GB2291322A

GB2291322A - Microwave oven

Info

Publication number: GB2291322A
Application number: GB9417885A
Authority: GB
Inventors: Yeon-Hag Seong; Jong-Chull Shon; Gweon-Jib Kim
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1994-07-12
Filing date: 1994-09-06
Publication date: 1996-01-17
Also published as: GB9417885D0; KR0140461B1; CN1124837A; DE4433105A1; RU94033105A; BR9500279A; FR2722559B1; JPH0845657A; FR2722559A1

Description

- - 1 - MICROWAVE OVEN

DESCRIPTION

2291322 The present invention relates to a microwave oven.

Typically, microwave ovens employs a magnetron which is. energized with a high voltage of, for example, 4 KV. In order to use a magnetron, the microwave ovens needs a high voltage transformer which results in safety problems, an increase in weight, and an increase in manufacturing cost.

An example of such a microwave oven employing a magnetron is illustrated in Figure 1.

In Figure 1, the reference numeral 10 denotes a power supply unit including a high voltage transformer and a high voltage capacitor. As a user manipulates a control panel (not shown) arranged on the right hand side of the front surface of the microwave oven, the power supply unit supplies electric power to a magnetron denoted by the reference numeral 20 in Figure 1 and a cooling fan not shown. The magnetron 20 is activated upon receiving a high voltage of 4 KV from the power supply unit 10. In its activated state, the magnetron 20 emits microwaves via an antenna 22. The microwaves emitted from the antenna 22 of - 2 the magnetron 20 are guided to a cooking chamber 50 via a waveguide 30 and then spread in the cooking chamber 50 by a stirrer 40. The spread microwaves are incident on a food contained in the cooking chamber 50, so that cooking is carried out.

The cooling fan, not shown, is typically arranged behind the magnetron 20 when viewed as in Figure 1. The cooling fan generates an air flow for cooling the magnetron 20. As the air flow cools the magnetron 20. it increases in temperature. The heated air is guided to an inlet 70 by a duct, not shown, so that it can be introduced into the cooking chamber 50.

The inlet 70 is constituted by at least one aperture having a diameter 1 less than 1/4 of the wavelength I of the microwaves (1 < 1/4) so as to prevent the incident microwaves leaking through the aperture.

In Figure 1, the reference numeral 60 denotes a housing as an enclosure of the microwave oven.

Figure 2 is a sectional view illustrating the magnetron 20 of the microwave oven shown in Figure 1. As shown in Figure 2, the magnetron 20 is a cylindrical bi-pole vacuum tube. A cathode 22 is arranged at the center of the magnetron 20. When the operating voltage is applied to an input terminal 21, the cathode 22 is heated to emit electrons. Arranged around the cathode 22 is an anode 23 which receives electrons emitted from the cathode 22.

A pair of cylindrical magnets 24a and 24b are disposed above and beneath the magnetron 20. The magnets 24a and 24b generate magnetic fluxes which are, in turn, guided by guide members 25a and 25b to pass through a cavity 26 defined between the cathode 22 and anode 23 and kept in vacuum.

Accordingly, the electrons emitted from the cathode 22 are deflected by the magnetic field formed in the cavity 26, so that they circulate between the cathode 22 and the anode 23.

Where a lot of electrons circulate in groups in the cavity 26, a resonant circuit is constructed in the anode 23. By means of this resonant circuit, microwaves are generated. The anode 23, which is heated by the impact of electrons, is cooled by cooling fins 29. The microwaves are outputted at the antenna 27 connected at one end to the anode 23.

The antenna 27 protrudes upwards through a hole, centrally provided, in the upper magnet 24a. The protruding portion of the antenna 27 is capped with a cap 28 which is mounted to surround the antenna 27.

4 - The microwaves, namely, radio frequency waves emitted from the antenna 27 reach the cooking chamber via the waveguide and the inlet both as typically equiping conventional microwave ovens and then heat the f ood contained in the cooking chamber.

However, since a high voltage of about 4 KV must be applied between the cathode 22 and the anode 23 in a magnetron having the above-mentioned construction, the conventional microwave oven presents a safety problem.

