchrotron
radiation, production of neutron beams by spallation, and for proton and light ion therapy.
V.2 The Early History of the Synchrotron
At the beginning of the Second World War, the skills of cyclotron builders in the US were
diverted to the task of the electromagnetic separation of uranium. Colleagues from the UK
joined them in this and, although some later regretted their role, the stimulating company,
coupled with an ample time to reflect, proved an ideal environment for creating new ideas. In
1943 Marcus Oliphant (See sidebar for Oliphant), an Australian physicist who had been working
in England at Birmingham University, found himself at Oak Ridge supervising the business of
transforming a laboratory experiment for isotope separation into a large scale industrial
process. As deputy to E.O. Lawrence he was often given the owl watch and “with little to do
unless troubles developed” occupied his time by speculating on plans for his return to
Birmingham when the war was over. He wrote a memo to the Directorate of Atomic Energy, UK
in which he proposed a new method of acceleration — the synchrotron. This contained the
essence of the idea:
“Particles should be constrained to move in a circle of constant radius thus
enabling the use of an annular ring of magnetic field… which would be
varied in such a way that the radius of curvature remains constant as the
particles gain energy through successive accelerations by an alternating
electric field applied between coaxial hollow electrodes.”
His new idea was not greeted with enthusiasm at a time when more important
business was afoot, but, as he left for England at the end of the war, he was encouraged by
Lawrence to pursue the idea further.
After his return to Birmingham, Oliphant read the comprehensive and beautiful papers
by McMillan (a student of Nobel prize-winning Lawrence) and Veksler (a Soviet physicist).
McMillan described Oliphant’s pulsed ring-magnet idea and announced his own plan to build
such a machine — without a single reference to Oliphant.
V.3 First Synchrotron
McMillan’s publication of plans to build a synchrotron was a challenge to others to get there
before him. Late in World War II the Woolwich Arsenal Research Laboratory in the UK had
bought a betatron to x-ray unexploded bombs in the streets of London, of which there were
many at the time. Frank Goward, a physicist at Woolwich, read of the synchrotron idea and
realized that he could become the first person to make it work by converting the betatron into a
synchrotron with the help of a rudimentary accelerating electrode. The influence of Lord
Rutherford’s “string and sealing wax” spirit of improvisation on a generation of UK physicists is
perhaps to be seen in Fig. 5.1, which shows this historic machine, with which they just scooped
their American rivals.
V.4 Electron Synchrotrons
A team constructing the first purpose-built synchrotron at the General Electric Co. at
Schenectady (the betatron factory) just failed to beat the Woolwich people to the post by a
month or two but they had the consolation that their 300 MeV machine, with a glass vacuum
chamber, was the first to produce a new phenomenon — synchrotron radiation in a visible
form. It had long been predicted that deflecting electrons in a magnetic field would emit
electromagnetic waves, but no one knew what their frequency would be. Witnesses report that
they had searched with a radio receiver scanning through the long and medium bands. Imagine
the surprise when an intense stream of visible light emerged from the glass donut that held the
electron beam in this early synchrotron. The synchrotron light is clearly to be seen in Fig. 5.2.
V.5 Early Proton Synchrotrons
The first synchrotrons had been electron machines but projects for proton synchrotrons aiming
at energies above 1 GeV were not far behind. Oliphant was back at the University of Birmingham
in the shadow of the replica of the Siena campanile that adorns that campus. He had made his
bid early to construct a 1 GeV proton machine but he was becoming bogged down in the red
tape and lack of imagination that abounded in post-war Britain. Europe was not yet used to big
science — after all, the Manhattan Project had been on US soil. Much of his workforce had to
come from the graduate students in his department.
There were technical problems to be solved too. Synchrotrons follow a pulsed “heartbeat.”
Particles are injected at low energy and then accelerated over several seconds before being
deflected onto a target at high energy. Acceleration in a synchrotron is provided not by the ring
of bending magnets, which merely keep the particles in a circular path, but by a copper cavity
driven by a radio transmitter. This must be tuned to follow the huge swing in revolution
frequency as the particles velocity around the ring increases. Such tuning had never been
tackled before and Oliphant’s solution involved plunging a large coil of copper wire, the
inductor of a resonant circuit, into a mercury bath. Filing the shape of a rotating disc — the
capacitor of this resonant circuit — proved an effective if irreversible means to make a fine
adjustment.
The strength of the electromagnets that guide the beam must also match the
momentum of the beam as it is accelerated. The pulsed current for the magnet excitation for
the Birmingham machine was produced via a direct current generator driven by an alternating
current motor. This output of the generator was servo controlled by light reflected from 120
mirrors on the periphery of the radio frequency capacitor.
