|
(11) | EP 0 316 092 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Magnetron Anodes |
| (57) A magnetron anode (3) has a main body (4) and a plurality of vanes (5), the main
body and vanes being formed respectively of two materials of differing thermal expansivities.
As a result the shift in frequency with temperature change can be controlled. |
[0002] One common type of magnetron anode is shown in plan view in Figure 1 and comprises an outer cylinder 1 bearing a number of vanes 2, the whole structure usually being formed from copper.
[0003] In operation, the cavities between the vanes resonate electrically, and the resonant frequency determines the operating frequency of the magnetron.
[0004] The resonance frequency of the cavities varies with the temperature of the anode due to thermal expansion of the anode. In many applications a magnetron may be expected to operate at any temperature in the range -50°C to + 100°C, so the change in the resonance frequency of the anode may be quite significant. As a result, the operating frequency of the magnetron will alter noticeably across its operating temperature range.
[0005] At present this change in frequency with temperature is usually controlled or compensated for by the use of tuning circuits which mechanically alter the dimensions of the anode so as to keep its resonant frequency constant. This is not very satisfactory, however, because the servo-mechanisms and control system required to achieve this are complex and relatively expensive.
[0006] If it is necessary to produce a magnetron having the lowest possible change in operating frequency with temperature, the whole anode can be constructed from a material having a low thermal expansivity. However, such an anode will always exhibit some change in resonant frequency with temperature also, such materials are often unsuitable for use in an anode because of other properties required for anode material, such as high termal and electrical conducitivity, nonmagnetism and no outgassing when heated in a vaccum.
[0007] In some uses it maybe desired to produce an magnetron having a resonant frequency which does alter with temperature, it is only possible to have the resonant frequency decrease as temperature increases.
[0008] This invention provides an anode for use in a magnetron, the anode comprising a main body portion and a plurality of vanes,characterised by the body being formed from a first material and the vanes being formed from a second material, the first and second materials having different thermal expansivities.
[0009] The resonant frequency f of a magnetron anode is related to its capacitance C and inductance L by the equation:
[0010] Referring to Figure 1 a known anode is shown, the inductance of the anode is proportional to the area A between adjacent vanes. The capacitance of the anode is inversely proportional to the separation T between the tips of adjacent vanes.
[0012] When the anode heats up and expands, the ratio of T and A changes and as a result the resonant frequency alters.
[0013] By using the invention, it is possible to arrange the rates of change of T and A with temperature to be such that the variation of the resonant frequency of the anode with temperature can be controlled without the use of external circuitry.
[0014] Preferably, the first material has a greater thermal expansivity than the second material. This allows the resonant frequency of the anode to be made proportional, rather than inversely proportional, to temperature or preferably independent of temperature.
[0015] It is preferred to form the main body of the anode from copper and the vanes from molybdenum because these materials have very good electrical and thermal conductivities and have widely differing thermal expansivities and are both non-magnetic.
[0016] An anode embodying the invention will now be described by way of example with reference to the accompanying figures, in which:
[0018] Referring to Figure 2 a magnetron anode 3 is shown in solid lines in a first low temperature position and in dashed lines 3/ in a second higher temperature position.
[0019] The anode is formed from an outer copper main body 4,4′ and eight molybdenum vanes 5,5′. The molybdenum vanes, 5,5′ are secured to the copper main body 4,4′ by plating the vanes 5,5′ with nickel and then copper and then brazing them to the main body, 4,4′.
[0020] As the temperature of the anode 3,3′ rises, all of its component parts expand. The expansion of the main body 4 to 4′ pulls the vanes 5 outward and the vanes 5 also expand into positions 5′. As a result of this expansion, the area A expands to A′ and the vane separation T expands to T′. As the vanes 5,5′ are formed of molybdenum and the main body 4,4. is formed of copper, they expand differentially and the relative rates of change of A and T with temperature can be selected to be the same by choosing an appropriate profile for the vanes 5.
1. An anode for use in a magnetron, the anode comprising a main body portion and a
plurality of vanes, characterised by the body being formed from a first material and
the vanes being formed from a second material, the first and second materials having
different thermal expansivities.
2. An anode as claimed in claim 1 in which the thermal expansivity of the first material
is greater than the thermal expansivity of the second material.
3. An anode as claimed in claim 2 in which the dimensions of the anode and the thermal
expansivities of the two materials are such that the frequency of the resonant cavities
defined by the anode is unaffected by changes in anode temperature.
4. An anode as claimed in any preceding claim in which the first material is copper
and the second material is molybdenum.
5. An anode for use in a magnetron comprising a cylindrical annular main body portion
formed of copper and a plurality of inwardly pointing radial vanes made of molybdenum.
6. A magnetron employing an anode as claimed in any preceding claim.