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:
Filament power supplyThe filament power supply serves to heat up the filament to a temperature high enough to emit a sufficient number of free electrons. This supply can provide an AC or DC voltage, with typical voltages ranging from 2.5 V to 15 V, and currents ranging from a few A to 100 A and above. The filament voltage has to be applied some time before the cathode voltage so the filament has sufficient time to pre-heat. Furthermore, due to an effect called "back-bombardment", the filament voltage might have to be reduced once the Magnetron is producing microwave energy, in Magnetrons with variable output power the filament voltage is therefore often controlled by an electronic circuit, keeping the filament at optimum temperature.
High tension power supplyThe high tension supply is the actual power supply of the Magnetron, as it provides the energy for the acceleration of the electrons. The high tension supply is always a DC supply, depending on the output power and application different types of power supplies are available. Typical voltages range from 2 kV (2000 V) to 15 kV and above, with supply currents ranging from a few 100 mA to several A.
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:
Handling & StorageThe filaments of Magnetrons, especially when made from thoriated tungsten, are rather brittle, and can be easily destroyed by impacts or strong vibrations. Furthermore, contamination of the filter box or the antenna with dirt or dust can cause Magnetrons to fail premature, they should therefore be handled with care and kept safely in their original packing until mounted inside a microwave generator.
Filament supplyMaximum lifetime of a Magnetron can only be reached if the filament temperature is kept constant under all modes of operation. Therefore the filament supply should be checked frequently, at least before the installation of a new Magnetron. This is especially important in case of electronic (variable) filament supplies.
Launcher sectionThe launcher section is responsible for coupling the microwave energy from the Magnetron into the waveguide system, wrongly designed or badly maintained launchers lead to insufficient coupling and overheating of the Magnetron.
Load matchingBadly matched loads cause overheating of the Magnetron due to reflected microwave energy. Loads should always be matched using suitable tuning elements, if the load impedance varies during operation auto-tuners or circulators should be installed in order to protect the Magnetron.
CirculatorsCirculators are the safest option to protect Magnetrons in high power applications. However, circulators require regular maintenance to ensure they perform according to specification and efficiently protect the Magnetron.
Cooling systemMagnetrons require cooling of the anode body, filter box and antenna. Especially the cooling air for the filter box and antenna must be clean, dry and free of dust, if the anode body is water-cooled make sure the water is of good quality and does not lead to scaling of the cooling ducts.
Control systemHigh power Magnetrons and Magnetrons with variable output power utilise electronic control systems to supervise and adjust the high tension supply, anode current, output power and filament heating. These control systems should be checked on a regular basis, latest before the installation of a new Magnetron.
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.