Oliphant’s machine reached just short of 1 GeV for the first time in July 1953 — a
few months later the Cosmotron at Brookhaven reached 3 GeV and closely followed by the 6
GeV Bevatron at Berkeley, which started up in 1954. Figure 5.3 shows the completed
synchrotron in Birmingham.
The race between the Berkeley and Brookhaven laboratories was keenly contested.
The Cosmotron team, which included Stan Livingston, John and Hildred Blewett, Ernest
Courant, Ken Green and N. Blackburn, came in first. It was in May 1952 that The New York
Times headlined their first “Billion Volt Shot” (See Fig. 5.4).
Early synchrotrons relied only upon the weak focusing fields produced by tapering the
gap between the poles of the guide magnets towards the inside. There was a lot of controversy
about the size of the gap necessary to allow for the mis-steering of the injected beam and the
effect of magnet imperfections. It was difficult to estimate regardless of whether one was an
optimist or a pessimist. A fall-back solution for the Bevatron had a huge magnet aperture, 4.3
× 1.2 m, which gave rise to rumors that it was destined to be the world’s most powerful
accelerator of Jeeps (which we are told could drive between the poles, provided their
windshield was down). The Cosmotron had boldly refined their aperture to a mere 1.2 × 0.22
m. Apart from such matters the Bevatron had other troubles, as its builders had been
distracted for a year or two in mid project to construct a large accelerator as part of a defense
project (see Section III.4). Fig. 5.5 shows the Bevatron as it was finally constructed. The
construction team was led by Edward Lofgren. (See sidebar for Lofgen)
V.6 Nimrod and Phasotron
The UK decided to follow the Birmingham machine with an 8 GeV proton synchrotron. Nimrod
had a huge magnet aperture and was powered by a motor generator-alternator whose load rose
from zero to 100,000 horsepower in 0.75 seconds. The energy to be used for the magnetic field
was stored in a huge flywheel which was driven from the mains and which speeded up and
slowed down as the magnet pulsed. One night a pole on the rotor of the alternator broke and it
was only the heroism of an operator racing towards the circuit breaker over a catwalk above the
monster, already writhing in its death throes that prevented the export of a large rotating
flywheel across the Channel to France.
The history of the synchrotron in the Soviet Union followed lines parallel to that in
the West. Their first application of the phase stability principle, discovered by Veksler and
independently by McMillan, was in the synchrocyclotron or “Phasotron” as Dubna called it.
The first operation of this machine was timed to be on Stalin’s 70th birthday, which we are
told was a Soviet tradition. Dubna’s 10 GeV weak focusing “Synchro-phasotron” see Fig.
5.6) (surpassed the 6 GeV Bevatron and, from 1957 until the 25 GeV Proton Synchrotron at
CERN was finished, offered the highest energy in the world.
V.7 Strong Focusing
The Cosmotron, a typical weak focusing synchrotron, had a “C” shaped magnet open to the
outside. Any imperfections in the magnetic fields or mis-steering of the beam entering the
accelerator would send it heading off to hit the sides of the vacuum chamber before it could be
accelerated. There had to be some magnetic focusing in both the horizontal and vertical
direction. To provide this focusing, the magnets were shaped to create a field gradient with a
radius that should be negative but not too strong. As the magnet was excited, the outer parts of
the poles tended to saturate and this made the field there weaker and the gradient stronger —
too strong for horizontal stability. The upper energy of the Cosmotron was limited by this effect.
Stan Livingston (see sidebar in Chapter II) had the idea of compensating saturation by
reinstalling some of the C magnets with their return yokes (normally to the inside of the ring)
towards the outside, but he was worried in case the focusing at low energy would be affected.
This led, almost by accident, to a much better way of focusing the beam thanks to
Courant, Livingston, Snyder and Christofilos — and thereby lies a story. Courant had been
given the task of checking the effect of alternating the yokes of the Cosmotron and reported
that — far from being harmful — the focusing seemed to improve as the strength of the
alternating component of the gradient increased. Snyder, as befits a good theorist, who should
always be ready with an a posteriori explanation, reminded them that in simple optics you can
make an alternating focusing system by equal convex and concave lenses. The Alternating
Gradient (AG) focusing idea was published near the end of 1952 by Courant, Livingston and
Snyder. Much to their surprise, then it was found that the idea had actually been patented
earlier by Nick Christofilos (see sidebar), who at the time was working in Greece as an
elevator engineer. (See sidebars for Courant, Snyder and Christofilos)
This was in 1952, just at the time that European countries were joining together to
form CERN in Geneva, Switzerland (See sidebar for CERN and Fig. 5.7). Their aim at first
was to build a 10 GeV scaled-up version of the Cosmotron, and much of the design work had
been done but, as if on cue, CERN visitors — Odd Dahl (see sidebar in Chapter I), Frank
Goward and Rolf Wideroe (see sidebar in Chapter III) — arrived to see the Cosmotron.