Moreover, a heavy transformer and capacitor are required for generating the high voltage. As a result, conventional microwave ovens are bulky and heavy. In addition, high manufacturing cost is involved.

Therefore, an aim of the invention is to solve the abovementioned problems encountered in the prior art and, thus, to provide a microwave oven capable of employing a low voltage oscillation tube, thereby eliminating the danger involved in the use of high voltage and achieving lighter construction.

According to the present invention, there is provided a microwave oven wherein the source of cooking microwave energy comprises a klystron.

Preferably, the klystron is a multi-beam klystron.

Embodiments of the present invention will now be described, by way of example, with reference to Figures 3 to 14 of the accompanying drawings, in which:

Figure 1 is a schematic view illustrating a conventional microwave oven; Figure 2 is a sectional view illustrating a magnetron of the microwave oven shown in Figure 1; Figure 3 is a schematic view illustrating a microwave oven in accordance with an embodiment of the present invention; Figure 4 is a side view of the microwave oven shown -in Figure 3; Figure 5 is a perspective view illustrating a klystron 15 employed in the microwave oven shown in Figure 3 in accordance with the present invention; Figure 6 is a plan view of the klystron shown in Figure 5; Figure 7 is a bottom view of the klystron shown in Figure 5; Figure 8 is a right side view of the klystron shown in Figure 5; Figure 9 is a left side view of the klystron shown in Figure 5; Figure 10 is a sectional view illustrating an internal construction of the klystron employed in the microwave oven in accordance with the present invention; Figure 11 is a perspective view illustrating a pole piece of 6 the klystron employed in a microwave oven in accordance with the present invention; Figure 12 is a perspective view illustrating a magnet of the klystron employed in a microwave oven in accordance with the present invention; Figure 13 is a plan view illustrating the drift channels of the klystron employed in a microwave oven in accordance with the present invention; and Figure 14 is a sectional view explaining the operation 10 principle of the klystron employed in a microwave oven in accordance with the present invention.

Referring to Figure 3, a microwave oven is denoted by the reference numeral 300.

Referring also to Figure 4, the reference numeral 310 denotes a power supply unit. As a user manipulates a control panel 500 arranged at the right hand side of the front surface of the microwave oven 300, the power supply unit 310 supplies electric power to a klystron 400 and a cooling fan 380. The klystron 400 is activated upon receiving power from the power supply unit 310. In its activated state, the klystron 400 emits microwaves via an antenna 322. The microwaves emitted from the antenna 322 of klystron 400 are guided to a cooking chamber 350 by a waveguide 330. The microwaves introduced are spread in the - 7 cooking chamber 350 by a stirrer 340, to be incident on food contained in the cooking chamber 350, so that the food is cooked.

The cooling fan 380 is arranged behind the klystron 400 when viewed as in Figure 4. The cooling fan 380 generates an air flow for cooling the klystron 400. As the air flow cools the klystron 400, it is heated. The heated air f low is guided to an inlet 370 by a duct 390 so that it can be introduced into the cooking chamber 350.

The inlet 370 is constituted by at least one aperture having a diameter P less than 1/4 of the wavelength 1 of microwaves (9 < 1/4) so as to prevent the incident microwaves from being leaked through the aperture.

The reference numeral 332 denotes a mounting member for fixedly mounting the klystron 400 to the waveguide 330 and the reference numeral 360 denotes a housing as an enclosure of the microwave oven.

Referring to Figure 5, the klystron 400 includes an input terminal 422 for receiving electric power, a klystron body 410 for generating microwave energy at a predetermined frequency upon receiving the electric power via the input terminal 422, an antenna 322 for transmitting the energy from the klystron body 410 to an external unit (in the 8 illustrated case, the waveguide 330 of Figures 3 and 4), and a cooling unit 430 for cooling the klystron body 410.

The input terminal 422 is electrically insulated from the 5 klystron body 410 by an insulator 424.

