(1) Each coil is wound identically and
(1) The dc electric field lines do not
each has the same amount of current flow. Since the
extend into the tube for any appreciable distance.
coils have the same direction of current flow, the
However, the beam electrons receive enough energy
magnetic field produced by one coil adds to the field
from the electric field to complete the trip through
produced by the adjacent coil. In this way, the
the tube.
magnetic field is continuous along the drift-tube axis
as shown in figure 51.
(2) With only the electric field present,
the beam electron at position A would tend to follow
(2) The magnetic field lines may extend
a field line. Notice that each field line's path is from
in either direction, that is with or against the
cathode-to-drift tube entrance. This means that the
direction of beam electrons flow. In the figure, the
beam electrons traveling from the cathode would
magnetic lines are opposite to the direction of beam
bombard the drift tube entrance. This would cause
electron flow.
excessive heat. This action, and the possibility of
excessive heat, is prevented by the field coil
c. High density electrons enter the drift
assembly.
tube and they have to travel the long length of the
drift tube. Because of the electrons' density and
d. In addition to the electric field lines, the
length of travel, they repeatedly repel each other.
magnetic field lines, generated by the field coil
This mutual repulsive force causes unwanted effects,
assembly, occur as shown in figure 53. The
beginning right at the drift tube entrance. For
magnetic field lines produce a force that counteract
example, assume that point A on figure 52
the upward vertical force on the electron (c above).
represents the position of one beam electron entering
Now the vertical force is downward and causes the
the drift tube. Because the electron is a negative
electron to move toward the drift tube axis.
charge, it is repelled by nearby electrons (also
negative charges). As shown by the vertical arrow,
e. In reality, the electric and magnetic flux
the repelling force is at right angles with respect to
provide primary control over the electron's path.
the drift tube axis. The horizontal arrow represents
The electric field imparts the energy that gives the
the direction of force caused by the dc electric field
electron its motion parallel to the drift tube axis;
lines (shown dotted) existing between cathode and
whereas, the magnetic field imparts the energy that
drift-tube entrance.
makes the electron move in a cyclotron (or spiral)
about the drift tube axis.
Figure 51. Continuous magnetic field lines along drift tube axis.
344 L1
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