US3428859A - Magnetron anode having temperature compensating members within the cavities of a different coefficient of thermal expansion from the cavities - Google Patents

Description

Feb. 18, 1969 v, HEATHCOTE 3,428,859

MAGNETRON ANODE HAVING TEMPERATURE COMPENSATING MEMBERS WITHIN THE CAVITIES OF A DIFFERENT COEFFICIENT 0F THERMAL EXPANSION FROM THE CAVITIES Filed March 15, 1966 v Fl 3. m- Henmo're- KMAQQCWLQ {$17k 'DTTOKMk United States Patent 3,428,859 MAGNETRON ANODE HAVING TEMPERATURE COMPENSATING MEMBERS WITHIN THE CAV- ITIES OF A DIFFERENT COEFFICIENT OF THERMAL EXPANSION FROM THE CAVITIES Vincent Albert Heathcote, London, England, assigno r t0 The M-O Valve Company, London, England, a British company Filed Mar. 15, 1966, Ser. No. 534,314 Claims priority, application Great Britain, Mar. 23, 1965,

12,353/65 US. Cl. 315-3951 3 Claims Int. Cl. H01j 25/50 ABSTRACT OF THE DISCLOSURE -A magnetron having a multicavity anode in each of which cavities there is disposed a member consisting of a material whose coefiicient of thermal expansion differs from that of the material of the anode so as to compensate for the thermal expansion of the cavity in which it is disposed, thereby to reduce the magnitude of changes in the frequency of operation of the magnetron with temperature.

This invention relates to m-agnetrons.

The invention relates particularly to magnetrons of the kind comprising an anode in which are formed a plurality of resonant cavities which open into a space in which is housed a cathode.

The frequency of oscillation of such a magnetron depends on the volume of the resonant cavities; variation in the frequency of oscillation of such a magnetron is thus liable to occur with changes in the temperature of its anode.

'It is an object of the present invention to provide a magnetron of the kind specified wherein this difliculty is alleviated.

According to the present invention, in a magnetron of the kind specified within each of at least the majority of the resonant cavities there is disposed an electrically conductive member arranged to be maintained at the same electric potential as the anode in operation, each said member being in good thermal contact with the anode and consisting of a material having a thermal coefi'lcient of expansion differing from that of the material of the anode, at least over a wide range of temperatures of the anode which may occur in normal operation of the magnetron, so that for any change of temperature within said range the change in the frequency of oscillation of the magnetron is less than the change in the frequency of oscillation that would occur for that change of temperature if said conductive members were formed of a material having the same thermal coefiicient of expansion as the material of said anode.

One arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing in which:

FIGURE-1 is a part-sectional elevation of a magnetron intended for operation in the frequency range 5,200 to 1 1,000 mc./s.;

FIGURE 2 is a view along the line II-II in FIGURE 1; and

FIGURE 3 is a perspective view of a part of the magnetron.

Referring to FIGURES -1 and 2, the magnetron has a hollow cylindrical sealed copper envelope 1 which houses an electrode structure comprising an anode 2 and a directly heated cathode 3.

The anode 2 comprises a copper-plated tubular molybdenum member 4 of internal diameter 0.435 inch and is sandwiched between one end of the curved wall of I the envelope 1 and the corresponding end of the envelope 1. To the inner curved surface of the anode tube 4 are fixed twelve rectangular vanes 8 which are equally spaced from one another and are also made from copper-plated molybdenum, each vane 8 having a length of 0.205 inch, a breadth of 0.185 inch and a thickness of 0.012 inch. The vanes 8 project radially inwards from the curved surface of the anode tube 4 so that their inner free edges lie on the surface of a cylindrical volume of diameter 0.093 inch coaxial with the tube 4, the ends of each vane 8 lying in the planes of the ends of the tube 4. The cathode 3, which is tubular in shape, tits coaxially within the volume delineated by the inner edges of the vanes 8. The copper plating upon the tube 4 and the vanes '8 serves a dual purpose; firstly, the electrical conductivity of the tube 4 and the vanes -8 is appreciably increased at the appropriate surfaces whilst, secondly, brazing of the tube 4 and the vanes '8 is facilitated.

[Extending into each vane 8 from each end are two adjoining rectangular slots 9 and 10 one of which has a width and a depth of 0.020 inch and the other of which has a width of 0.055 inch and a depth of 0.035 inch, the deeper slot 9 being nearer the inner edge at one end of each vane '8 and the shallower slot 10 being nearer the inner edge at the other end of each vane 8, and the inner edge of the inner slot at each end of each vane 8 being spaced 0.030 inch or 0.015 inch from the inner edge of the vane 8, depending on whether the inner slot is a deeper slot 9 or a shallower slot 10. The vanes 8 are arranged so that at each end of the vanes 8 the shallower slots 10 are nearer the inner edge in alternate vanes 8, and at each end of the vanes 8 two circular metal rings 11 of 0.020 inch square cross-section are arranged to run one within the inner slot in each vane 8 and the other within the outer slot in each vane 8, each ring 11 being brazed to the floor of each shallower slot 10 through which it passes. Thus, each set of alternate ones of the vanes 8 is electrically connected by the outer one of the two rings 1'1 at one end, and by the inner one of the two rings 11 at the other end this serving to suppress unwanted modes of oscillation in operation.