The klystron body 410 includes a yoke 402 forming an enclosure, a pair of spaced magnets 450a and 450b disposed in the yoke 402, and a tube 440 interposed between the magnets 450a and 450b.

A plurality of clamping pieces 412 protrude from opposite edges of the upper portion of the klystron body 410. Each of the clamping pieces 412 has a clamping hole 414. It is preferred that the clamping pieces 412 are positioned to balance the weight of klystron 400.

The antenna 322 protrudes upwards from the klystron body 410 and includes a coaxial line which will be described below, an insulating member 322a and a cap 322b. The insulating member 322a is made of an insulator such as a ceramic for providing insulation from the yoke 402 of klystron body 410. The cap 322b is made of a material such as stainless steel.

The cooling unit 430 includes a plurality of cooling fins 432 for dispersing heat generated in the klystron body 410, - 9 a cooling rod for transferring the heat from the klystron body 410 to the cooling fins 432, and a cooling member 434 surrounding the cooling fins 432 and forming an enclosure for the cooling unit 430.

Referring to Figures 6 to 9, the input terminal 422 is arranged at a right hand side portion of the klystron 400. The input terminal 422 is electrically insulated from the yoke 402 forming the enclosure of the klystron body 410. As mentioned above, the clamping pieces 412, each having a clamping hole 414, protrude f rom opposite edges of the upper portion of the klystron body 410 and are preferably positioned to balance the weight of the klystron 400. The antenna 322 protrudes upwards from the klystron body 410 and includes the insulating member 322a and the cap 322b.

The cooling unit 430 is disposed at a left hand side portion of klystron body 410.

Referring to Figure 10, as mentioned above in conjunction with Figure 5, the klystron 400 includes an input terminal 422 adapted to receive electric power, the klystron body 410 for generating microwave energy upon receiving electric power from the input terminal 422, the antenna 322 adapted to transmit the energy from the klystron body 410 to the external unit (in the illustrated case, the waveguide 330 of Figures 3 and 4), and the cooling unit 430 adapted to cool the klystron body 410.

As shown in Figure 10, the klystron body 410 includes an electron gun 460 for receiving electric power from the input terminal 422 and generating electrons, a tube 440 having a plurality of cavities (preferably, two to eight in number and, in the illustrated case, four cavities 440a to 440d) and a plurality of channels which will be described hereinafter, and an anode, namely, collector 490 for collecting electrons emerging from the tube 440. A pair of magnets 450a and 450b are disposed around the electron gun 46 and the collector 490 respectively. The magnets 450a and 450b serve to maintain the orientation of electrons toward the collector 490 and the moving centre of the beams of electrons. The klystron body 410 further includes a pair of pole pieces 470a and 470b for guiding magnetic flux, generated by the magnets 450a and 450b, to the interior of the tube 440 and uniformly distributing the magnetic flux in the tube 440, and a yoke 402 serving as a guide for forming a closed loop of the magnets 4501 and 450b, the pole pieces 470a and 470b, the tube 440 and the magnetic flux paths.

The magnets 450a and 450b are arranged such that the directions of magnetization thereof are oriented axially, namely perpendicular, to the facing surfaces thereof.

Alternatively, the magnets 450a and 450b may be arranged such that the directions of magnetization thereof are oriented radially. In the latter case, one of the magnets 450a and 450b has a magnetization direction oriented radially inwardly whereas the other magnet has a magnetization direction oriented radially outwardly.

As mentioned above, the antenna 322 includes the coaxial line 424, the insulating member 322a and the cap 322b. The coaxial line 424 has a loop coupling 424a disposed in the cavity 440d of the tube 440. The loop coupling 424a receives microwave energy from a magnetic field formed in the cavity 440d.

The insulating member 322a is made of an insulator such as a ceramic for providing insulation from the yoke 402 of klystron body 410. The cap 322b is made of a material such as stainless steel.

As mentioned above, the cooling unit 430 includesthe cooling fins 432 adapted to disperse heat generated in the collector 490 of klystron body 410, the cooling rod 436 adapted to support the cooling fins 432 and transfer the heat from the collector 490 to the cooling fins 432, and the cooling member 434 disposed to surround the cooling fins 432 and adapted to form the enclosure of the cooling unit 430. The cooling rod 436 is brazed to the collector 490 so as to be integral therewith.

