transverse dynamics 23
as they require a better vacuum for their operation and are
vulnerable to poisoning by heavier elements present in the
residual gas.
In thermionic guns, a grid can be installed between the
anode and cathode. Applying pulsed voltage to the grid allows for the generation of a train of pulses suitable for consequent RF acceleration.
In an electron gun, the electrons generated by thermionic
emission tend to repel from each other as illustrated in
Fig. 2.1, resulting in reduced beam quality. The additional
focusing electrodes can help to maintain the quality of the
beam. In particular, it was shown by J.R. Pierce in 1954 that
in planar geometry, an electrode at the potential of the cathode inclined at 67.5 ◦ (called the Pierce angle), as shown in
Fig. 2.2, will help to maintain a parallel flow of electrons.
Any intermediate accelerating electrodes or the anode would
need to be placed along the equipotential lines, as shown in
Fig. 2.2, to maintain the parallel flow.
FIGURE 2.2
Electron gun with Pierce electrode and collector made in the
form of a Faraday cup.
The same Fig. 2.2 also illustrates the concept of the beam
anode (collector) made in the shape of a Faraday cup — a useful device for the accurate measurement of electron current.
Any secondary charged particles emitted from the walls are
eventually absorbed and do not affect the measurements of
the current.
Controlling the beam shape and beam quality in high
density electron guns often requires the use of an accompanying solenoid magnetic field. Let us now consider equations
for the motion of charged particles in electromagnetic fields.
as they require a better vacuum for their operation and are
vulnerable to poisoning by heavier elements present in the
residual gas.
In thermionic guns, a grid can be installed between the
anode and cathode. Applying pulsed voltage to the grid allows for the generation of a train of pulses suitable for consequent RF acceleration.
In an electron gun, the electrons generated by thermionic
emission tend to repel from each other as illustrated in
Fig. 2.1, resulting in reduced beam quality. The additional
focusing electrodes can help to maintain the quality of the
beam. In particular, it was shown by J.R. Pierce in 1954 that
in planar geometry, an electrode at the potential of the cathode inclined at 67.5 ◦ (called the Pierce angle), as shown in
Fig. 2.2, will help to maintain a parallel flow of electrons.
Any intermediate accelerating electrodes or the anode would
need to be placed along the equipotential lines, as shown in
Fig. 2.2, to maintain the parallel flow.
FIGURE 2.2
Electron gun with Pierce electrode and collector made in the
form of a Faraday cup.
The same Fig. 2.2 also illustrates the concept of the beam
anode (collector) made in the shape of a Faraday cup — a useful device for the accurate measurement of electron current.
Any secondary charged particles emitted from the walls are
eventually absorbed and do not affect the measurements of
the current.
Controlling the beam shape and beam quality in high
density electron guns often requires the use of an accompanying solenoid magnetic field. Let us now consider equations
for the motion of charged particles in electromagnetic fields.