Referring now also to FIGURE 3, between each adjacent pair of vanes 8, except one, there is positioned a wedge-shaped copper member 12 in the form of a 30 segment of a tube of length 0.235 inch and internal and external diameters 0.332 inch and 0.441 inch respectively Each copper wedge 12 has each of its two rectangular faces disposed parallel to and spaced 0.014 inch from the nearest vane 8 whilst the arcuate face having the larger radius of curvature is spaced 0.022 inch from the interior wall of the anode tube 4. Each copper wedge 1-2 constitutes a projection from one end of a tubular copper support 13 which is mounted in good electrical and thermal contact with the tubular member 6 which carries the anode 2, thus the wedges 12 are in good electrical and thermal contact with the anode 2.

An output is derived from the magnetron via a short section of a circular waveguide 14 sealed through the side wall of the envelope 1, the waveguide 14 having a cylindrical ceramic plug 15 sealed into it. At its inner end the plug 15 is provided with a slot 16 into which fits an E-shaped metal coupling member 17 the central limb of which is secured to the anode tube 4 and the outer limbs of which extend through the space between the pair of adjacent vanes 8 which are not separated by a wedge 12 and are respectively connected at their ends to the outer rings 11. Diametrically opposite the output waveguide a copper pinch tube 18 is provided which is sealed off at its outer end during manufacture of the magnetron.

The spaces within the envelope 1 on each side of the anode 2 are occupied by the two frusto-conical magnetic pole pieces 19 which are associated with a permanent magnet (not shown) external of the envelope 1 in operation of the magnetron. The narrower end of each pole piece 19 is disposed adjacent the anode 2 and at its wider end, each pole piece 18 is sealed through an aperture formed centrally in the corresponding end of the envelope 1. A support stem and leads 20 for the cathode 3 extend through an aperture which extends coaxially through one of the pole pieces 19.

It will be appreciated that in FIGURE 2 the pole pieces 19 the cathode 3 and the cathode support stem and leads 20 have been omitted for the sake of clarity.

It will be understood that the frequency of oscillation of the magnetron depends on the volume of the resonant cavities defined by the anode tube 4, the vanes 8 and the wedges 12, the frequency of oscillation decreasing with increase in the volume of the cavities and vice versa. It will be appreciated that the volume of the resonant cavities is the volume contained between the vanes 8 and the anode tube 4 minus the volume of the wedges 12. The coefficients of thermal expansion for molybdenum and copper are 5.1 l C. and l6.5 l0 C, over the range of temperatures of the anode 2 which may occur in operation. Hence, for a given change in temperature within the relevant range, the volume of the copper wedges 12 changes by a larger fraction than does the volume contained by the vanes 8 and the anode tube 4. In the magnetron, described by way of example, the relative dimensions of the wedges 12 and of the anode tube 4 and the vanes 8 are chosen in relation to the difierent thermal coefficients of expansion of the materials of which these members consist so that for a given change in temperature the change in volume of the wedges 1 2 is approximately equal to the change in the volume contained by the vanes 8 and the anode tube 4. The volume of the resonant cavities thus remains approximately constant in operation with changes in temperature. It will be appreciated that the copper plating on the anode tube 4 and the vanes 8 does not appreciably atfect the thermal expansion of these members.

The particular magnetron described above, by way of example, has a thermal coefficient of frequency of oscillation of about 7 kc./s./ C.; a similar magnetron designed for operation at the same frequency and having a molybdenum anode but no copper wedges disposed in its resonant cavities had a thermal coefficient of frequency of oscillation of about 50 kc./s./ C.

It will be appreciated that in other arrangements in accordance with the invention materials other than molybdenum and copper may be used for the anode and electrically conductive members disposed in the resonant cavities in the anode, but of course the materials used must have suitably diifering thermal coefiicients of expansion. It will be understood that the number of electrically conductive members employed is not critical. However, in order for an appreciable degree of temperature compensation to be achieved, it is necessary for there to be a conductive member in each of at least the majority of the resonant cavities.

I claim:

1. A magnetron comprising: an anode in which is formed a plurality of resonant cavities; a cathode housed in a space into which the resonant cavities open; and a number of electrically conductive members, each of which is disposed in a different one of the resonant cavities, said conductive members being in good electrical and thermal contact with the anode and consisting of a material having a thermal coeificient of expansion differing from the thermal coefiicient of expansion of the material of the anode at least over a wide range of temperatures of the anode which may occur in normal operation of the mag netron, so that for any change of temperature within said range, the change in the frequency of oscillation of the magnetron, is less than the change in the frequency of oscillation of the magnetron that would occur for that change in temperature if said conductive members were formed of a material having the same thermal coeflicient of expansion as the material of the anode.

2. A magnetron according to claim 1, wherein the anode consists of molybdenum and said conductive members consist of copper.

3. A magnetron according to claim 1, wherein the anode comprises a tubular member and a plurality of planar vanes which project radially inwardly from the inner curved surface of said tubular member, and each of said conductive members is in the form of a segment of a tube and is disposed between an adjacent pair of said vanes with its two planar sides respectively parallel to the twoadjacent vanes and its curved face having the larger radius of curvature facing and parallel to the inner curved surface of said tubular member.

References Cited UNITED STATES PATENTS 2,810,094 10/1957 Derby et al. 315-3959 X 3,289,037 11/1966 Whitmore 31539.59 X 2,520,955 9/1950 Okress et al 313311 X 2,608,673 8/1952 Brown 31539.51 X 2,811,670 10/1957 Amsdem et al 31346 X 2,852,720 9/1958 Crapuchettes 313-311 X 3,297,905 '1/1967 Fiedor et al. 313-311 X ELI LIEBERMAN, Primary Examiner.

S. CHATMON, In, Assistant Examiner.

US. Cl. X.R.

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