Engineering Question #82

Yogesh Patil, a 25 year old male from the Internet asks on October 20, 1999,

Can you provide a detailed structure and working of the magnetron tube used in microwave ovens?

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The answer

Barry Shell answered on October 20, 1999

Chapter 11 of the book Driving Force: The Natural Magic of Magnets by James D. Livingston is all about this. More detailed descriptions of how magnetrons work can be found in many places, including the Encyclopedia Britannica (under Electronics in their Macropedia) which states: "Electron tubes of this type are primarily used to generate power at microwave frequencies for radar systems and microwave ovens. Magnetrons can be manufactured relatively inexpensively, because they require so few parts--namely, a cathode, anode, tank circuit, and magnet.The cylindrical anode structure contains a number of equally spaced cavity resonators with slots along the anode surface adjacent to the cylindrical cathode. A permanent magnet is usually used to provide the necessary magnetic field. The power output can be coupled through a slot in the cavity or by means of a coupling loop. As in other types of oscillators, the oscillation originates in random phenomena in the electron space charge and in the cavity resonators. The cavity oscillations produce electric fields that spread outward into the interaction space from the slots in the anode structure, as shown in Figure 2. Energy is transferred from the radial DC field to the rf field by interaction of the electrons with the fringing rf field. The first orbit of an electron occurs when the rf field across the gap is in a direction to retard the electron. The transfer of energy is from the electron to the tangential component of the rf field. The electron comes to a stop and is again accelerated by the radial DC field, making an orbit adjacent to the next cavity slot. If the rf field across the next cavity slot has changed phase by 180 degrees, the direction of the rf field is in the same direction as that of the electron, which moves in synchrony with the rf field. The electron gives up most of its energy to the cavities before it finally terminates on the anode surface. There is a net delivery of energy to the cavity resonators because electrons that absorb energy from the rf field are quickly returned to the cathode. By contrast, the energy in the rotational component of motion of the electrons in the retarding rf field remains practically unaffected, and the electrons may orbit around the cathode many times. Magnetrons have a wide range of output powers--from that used for cooking, which is about 600 watts, to special ones capable of generating pulsed power levels over 100 megawatts. The DC-to-rf power-conversion efficiency typically ranges from 50 to 85 percent." There is a book by Kettlewell called The Magnetron Oscillator (now out of print) that will probably tell you much more than you want to know. Other books that have a bit more than the encyclopedias are Introduction to Microwaves by Fred E. Gardiol, Microwave Engineering by David M.Pozar and the monster Handbook of Microwave Technology: Applications edited by T. Koryu Ishii.

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