Notes

Since the cathode is kept at negative voltage, the static electric field is in
radial direction from (grounded) anode block to the cathode. When no
magnetic field exists, heating the cathode results in a uniform and direct
movement of the electron from the cathode to the anode block (the blue
path in figure 5). A weak permanent magnetic field B perpendicular to the
electric field bends the electron path as shown with the green path in figure
5. If the electron flow reaches the anode, so a large amount of plate current
is flowing. If the strength of the magnetic field is increased, the path of the
electron will have a sharper bend. Likewise, if the velocity of the electron
increases, the field around it increases and the path will bend more sharply.
However, when the critical field value is reached, as shown in the figure 5 as
a red path, the electrons are deflected away from the plate and the plate
current then drops quickly to a very small value. When the field strength is
made still greater, the plate current drops to zero.
These values of the anode voltage and magnetic field strength that prevent
an anode current are called Hull cut-off magnetic field and cut-off voltage.
When the magnetron is adjusted to the cut-off, or critical value of the plate
current and the electrons just fail to reach the plate in their circular motion,
it can produce oscillations at microwave frequencies.