2
Introduction: The optical nature of a charged particle beam
the intrinsic charge of electrons and ions through the force known
as the Lorentz force, after the physicist who first identified it in
the nineteenth century. By bombarding with a very high energy
beam, the atomic nucleus can dissociate into its constituent elementary particles. This is the mechanism by which a high energy
particle accelerator is used to probe the fundamental makeup of
matter.
Many examples of free charged particles exist in nature. Energetic ions appear as cosmic rays which pervade interstellar space,
and bombard the earth’s atmosphere in large numbers. A large variety of subnuclear particles are produced in high energy particle
accelerators. Many of these also appear as cosmic rays. The beam
inside an electron microscope or a cathode ray tube consists of
free, energetic electrons in a vacuum. Indeed, it is not difficult to
form a beam of charged particles in a vacuum by making use of the
intrinsic properties of matter, together with electric and magnetic
fields to focus and steer the beam.
According to the laws of classical physics, a single charged particle traces out a path of motion under the influence of electric
and magnetic fields. A collection of many particles emitted from
a source, each with its own trajectory, form a beam.
Two common sources are shown schematically in Figure 1.1. In
(a) a hot tungsten wire at the top of the figure, with a temperature of about 2000 degrees Kelvin is placed opposite a planar electrode called the anode. The anode is typically electrically
grounded. Electrons are spontaneously emitted from the hot wire
by the process of thermionic emission. By means of an external
power supply, the tungsten wire is elevated to a negative voltage
which can be anywhere between a few volts to a few millions of
volts relative to the anode. This voltage is called the accelerating
voltage, because the resulting electric field accelerates the particles. This forms a beam, which is analogous in several fundamental
ways to a beam of light. Each trajectory in the figure corresponds
to the path of a single charged particle.
Introduction: The optical nature of a charged particle beam
the intrinsic charge of electrons and ions through the force known
as the Lorentz force, after the physicist who first identified it in
the nineteenth century. By bombarding with a very high energy
beam, the atomic nucleus can dissociate into its constituent elementary particles. This is the mechanism by which a high energy
particle accelerator is used to probe the fundamental makeup of
matter.
Many examples of free charged particles exist in nature. Energetic ions appear as cosmic rays which pervade interstellar space,
and bombard the earth’s atmosphere in large numbers. A large variety of subnuclear particles are produced in high energy particle
accelerators. Many of these also appear as cosmic rays. The beam
inside an electron microscope or a cathode ray tube consists of
free, energetic electrons in a vacuum. Indeed, it is not difficult to
form a beam of charged particles in a vacuum by making use of the
intrinsic properties of matter, together with electric and magnetic
fields to focus and steer the beam.
According to the laws of classical physics, a single charged particle traces out a path of motion under the influence of electric
and magnetic fields. A collection of many particles emitted from
a source, each with its own trajectory, form a beam.
Two common sources are shown schematically in Figure 1.1. In
(a) a hot tungsten wire at the top of the figure, with a temperature of about 2000 degrees Kelvin is placed opposite a planar electrode called the anode. The anode is typically electrically
grounded. Electrons are spontaneously emitted from the hot wire
by the process of thermionic emission. By means of an external
power supply, the tungsten wire is elevated to a negative voltage
which can be anywhere between a few volts to a few millions of
volts relative to the anode. This voltage is called the accelerating
voltage, because the resulting electric field accelerates the particles. This forms a beam, which is analogous in several fundamental
ways to a beam of light. Each trajectory in the figure corresponds
to the path of a single charged particle.