Magnetron tube with improved low cost structure

A magnetron electron discharge device preferably for use in microwave heating or cooking apparatus has a cylindrical resonant anode structure surrounding a concentric electron emitting filament which is supported directly between re-entrant end closures housing magnet members which are external to the vacuum envelope but are so located as to achieve high magnet circuit efficiency, rugged construction and low spurious noise output.

BACKGROUND OF THE INVENTION 
Since 1947, magnetrons have undergone extensive development directed 
principally toward their evolution from the very expensive transmitters 
used in World War II radar to the present low cost and highly reliable 
units applicable to use in a kitchen appliance. This effort has reduced 
the magnetrons cost from several hundreds of dollars for the early cooking 
magnetrons to less than $20.00 wholesale cost for the present high 
production tubes. This downward trend continues, although at a lower rate, 
in spite of steadily increasing labor and materials costs. Along with 
price reduction, great strides have also been made in compactness and 
reliability, resulting in greater design freedom and longer life. 
Recently, however, because of world affairs impacting the cost of cobalt 
metal used in the magnet circuit of most magnetrons, much development 
effort has been expended in reducing the amount of magnetic material 
required, and hence the cost, of magnetrons using permanent magnets. A 
substitute for cobalt bearing magnets has been found in ferrite materials, 
but these have thermal temperature cooefficients almost one order of 
magnitude (nine times) greater than the cobalt based materials and so are 
not stable in microwave ovens which have high or changing ambient 
temperatures such as those using resistance heated browning elements. In 
addition, ferrite magnets have greater bulk and are subject to cracking 
due to temperature gradients. 
It has become well known that the efficiency of the magnetic circuit of a 
magnetron is improved as the magnetic material is moved closer to the 
working gap, since leakage flux is thereby reduced. Early cooking 
magnetrons had magnetic circuit figures of merit of only 0.3% but due to 
the development trend to minimize cobalt costs, this figure has been 
extended over ten times in the last ten years. Recently, in order to 
optimize this figure, it has been considered necessary to include the 
cobalt-containing material (such as the Alnico or samarium-cobalt alloys) 
within the vacuum envelope of the magnetron. This practice has 
dissadvantages in that the outgassing (pumping) time of the device is 
considerably increased with attendant cost increase. 
Another dissadvantage of the conventional magnetron design is that the 
magnetic pole pieces adjacent to either end of the electron emitting 
filament are at positive potential with respect to the electron emitter. 
The result is that some electrons are therefore attracted axially to the 
positive pole pieces, reducing the efficiency of the magnetron and also 
causing excess noise generation during the build-up of coherent 
oscillations. Such noise is very broad spectrally, causing interference to 
other services such as televisin, microwave communications, and radio 
receivers. To prevent the radiation of such noise components, as required 
by governmental agencies, necessitates the addition of radiation filter 
components which add appreciably to the manufacturing cost of the 
magnetron, as well as to its bulk. 
Another problem with the prior art magnetron structure is that the filament 
is cantilever supported from one end and constitutes an assembly having 
two distinct mechanical resonances the vibrating masses of which are 
joined by the thoriated tungsten emitting material which, after conversion 
to tungsten carbide, in the process known as carburizing, is extremely 
brittle. As a consequence, any mechanical shock or vibration during 
shipping or rough handling, subjects the filament to stresses which often 
result in breakage. Also, since the filament and both end hats and the 
center rod operate at very high temperatures, they must be made of exotic 
and expensive metals such as tungsten and molybdenum and joined with 
expensive materials such as platinum, ruthenium, or rutanium-molybdenum 
alloys which add greatly to the expense of manufacture. Also, because of 
the high operating temperatures, cantilever supported filaments are 
usually restricted to operation only in the vertical position to prevent 
sagging, or mechanical deformity due to gravity. 
SUMMARY OF THE INVENTION 
It is therefore an overall objective of the present invention to provide an 
improved magnetron for microwave ovens which substantially overcomes the 
above limitations of the prior art devices: (a) It is an objective to 
provide a magnetron electron tube for microwave ovens which is more 
compact than the current conventional design. (b) It is another objective 
to provide such a magmetron tube for microwave cooking and other heating 
applications which is more economical to manufacture because of simplified 
assembly, the elimination of expensive metals such as molybdenum, 
tungsten, platinum, monel, and Kovar, and the elimination of costly 
components required with the conventional design for the supression of 
spurious radiation. (c) It is yet another objective of the present 
invention to minimize the generation of spurious radiation, including 
harmonic radiation, through the use of a unique geometry. (d) The electron 
discharge device of the present invention also provides a structure which 
is extremely rugged against breakage caused by shock and vibration. (e) A 
yet further purpose of the disclosed structure is the realization of 
increased magnet circuit efficiency by permitting the symmetrical magnets 
to be inserted to a position immediately adjacent to the working gap, yet 
not included within the vacuum envelope of the magnetron. (f) A further 
objective is the provision of a magnetron tube which can be used in any 
mounting position.