Hearing from the Cosmotron team of their new strong focusing idea, they immediately
abandoned plans for a 10 GeV weak focusing machine in favor of a 25 GeV strong focusing
proton synchrotron.
They were convinced that the reduction in the dimensions of the magnet gap and
beam chamber would allow them to go to nearly three times the energy for the same price.
Brookhaven had already planned such a machine as their next step — the Alternating
Gradient Synchrotron or AGS (Fig. 5.8).
The first people to take up the idea of AG focusing were Bob Wilson and Boyce
McDaniel (See sidebars for R.R. Wilson and McDaniel), who were then in the process of
building one of the first electron synchrotrons-at the Laboratory for Nuclear Physics, Cornell.
They quickly modified the magnet design to try out the idea. Meanwhile the Brookhaven
team wanted to satisfy themselves that the idea would work before embarking on the AGS,
and built an electrostatic model. The leading figure in building the AGS was Kenneth Green
(See sidebar for Green). Waiting for the results of this model delayed the AGS somewhat and
enabled CERN to overtake them. In Fig. 5.9 we see the first CERN magnet unit being
installed.
V.8 Brookhaven’s AGS and CERN’s PS
By 1959 CERN’s PS was ready for testing, somewhat ahead of Brookhaven’s AGS, but there had
always been some doubts about what would happen to the beam at “transition,” a sort of
watershed in the acceleration process when an effect that kept the beam in step with the
accelerating field changed from stable to unstable. Indeed the CERN PS faltered at transition
until a bright young radio engineer, Wolfgang Schnell (see sidebar in Chapter IX), produced a
circuit he had built in a Nescafe tin to change the phase of the accelerating wave at the moment
of transition. Schnell, his box, and a few coaxial connectors brought the beam through transition
to full energy.
John Adams (See sidebar for Adams), who had led the CERN team with a nice
mixture of courage and caution, was able to announce their success on 24 November 1959.
Key members of the team were Mervin Hine and Hugh Hereward (See sidebars for Hine and
Hereward). Colleagues at Dubna in the USSR had sent CERN a bottle of vodka with which to
celebrate. The bottle was dutifully returned empty, with a message of thanks.
The AGS followed rather soon afterwards and for a short time better prepared to
launch into a rich programme of experiments than CERN.
V.9 Fermilab and SPS
After the AGS and CERN’s PS there had to be a significant leap in beam energy. A factor of 10
would raise the “fixed target” energy by a factor of 3. Somewhere between 200 and 300 GeV
seemed to be a reasonable aim, provided economies in the construction could be implemented.
The mastermind behind the first of these second-generation proton synchrotrons was Robert
(Bob) Wilson, who had worked with Lawrence in the cyclotron era and had built a number of
successful electron synchrotrons at Cornell. He had no hesitation in adopting the AG principle
for these machines and he was to add his own particular flavor to the construction of his new
200 GeV machine at the Fermi National Accelerator Laboratory (FNAL) near Chicago (See
sidebar on Fermilab and Figs. 5.10 and 5.11). As director and project leader of this new
laboratory he brought to the enterprise a flavor of economy and innovation he had inherited
from his mentor Lawrence. By separating the functions of focusing and bending, which had been
combined in the magnets of the AGS and PS, and using pure quadrupole and dipole units, he
found he could squeeze more bending power into each kilometer of the ring.
Wilson’s team were encouraged to use lateral thinking to emulate their leader’s
unconventional approach. Key members of Wilson’s team were Boyce McDaniel, Paul
Reardon and Roy Billinge. (See sidebars for Reardon and Billinge.) One such innovation, due
to Tom Collins (See sidebar for Collins), was to use a “telephoto” system of magnetic
quadrupoles to shoot the beam across a 60-meter-long straight section, catching it at the other
end with the mirror image of this optical system. These modifications were made at six points
in the ring to house crucial components for injection, acceleration and extraction, which
needed plenty of space to do their job. With this and other bold economies such as reducing
as many as possible of the gauges, taps and switches that threatened to adorn each of the
ring’s 1000 magnets, not to mention applying the production line techniques of Henry Ford to
the construction, Wilson was able to double the energy target to 400 GeV and propose its
completion in a mere 5 years. The first bold innovators in such ventures are perilously
exposed, but he kept his promise to complete the machine in only 5 years and thereby gain a
march on the rival SPS (see Fig. 5.12) at CERN, which had become stuck in a political
quagmire and which started construction only as Wilson’s machine began to run. Later the
addition of a superconducting ring, the Tevatron, was to complete the world’s first
superconducting hadron collider at Fermilab.
