Abstract:
A device is proposed for producing high-frequency microwaves, having a cathode arrangement with heatable cathodes for emitting electrons, two grating arrangements for controlling and focusing the electrons flow and an anode for recaiving the electrons passing through the grating arrangements. The cathode arrangement and the first grating arrangement and also a blocking or choke element define an output cavity forming a resonance cavity and the anode and the second grating arrangement define an output cavity likeeise forming a resonance cavity. The cathode arrangement has a monuting for the cathode such that deformation of the cathode with reduction of the spacing between the heatable cathode and grating is avoided.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a device for producing high-frequency microwaves according to the preamble of the main claim. 
   A device for producing high-frequency microwaves is disclosed in the U.S. Pat. Nos. 5,883,367, 5,883,369 and 5,883,386. This device has two resonance cavities, an input cavity and an output cavity, the input cavity comprising a cathode for emitting a linear electron beam, a blocking or choke structure for blocking a direct current and for transmitting a weak oscillation and a grating for focusing the electron beam and for modulating the same with respect to its density. The output cavity has a grating and an anode which receives the electron beam or the electrons thereof modulated in density, a microwave oscillation being produced. A feedback bar, by means of which the resonance cavities are coupled to each other, is connected to the input cavity and protrudes into the output cavity, as a result of which a part of the microwave energy is fed back into the input cavity. The microwave energy is directed out of the device by means of an antenna coupled to the output cavity. 
   SUMMARY OF THE INVENTION 
   This known device is used essentially for microwave ovens, a cylindrical magnetron being used frequently in microwave ovens as microwave source. The above-described device has the advantage relative to the magnetron that no magnets are required in order to focus electrons. The operating voltage at approximately 500 to 600 volts is lower than in the case of a microwave source with a magnetron and a transformer is not required. The output power can be varied by using a resistor between the grating and the cathode. The electromagnetic noise level of the device is very low since the microwave energy is produced by a linear movement of the electrons. 
   In the case of the known device, a precise alignment of the components, i.e. of the cathode, two gratings and an anode, is important. The intermediate spacings are in the range of 0.1 to 1 mm which normally does not present a problem in the case of a cold arrangement. However, the temperature of the cathode faces is in the range of 600° C. to 1,000° C. At such high temperatures, it is difficult because of the thermal deformations to maintain the precise alignment, which results in for example a contact between the grating and the cathode but also between the gratings themselves or between the grating and the anode. This is a critical problem for operating the above-mentioned device. 
   The object therefore underlying the invention is to produce a device for producing high-frequency microwaves, in which electrical short circuits, in particular between cathode and grating, due to thermal deformations, are extensively avoided. 
   This object is achieved according to the invention by the characterising features of the main claim in conjunction with the features of the preamble. Advantageous developments and improvements are possible due to the measures indicated in the sub-claims. 
   By means of the precise positioning of at least the first grating arrangement and the cathode arrangement via positioning means and also the provision of a mounting for the cathode, which avoids the deformation of the cathode with reduction of the spacing between the grating arrangement and the cathode arrangement, a thermally stable arrangement is produced which permits small spacings between the cathode and the grating without short circuits. 
   The mounting comprises a cathode housing, on or in which the cathode is disposed as a part which is separate from the housing with a spacing from the housing wall, as a result of which deformation of the cathode arrangement because of different heat expansion coefficients between the heatable cathode and surrounding housing, is avoided. The mounting comprising the cathode housing holds the cathode if necessary by means of a cathode body whilst maintaining a gap between the parts. The gap serves as a buffer for the expansion due to heat. 
   The cathode housing insulates&#39; the cathode from the input resonance cavity and is used for an arrangement of the cathode face and of the first grating in the micrometer range. It minimises a radial loss of heat energy from the cathode and reduces radial expansion of the cathode which could influence the dimension of the input resonance cavity. 
