10
An Introduction to Beam Physics
mechanism schematically. The chamber on the left holds the plasma where
ions are produced. First, the gas of the element of interest is injected to
the chamber. Collisions among the atom generates electrons and ions. The
magnetic field (see top part of Fig. 1.6) forms a magnetic mirror that confines the ions and the electrons. For the example shown, this time is around
100 μs, and for more modern and advance versions for around 10 ms. High
frequency (2.45 to 28 GHz) microwaves are injected into the chamber and
electrons with rotation frequency matching that of the microwaves are accelerated to between 1 and 20 keV. This process of heating up the electron gas is
called electron cyclotron resonance heating. The resulting hot electrons
collide with ions and neutral atoms and generate more ions. Furthermore,
through step-by-step ionization, even multiply charged ions (e.g., Xe
38+ ) can
be produced.
In order to produce sufficient quantities of multiply charged ions, the ion
confinement has to relatively long (∼ 10 ms). The major improvement in this
aspect is the addition of a sextupole magnet, which ensures that the magnetic
field at the center of the chamber is at the minimum, which prevents the ions
from drifting to the side wall. Another consequence of this configuration of
the magnetic field is that the surface on which electron cyclotron resonance
heating takes place is now closed, which significantly reduces hot electron
loss. This configuration of the magnetic field also makes ion production more
efficient, since electrons are confined longer and can collide with ions and
be reheated many times. The fact that the ECR ion source does not use a
cathode to generate electrons makes it a much more reliable source compared
to other varieties.
The ECR ion source was first developed in the mid-1960s and, by the mid1970s, many had been built around the world. It is probably the best source to
produce multiply charged ions and has become the main choice of ion sources
for nuclear physics facilities.
1.3 Acceleration of Beams
We now assume that an ensemble of particles occupying a small volume of
phase space has been created, and we thus have what is called a beam. In
many if not most of the practical cases, the energy that the beam has after
being produced by the source is not sufficient for the purpose it is to be used
for, which frequently amounts to furnishing the energy necessary for atomic,
nuclear, or particle processes of interest.
In most cases, the motion is best studied by first considering the motion
of the reference particle, and once this motion is understood satisfactorily,
to study the relative motion of the other particles. For a simple analysis
An Introduction to Beam Physics
mechanism schematically. The chamber on the left holds the plasma where
ions are produced. First, the gas of the element of interest is injected to
the chamber. Collisions among the atom generates electrons and ions. The
magnetic field (see top part of Fig. 1.6) forms a magnetic mirror that confines the ions and the electrons. For the example shown, this time is around
100 μs, and for more modern and advance versions for around 10 ms. High
frequency (2.45 to 28 GHz) microwaves are injected into the chamber and
electrons with rotation frequency matching that of the microwaves are accelerated to between 1 and 20 keV. This process of heating up the electron gas is
called electron cyclotron resonance heating. The resulting hot electrons
collide with ions and neutral atoms and generate more ions. Furthermore,
through step-by-step ionization, even multiply charged ions (e.g., Xe
38+ ) can
be produced.
In order to produce sufficient quantities of multiply charged ions, the ion
confinement has to relatively long (∼ 10 ms). The major improvement in this
aspect is the addition of a sextupole magnet, which ensures that the magnetic
field at the center of the chamber is at the minimum, which prevents the ions
from drifting to the side wall. Another consequence of this configuration of
the magnetic field is that the surface on which electron cyclotron resonance
heating takes place is now closed, which significantly reduces hot electron
loss. This configuration of the magnetic field also makes ion production more
efficient, since electrons are confined longer and can collide with ions and
be reheated many times. The fact that the ECR ion source does not use a
cathode to generate electrons makes it a much more reliable source compared
to other varieties.
The ECR ion source was first developed in the mid-1960s and, by the mid1970s, many had been built around the world. It is probably the best source to
produce multiply charged ions and has become the main choice of ion sources
for nuclear physics facilities.
1.3 Acceleration of Beams
We now assume that an ensemble of particles occupying a small volume of
phase space has been created, and we thus have what is called a beam. In
many if not most of the practical cases, the energy that the beam has after
being produced by the source is not sufficient for the purpose it is to be used
for, which frequently amounts to furnishing the energy necessary for atomic,
nuclear, or particle processes of interest.
In most cases, the motion is best studied by first considering the motion
of the reference particle, and once this motion is understood satisfactorily,
to study the relative motion of the other particles. For a simple analysis