In order to reduce the number of electrons reflected by the collector 490, a material, such as molybdenum, exhibiting a high work function may coat collector 490. Alternatively, the collector 490 may have a construction such that its centre is positioned apart from the tube 440 whereas its periphery is positioned adjacent to the tube 440.

Preferably, the tube 440 is made of copper for inhibiting chemical reactions thereon.

It is also preferred that the opposite end portions of tube 440, respectively positioned adjacent to the electron gun 460 and the collector 490, are made of a magnetic material in order to uniformly maintain the magnetic flux density in the tube 440. In this case, the magnetic bodies are coated with copper so as to prevent a corrosion thereof and maintain a vacuum characteristic thereof.

Referring to Figure 11, a pole piece is denoted by the reference numeral 470. The pole piece 470 has a cylindrical construction, closed at one end. At the closed end of the pole piece 470, a plurality of apertures 472 are provided.

These apertures 472 constitute drift channels for passing electron beams, as will be described hereinafter.

Referring to Figure 12, the magnetron is denoted by the reference numeral 450. The magnet 450 has a polygonal construction having a predetermined thickness t. A circular hole 452 is centrally provided in the magnet 450. The pole piece 470 is fitted in the central hole 452, as shown in Figure 10. This form of magnet 450 is arranged around each of the electron gun 460 and the collector 490.

Referring to Figure 13, the drift channels are denoted by the reference numeral 600. The drift channels 600 are passages for passing beams of electrons generated by the electron gun 460. The drift channels 600 extend along the pole piece 470a, the tube 440 and the pole piece 470b.

Preferably, each drift channel 600 has a diameter of 0.3 mm to 5 mm.

Referring to Figure 14, as electric power is applied to the input terminal 422 of the klystron 400, hot electrons are generated by the electron gun 460 and concentrated into points, thereby producing electron beams 462. These electron beams 462 are then accelerated to a velocity v by the potential difference VO between the electron gun 460 and the collector 490. The velocity v corresponds to /2 1/1 S-1 (2eVO/m)l, that is 5.93 x 10"(V0) - m Electrons passing the first gaps 442a, respectively - 14 communicating with the drift channels 600, at different times have different velocities in the drift channels 600. On the basis of this fact, electrons leaving the first gaps 442a can travel along the gaps 442b, 442c and 442d positioned downstream of the first gaps 442a with electrons having been emitted earlier at a lower velocity than the average velocity, . As a result, groups of electrons are formed in the electron beams 462.

The first cavity 440a is kept at a predetermined DC voltage level. The first gaps communicating with the first cavity 440a maintain a short circuit state at one end thereof and a connected state at the other end thereof at the point in time just before an optional electron group is introduced therein.

The value obtained by integrating electric fields present among the gaps 442a to 442d has the form of a voltage of V.elwt. By this voltage, the electrons forming electron beams are accelerated and decelerated as they pass through the gaps 442a to 442d.

This phenomenon is called velocity modulation. The periodical variation in voltage at the gaps 442a to 442d means a periodical variation in velocity of electrons constituting the electron beams 462. On the basis of the velocity, the electron beams 462 are bunched into electron groups. When the electrons bunched by the velocity modulation in the first cavity 440a reach the next gaps 442b kept at a voltage of V.ejOt, bunching of the electron beams occurs more intensively by the coaction between the electron beams and the gaps 442b. At this time, the dense groups of electrons have high energy whereas the sparse groups of electrons have low energy.

A kinetic phenomenon of the electrons forming the electron beams 462 will be described. Although the electrons introduced into the tube 440 move at a constant velocity, the electron beams 462 tend to be diffused in the tube 440. This is because electrostatic repulsion occurs between the electrons present in the tube 440. As the electron beams 462 are diffused, they strike against the wall of tube 440, thereby causing the kinetic energy of electrons to be converted into thermal energy.