   Preferably, the cathode housing is configured as a cylinder with a flange fixed to the circumferential face of the cylinder, the cathode being disposed in the cylinder with a gap. In this manner, a clear separation between the face emitting electrons and the resonance face is prescribed in the input cavity corresponding to the invention. The grating arrangement comprises advantageously an annular grating holder with spoke-shaped webs, i.e. an inner ring and an outer ring are provided which are connected by spokes, and the grating is supported on the edge and on the webs of the grating holder and is fixed to the latter in a frictional and/or form fit. 
   The configuration of the cathode as a combination of a cathode body and metal plate emitting electrons minimises thermal deformation due to high operating temperatures. 
   Advantageously, the cathode housing is an annular blocking or choke element disposed between the cathode housing and the grating holder of the first grating arrangement, and the grating holders of the two grating arrangements are aligned relative to each other by means of alignment pins and fixed in their position relative to each other as a result of which the output cavity is aligned securely above the input cavity and parallel thereto, the electrical insulation between the two cavities being produced by using ceramic spacing elements which screen the alignment pins. 
   Due to the above arrangement, an optimal design and an optimal arrangement of the components is ensured and thermal deformation, such as sagging of the gratings, is successfully reduced because of the bridges or web structure, short circuits between the components being avoided due to the clean spacing and alignment of the components relative to each other and as a result of which a good focusing of the electron beams is ensured. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are illustrated in the drawing and are described more fully in the subsequent description. There are shown 
       FIG. 1  a section through the device for producing microwaves according to an embodiment of the present invention, 
       FIG. 2  a section through the lower part of the device according to  FIG. 1  with input cavity and output cavity, 
       FIG. 3  an enlarged section through parts of the device according to FIG.  1  and  FIG. 2  with input cavity, 
       FIG. 4  a view from below of a cathode housing and a side view of the cathode housing, 
       FIG. 5  a view of a cathode body and a section view and a view of the plate emitting electrons, 
       FIG. 6  an enlarged section illustration of the feedback arrangement, 
       FIG. 7  a view of a blocking or choke element, 
       FIG. 8  a view of and a section through an embodiment of the first grating arrangement, 
       FIG. 9  a view of an embodiment of the second grating arrangement, and 
       FIG. 10  a view of the anode, observed from below. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The device  1  illustrated in  FIG. 1  has a vacuum chamber  2  surrounded by a housing  32 , in which device a cathode arrangement, a grating arrangement and in part an anode arrangement are contained, which can be detected in more detail in FIG.  2 . One part of the anode  3  fixed on the housing  32  of the vacuum chamber  2  protrudes into a cooling chamber  4 , in which cooling ribs  5  are disposed between the anode  3  and the housing  6  for dissipating the heat from the anode  3 . A bar-shaped antenna  7  is aligned centrally relative to the anode  3  and is insulated from the anode  3  by a ceramic disc  8 . It terminates on the anode side in a coupling element  9 , whilst the other end is contained in a cap  10 , a ceramic cylinder  11  insulating the antenna  7  from the remaining housing. 
   In  FIG. 2 , the components which are contained in the vacuum chamber  2  are illustrated more precisely. Two resonance chambers or resonance cavities are disposed one above the other and parallel, an input cavity  12  and an output cavity  13 . The input cavity  12  configured as an annular chamber is delimited by a ring arrangement which is formed by a cathode housing  14 , a blocking or choke arrangement  16  and a grating holder  17 . A cathode  15  is inserted in the cathode housing  14  and a grating  18  is disposed on the grating holder  17 . A feedback arrangement  19  is provided in the central region within the cathode housing  14 . The input cavity  12  is dimensioned to be very narrow in the region between the grating  18  and the cathode  15 , i.e. the spacing between the components is approximately in the region of 0.1 mm. Hence the spacings must also be maintained during operation in order that no short circuits occur. In the illustration, the spacing between the grating  18  and the cathode  15  was chosen very much larger, in reality for example the lower face of the grating holder lies in the region of the upper end of the cathode housing  14  and thereunder, as is shown in FIG.  1 . 
