A new, radial B-field magnetron for use in microwave ovens

A new microwave tube design, suitable for application in microwave ovens, is described. It is a development of the basic planar magnetron in which the cathode and anode are two annuli and the magnetic field is radial. Potential advantages over existing designs are: higher power density, higher efficiency, easier tuning, the probability of reduced harmonics, and lower manufacturing costs. The work reported here was directed towards reducing the cost f the magnetron power source which is a major item in the cost of microwave ovens. Most similar exercises (e.g. Hisada et a1 1971, Fukatsu et a1 1979) have been concerned with modifications to the long-established cylindrical cavity magnetron design. The following account describes a radically new design (British Patent Application No 8314167) that should result in a microwave tube having twice the power density of existing magnetrons, a significantly higher efficiency, easier tuning, the probability of reduced harmonics, and lower manufacturing costs. The new design is a third development of the basic planar magnetron, schematically drawn in figure l(a). In this basic system, electrons are emitted from a cathode in the xz plane and are accelerated by an electric field E , in the y direction, towards the anode. The electron trajectory becomes cycloidal by the presence of a magnetic field B in the z direction, which generates a force uB in the x direction, U being they velocity. In the conventional cavity magnetron, a planar system is wrapped around an axis in the z direction, to become a re-entrant cylindrical system. In a second development, proposed by Buneman (1950), the planar system is wrapped around an x axis below the cathode plane. The B-field then becomes circular (about thex axis) and can be produced by cathode current in the x direction. If this new configuration is then wrapped around either a y or z axis, Buneman’s toroidal magnetron results. A serious difficulty with this arrangement is that a cathode current of more than 1000 A is required in order to produce an adequate magnetic field-a condition perhaps acceptable for pulsed operation but not for continuous operation in a microwave oven. The third possible development, wrapping around an axis in the y direction, does not seem to have been proposed elsewhere. It is shown schematically in figure l (b) , in which the cathode and anode are now two annuli, still separated by a gap d. The magnetic field has now become radial. Figure 2 is a notional diagram illustrating the essentials of a practical magnetron based upon the new concept, this particular version having a 0022-3727/84/030043 + 04 $02.25