In order to inhibit this phenomenon, an electromagnetic field is applied to the spaces through which the electron beams 462 pass. For forming the electromagnetic field in the electron beam-passing spaces, the magnet system is provided in the klystron 400.

The magnet system includes the following four parts:

1) magnets 450a and 450b which are permanent magnets and serve as a magnetic flux source, 2) pole pieces 470a and 470b which is adapted to guide the 5 magnetic flux generated by the magnets 450a and 450b into the spaces where the electron beams 462 are present and force the magnetic flux to be uniformly distributed in the channels of tube 440, 3) channels of the tube 440 which are spaces where the electron beams 462 are present and should be maintained at a predetermined magnetic flux density, and 4) a yoke 402 which serves as a guide for obtaining a closed magnetic flux loop.

As the above four parts constitute a magnetic circuit, the electron beampassing spaces are maintained at a uniform and proper magnetic flux density.

The above-mentioned construction is very advantageous for a decrease in volume because it enables the magnet system to be simplified.

Factors for determining the electromagnetic field include the electromagnetic field, the perveance, the radius

3 - 17 of the electron beam and the number of electron beams. Magnetic flux density B in a multi-beam klystron can be expressed by the following equation:

B = Q 1-) 2rb 1 ([,p X V0) 2 N where, "rb" represents the radius of electron beam, gP the micro-perveance, VO ', the drive voltage between the electron gun 460 and the collector 490, and "N" the number of electron beams.

Where a single beam klystron is employed, the required magnetic flux density is about 14 082 Gauss. This value corresponds substantially to 12 times that required where the multi-beam klystron is employed.

When the electromagnetic field generated by the abovementioned magnet system is applied such that its applied direction corresponds to the direction of travel of the electron beams 462, constantly advancing electrons move without being subjected to any force. However, electrons tending to be diffused radially outwardly are subjected to a tangential force, so that they may advance while spiraling. Consequently, the diffusion of the electron beam 462 is inhibited.

The electron beams 462 advancing in the above-mentioned manner then reach the first cavity 440a. As electron waves with small energy are inputted or fed back from external or other cavity into the first cavity 440a, the electrons are modulated in velocity due to the electron waves applied thereto.

The velocity modulation is determined by the time taken for the electrons to pass the first cavity 440a and the intensity of electromagnetic field of the electron waves present in the gaps 442a of the first cavity 440a.

The intensity of electromagnetic field varies in accordance with a sine function. The number of incident electrons varies at a certain rate. Accordingly, the bunching cycle of electrons accords with the cycle of the electron waves.

Therefore, electron beams emerging from the first cavity 440a have a nonuniform electron density. Although they take a more or less bunched form, it is insufficient to obtain a satisfactory output. For enhancing the electron density, the above procedure, therefore, is required to be repeated.

In other words, at the moment a group of electrons more or less bunched reaches the second cavity 440b, leading 19 electrons of the electron group lose their energy which is, in turn, transferred to electrons following the leading electrons. As a result, the electron group has a further increased density.

This result may be obtained in the third cavity 440c. As a result, sufficient bunching can be obtained.

As the electron beams 462 subjected to repeated bunching actions reach the fourth cavity 440d, an induction current is generated. Such an induction current is repeatedly generated in the above-mentioned manner as groups of electrons subjected to the bunching action are sequentially introduced into the fourth cavity 440d.

The induction current serves to induce and distribute an electromagnetic field in the opposing wide spaces of each of the cavities 440a to 440d. In the central gaps 442a to 442d, the induction current serves to exert a repeated 20 action to alternate the electromagnetic field.

The energy of the electron waves (in the illustrated case, electron waves having a frequency f of about 2 450 MHz) can be externally outputted from the fourth cavity 440d via the coaxial line 424 which has the loop coupling 424a to the electromagnetic field in the fourth cavity 440d.

The charge density of each electron group should be increased for obtaining a high power electron wave energy. However, such an increase in charge density results in a great increase in repulsion among electrons. It, therefore, is required to increase magnetic flux density and voltage also.