DESCRIPTION OF THE INVENTION 
FIG. 1 shows a perspective view of a finished magnetron constructed in 
accordance with the teachings of my invention. The antenna consists of a 
metallic end cap 10, an insulating cylinder 11 of glass or, preferably, 
ceramic 11 and a metallic base cylinder 12, all hermetically sealed to one 
another and to anode body 13, not visable in FIG. 1. A mounting plate 14 
is constructed of metal and fitted with mounting screws 15 for attaching 
the megnetron ot a waveguide or other device for removing the useful 
energy generated by the magnetron. A pair of ferrous extensions 
perpendicular to the mounting plate 14 form magnet return path 16 which 
may be an integral part of mounting plate 14, or may be a seperate part, 
provide a return path for the magnetic flux produced by the magnets 26, 
not visable. A channel shaped member 17 is attached to each end of 
mounting plate 14 to form an air duct for guiding the required cooling 
air. Cooling fins 18 are constructed of a highly thermally conducting 
material such as aluminum and are supported in intimate thermal contact 
with the magnetron anode body 13 by air duct 17 as shown in FIG. 7. 
Prior to proceeding to a detailed discription of the present invention as 
illustrated in FIG. 2, it will be helpful to examine the construction of a 
typical magnetron of the prior art as shown in FIG. 3. A cylindrical anode 
structure 13 supports a radial array of vanes 19 which extend inwardly to 
form a multi-cavity resonator circuit surrounding electron emitting 
filament 20 and is concentric therewith. Filament 20 is supported by 
cantilever structures 21 and 30 connected to electrical terminals 22 and 
23 and is electrically isolated from anode 13 by insulator 24 and 
terminals 23 and 24 are isolated from each other by insulator 25. Magnets 
26 cooperate with pole-pieces 27 to produce a magnetic field in the 
electron interaction space between anode vanes 19 and the emitting 
filament 20. A ferrous metal yoke 14 shown in phantom lines in FIG. 3 
provides a magnetic return path between the outer extremities of magnets 
26. Antenna 28 is connected to one or more vanes 19 for the purpose of 
removing generated microwave power from the resonator and delivering it to 
a useful load. Since in this known structure, end hats 29, center rod 30 
and support member 21 all operate at elevated temperatures approaching 
that of emitting filament 20, they must be constructed of expensive high 
temperature metals such as molybdenum or tungsten and assembled with 
alloys containing platinum, molybdenum, ruthenium or rutanium. Operation 
at such elevated temperatures also gives rise to unwanted emission from 
end hats 29 and mechanical sagging of the structure when used with the 
axis horizontal. 
Now in FIG. 2, wherein similarly functioning parts bear symbol designations 
corresponding to the prior art device of FIG. 3, the advantages of the 
improved structure of the present invention are illustrated. The emitting 
filament 20, which consists of a helix of thoriated tungsten wire, is 
directly affixed at both ends to metallic support cylinders 29 (designated 
"end hats" 29 in FIG. 3) preferably by an interference fit, frictional 
joint according to the teachings of my prior U.S. Pat. No. 3,566,179 
"CATHODE AND HEATER CONSTRUCTIONS AND MOUNTINGS IN ELECTRON DISCHARGE 
DEVICES" issued Feb. 23, 1971. No other means of attachment is usually 
necessary since the temperature of support cylinders 29 do not reach the 
mechanical yield temperature of the end turns of tungsten filament wire 
20, although brazing or welding of filament 20 to support cylinders 29 can 
be employed, although at additional manufacturing cost. Filament support 
cylinders 29 are in turn brazed, welded or are formed as integral parts of 
end closure 33 and are coaxial therewith. As in FIG. 3, anode structure 13 
consists of circular array of resonators 19 which may be of any type known 
to the prior art such as vane, hole-and-slot, interdigital or rising sun 
types (See M.I.T. Radiation Laboratories Radar Series, G. B. Collins, Vol. 
