Invention of MagnetronLimitations of Ordinary Gridded Tubes-Skin
Effect-Transit Angle EffectNeed of High-power microwaves
INTRODUCTIONMagnetron is a cross field device as the electric
field between the anode and the cathode is radial whereas the
magnetic field produced by a permanent magnet is axial. A high DC
potential can be applied between the cathode and anode which
produces the radial electric field. Depending on the relative
strengths of the electric and magnetic fields, the electrons
emitted from the cathode and moving towards the anode will traverse
through the interaction space.Cavity Magnetron
Effect of electric fieldEffect of magnetic fieldEffect of
Crossed-FieldsNS
Fm
B
e-
e-
B
E
e-
CONSTRUCTIONAs shown in the figure, a cavity magnetrons consist
of a hot filament (cathode) kept at a high negative potential by a
high-voltage, direct-current power supply. The cathode is built
into the center of an evacuated, lobed, circular chamber. A
magnetic field parallel to the filament is imposed by a
electro-magnet. The magnetic field causes the electrons, attracted
to the (relatively) positive outer part of the chamber, to spiral
outward in a circular path rather than moving directly to this
anode.
OPERATIONTo understand the operation of cavity magnetron, we
must first look at how the electrons behave in the presence of a
closed electric and magnetic fields.This are explained in phase one
and two.PHASE 1In the absence of magnetic field (B = 0), the
electron travel straight from the cathode to the anode due to the
radial electric field force acting on it.
Phase 2If the magnetic field strength is increased slightly, due
to lateral force the electron path bends. If the strength of the
magnetic field is made sufficiently high then the electrons can be
prevented from reaching the anode. The magnetic field required to
return electrons back to the cathode just grazing the surface of
the anode is called the critical magnetic field (Bc) or the cut off
magnetic field.
If the magnetic field is larger than the critical field (B >
Bc), the electron experiences a greater rotational force and may
return back to the cathode quite faster.
The electron path under the influence of different strength of
the magnetic fieldMagnetic field equal to zero
Slightly increased magnetic field
Critical magnetic field
Excessive magnetic field
Cathode -veAnode blocks +veInteraction Space
All the above explanation is for a static case in the absence of
the RF field in the cavity of magnetron.