For obtaining the required magnetic flux density, however, a bulky magnet system is needed. Furthermore, the low voltage oscillation by the klystron is not expected when using the increased voltage.

For these reasons, the present invention employs a multibeam klystron.

Although the perveance of each electron beam may be decreased where the multi-beam klystron is used, it is possible to improve the efficiency and obtain high output at a low voltage because the perveance of the overall system corresponds to the sum of the perveances of the individual electron beams and, thus, has a large value.

Where the multi-beam klystron is employed, therefore, the microwave oven can operate with a simple magnet system and at a low operating voltage in which the perveance of each electron beam is maintained at a low level.

- 21 Simultaneously, the microwave oven can generate a high output in that the perveance of the overall system is maintained at a high level.

The minimum number N of electron beams in the multi-beam klystron corresponds to (V0m/V0s)2/5 ("VOM" is the operating 11 voltage of the multi-beam klystron and "V,, is the operating voltage of the single beam klystron corresponding to 4 KV). In practice, the number of electron beams in the multi-beam klystron should be determined to satisfy the geometrical arrangement of drift channels (denoted by the reference numeral 600 in Figure 13). Accordingly, it is preferred that the number of drift channels of the multibeam klystron is less than 500. For example, where the multi-beam klystron is to operate at an operating voltage of 600 volts, 127 electron beams are needed. For operating the multi-beam klystron at an operating voltage of 400 volts, 337 electron beams are needed.

In accordance with the present invention, the radius of each electron beam 462 is determined to be a predetermined proportion of the radius of each drift channel 600. When the electron beams 462 have the determined radius, they are partially lost in the drift channels 600, thereby causing a loss of energy.

22 - The electron beams 462 are formed by electrons generated from the surface of an electron gun 460 and concentrated on points. As these electron beams 462 strike against the collector 490 after passing through the drift channels 600, they disappear.

The electron beams 462 emitted from the electron gun 460 are accelerated by the intensity of the electromagnetic field until they reach the pole piece 470b. Thereafter, the electron beams 462 move at a constant velocity.

As mentioned above, the multi-beam klystron splits an electron beam into a plurality of electron beams which have no affect thereamong, thereby enabling the electrons to act independently. As many electron beams are provided by the split, the quantity of charge in each electron beam is relatively small. As a result, the repulsion of electrons is not so high even though the electron beams are bunched. It is, therefore, possible to considerably reduce the intensity of electromagnetic field and the voltage of collector 490.

As apparent from the above description, where the multibeam. klystron is employed in a microwave oven for cooking, the requirement of a high voltage transformer is eliminated. This results in simplicity of construction and, thereby, a - 23 reduction in weight and volume. In place of the high voltage transformer, a simple back-voltage circuit may be used to obtain a voltage of a desired level.

Although the present invention has been described in conjunction with the microwave oven wherein a stirrer is equipped in the cooking chamber, it can be equivalently applied to a case equipped with a turntable.

The magnets may have an annular shape, although they have been described as having a polygonal shape. Alternatively, the magnets may have a polyhedron lattice shape.

Although not described hereinbefore, spaces through which electron beams pass are kept at a resonation state, as in conventional magnetrons wherein a resonant state is made upon forming an antenna.

The klystron described herein is suitable for applications other than the microwave oven described.

a -zi- -

Claims

CLAIMS 1. A microwave oven comprising: a klystron f or receiving an

electric power and thereby generating microwaves; a cooking chamber for receiving the microwaves and perf orming a cooking for a f ood by use of the received microwaves; and control means for controlling control of a user.

