8
An Introduction to Beam Physics
FIGURE 1.4: Layout (left) and field distribution (right) of the RF gun at
the Linac Coherent Light Source (LCLS). (From J. Arthur, et al., SLAC-R593, 2002 [2]. Courtesy SLAC National Accelerator Laboratory.)
1.5 cells (1.6 for this example) with the field in the two cells in opposite phase
to ensure that electrons emitted into high extracting field end up with high
energy and low emittance at the exit. In addition, materials such as cesiated
GaAs with strained lattice can also produce polarized electrons, which has
been used at SLAC National Accelerator Laboratory, California, USA, for
high energy physics experiments on the one hand and in spin polarized low
energy electron microscope (LEEM) on the other.
1.2.2 Proton Sources
In most proton accelerators, the protons are produced through stripping
the two electrons in a negative hydrogen ion H
− at the entrance, mostly by
passing the ion through a thin carbon foil, although lasers have also been experimented with recently. In some applications, such as medical accelerators,
H
+
2 ions instead are produced and accelerated.
Although the stripping process makes the technology a little more complicated, it has one distinct advantage over injecting protons directly. For
injection into circular accelerators over many turns, the protons generated
through stripping can easily be aligned with earlier produced protons already
in the accelerator, because before stripping, both types of particles follow different paths due to the differing charge. As a result, the density of protons in
the beam can increase more and more over an extended injection period. On
the other hand, protons injected directly require much more care and can only
be injected with positions and directions that are different from those of other
protons already in the ring, since because of time reversal, all protons with the
same position and direction must have followed the same earlier trajectory.
The H
− ions can be produced by a variety of different methods. Here, only
the surface plasma source of the magnetron type is discussed. It obtained
its name due to the fact that it is similar in configuration to the ubiquitous
An Introduction to Beam Physics
FIGURE 1.4: Layout (left) and field distribution (right) of the RF gun at
the Linac Coherent Light Source (LCLS). (From J. Arthur, et al., SLAC-R593, 2002 [2]. Courtesy SLAC National Accelerator Laboratory.)
1.5 cells (1.6 for this example) with the field in the two cells in opposite phase
to ensure that electrons emitted into high extracting field end up with high
energy and low emittance at the exit. In addition, materials such as cesiated
GaAs with strained lattice can also produce polarized electrons, which has
been used at SLAC National Accelerator Laboratory, California, USA, for
high energy physics experiments on the one hand and in spin polarized low
energy electron microscope (LEEM) on the other.
1.2.2 Proton Sources
In most proton accelerators, the protons are produced through stripping
the two electrons in a negative hydrogen ion H
− at the entrance, mostly by
passing the ion through a thin carbon foil, although lasers have also been experimented with recently. In some applications, such as medical accelerators,
H
+
2 ions instead are produced and accelerated.
Although the stripping process makes the technology a little more complicated, it has one distinct advantage over injecting protons directly. For
injection into circular accelerators over many turns, the protons generated
through stripping can easily be aligned with earlier produced protons already
in the accelerator, because before stripping, both types of particles follow different paths due to the differing charge. As a result, the density of protons in
the beam can increase more and more over an extended injection period. On
the other hand, protons injected directly require much more care and can only
be injected with positions and directions that are different from those of other
protons already in the ring, since because of time reversal, all protons with the
same position and direction must have followed the same earlier trajectory.
The H
− ions can be produced by a variety of different methods. Here, only
the surface plasma source of the magnetron type is discussed. It obtained
its name due to the fact that it is similar in configuration to the ubiquitous