JPH03230456A - Magnetron for microwave oven - Google Patents

Abstract

PURPOSE:To surely prevent the occurrence of a high-frequency discharge when a magnetron is operated under severe conditions by sticking a film with the secondary electron emission ratio smaller than 1 on the inner face of a metal cylinder for a high-frequency choke. CONSTITUTION:A metal cylinder 30 for the first high-frequency choke having the diameter smaller than that of a metal container 19 and surrounding an antenna lead 25 apart from it is provided in the metal container 19. A film 32 made of a material with the secondary electron emission ratio smaller than 1 is stuck on at least the inner periphery of the cylinder 32. The film 32 is made of carbon, titanium carbide or titanium nitride, and it is formed on the whole inner periphery of the cylinder 30. The occurrence and continuous action of a multipactor discharge between the lead 25 and cylinder 30 are suppressed, and the heating and melting phenomena of constituting parts do not occur. The occurrence of a high-frequency discharge is surely prevented, and a reliable and stable operation can be obtained with a simple structure.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a magnetron for a microwave oven, and particularly relates to an improvement in its high frequency output section.

(Prior Art) The main parts of a magnetron for a microwave oven have conventionally been constructed as shown in FIG. In the same figure, reference numeral 11 denotes the main body of the full vibration part, 12 an anode cylinder, 13 a plurality of anode vanes constituting a part of the cavity resonator, 14 a strap ring,
15 is a filament cathode, 16 is its end shield, 17.18 is a pole piece fixed to the open end of the anode cylinder, 19 is a cylindrical metal container, 20 is a high frequency output part,
21 is a ceramic cylinder for the output part, 22 is a sealing ring for the tip of the blade, 23 is a metal exhaust pipe hermetically sealed thereto, 24
25 is an output metal cap, and 25 is an output antenna lead, one end 25a of which is electrically connected to one of the vanes, passes through the through hole 17a of the pole piece, penetrates the metal container and the ceramic cylinder, and extends the tip. The portion 25b is held between the metal exhaust pipes and hermetically sealed. Further code 2
B is a ring-shaped permanent magnet coaxially arranged around the outer periphery of the metal container, 27 is a ferromagnetic yoke, 28 is a ferromagnetic thin plate,
29 is a mesh conductor gasket. A first metal cylinder 30 for a harmonic choke with a small diameter is hermetically soldered to the lower end of the ceramic cylinder 21, and a second metal cylinder 31 for a harmonic choke is located on the outer periphery of the metal cylinder 30 and is soldered in an airtight manner. being touched. The second harmonic choke metal cylinder 31 is hermetically soldered to the tip of the metal container 19, and the tip 31a holds the inner peripheral portion of the gasket 29.

With this structure, the exhaust pipe part has a quarter-wavelength chock groove C2 for the second harmonic, a chock groove 04 for the fourth harmonic, and a metal container 19 and two metal cylinders 30 and 31 in its inner space. The third harmonic groove C
Third and fifth harmonic grooves C5 are formed, respectively. In addition, a metal container 19, a metal cylinder 30 for both chokes.
31 is composed of a thin ferromagnetic cylinder made of iron or iron alloy. In order to obtain a necessary and sufficient choke effect, the first harmonic choke metal cylinder 30 has an inner diameter dimension D
1 is set to be smaller than the inner diameter dimension D2 of the ceramic cylinder 21 and smaller than 1/2 of the fifth harmonic wavelength.

In such a magnetron, during operation, a fundamental wave of, for example, 2450 MHz is efficiently radiated from the output section, while external radiation of each harmonic component is suppressed by the choke action of the choke groove.

(Problems to be Solved by the Invention) As described above, in order to obtain a reliable choke effect on high-order harmonic components such as the fifth harmonic, the inner diameter of the harmonic choke metal cylinder 30 is reduced to a certain extent. There is a need to. When the diameter is reduced in this way, the distance S between the choke metal cylinder 30 and the antenna lead 25 passing inside it is naturally shortened. A high high frequency voltage is applied between these, and there is a risk that high frequency discharge will occur due to the relationship with reflected waves from a high frequency load such as a microwave oven. Assuming an extreme case, if the stirrer fan stops rotating for some reason while the microwave oven is in use, and there is little or no object to be heated that is a high-frequency load, the high-frequency reflection to the magnetron will be lower than the standing wave ratio. (VSWR) may be 30 or more. In such a case, a discharge will occur between the antenna lead and the metal cylinder for the harmonic choke, and in extreme cases, the antenna lead 25
Alternatively, it is also possible that a part of the choke metal cylinder 30 generates heat due to high frequency discharge and melts. And if-
If the parts melt, the resulting gas will cause local gas discharges, which will further cause high-frequency short circuits and reflections, leading to chain discharges in the output area and fatal melting of each part. It is also possible that damage may occur.