   Above the input cavity  12 , the output cavity  13  is provided in a parallel arrangement, said output cavity being configured as a toroidal chamber and is delimited by the anode  3 , by a grating holder  20  for a grating  21  and also by a wall  22  surrounding the output cavity  13  in an annular form, which wall is a component of the anode  3 . The coupling element  9  connected to the antenna  7  protrudes into a central chamber between the anode  3  and the grating holder  20 . Furthermore, a tuning pin  23  which serves for changing the resonance frequency in the output cavity  13 , engages through the surrounding wall  22 . 
   In  FIG. 3 , the cathode arrangement, which has the cathode housing  14  and the cathode  15 , the choke arrangement  16  and the first grating arrangement with grating holder  17  and grating  18 , is illustrated in more detail. It should be noted in this respect that, for clarity, the spacing between the cathode  15  and the grating  18  is illustrated very much larger, just as in  FIG. 2 , than if it were true to scale. 
   The cathode  15  is configured as a thermoionic cathode, thus a heating device  24  is disposed underneath the cathode  15  and has a helical heating wire  25 . The heating device  24  is contained in a cylindrical housing  26  which has a member parallel to the cathode  15 , a cylinder  76 , which is connected to the cathode housing  14 , for example by welding, presses the housing  26  upwardly with the bent-over member. Preferably, the housing  26  and the cylinder  76  are made of tantalum. The helical heating wire  25  is secured to the heating housing  26  via ceramic rings  27 , the electrical connections  28  for the heating wire  25  being produced by means of a ceramic duct  29  with two borings. The heating housing  26  has in the region of the duct  29  a cylinder extension  30  which supports the duct  29 . The electrical connections  28  are connected to a plug  31  which is secured to the housing  32  surrounding the vacuum chamber  2  (see FIG.  1 ). 
   The housing  26  of the heating device  24  is encompassed on the external circumference by the cathode housing  14 , the cathode housing being illustrated in more detail in FIG.  4 . The cathode housing  14  has an inner cylinder  33 , to which a flange  34  is fixed. The flange is a plurality of through-holes  35  which, as described later, serve for alignment via alignment pins. The inner cylinder  33  has four incisions  36 , observed across its circumference, which cooperate with the grating holder  17 . As can be detected in  FIG. 4 , the cylinder has an inwardly directed bend  37 . 
   The cathode  15 , which is illustrated in  FIG. 5 , is contained in the cylinder  33  of the cathode housing  14  and has a cathode body  38  and a face  39  which emits electrons or is sensitive. In  FIG. 5 , the face  39  emitting electrons is configured as annular segment-like plates which can be secured on the cathode body  38  by means of pins  40 . The cathode body  38 , which is likewise configured annularly, has gradations  41 , which serve for fixing with respect to the cathode housing  14 , on its inner and outer circumference. For this purpose, the bend  37  engages via the gradation. 
   The cathode  15  is inserted into the cathode housing  14 , the cathode body  38  being supported on the one hand on the cylindrical heating housing  26  and being supported on the other hand by a cylinder  42  which is supported on a gradation of a centrally disposed feedback body  43 . The feedback body  43  is a component of the feedback arrangement  19  which is described further on. Furthermore, a cover  44  is connected to the feedback body  43 , e.g. by welding, the cover  44  surrounding the cathode body  38  and overlapping the gradation  41  on the inner diameter of the cathode body  38 . Between the outer circumference of the cathode body  38  and if necessary the sensitive face  39  and the internal circumference of the cylinder  33 , also in the region of the bend  37  of the cathode housing and also the corresponding circumferential faces of the cover  44 , a gap or a break is provided so that the cathode can expand when heated by the heating device  24  without said cathode bending. The gap is a buffer for equalising the differences in the thermal expansion coefficient between the cathode housing  14  and the cathode  15 . At the bends  37 , the cathode housing is connected electrically to the cathode body  38 . 
