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Introduction: The optical nature of a charged particle beam
Figure 1.1: (a) electron source, and (b) positive ion source.
In (b) a tungsten wire is formed into a very sharp tip. The tip
is elevated to a positive voltage, typically a few thousand to a few
tens of thousands of volts, relative to the planar electrode. A small
amount of helium gas is admitted into the system. Helium atoms
diffuse toward the vicinity of the tip, where they are ionized in
the very high electric field. This is known as a gas field ionization
source. Ion sources of other chemical species exist as well. Practically any material which can be ionized can be used to form an
ion beam. This enables a rich variety of species of ion beams to be
formed.
In all cases, an electric field accelerates the charged particles. Each
particle acquires an energy equal to its charge times the accelerating voltage. A natural unit of energy is the electron-Volt, abbreviated as eV. It is the energy which a particle with one electronic
charge acquires when accelerated through one volt. The beam energy is thus easily tuned to almost any desired value by simply
controlling the accelerating voltage. This turns out to have considerable practical utility. Practical charged particle beams range
in energy from a few eV to about fourteen trillion eV. This is the
Introduction: The optical nature of a charged particle beam
Figure 1.1: (a) electron source, and (b) positive ion source.
In (b) a tungsten wire is formed into a very sharp tip. The tip
is elevated to a positive voltage, typically a few thousand to a few
tens of thousands of volts, relative to the planar electrode. A small
amount of helium gas is admitted into the system. Helium atoms
diffuse toward the vicinity of the tip, where they are ionized in
the very high electric field. This is known as a gas field ionization
source. Ion sources of other chemical species exist as well. Practically any material which can be ionized can be used to form an
ion beam. This enables a rich variety of species of ion beams to be
formed.
In all cases, an electric field accelerates the charged particles. Each
particle acquires an energy equal to its charge times the accelerating voltage. A natural unit of energy is the electron-Volt, abbreviated as eV. It is the energy which a particle with one electronic
charge acquires when accelerated through one volt. The beam energy is thus easily tuned to almost any desired value by simply
controlling the accelerating voltage. This turns out to have considerable practical utility. Practical charged particle beams range
in energy from a few eV to about fourteen trillion eV. This is the