Such high frequency discharge can be estimated to be a multipactor discharge phenomenon in many cases. That is, in the space between the antenna lead and the harmonic choke metal cylinder, the leakage DC magnetic flux from the permanent magnet 26 extends approximately parallel to the tube axis Z, as indicated by the symbol F in FIG. Furthermore, the magnetic flux in this space has a roughly axis-symmetric distribution. Since these metal parts usually have a secondary electron emission ratio greater than 1, when floating electrons or the like collide with the antenna lead or the inner surface of the metal cylinder for choke, secondary electrons are generated. These electrons are accelerated or decelerated by the high frequency electric field between the antenna lead and the metal cylinder. When electrons emitted from one of the conductors encounter a high-frequency accelerating electric field, they are accelerated and collide with the other conductor, emitting many secondary electrons. At this time, if the high-frequency electric field is reversed and becomes an electric field that accelerates these secondary electrons in the direction of the original conductor, these secondary electrons will still be accelerated and collide with the original conductor, emitting many secondary electrons again. do.

When electrons and high-frequency electric fields are synchronized in both directions in this way,
As the secondary electrons increase exponentially and gain energy, both conductors will heat up and eventually cause melting. This phenomenon is a discharge called a double-sided multipactor.

On the other hand, electrons undergo a swirling motion due to the DC magnetic field that exists in this region. When the period of the high-frequency electric field and the period of the swirling motion of the electrons are synchronized, secondary electrons e are cumulatively generated as schematically shown in Figure 4, and the metal cylinder material rapidly generates heat due to the collision energy, and finally It is estimated that melting will occur. This phenomenon is called one-sided multipactor discharge.

An object of the present invention is to provide a magnetron for a microwave oven that can reliably suppress the occurrence of high-frequency discharge within the high-frequency output section even when operated under the above-mentioned severe conditions.

[Structure of the Invention (Means for Solving the Problems) This invention provides a metal cylinder for a harmonic choke, which has a diameter smaller than that of the metal container of the output section and surrounds the antenna lead at a distance. This is a microwave oven magnetron to which a coating having a secondary electron emission ratio of less than 1 is attached.

(Function) According to the present invention, the cumulative generation of secondary electrons on the inner surface of the harmonic choke metal cylinder surrounding the antenna lead is suppressed, and high frequency discharge does not occur or persist, causing heat generation in the component parts. No melting phenomenon occurs. Therefore, highly reliable and stable operation can be obtained with a relatively simple configuration.

(Example) The embodiment will be described below with reference to FIG.

Note that the same parts as in Figure 2 are designated by the same symbols.

Inside the metal container 19, at least the inner peripheral surface of the first harmonic choke metal cylinder 30, which has a diameter smaller than this and surrounds the antenna lead 25 at a distance, has a secondary electron emission ratio of 1.
A coating 32 of a smaller material is deposited. This coating 32 may be made of carbon (C), titanium carbide (
Materials such as TiC) or titanium nitride (T i N) are suitable. The film thickness may be approximately 30 μm or more.

Although it is more preferable that the coating 32 be formed on the entire inner circumferential surface of the metal cylinder 3o for harmonic choke, the coating 32 is not limited thereto, and may be partially attached, for example, in a plurality of areas in a striped manner along the axis Z direction. Good too.

Furthermore, a coating having a low secondary electron emission ratio may be similarly formed on the surface of the region of the antenna lead 25 located inside the metal cylinder 30 for harmonic choke.

[Effects of the Invention] As explained above, according to the present invention, the occurrence and persistence of multipactor discharge in the space between the antenna lead and the relatively small-diameter harmonic choke metal cylinder surrounding the antenna lead is suppressed, No heat generation or melting of component parts occurs. Therefore, highly reliable and stable operation can be obtained with a relatively simple configuration.

[Brief explanation of drawings]

Fig. 1 is a longitudinal sectional view of the main part showing an embodiment of the present invention, Fig. 2 is a longitudinal sectional view of the main part showing the conventional structure, Fig. 3 is a schematic diagram showing the magnetic flux distribution of the main part, and Fig. 4 is a longitudinal sectional view of the main part showing an embodiment of the present invention. It is a schematic diagram explaining a high frequency discharge phenomenon. 12... Anode cylinder, 13... Anode vane, 17... Pole piece, 19... Cylindrical metal container, 21... Ceramic cylinder, 25... Antenna lead, 30... Harmonic wave Metal cylinder for chalk, 2
6...Permanent magnet, F...DC magnetic flux, Z...
...Tube axis, 32...Coating.

Claims (1)

[Claims] (1) A plurality of anode vanes are fixed inside an anode cylinder to form a cavity resonator, a pole piece is fixed to the open end of the anode cylinder, and a cylindrical metal container is joined to the metal container. An output ceramic cylinder is joined to the open end of the antenna lead, one end of which is electrically connected to the cavity resonator, and an antenna lead is extended through the inside of the metal container and the ceramic cylinder. In the microwave oven magnetron, a harmonic choke metal cylinder having a diameter smaller than that of the antenna lead and surrounding the antenna lead at a distance is fixed, and a ring-shaped permanent magnet is arranged around the outer periphery of the metal container. A magnetron for a microwave oven, characterized in that a coating having a secondary electron emission ratio of less than 1 is attached to the inner surface of a metal cylinder.