the klystron under a
2. A microwave oven in accordance with claim 1, further comprising a waveguide for guiding the microwaves from the klystron to the cooking chamber.
3. A microwave oven in accordance with claim 1 or 2, further comprising a stirrer for dispersing the microwaves introduced in the cooking chamber.
4. A microwave oven in accordance with claim 1, 2 or 3, 20 further comprising a fan for cooling the klystron.
5. A microwave oven in accordance with claim 4, further comprising at least one aperture for introducing an air flow generated by the fan and exhausted after cooling the 25 klystron into the cooking chamber.
6. A microwave oven in accordance with claim 5, further comprising a duct for guiding the air flow exhausted after cooling.the klystron to the aperture.
7. A microwave oven in accordance with claim 1, wherein the klystron comprises: an input terminal for receiving the electric power from an external; a klystron body for generating the microwaves upon receiving the electric power via the input terminal; and an output unit for emitting the microwaves from the klystron body.
8. A microwave oven in accordance with claim 7, wherein the klystron further comprises cooling means for outwardly discharging heat generated in the klystron body.
9. A microwave oven in accordance with claim 8, wherein the cooling means comprises: a plurality of cooling fins adapted to disperse a heat generated from a collector equipped in the klystron's body; a cooling rod adapted to support the cooling fins and transfer the heat from the collector to the cooling fins; and a cooling member adapted to surround the cooling fins and constitute an enclosure of the cooling means.

-2t -
10. A microwave oven in accordance with claim 9, wherein the cooling rod is brazed to the collector so as to be integral with the collector.
11. A microwave oven in accordance with claim 7 or 8, wherein the input terminal is electrically insulated from the klystron body by an insulator.
12. A microwave oven in accordance with claim 7 or claim 8, wherein the klystron body comprises a yoke constituting an enclosure of the klystron body.

is
13. A microwave oven in accordance with claim 7 or claim 8, wherein the klystron body further comprises a plurality of clamping means.
14. A microwave oven in accordance with claim 7 or claim 8, wherein the klystron further comprises a pair of magnets respectively disposed around an electron gun and a collector both equipped in the klystron body, the magnets constituting a closed magnetic circuit serving to maintain the orientation of electrons generated by the electron gun toward the collector and providing a moving center of the electrons.
15. A microwave oven in accordance with claim 14, wherein 1 1 the collector is coated with a material exhibiting a high work function.
16. A microwave oven in accordance with claim 14, wherein the magnets are arranged such that the magnetization directions thereof are oriented perpendicular to facing surfaces thereof.
17. A microwave oven in accordance with claim 14, wherein the magnets arranged such that the magnetization directions thereof are oriented radially, one of the magnets being oriented radially inwardly and the other magnet being oriented radially outwardly.
18. A microwave oven in accordance with claim 14, wherein the magnets have an annular shape.
19. A microwave oven in accordance with claim 14, wherein the magnets have a polyhedral lattice shape.
20. A microwave oven in accordance with claim 1, wherein the cooking chamber is provided at a bottom surface thereof with a turntable for rotating a food contained in the cooking chamber.
21. A microwave oven in accordance with claim 1, wherein the klystron further comprises a tube including a plurality of channels for moving a beam of electrons generated from an electron gun toward a collector in a split manner, both the electron gun and the collector being equipped in the 5 klystron.
22. A microwave oven in accordance with claim 21, wherein the number of channels of the tube is less than 500.
23. A microwave oven in accordance with claim 21, wherein the tube further includes 2 to 8 cavities.
24. A microwave oven in accordance with claim 21, wherein each of the channels has a diameter of 0.3 mm to 5 mm.
25. A microwave oven in accordance with claim 21, wherein the tube has a leading end and a trailing end each comprised of a magnetic body capable of obtaining a uniform magnetic flux density in the tube.
26. A microwave oven in accordance with claim 25, wherein the magnetic body is coated with copper.
27. A microwave oven in accordance with claim 21, wherein 25 the tube is made of copper.

jI - 1.1m P -
28. A microwave oven in accordance with claim 1, wherein the klystron includes a coaxial line electrically connected to a cavity disposed to a collector equipped in the klystron, the coaxial line serving to externally transmit 5 the microwaves from the klystron.
29. A microwave oven wherein the source of cooking microwave energy comprises a klystron.
30. A microwave oven according to claim 29, wherein the klystron is a multi-beam klystron.
31. A microwave oven substantially as hereinbefore described with reference to Figures 3 to 14 of the 15 accompanying drawings.
32. A klystron substantially as hereinbef ore described with reference to Figures 5 to 14 of the accompanying drawings.