V.10 Superconducting Magnets
Over the decades, as synchrotrons have become larger, physicists have continued to ask for
beams of higher and higher energy. Their size is roughly proportional to the energy (strictly the
momentum) of the particles they accelerate and is limited by the maximum field (about 2 T)
that an electromagnet can produce before the iron parts saturate. An alternative is to build a
magnet in which only coils, and not the poles, determine the field shape, but this would require
very high currents and power dissipation due to losses (as given by Ohm’s law) in the copper
windings. On the other hand, a coil made of superconducting material has no resistance and will
pass huge current without any ohmic loss. Once established, the current will flow forever. Of
course the technology is somewhat more complicated than it sounds. The coil must be wound to
a very precise shape if the field is to be pure and uniform; and the magnetic forces trying to
blow the magnet apart are huge. Each magnet has to be mounted within a “dewar,” a huge
cryostat, to keep its temperature below the critical temperature for superconductivity, which,
for the superconductors used, is only a few degrees above absolute zero. (See sidebar on
Superconducting Magnets)
The development of a method of embedding the fine superconducting filaments, some
only ten microns in diameter, into a copper wire that could be woven into a cable, was crucial
to building practical superconducting magnets.
Martin Wilson at the Rutherford laboratory in the UK was instrumental in developing
a cable that could withstand the pulsing of a synchrotron while Alvin Tollestrup and Helen
Edwards solved the challenge of using superconductors for the first superconducting
synchrotron: the Energy Doubler at Fermilab (Fig. 5.13). (See sidebars on M. Wilson,
Tollestrup, and Edwards)
Martin Wilson’s cabling activities led directly to a valuable spin-off to the medical
field, where superconducting cable was used to build the solenoids that are the principal
component of the Magnetic Resonance Imaging (MRI) whole body scanners, now to be
found in most large hospitals in developed countries.
V. 11 Injecting and Extracting Beams
The beam entering the synchrotron must thread its way between the yokes of bending and
focusing magnets to eventually find its place on the centre line of the machine. Once there a
pulse of current applied to a dipole magnet will deflect it to send it in the direction of the axis of
the beam pipe. The pulse must rise and fall as the beam passes on its first turn in a matter of
tens of nanoseconds. Such a fast single turn injection process cannot be repeated without
disturbing the beam that is already there.
The reverse procedure can be applied to extract a beam within one turn and a multiturn extraction procedure can be used to peel off a circulating beam over a longer time. The
beam is encouraged to grow in size by exciting a resonant condition in the focusing fieldsnormally an effect to be avoided. As the beam grows a thin septum carrying a high electric
current peels away the edge of the beam rather like a bacon slicer. Such a resonant extraction
can be prolonged to extract a thin beam over many hundreds of turns and is used when a very
intense beam would saturate scintillation counters or when a beam is used to paint the surface
of a tumor in therapy. Unfortunately the reverse process cannot be applied to inject a beam
over many turns but a clever solution is at hand. People have found ways to produce a source
of negatively charged hydrogen ions (protons with two electrons attached) such an H-minus
beam can be accelerated in a linac or even in a small synchrotron feeding a larger machine.
The stream of negative charges passes through a constant field dipole magnet on the path of
the circulating beam deflecting the beam onto its circulating path. Passing briefly through a
thin foil, the electrons are stripped away leaving positively charge protons to be deflected the
opposite way when they return after a turn of the machine. This small constant deflection of
the circulating beam on each turn can be easily compensated by other deflections elsewhere
in the ring. In theory the process can go on indefinitely building up current until instability or
space charge places a limit on the current. This technique, widely used today, was first
proposed by G. I. Budker and G.I. Dimov in 1963. (See sidebar for Budker in Chapter VI)
Another Russian idea which can help the process of extracting the beam is to make
use of the large magnetic fields within a crystal. The beam is deflected from its central path
through a crystal placed like a septum just outside the normal envelope of the beam.
Although it is slightly scattered in the crystal the deflection it receives is enough to bend it
clear of the magnets and other hardware downstream. Yet another kind of multi-turn injection
is used in electron synchrotrons. The beam is injected into the edge of the circulating beam
distribution where synchrotron radiation damping causes it to eventually settle on the centre
line of the machine.