10
E. Wilson and B. J. Holzer
revolution—just the length of one turn divided by the particle’s velocity
f =
v
2ππ
=
v
2π
·
eB
mv
.
(1.4)
has a numerator and denominator which are both proportional to v. This frequency
remains constant as the particle is accelerated in the low energy, classical, regime.
Thus, the circulating particles stay in synchronisation with the oscillating RF field
and a continuous stream of ions injected in the centre will follow a spiral path to
reach their highest energy at the rim of the poles.
Unfortunately, the synchronism between r.f. voltage and revolution frequency
breaks down as the particles velocity begins to approach that of light and the
relativistic mass in the above equation is no longer constant. This happens over
30 MeV for protons and at double this energy for deuterons. Electrons are much too
light and relativistic to be accelerated in a cyclotron to any significant energy. For
them other acceleration concepts are more adequate, like the disk loaded travelling
wave linac or the betatron that both were described before.
The possible remedy of making the field stronger at the edge of the poles would
have preserved synchronism and continuous beams but, as we shall see, was in
conflict with the need to have a negative radial gradient to the field to provide vertical
weak focusing. As a consequence a more powerful concept had to be developed to
achieve highest particle beam energies: The synchrotron.
1.2.6 The Synchrotron
Meanwhile, in the 1940s, still higher energies were needed to pursue the aims of
physics and the stage was set for the discovery of the synchrotron principle which
opened the way to the series of circular accelerators and storage rings which have
served particles physics up to the present day. It was Australian physicist Mark
Oliphant who synthesized three old ideas into a new concept—the synchrotron. The
ideas were: accelerating between the gaps of resonators, varying the frequency, and
pulsing the magnet. In 1943 he described his invention in a memo to the UK Atomic
Energy Directorate (see [11]).
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.
Unlike the cyclotron, the synchrotron accelerates the beam as a series of discrete
pulses or “bunches” as they are called. Each short pulse is injected at low field
and then the field rises in proportion to the momentum of particles as they are
accelerated. This ensures that the radius of the orbit remains constant. In contrast
to cyclotrons and betatrons, the synchrotron needs no massive poles to support a
E. Wilson and B. J. Holzer
revolution—just the length of one turn divided by the particle’s velocity
f =
v
2ππ
=
v
2π
·
eB
mv
.
(1.4)
has a numerator and denominator which are both proportional to v. This frequency
remains constant as the particle is accelerated in the low energy, classical, regime.
Thus, the circulating particles stay in synchronisation with the oscillating RF field
and a continuous stream of ions injected in the centre will follow a spiral path to
reach their highest energy at the rim of the poles.
Unfortunately, the synchronism between r.f. voltage and revolution frequency
breaks down as the particles velocity begins to approach that of light and the
relativistic mass in the above equation is no longer constant. This happens over
30 MeV for protons and at double this energy for deuterons. Electrons are much too
light and relativistic to be accelerated in a cyclotron to any significant energy. For
them other acceleration concepts are more adequate, like the disk loaded travelling
wave linac or the betatron that both were described before.
The possible remedy of making the field stronger at the edge of the poles would
have preserved synchronism and continuous beams but, as we shall see, was in
conflict with the need to have a negative radial gradient to the field to provide vertical
weak focusing. As a consequence a more powerful concept had to be developed to
achieve highest particle beam energies: The synchrotron.
1.2.6 The Synchrotron
Meanwhile, in the 1940s, still higher energies were needed to pursue the aims of
physics and the stage was set for the discovery of the synchrotron principle which
opened the way to the series of circular accelerators and storage rings which have
served particles physics up to the present day. It was Australian physicist Mark
Oliphant who synthesized three old ideas into a new concept—the synchrotron. The
ideas were: accelerating between the gaps of resonators, varying the frequency, and
pulsing the magnet. In 1943 he described his invention in a memo to the UK Atomic
Energy Directorate (see [11]).
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.
Unlike the cyclotron, the synchrotron accelerates the beam as a series of discrete
pulses or “bunches” as they are called. Each short pulse is injected at low field
and then the field rises in proportion to the momentum of particles as they are
accelerated. This ensures that the radius of the orbit remains constant. In contrast
to cyclotrons and betatrons, the synchrotron needs no massive poles to support a