Miscellanea
Miscellanea
I am grateful to my colleague Jim Colville for bringing the following article from the Autumn 1962 edition of "The Locking Review" to my attention.
THE LIFE OF A MAGNETRON
by Sqn. Ldr. G. D. BOLAM, M.A., B.Sc., Dip.EI., A.M.I.E.E.
One of the most important of the secret weapons developed during the war was the multi-cavity magnetron. It is not generally realised that the cathode of a magnetron is very quickly stripped of its coating and after passing current for a total time of only a few hours a magnetron is quite useless. Fortunately, a magnetron passes current for only a very small fraction of the time a radar is working.
Multi-cavity magnetrons are designed to work on centimetric wavelengths and must therefore be small (in physical size). In order to provide a high power pulse output the cathode must be capable of a very large peak emission - of the order of 1 A/cm2.
In order to secure such a high peak emission the cathode has to be oxide coated. With such a coating the electric field near the surface of the cathode should really be less than a certain value to avoid stripping the cathode a value corresponding to anode potentials of a few kilovolts. However, to provide a high power pulse the valve must not only pass a high peak current (about 20 A) it must also have a high anode potential of the order of 20 kV. This sets up an electric field which will strip the cathode in a few hours.
Magnetron cathodes are made to give as long a life as possible by special surface treatments but the life is still short only a few hours. Fortunately, pulse-working means that for each hour a radar is in operation the magnetron is in fact passing current for only a fraction of a second. Thus, although the working life of a magnetron totals only a few hours it can be used in an equipment for a few thousand hours.
Since the life of a magnetron is limited it is important to have a convenient way of testing magnetrons in use. This is done by using a spectrum analyser and rejecting a magnetron when the side lobes in the spectrum exceed 40 per cent. of the main lobe.
