Magnetron and klystron are the main microwave sources used in electron linac.



Magnetron is an electric vacuum device used to generate microwave energy. It's essentially a diode in a constant magnetic field. The electrons in the tube interact with the high-frequency electromagnetic field under the control of the constant magnetic field and the constant electric field, and convert the energy obtained from the constant electric field into microwave energy, so as to achieve the purpose of generating microwave energy. At the same time, the magnetron is a consumable and easy to age and demagnetize.

The magnetron consists of a sealed vacuum tube containing a cylindrical central cathode (electron source), which is placed in a cylindrical anode, and electrons are attracted by an electrostatic field to flow to the anode. A stable magnetic field along the tube axis causes the electrons to deviate from their radial path and rotate around the cathode, producing oscillations at microwave frequencies.
Magnetron is an electric vacuum device used to generate microwave energy. It's essentially a diode placed in a constant magnetic field. The electrons in the tube interact with the high-frequency electromagnetic field under the control of perpendicular constant magnetic field and constant electric field, and convert the energy obtained from the constant electric field into microwave energy to achieve the purpose of generating microwave energy.

Magnetrons can be divided into pulse magnetrons and continuous wave magnetrons because of their different working states.

The magnetron consists of a tube core and a magnetic steel (or electromagnet). The structure of the tube core includes four parts: anode, cathode, energy output and magnetic circuit system. A high vacuum is maintained inside the tube.



Magnetrons usually operate in π mode, and the phase of the microwave electric field at the mouth of the adjacent two resonators is exactly 180°, that is, the direction of the microwave electric field is opposite. Although this microwave field is a standing wave field, in the case of PI mode, it is equivalent to two identical microwave fields moving in opposite directions on the circumference, and the phase velocity values of the two fields are equal. The electrons emitted from the cathode move in cycloidal motion under the action of an orthogonal electromagnetic field. By adjusting the DC voltage and the constant magnetic field, the average drift velocity of the electrons in the circular direction is exactly equal to the phase velocity of a microwave field moving in its direction, and the electrons can move synchronously with the microwave field. In the process of synchronous motion, the part of electrons in the microwave deceleration field gradually gives their DC potential energy to the microwave field, and moves closer to the anode, and finally is collected by the anode.



These electrons transfer energy to the microwave field. The part of the electron in the microwave accelerating field receives energy from the microwave field and moves towards the cathode, finally hitting the cathode. These electrons ejected secondary electrons when bombarding the cathode, increasing the number of interacting space electrons. Most of the favorable electrons in the interaction space are in the process of moving towards the anode, and the favorable electrons have a longer rotation time, and they can fully rotate the DC potential energy into microwave energy. Fewer electrons are returned to the cathode, and they hit the cathode shortly after they are emitted from the cathode, so less energy is absorbed from the microwave field. In this way, the total effect of all the electrons in the interaction space interacting with the microwave field is that the electrons give DC potential energy to the microwave field, establishing a stable microwave oscillation in the magnetron.