   As can be detected in  FIGS. 2 and 3 , there are located in position one on top of the other on the flange  34  of the cathode housing  14  the annular blocking or coupling element  16 , which is illustrated in more detail in  FIG. 7 , and thereabove the outer edge region of the grating holder  17 , which is illustrated in more detail in FIG.  8 . The blocking or coupling element  16  is made of a ceramic disc  45 , having a central hole and a metal coating  46  around the outer edge and side region, the metal coating  46  having no contact with the cathode housing  14  or with the grating holder  17 . Corresponding to the cathode housing  14 , the choke element  16  or the ceramic disc  45  has no through-holes  55  for alignment pins. 
   The grating holder  17  corresponding to  FIG. 8  has an inner ring  47  and an outer ring  48  which are connected via four spokes or bridge members  49 . The outer ring  48  is provided with a gradation in order to ensure the spacing from the cathode arrangement. Through-holes  50  for the alignment pins are provided in the outer ring  48 . The grating  18  with a multiplicity of holes is supported on the grating holder  17 , the spokes  49  preventing sagging of the grating  18  at high temperatures of the cathode  15 . The spacing between the grating  18  and the cathode  15  lies approximately between 0.1 and 1 mm and the diameter of the cathode and of the grating is approximately 40 mm. The grating  18  is positioned and fixed on the grating holder  17  by four rectangular cut-outs  51  and pins  52 . 
   As can be detected in  FIG. 3 , alignment pins  53 , which are surrounded with an electrically insulating sleeve, e.g. a ceramic sleeve  54 , reach through the alignment holes  50  of the grating holder  17 , the through-holes  55  of the blocking element  16  and the through-holes  35  of the flange  34  of the cathode housing  14 . The alignment pins  53  are screwed in respectively with interposition of a spacing ring  57  and an insulation ring  58 . For the alignment of the cathode housing  14  with cathode  15  and of the grating holder  17  with grating  18 , notch marks  59  are provided on the circumference of the flange  34  of the cathode housing and of the grating holder  17 , with the superimposition of which marks it is ensured that the webs  49  of the grating holder  17  can engage in radial recesses  60  in the cathode body  38  (see  FIG. 5 ) whilst maintaining a spacing for the electrical insulation therebetween. The webs  49  likewise engage in the rectangular incisions  36  of the cathode housing  14  but do not come into electrical contact with the latter due to the precise positioning. 
   The second grating arrangement, which has the grating holder  20  and the grating  21 , is situated above the first grating arrangement. The second grating arrangement, which is illustrated in  FIG. 9 , is constructed similarly to the first grating arrangement according to FIG.  8  and has an outer ring  61  provided with through-holes  77  and an inner ring  62 , the two being connected by spokes  63 . The grating  21  is supported on the spokes  63  in order to avoid sagging thereof, and is likewise fixed via rectangular incisions  64  and pins  65 . A notch mark  66  serves for positioning with respect to the other components. The alignment pins  53  with the ceramic sleeves also reach through the through-holes  77 . The grating holder  20  is connected securely to the anode wall  22  and the alignment pins  53  are connected securely to the grating holder  20 . 
   The ceramic sleeves  54  surrounding the alignment pins  53  serve at the same time as spacing elements between the grating holder  20  and the grating holder  17 , as a result of which the output cavity and the input cavity are disposed parallel to each other whilst maintaining a precise spacing. 
   The anode  3  is illustrated in  FIG. 10 , observed from below. It has four segment-like projections  67 , as a result of which an outer annular chamber  68  which represents the output cavity, and an inner annular chamber  69  are formed. In the anode wall surrounding the outer annular chamber  68 , three through-holes  75  are provided for the tuning pins  23 . 
