magnetron at higher temperatures than 300 degrees Celsius without a serious reduction in the magnetic field and therefore an accompanying drop in operating anode voltage and efficiency. The rare earth cobalt magnets that were used were ring magnets 2 inches in outside diameter and 3/8 of an inch in thickness. They are made from grade 18 material, which has a specified "maximum practical operating temperature" of 225 degrees Celsius. They may therefore be marginal, or even inadequate, depending upon the actual microwave power required from the tube. Thus far, it has been impractical to operate the magnetron with integral magnets in a vacuum because of the microwave power disposal problem. Even though the currently used magnets may present a potential problem there is a new generation of rare earth magnets that are becoming available and which have a maximum useable temperature range of 300 to 350 degrees Celsius, a full 100 degrees greater than the previous generation. This new material also has a higher energy product which will reduce the size of the magnet that is needed. These new magnets are described on page 930 of the June 1990 Special Issue of the Proceedings of the IEEE on Magnetics in an article by Karl Strant.
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