Magnetrons differ from the famous tubes in that they are cross-field tubes where the electric and magnetic fields intersect at a perpendicular angle. The TWT experiment revealed that electrons interacting with RF for an extended period produced higher efficiency than in Klystron, and Magnetrons employ the same approach.
There are three main types of Magnetrons.
Negative Resistance Type
The resistance between two anodes are used having low frequencies. They are used at low frequencies <500MHz.
Cyclotron Frequency Magnetrons
The synchronism between the electric component and oscillating electrons is considered.Useful for frequencies higher than 100MHz.
Travelling Wave or Cavity Type
The interaction between electrons and rotating EM field is considered. High peak power oscillations are provided. Useful in radar applications.
The Magnetron is called as Cavity Magnetron because the anode is made into resonant cavities and a permanent magnet is used to produce a strong magnetic field, where the action of both of these make the device work.
Making of Cavity Magnetron
A thick cylindrical cathode is present at the center and a cylindrical block of copper, is fixed axially, which acts as an anode. This anode block is made of a number of slots that acts as resonant anode cavities. The space present between the anode and cathode is called as Interaction space. The electric field is present radially while the magnetic field is present axially in the cavity magnetron. This magnetic field is produced by a permanent magnet, which is placed such that the magnetic lines are parallel to cathode and perpendicular to the electric field present between the anode and the cathode. The following figures show the constructional details of a cavity magnetron and the magnetic lines of flux present, axially.