It is a diode usually of cylindrical configuration with a thick cylindrical cathode at the center and a coaxial cylindrical block of copper as anode. In the anode block are cut a number of holes and slots which act as resonant anode cavities. The space between the anode and cathode is the interaction space and to one of the cavities is connected a coaxial line or waveguide for extracting the output.

It is a cross field device as the electric field between anode and cathode is radial whereas the magnetic field produced by a permanent magnet is axial. The permanent magnet is placed such that the magnetic lines are parallel to the vertical cathode and perpendicular to the electric field between cathode and anode. The construction is shown in Fig.

Operation

The cavity magnetron shown in Fig. has 8 cavities, that are tightly coupled to each other. We know, in general that a N-cavity tightly coupled system will have N-modes of operation each of which is uniquely characterized by a combination of frequency and phase of oscillation relative to the adjacent cavity.

In addition, these modes must be self-consistent so that the total phase shift around the ring of cavity resonators is 2π where n is an integer. For example, a phase shift should be 40° between cavities of an 8-cavity magnetron will mean that the first cavity is out of phase with itself by 320°! The correct minimum phase shift should be 45° (45 x 8 = 360°). Therefore if $φ_U$ represents the relative phase change of the ac electric field across adjacent cavities, then,

$$φ_U=\frac{2πn}{N} \hspace{0.5cm} where \space n=0,±1,±2,± \bigg(\frac{N}{2}-1\bigg),±N/2$$

i.e., N/2 mode of resonance can exist if N is an even number.

$$If n=N/2, φ_U=π$$

This mode of resonance is called the π mode,

$$If n=0, φ_U=0$$

This is the zero mode, meaning there will be no RF electric field between anode and cathode (called the fringing field) and is of no use in magnetron operation.

To understand the operation of cavity magnetron, we must first look at how the electrons behave in the presence of closed electric and magnetic fields.

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 as shown in Fig.

In the absence of magnetic field (B = 0), the electron travels straight from the cathode to the anode due to the radial electric field force acting on it (indicated by the trajectory 'a' in Fig).

If the magnetic field strength is increased slightly (i.e., for moderate value of B) it will exert a lateral force bending the path of the electron as shown by path 'b' in Fig. The radius of the path is given by $R=\frac{mv}{eB}$, that varies directly with electron velocity and inversely as the magnetic field strength.

If the strength' of the magnetic field is made sufficiently high so as to prevent the electrons from reaching the anode (as shown by path 'c' and those inside in Fig.) the anode current becomes zero. The magnetic field required to return electrons back to cathode just grazing the surface of the anode is called the critical magnetic field $(B_C)$ the cut-off magnetic field. If the magnetic field is made larger than the critical field $(B \gt B_e)$, the electron experiences a greater rotational force and may return back to cathode quite faster. All such electrons may cause back heating of the cathode.

This can be avoided by switching off the heater supply after commencement of oscillation. This is done to avoid fall in the emitting efficiency of the cathode.

All the above explanation is for a static case in the absence of the RF field in the cavity of magnetron.

Performance Characteristics:

• Frequency range : 500 MHz to 12 GHz
• Power output : In excess of 25kW (pulsed) 10 mW (UHF) 2mW (X-band) 8 kW (at 95 GHz)
• Duty cycle : 0.1%
• Efficiency : 40% to 70%