4
An Introduction to Beam Physics
refer to eq. (1.149) in [5] for details of the derivation.
When studying the evolution of the beam from the time it is born until it is
used, there are usually four steps involved. First, there must be a way for the
production of the beam, and for the sake of efficiency if possible in such a
way that its emittance is small. Second, in most cases the energy of the beam
has to be increased; there has to be a mechanism of acceleration. Because
of the outstanding importance of this process, the whole field is often called
accelerator physics. Then it is necessary to transport the beam to where
it is being used. And finally, there is often a need for storage of the beam
for use at a later time or reuse. Lastly, often there is a need for analysis of
the beam, in particular after the beam has been used for its purpose, which
frequently is the facilitation of certain nuclear or high energy reactions. The
field of beam physics spans all these steps, and each of the steps has it own
unique problems to be solved.
1.2 Production of Beams
The mechanisms used for the production of the beam depend very much
on the particular kind of particles and the characteristics of the beam that
is needed, and they include mechanisms from a variety of different fields including thermal, electrical, atomic, nuclear, and even high energy physics
processes. Common beams consist of electrons, protons, or H
− , and some
of the beams produced through nuclear and high energy physics processes include positrons, antiprotons, pions, kaons and radioactive nuclei. Overall, due
to the diversity of the species of the particles and the required properties of
the beam, there are dozens of different ways of producing various beams. We
here restrict ourselves to some of the source types that are most commonly
used in particle accelerators and electron microscopes.
1.2.1 Electron Sources
Electrons exist in abundance in metals, and forming them into beams requires their extraction from the metal, called the cathode. For this the
electrons need to overcome the potential barrier, i.e., the work function,
at the boundary between the metal and the environment. The work function usually ranges from a fraction of an electron Volt (eV) to a few electron
Volts; for comparison, the average kinetic energy of gas molecules at room
temperature amounts to approximately 1/40 eV. This can be achieved by either supplying additional energy to the electrons so that they can leave the
material, or by lowering the work function. In the following we discuss some
common approaches based on these methods.
An Introduction to Beam Physics
refer to eq. (1.149) in [5] for details of the derivation.
When studying the evolution of the beam from the time it is born until it is
used, there are usually four steps involved. First, there must be a way for the
production of the beam, and for the sake of efficiency if possible in such a
way that its emittance is small. Second, in most cases the energy of the beam
has to be increased; there has to be a mechanism of acceleration. Because
of the outstanding importance of this process, the whole field is often called
accelerator physics. Then it is necessary to transport the beam to where
it is being used. And finally, there is often a need for storage of the beam
for use at a later time or reuse. Lastly, often there is a need for analysis of
the beam, in particular after the beam has been used for its purpose, which
frequently is the facilitation of certain nuclear or high energy reactions. The
field of beam physics spans all these steps, and each of the steps has it own
unique problems to be solved.
1.2 Production of Beams
The mechanisms used for the production of the beam depend very much
on the particular kind of particles and the characteristics of the beam that
is needed, and they include mechanisms from a variety of different fields including thermal, electrical, atomic, nuclear, and even high energy physics
processes. Common beams consist of electrons, protons, or H
− , and some
of the beams produced through nuclear and high energy physics processes include positrons, antiprotons, pions, kaons and radioactive nuclei. Overall, due
to the diversity of the species of the particles and the required properties of
the beam, there are dozens of different ways of producing various beams. We
here restrict ourselves to some of the source types that are most commonly
used in particle accelerators and electron microscopes.
1.2.1 Electron Sources
Electrons exist in abundance in metals, and forming them into beams requires their extraction from the metal, called the cathode. For this the
electrons need to overcome the potential barrier, i.e., the work function,
at the boundary between the metal and the environment. The work function usually ranges from a fraction of an electron Volt (eV) to a few electron
Volts; for comparison, the average kinetic energy of gas molecules at room
temperature amounts to approximately 1/40 eV. This can be achieved by either supplying additional energy to the electrons so that they can leave the
material, or by lowering the work function. In the following we discuss some
common approaches based on these methods.