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The donutron is an all-metal squirrel-cage magnetron. It is a multisegment magnetron with a single resonant structure. It is tuned by the relative axial displacement of alternate anode segments, through flexure of one wall of the cavity in which the anode structure is supported. During the testing of some sixty models, the operating efficiency and the output power have been increased to 40 to 50 per cent and around 50 watts, respectively (6- to 12-centimeter range). The best model to date tunes over a 1.5 to 1 ratio with power flat to 3 decibels. A single value of voltage and magnetic field is adequate for the entire tuning range. Of the various modes of operation, two are important, a long-wave tunable cavity mode and a shortwave resonant re-entrant-line mode. The former can be entirely suppressed and the latter enhanced by a special phase-reversing anode. In the line mode the highest output powers have been observed. In cold tests, modes around 4 centimeters as well as indication of even shorter resonance wavelengths have been found. A wide range of design parameters has been studied. Rieke diagrams have been taken for the best tubes.