Beams and Beam Physics
15
FIGURE 1.10: The principle of the tandem Van de Graaff accelerator.
(Reprinted from Nucl. Instrum. Methods, v. 8, R. J. Van de Graaff, Tandem
electrostatic accelerators, p. 195–202, Copyright (1960), with permission from
Elsevier [19].)
The Van de Graaff accelerator has several desirable features; for example, it
can produce a fully continuous beam (often denoted by the term “cw” for continuous wave) and at high beam current. Its main limitation is the relatively
low energies that it can produce, which seldom exceed about 20 MeV.
The tandem Van de Graaff is an efficient modification of the Van de Graaff
concept, in which both the source and the target are kept at ground potential
and which can efficiently increase the energy that can be obtained. For
this purpose, a source is chosen that produces negatively charged ions, which
are then sent through a regular Van de Graaff. At the end of the accelerating section, the ions are sent through a thin foil, in which many of them are
stripped of some of their electrons, resulting in positive ions. Because the
particles already have substantial energy when hitting the foil, often much
higher charge states can be produced than in the ion source itself. These
positive ions are then sent through a second stage Van de Graaff, which is
essentially a reversion of the first stage. At the location of the target, depending on their charge state after stripping, their energy is increased by a factor
of two or more. The mechanism of the tandem Van de Graaff accelerator is
shown in Fig. 1.10. Having very similar characteristics to the original Van de
Graaff, the energies that can be achieved in this way are in the range of up
to 60 MeV.
1.3.2 Linear Accelerators
It is an important observation that the field strength that can be obtained
in quickly oscillating (radio frequency, RF) electric fields can be substantially higher than what can be obtained statically in devices of similar
15
FIGURE 1.10: The principle of the tandem Van de Graaff accelerator.
(Reprinted from Nucl. Instrum. Methods, v. 8, R. J. Van de Graaff, Tandem
electrostatic accelerators, p. 195–202, Copyright (1960), with permission from
Elsevier [19].)
The Van de Graaff accelerator has several desirable features; for example, it
can produce a fully continuous beam (often denoted by the term “cw” for continuous wave) and at high beam current. Its main limitation is the relatively
low energies that it can produce, which seldom exceed about 20 MeV.
The tandem Van de Graaff is an efficient modification of the Van de Graaff
concept, in which both the source and the target are kept at ground potential
and which can efficiently increase the energy that can be obtained. For
this purpose, a source is chosen that produces negatively charged ions, which
are then sent through a regular Van de Graaff. At the end of the accelerating section, the ions are sent through a thin foil, in which many of them are
stripped of some of their electrons, resulting in positive ions. Because the
particles already have substantial energy when hitting the foil, often much
higher charge states can be produced than in the ion source itself. These
positive ions are then sent through a second stage Van de Graaff, which is
essentially a reversion of the first stage. At the location of the target, depending on their charge state after stripping, their energy is increased by a factor
of two or more. The mechanism of the tandem Van de Graaff accelerator is
shown in Fig. 1.10. Having very similar characteristics to the original Van de
Graaff, the energies that can be achieved in this way are in the range of up
to 60 MeV.
1.3.2 Linear Accelerators
It is an important observation that the field strength that can be obtained
in quickly oscillating (radio frequency, RF) electric fields can be substantially higher than what can be obtained statically in devices of similar