M-Press Systems (M) Sdn. Bhd.
Industrial Microwave-, Plasma-, RF- and Control-Systems

Magnetron Basics

Most industrial microwave equipment uses Magnetrons for the generation of the required microwave energy. This is due to the facts that Magnetrons are relatively low-cost, compact, easy to operate and have a good efficiency. Only applications with high demands regarding frequency and phase stability are using other types of vacuum tubes, e.g. Gyrotrons or Klystrons.

Working principle of Magnetrons

A Magnetron consists of a filament in the centre of the tube, acting as the cathode, with the anode body surrounding the filament. The filament and anode body are packed into a single device together with permanent magnets and, is some cases, additional electromagnetic coils, which permit to control and vary the output power of the Magnetron. The inside of the anode body, containing the filament, is then evacuated to a high vacuum and sealed.

The filament is made from a special material, e.g. thoriated tungsten, which, when heated to approximately 2400 °C, starts to emit free electrons. Because the filament is connected to the negative pole of the high-tension DC supply and the anode body to the positive pole, the electrons are accelerated by the electrical field towards the anode. However, due to the magnetic field orientated perpendicular to the path of the accelerated electrons, they are forced to follow a spiral path leading from the filament to the anode body. The anode body contains a number of cavities machined into it, and as the stream of electrons is passing by these cavities they are "bunched" together due to resonant effects. One of the cavities is coupled to the antenna located outside the Magnetron, and converts a portion of the kinetic energy of the electron bunches into RF (microwave) energy, which is coupled from the antenna into the waveguide through a device called the launcher. Please note that the output frequency of a Magnetron is directly depending on the mechanical dimensions of the cavities machined into the anode body, therefore Magnetrons are getting smaller with increasing output frequency.

Operation of Magnetrons

In order to operate, the Magnetron requires 2 power supplies:

Lifetime of Magnetrons

While some other factors might affect the lifetime of a Magnetron, e.g. insufficient pre-heating time for the filament or voltage spikes on the high tension supply, under normal operation it is limited mainly by the lifetime of the filament. Due to Thorium evaporation and a "sandblasting-effect" caused by the back-bombardment of the electrons the filament is wearing off, giving the Magnetron a limited lifetime, which typically ranges somewhere between 2000 h and 10.000 h. To maximise the lifetime, the following points must be taken into consideration:

In case your microwave system under-performs or your Magnetrons only reach a short lifetime please feel free to contact us, we have the required expertise and equipment to check your system thoroughly and put it back into an "as new" condition.