1 Accelerators, Colliders and Their Application
7
Fig. 1.3 left: The concept of the drift tube linac (from [8]); right: CERN’s Linac 1
gaps is 360 ◦ . Each gap appears to the particle to be an identical field gradient
which accelerates particles from left to right. The particles are protected from the
decelerating phase while inside the metallic drift tubes. Although the particle gains
energy steadily as it passes each gap, the total voltage between parts of the assembly
and ground does not become larger along the length of the device as it would for an
electrostatic machine.
The distances between gaps, or the lengths of the tubes, increase as the particle
is accelerated since it travels an ever increasing distance during one swing of
the radio frequency oscillation. At low energy, we would expect this distance to
increase with the velocity or the root of the kinetic energy but when the energy is
large we find the length of the drift tubes and their spacing no longer increases—
a practical demonstration of special relativity. The Alvarez structure is still widely
used, especially for non-relativistic proton and ion beams.
It was well known at the time that waves might be propagated along a much
simpler smooth waveguide and that some of the modes have an accelerating electric
field in the direction of propagation. Closer examination however shows that the
stumbling block is that the phase velocity of these modes in a wave guide is
always greater than that of light and hence the particle sees a field which sometimes
accelerates and then decelerates as the wave overtakes the particle. It was later found
that the phase velocity could be reduced by a series of iris diaphragms in the pipe.
Such a structure (Fig. 1.4) is very popular in electron linacs and also in storage rings
in which the particle is close to the velocity of light and cavities need not be tuned
to follow the acceleration cycle.
These diaphragm-loaded linac structures have been commonly used as injectors
for circular accelerators to accelerate electrons and protons to energies in the range
10 to 1000 MeV. As compact high frequency structures they have also been widely
used to accelerate electrons to, typically 10 MeV, as a source of X-rays for cancer
therapy. An early and very successful adventure in the electron linac development
was the “two-mile long” Stanford Linear Accelerator at SLAC in California which
has been the work horse for a number of ground breaking fixed target experiments
and circulating beam storage ring projects at 20 to 50 GeV. With the help of two
semi-circular arcs it was used to bring beams of electrons and positrons into headon collision in the Stanford Linear Collider Project. This project, is forerunner for
7
Fig. 1.3 left: The concept of the drift tube linac (from [8]); right: CERN’s Linac 1
gaps is 360 ◦ . Each gap appears to the particle to be an identical field gradient
which accelerates particles from left to right. The particles are protected from the
decelerating phase while inside the metallic drift tubes. Although the particle gains
energy steadily as it passes each gap, the total voltage between parts of the assembly
and ground does not become larger along the length of the device as it would for an
electrostatic machine.
The distances between gaps, or the lengths of the tubes, increase as the particle
is accelerated since it travels an ever increasing distance during one swing of
the radio frequency oscillation. At low energy, we would expect this distance to
increase with the velocity or the root of the kinetic energy but when the energy is
large we find the length of the drift tubes and their spacing no longer increases—
a practical demonstration of special relativity. The Alvarez structure is still widely
used, especially for non-relativistic proton and ion beams.
It was well known at the time that waves might be propagated along a much
simpler smooth waveguide and that some of the modes have an accelerating electric
field in the direction of propagation. Closer examination however shows that the
stumbling block is that the phase velocity of these modes in a wave guide is
always greater than that of light and hence the particle sees a field which sometimes
accelerates and then decelerates as the wave overtakes the particle. It was later found
that the phase velocity could be reduced by a series of iris diaphragms in the pipe.
Such a structure (Fig. 1.4) is very popular in electron linacs and also in storage rings
in which the particle is close to the velocity of light and cavities need not be tuned
to follow the acceleration cycle.
These diaphragm-loaded linac structures have been commonly used as injectors
for circular accelerators to accelerate electrons and protons to energies in the range
10 to 1000 MeV. As compact high frequency structures they have also been widely
used to accelerate electrons to, typically 10 MeV, as a source of X-rays for cancer
therapy. An early and very successful adventure in the electron linac development
was the “two-mile long” Stanford Linear Accelerator at SLAC in California which
has been the work horse for a number of ground breaking fixed target experiments
and circulating beam storage ring projects at 20 to 50 GeV. With the help of two
semi-circular arcs it was used to bring beams of electrons and positrons into headon collision in the Stanford Linear Collider Project. This project, is forerunner for