This invention relates to a magnetron and in particular an improvement in a magnetic device for a magnetron.
Recently a magnetron has been used for quantity-production type microwave ovens etc. and a growing demand is made for a compact and low-cost magnetron. A conventional magnetron, however, can not adquately satisfy design and sales requirements from the standpoint of its characteristic as well as its construction.
A conventionally known magnetron is of an external magnet type in which a magnet device is disposed outside of a case which hermetically seals an anode, cathode etc. Recently, however, an internal magnet type magnetron in which a magnet device hermetically sealing an anode, cathode etc. is disposed outside of the case dominates over the external magnet type magnetron from the miniaturization requirements as well as the ease with which the magnetron is handled.
For the internal magnet type magnetron a permanent magnet should be made compact in size and requires a strong magnetic field. For this reason, a rare earth magnet made of, for example, samarium cobalt or cerium cobalt is generally used instead of a ferrite or alnico magnet. However, the rare earth magnet presents the following various problems.
In the manufacturing process of an ordinary magnetron the rare earth magnet is beforehand strongly magnetized to a value greater than a predetermined value required from the standpoint of an operational characteristic and after the magnetron is operated an output from the magnetron is measured. Then, the slight demagnetization adjustment of the magnet is effected to obtain a desired output. Since the rare earth magnet has a coercive force greater than the ferrite magnet etc., difficulty is encountered in obtaining a predetermined output through the demagnetization adjustment. Furthermore, an expensive large-sized device will be required during the demagnetization adjusting process. The rare earth magnet has more than four times the "energy product" of, for example, the alnico magnet and provides, even in the case of a small magnet, a magnetic field of 1500 to 2000 gauses which is required for a microwave oven magnetron. However, the cost per unit weight of the magnet is high and it is desired that the amount used be reduced to a minimum possible extent. It is because that the use of the rare earth magnet in the magnetron results in high-cost magnetron.