6) Anode structure 13 surrounds and is concentric with filament 20. 
Insulators 24 are brazed or otherwise hermetically sealed to anode 
structure 13 and end closures 33 to complete the vacuum enclosure and to 
maintain these parts in concentric relationship. End closures 33 are 
formed of a non-ferrous material to provide deeply re-entrant hollow 
cylindrical receptacles for magnets 26 and field shaping pole-pieces 35 
where needed. In the preferred embodiment depicted in FIG. 5(a) further 
improvement in magnetic circuit efficiency is achieved by incorporating 
field shaping pole-pieces 35 in the inner ends of end closures 33 in a 
manner such that the magnetic gap is reduced by two times the thickness of 
the non-ferrous material comprising end closures 33. This location of 
magnets 26 immediately adjacent to the electron interaction space defined 
by the inner diameter of anode circuit 13 and the outer diameter of 
electron emitting filament 20 serves to maximize magnetic circuit 
efficiency while at the same time maintaining the magnets 26 outside of 
the vacuum envelope. Such mounting and location of magnets 26 renders them 
easily removable from the magnetron structure for re-use. By the present 
design a magnetic circuit figure of merit of over 5% is achieved as 
compared with approximately 0.9% for the prior art design of FIG. 3. 
A magnetic return path is provided by plates 16 made of a low reluctance 
ferrous material such as cold rolled steel connecting the outer 
extremities of magnets 26 as shown as phantom lines in FIG. 2 and may in 
the preferred embodiment, also provide a mounting surface 14 for the 
magnetron and a ground plane for antenna 10. Magnets 26 which are at high 
negative potential with respect to grounded magnetic return path 16 are 
isolated therefrom by means of dielectric insulators 36 which may be thin 
sheets of high voltage, high temperature material such as Teflon or 
ceramic. 
As shown in FIG. 6, antenna wire 28 connects antenna cap 10 to resonator 
vanes 19 and forms an inductive loop for coupling out microwave energy. 
Antenna cap 10 is otherwise isolated from anode 13 by means of cylindrical 
insulator 11 which may be of glass or ceramic hermetically sealed at each 
end. 
Microwave leakage from the filament 20 and end closures 33 through 
insulators 24 is minimized by a cylindrical capacitative member 32 
supported and integral with annulus 34 which is supported in turn by anode 
body 13 and maintained concentric with and in close proximity to end 
closures 33 to form microwave chokes approximately one-quarter wavelength 
long at the operating frequency. 
Thus, the structure, which is symmetrical about its axial midplane, 
provides an extremely rugged, simple assembly which eliminates the use of 
most exotic, expensive, high temperature metals in the filament structure. 
Furthermore, noise generation is reduced by virtue of the elimination of 
leakage currents to pole pieces 27 (FIG. 3) since in my improved structure 
end closures 33 operate at the same potential as the emitting filament 20. 
An alternative method of supporting helical filament 20 and cylindrical 
metallic supports 29 is depicted in FIG. 5(b). In this embodiment, 
adjustment of the operating temperature of supports 29 is made possible by 
providing a longer heat conducting path to permit supports 29 to operate 
closer to the yield point of emitting filament 20 so as to reduce the 
effects of end cooling. A temperature of 650.degree. Centigrade at the end 
turns of helical emitting filament 20 can be tolorated before contact 
tension is lost. Small diameter extensions 37 of support cylinders 29 are 
welded or otherwise hermetically affixed to re-entrant cylinders 38 which 
in turn, are hermetically affixed to end closures 33 to support emitting 
filament 20 essentially concentric with anode structure 13. To permit 
support cylinders 29 to operate at temperatures above 250.degree. 
Centigrade, filament 20 may be welded or brazed to support cylinders 29 
which is then constructed of a metal such as molybdenum. 
To further reduce microwave leakage through insulators 24, and to increase 
the high voltage breakdown capacity between the outer surfaces of end 
closures 33 and other parts at ground potential, such as anode 13 and 
magnet return path 16, the space between these parts is filled with a 
mixture of a high voltage, high temperature material such as RTV silicone 
rubber and particals of a microwave absorptive material such as ferrite 
granules or powder as shown as a stippled area 40 in FIG. 4 which also 
shows the details of the magnetic circuit in greater particularity. The 
ends of filament connections 30 which apply filament voltage and negative 
high voltage to end closures 33 for the operation of the magnetron, are 
also encapsulated in the high voltage insulating material 40.