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Construction and Operation of Magnetron

Microwave Tubes

Homepage Introduction History Magnetron Klystron Traveling Wave Tubes Conclusion Discussion Definition Reference

Construction

        Magnetron is the combination of a simple diode vacuum tube with built in cavity resonators and an extremely powerful permanent magnet. The typical magnet consists of a circular anode into which has been machined with an even number of resonant cavities. The diameter of each cavity is equal to a one-half wavelength at the desired operating frequency. The anode is usually made of copper and is connected to a high-voltage positive direct current. In the center of the anode, called the interaction chamber, is a circular cathode.

Operation of Magnetron

        All cavity magnetrons consist of a hot filament (cathode) kept at, or pulsed to, a high negative potential by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. Circular cathode that emits electrons when heated. In a normal diode vacuum tube, the electrons would flow directly from the cathode straight to the anode, causing a high current to flow. In a magnetron tube, however, the direction of the electrons is modified because the tube is surrounded by a strong magnetic field. The field is usually applied by a C-shape permanent magnet centered over the interaction chamber.

 

        The magnetic fields of the moving electrons interact with the strong field supplied by the magnet. The result is that the path for the electron flow from the cathode is not directly to the anode, but instead is curved. By properly adjusting the anode voltage and the strength of the magnetic field, the electrons can be made to bend such that they rarely reach the anode and cause current flow. The path becomes circular loops. Eventually , the electrons do reach the anode and cause current flow. By adjusting the dc anode voltage and the strength of the magnetic field, the electron path is made circular. In making their circular passes in the interaction chamber, electrons excite the resonant cavities into oscillation. A magnetron, therefore, is an oscillator, not an amplifier. A takeoff loop in one cavity provides the output.

         Magnetros are capable if developing extremely high levels of microwave power. Thousand and even millions of watts of power can be produced by a magnetron. When operated in a pulse modo, magnetron can generate several megawatts of power in the microwave region. Pulsed magnetron are commonly used in radar systems. Continuous-wave magnetrons are also used and can generate hundreds and even thousands of watts of power. A typical application for a continuous magnetron is for heating purposes in microwave ovens.