The present invention relates to a magnetron for microwave oven.
An ordinary magnetron for microwave oven oscillating radio waves of 2450 MHz band is provided with an anode cylinder and a plurality of vanes. The vanes are radially arranged inside the anode cylinder. The vanes are alternately connected together in the direction of circumference by a pair of large and small strap rings soldered with the top and bottom ends of the vanes. A spiral cathode is located along the axis of the anode cylinder in the electronic interaction space surrounded by the free ends of the vanes. Both ends of the spiral cathode are fixed to the output side end hat and the input side end hat respectively. Furthermore, both ends of the anode cylinder are fixed to pole pieces formed like funnels of output and input side respectively.
In view of miniaturization of heating appliances per se, resource saving and cost reduction, it is required to miniaturize the magnetron for microwave oven. However, merely miniaturization would cause degradation of various characteristics of the magnetron.
For example, if the height of the vane in the direction of the axis and the length of the input stem portion are decreased, cathode back heat by electrons or deterioration of load stability may occur. If the distance between both pole pieces is merely narrowed for the purpose of effective utilization of the magnet in order to reduce the height of the magnetron, electromagnetic coupling between the pole piece and the strap ring is increased and result in temperature rising of the cathode due to increase of cathode back heat by electrons. On the other hand, if the height of the vanes in the direction of the axis is shortened to assure the distance between the pole piece and the strap ring to some extent, load stability may be deteriorated. In addition, if the input stem portion is shortened, cathode back heat by electrons is extremely increased and temperature of the cathode is raised. In extreme cases, a part of the cathode may melt. Therefore, a magnetron which height of the vanes in the direction of the axis is 9 to 10 mm has ever been estimated suitable for miniaturization and favorable characteristics.
Japanese Patent Application Publication No. 1993-035531 gives attention especially to the distribution of magnetic field in the interaction space and discloses a magnetron that has smaller height of vanes in the direction of the axis by improving the shape, dimensions and electromagnetic coupling with the strap of the pole piece. According to this magnetron, change of the shape and the dimensions of pole piece makes that strength difference in the direction of the axis at the inner end surface of the vanes is less than a predetermined ratio, and equalizes distribution of the magnetic field in the interaction space. Furthermore, by digging the strap ring into inside the groove from the side end of the vane to decrease the electromagnetic coupling with the pole piece, cathode back heat by electrons and deterioration of the load stability rarely take place even if the height of the vanes in the direction of the axis or the input stem portion is diminished.
However, the magnetron disclosed in Japanese Patent Application Publication No. 1993-035531 has a vane of more than of equal to 8.5 mm in height in the direction of the axis. If the height of the vane in the direction of the axis is further decreased, it is conceivable that the load stability will be extremely deteriorated, so that the magnetron will not be suitable for practical use. Additionally, if the height of the vane in the direction of the axis is diminished, electrons leaking from the interaction space increase (i.e. dark current increases) because the gap between the end hats at the both ends of the cathode and the ends of the vane in the direction of the axis becomes large. Thus, there is a possibility of deterioration of the output efficiency and melting of the pole piece, etc.
The distance between the end hats at the both ends of the cathode, i.e. the effective length of the filament must be shortened in order to avoid deterioration of the output efficiency and melting of the vane, etc. However, if the effective length of the filament is shortened, the load stability will deteriorate further and the cathode back heat by electrons will increase.