   With reference to  FIGS. 2 ,  3  and  6 , the feedback arrangement  19  is now described. The feedback arrangement  19  has the centrally disposed feedback body  43 , into which a cylinder  73  and a screw sleeve  74  are inserted centrally, all three elements being made preferably from molybdenum. A feedback bar  70  made of copper is screwed into the screw sleeve  74 , the feedback bar being supported on a first ceramic disc  71  which is disposed on the end faces of the cylinder  73  and of the screw sleeve  74 , a second ceramic disc  72  abutting against the other end faces and the feedback body  43 . 
   As indicated in  FIG. 1 , earth potential or a positive voltage is applied to the anode and a negative voltage to the cathode housing via the plug  31 , a not-illustrated trimming resistor being provided between the grating holder  17  and the cathode housing  14 . The trimming resistor leads to a potential block in the grating  18  for electrons, as a result of which the quantity of electrons passing through the holes in the grating  18  is limited. Hence a power control is possible. 
   The mode of operation of the device is as follows. An initial microwave oscillation is produced in the input cavity  12 , this oscillation modulating an electron flow in density. The electron flow  78  (FIG.  3 ), which is modulated in density, is focused by means of the gratings  18 ,  21  and accelerated towards the anode  3  by means of the voltage existing between the cathode and anode. The output cavity  13  transforms the kinetic energy of the electrons into microwave energy. A part of the microwave energy is fed back to the input cavity  12 . This leads to the fact that the oscillations in the input cavity and in the output cavity are harmonised. 
   The choke or blocking arrangement  16  has the effect that an initial microwave oscillation is produced in the input cavity  12 . When the thermionic cathode  15  is heated by the heating device to a specific operating temperature, e.g. between 800 and 1000° C., it emits electrons. Due to the high voltage, e.g. a direct voltage of 550 V, between the cathode  15  and the anode  3 , the electrons flow through the aligned holes in the grating  18  and the grating  21  towards the anode. A small proportion of electrons is trapped by the grating  18 , as a result of which a negative potential is formed relative to the cathode  15 . A small flow flows on the surface in the input cavity and the flow direction is changed by means of the choke arrangement  16  which induces a weak oscillation. The choke arrangement thereby has the function of blocking a direct current between the grating holder  17  and the cathode housing  14 . The negative potential on the grating  18  increases to a stabilised value which is prescribed by the trimming resistor. As a result, the oscillation amplitude is stabilised and an electron flow is modulated in density by the grating  18  due to the oscillation. The negative potential on the grating  18  induces an electrostatic field which focuses the flow of the electrons. The electrons which are modulated in density are accelerated towards the projections  67  of the anode  3  via the grating  18  and the grating  21 . In the outer annular chamber  68 , the kinetic energy of the electrons in transformed into microwave energy. The coupling element protruding into the inner annular chamber  69  transmits the predominant proportion of microwaves to the antenna  7  which decouples the energy to a not-illustrated waveguide. The feedback bar  70  protruding into the inner annular chamber  69  transmits a part of the microwave energy to the input cavity  12  via the ceramic discs  71 ,  72 , as a result of which a coherence of the oscillations is ensured. 
   The cathode  15  according to  FIG. 5  is a combination of a cathode body  38  with pins  40  and metal plates  39 , in which the pins  40  are used in order to align the metal plates relative to the cathode body  38 . The cathode body  38  which is produced from metal with a relatively low heat expansion coefficient, serves for reducing the thermal deformation due to the high operating temperatures. If a metal oxide cathode is used, the plates are made of a nickel sheet on which a thick layer of a BaO mixture is deposited. 
   The thick layer is produced by spraying or screen printing. The operating temperature is approximately 850° C. If a metal alloy cathode is used, the metal plate is an alloy metal, e.g. Pd—Ba, Pt—Ba. This cathode enables the emission of electrons at a relatively low operating temperature (approximately 650° C.) but it is very expensive.