Beams and Beam Physics
7
momenta is rather small as well. Fig. 1.3 shows the basic principle.
The small emitting area imposes a severe limit on the total current extracted, but it is the standard electron source for transmission electron microscopes discussed in more detail below. Here the current need is low, but
the origination from a small area and with small energy spread amounting to
what is called high brightness is very useful. The use of sharp needles (point
filaments as they were historically called) was pioneered in the 1950s in order
to increase brightness of the electron beam in microscopes.
Particularly fruitful is combining needle geometries, resulting in an effective
lowering of the work function due to stronger electric field at surface, with
heating, which results in an increase of electrons of higher energy than the
work function that can thus traverse it. This kind of thermionic emission with
significant external field is called Schottky emission. In the 1960s, a new
generation of cathodes (mainly ZrO/W, which is a tungsten tip covered with a
thin layer of ZrO) were developed with stronger field at the surface, where field
emission plays a significant role and complements Schottky emission. This
regime is called the extended Schottky emission. This kind of emitters
are the main sources of electrons for electron microscopes. The emitters that
produce electrons through only the field emission process, called cold field
emission gun (CFEG), have been studied since the 1970s, and recently
have been able to produce high brightness beams.
The third process to produce electrons is photoemission where electrons
are produced via the photo effect. Exposing a surface to a large flux of
photons leads to some of the photons being absorbed by electrons within the
material, which consequently increase their kinetic energy. This additional
kinetic energy acts very similar to intense local heating in the thermionic gun
and leads to electrons with energies exceeding the work function and which
can consequentially escape. In certain cases photo-energized electrons can also
leave the material directly without colliding with other electrons, in a ballistic
process. In practice, photons are supplied through laser pulses, of which the
intensity, spot size and duration are relatively easy to adjust. This leads to
the photocathode gun. This approach has greatly facilitated the advance
of free electron lasers (FELs) in the last few decades and is an important
component in efforts of time-resolved spectroscopy and microscopy.
Again the extracted current is limited by the Child-Langmuir law (1.8),
despite the fact that an intense laser pulse can often produce large numbers of
electrons. Different classes of materials have been developed for the cathode,
including GaAs that has been cesiated, i.e., covered by less than a mono-layer
of cesium. This has allowed the production of electrons with energy spreads
down to fractions of 10 −1 eV, which is the range of thermodynamic energies
encountered at room temperature.
Other materials such as copper are able to withstanding the harsh environment of a radio frequency (RF) gun, where very high extraction fields can
be produced that significantly exceed those of the electrostatic case. Fig. 1.4
shows the layout of an RF gun and the field distribution. It consists of roughly
7
momenta is rather small as well. Fig. 1.3 shows the basic principle.
The small emitting area imposes a severe limit on the total current extracted, but it is the standard electron source for transmission electron microscopes discussed in more detail below. Here the current need is low, but
the origination from a small area and with small energy spread amounting to
what is called high brightness is very useful. The use of sharp needles (point
filaments as they were historically called) was pioneered in the 1950s in order
to increase brightness of the electron beam in microscopes.
Particularly fruitful is combining needle geometries, resulting in an effective
lowering of the work function due to stronger electric field at surface, with
heating, which results in an increase of electrons of higher energy than the
work function that can thus traverse it. This kind of thermionic emission with
significant external field is called Schottky emission. In the 1960s, a new
generation of cathodes (mainly ZrO/W, which is a tungsten tip covered with a
thin layer of ZrO) were developed with stronger field at the surface, where field
emission plays a significant role and complements Schottky emission. This
regime is called the extended Schottky emission. This kind of emitters
are the main sources of electrons for electron microscopes. The emitters that
produce electrons through only the field emission process, called cold field
emission gun (CFEG), have been studied since the 1970s, and recently
have been able to produce high brightness beams.
The third process to produce electrons is photoemission where electrons
are produced via the photo effect. Exposing a surface to a large flux of
photons leads to some of the photons being absorbed by electrons within the
material, which consequently increase their kinetic energy. This additional
kinetic energy acts very similar to intense local heating in the thermionic gun
and leads to electrons with energies exceeding the work function and which
can consequentially escape. In certain cases photo-energized electrons can also
leave the material directly without colliding with other electrons, in a ballistic
process. In practice, photons are supplied through laser pulses, of which the
intensity, spot size and duration are relatively easy to adjust. This leads to
the photocathode gun. This approach has greatly facilitated the advance
of free electron lasers (FELs) in the last few decades and is an important
component in efforts of time-resolved spectroscopy and microscopy.
Again the extracted current is limited by the Child-Langmuir law (1.8),
despite the fact that an intense laser pulse can often produce large numbers of
electrons. Different classes of materials have been developed for the cathode,
including GaAs that has been cesiated, i.e., covered by less than a mono-layer
of cesium. This has allowed the production of electrons with energy spreads
down to fractions of 10 −1 eV, which is the range of thermodynamic energies
encountered at room temperature.
Other materials such as copper are able to withstanding the harsh environment of a radio frequency (RF) gun, where very high extraction fields can
be produced that significantly exceed those of the electrostatic case. Fig. 1.4
shows the layout of an RF gun and the field distribution. It consists of roughly