Beams and Beam Physics
9
FIGURE 1.5: Drawing of a surface plasma source of the magnetron geometry. (From J. Ishikawa, Negative ion sources, in I. G. Brown, ed., The Physics
and Technology of Ion Sources, 2nd ed. [31]. Copyright (c) 2005 Wiley-VCH.
With permission.)
microwave source called the magnetron, which is used in common appliances
such as the microwave oven. The main process of producing the negative
hydrogen ions is through electron capture of neutral hydrogen atoms on the
cathode surface, where electrons penetrate the potential barrier through quantum tunneling. The presence of the electric and magnetic field creates a dense
plasma near the surface of the cathode where ions, most of which are positive,
are produced. Those positive ions and neutral particles bombard the cathode
partially covered with cesium and H
− ions are produced. The presence of cesium lowers the work function and greatly increases the probability of barrier
penetration and hence H
− production.
Some of the produced H
− ions are neutralized shortly after production.
An electric field between the cathode and the anode is used to accelerate
the remaining H
− ions towards the exit. However, electrons are accelerated
towards the anode as well. But since electrons are much lighter than the H
−
ions, they are bent much more easily than the H
− ions, and are absorbed by
the electron collector (Fig. 1.5).
The magnetron type of negative hydrogen ion source was first developed
in the former Soviet Union in the early 1970s and quickly spread to Europe
and the United States. It has become the main choice for high energy proton
accelerators.
1.2.3 Ion Sources
There are a large variety of different sources for ions which found applications in many fields. Presenting even a brief overview is already beyond the
scope of this book; for more comprehensive reviews, see [77, 8]. As in the
previous subsection, we limit our discussion to one important kind, which is
the electron cyclotron resonance (ECR) ion source. Fig. 1.6 shows the
9
FIGURE 1.5: Drawing of a surface plasma source of the magnetron geometry. (From J. Ishikawa, Negative ion sources, in I. G. Brown, ed., The Physics
and Technology of Ion Sources, 2nd ed. [31]. Copyright (c) 2005 Wiley-VCH.
With permission.)
microwave source called the magnetron, which is used in common appliances
such as the microwave oven. The main process of producing the negative
hydrogen ions is through electron capture of neutral hydrogen atoms on the
cathode surface, where electrons penetrate the potential barrier through quantum tunneling. The presence of the electric and magnetic field creates a dense
plasma near the surface of the cathode where ions, most of which are positive,
are produced. Those positive ions and neutral particles bombard the cathode
partially covered with cesium and H
− ions are produced. The presence of cesium lowers the work function and greatly increases the probability of barrier
penetration and hence H
− production.
Some of the produced H
− ions are neutralized shortly after production.
An electric field between the cathode and the anode is used to accelerate
the remaining H
− ions towards the exit. However, electrons are accelerated
towards the anode as well. But since electrons are much lighter than the H
−
ions, they are bent much more easily than the H
− ions, and are absorbed by
the electron collector (Fig. 1.5).
The magnetron type of negative hydrogen ion source was first developed
in the former Soviet Union in the early 1970s and quickly spread to Europe
and the United States. It has become the main choice for high energy proton
accelerators.
1.2.3 Ion Sources
There are a large variety of different sources for ions which found applications in many fields. Presenting even a brief overview is already beyond the
scope of this book; for more comprehensive reviews, see [77, 8]. As in the
previous subsection, we limit our discussion to one important kind, which is
the electron cyclotron resonance (ECR) ion source. Fig. 1.6 shows the