US 3896332 A
Description (OCR text may contain errors)
United States Patent Heathcote 1 July 22, 1975  HIGH POWER QUICK STARTING 2,647,216 7/1953 Brown SIS/39.63 2,736,838 2/1956 Nelson... 315/39.63 2,818,528 12/1957 Feinstein... 313/338 X  Inventor: Vincent Albert Heathcote, London, 22,499 2 958 Lynch 313 338 x England 3,403,281 9/1968 Weaver 315/39.63 3,505,557 4 1970 S h 'dt 313 338 X  Assignee: The M-O Valve Company Limited, C m
London England Primary Examiner-Saxfield Chatmon, Jr.  Filed: May 29, 1974 Attorney, Agent, or Firm-Kirschstein, Kirschstein, 21 Appl. No.: 474,240 Frank  ABSTRACT  Foreign Appllcatlon Prlorlty Data A k t t t d qu1c s ar magne ron compnsmg: an ano e; a mam June 4, 1973 Unlted Kmgdom 26552/73 cathode positioned adjacent the anode and providing 52 US. Cl. 315/3951; 313/338; 315/3963 by Secondary electron due back 1 Int Cl 2 H0 25/50 bombardment at least a ma or part of the electron cur-  Field or Search 313/337, 338; 315/3951, rent requred T of W and an electron gun employlng a therm1on1c cathode of low 315/3963 thermal 1nert1a arranged to d1rect a stream of elec- References Cited trons Into the apace between the anode and the mam cathode to 1n1t1ate operat1on of the magnetron. UNITED STATES PATENTS 0 2,574,562 11/1951 Hansell 315/39.63 x 9 Clam, 1 D'awmg HIGH POWER QUICK STARTING MAGNETRON This invention relates to magnetrons.
The invention relates particularly to quick starting magnetrons of high power capability.
It is known how to obtain quick starting properties in a magnetron by providing the magnetron with a thermionic cathode structure of low thermal inertia so that the cathode will heat up rapidly to its emission temperature from cold. Such a cathode structure inevitably exhibits poor heat dissipation properties, and in consequence, the temperature of such a cathode structure tends to rise in operation due to electron back bombardment, between five and ten percent of the input power to a magnetron being typically dissipated at the cathode.
As a result, the maximum power capability of known quick starting magnetrons is limited.
The present invention seeks to overcome this difficulty and provide quick starting magnetrons of higher power capability than are at present available.
According to the present invention a magnetron comprises: an anode; a main cathode positioned adjacent the anode and adapted to provide by secondary electron emission due to electron back bombardment in operation of the magnetron at least a major part of the electron current required for operation of the mag netron; and an electron gun employing a thermionic cathode of low thermal inertia arranged to direct a stream of electrons into the space between the anode and the main cathode to initiate operation of the mag netron.
Where it is desired to cool the main cathode, the main cathode suitably comprises one end of a solid rod of material of good thermal conductivity, the other end of the rod being provided with means for cooling the rod.
Where it is not desired to cool the main cathode, the main cathode suitably comprises a body of an impregnated porous refractory material.
In one particular arrangement in accordance with the invention the anode and the main cathode define between them a generally annular space and the electron gun is arranged to direct an annular stream of electrons coaxially into said space.
In such an arrangement the electron gun thermionic cathode preferably comprises a thin-walled metal tube having a coating of electron emissive material on its outer surface, the tube being disposed coaxially with the magnetron anode/cathode space and axially spaced therefrom. The metal tube is preferably arranged to be electrically heated by the passage of an electric current directly through it. Such an electron gun preferably includes a control electrode in the form of a nonmagnetic annular member coaxially surrounding the end of the electron gun cathode adjacent the magnetron anode/cathode space.
One arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing which is a diagrammatic sectional view of a high-power, quick-starting magnetron.
Referring to the drawing, the magnetron includes a rising sun type copper anode resonator of conventional form comprising an outer annular wall 1 from the inner surface of which a plurality of radial vanes 2 project inwardly to form a plurality of resonant cavities. An output is fed from the magnetron to a rectangular waveguide 3 via a slot 4 in the outer wall 1 of the anode, the waveguide being sealed by a ceramic window 5.
The necessary magnetic field is provided by a permanent magnet (not shown) co-operating with two conical, iron pole pieces 6 and 7 disposed coaxially with the anode. The pole pieces 6 and 7 are disposed one at each end of the anode with their narrower ends sealed through apertures formed centrally in respective copper discs 8 sealed to opposite ends of the anode.
The main cathode of the magnetron is constituted by a platinum clad end portion 9 of a tungsten rod 10, which end portion is disposed coaxially within the anode. Adjacent the anode vanes 2 the rod carries an end hat 11 and beyond this end hat 11 the rod 10 extends axially through a central bore in the pole piece 6. The rod 10 is supported from the outer end of the pole piece 6 by means of a tubular ceramic insulator 12, and is provided with a set of cooling fins 13 at its end remote from the anode.
The magnetron further includes an electron gun arranged to project an annular stream of electrons into the space between inner ends of the anode vanes 2 and the main cathode 9. The electron gun comprises a cathode in the form of a coating of thermionic electron emissive material on the outer surface of a thin-walled tube 14 of highly resistive material, for example of 30% tungsten and nickel. The tube 14 is disposed for the most part coaxially within a bore extending centrally through the pole piece 7, but projects slightly from the narrower end of the pole piece 7. The projecting part of the tube 14 is surrounded by a nommagnetic metal ring 15 secured to and in electric contact with the narrower end of the pole piece 7, the ring 15 serving as a control electrode of the electron gun.
The tube 14 is supported between a metal ring 16 and an outwardly extending flange 17 formed at one end of a metal tube 18, the flange serving as a second end hat for the main cathode 9. The metal tube 18 extends axially through the tube 14 at whose outer end the tube 18 coaxially adjoins a metal rod 19 which is supported from the outer end of the pole piece 7 by means of two tubular ceramic insulators 20 and 21. The metal ring 16 is secured to one end of a perforated metal tube 22 which coaxially surrounds the rod 19 and at its other end is connected to a metal contact ring 23 sealed between the two ceramic insulators 20 and 21.
In operation of the magnetron the electron gun cathode is energised by applying a suitable voltage between the rod 19 and the contact ring 23 to cause an electric current to pass through the tube 14. Due to the thinness of the wall of the tube 14, the cathode has a low thermal inertia and therefore rapidly heats up to its electron emission temperature, thus conferring quick starting properties on the magnetron.
When the magnetron is required to start oscillating the main cathode 9 and the electron gun cathode are pulsed negatively with respect to the anode by application of a suitable potential to the rod 10 and to the rod 19 or the contact ring 23, the magnitude of the applied potential depending on the required magnetron operating conditions. Under the action of the applied potential the electrons produced at the electron gun cathode are formed into an annular stream directed into the anode/cathode space of the magnetron, the control electrode 15 assisting in the formation of the electron stream. On arrival of the electron stream in this space the magnetron immediately starts to oscillate and a sufficient supply of electrons for further operation of the magnetron is produced by secondary emission at the surface of the main cathode 9 due to electron back bombardment.
For continuous operation of the magnetron the electric supply electron gun cathode may be switched off after oscillation has started, but for pulsed operation it is normally convenient for the electron gun cathode to be energised continuously.
Due to its relatively massive form, the main cathode 9 has a high thermal inertia and the heat produced at the main cathode by electron bombardment is rapidly conducted along the rod 10 to the fins 13 where it is disposed of by radiation. As a result, the temperature of the main cathode 9 does not rise unduly in operation. It will be appreciated in this connection that since the main cathode 9 does not require to be heated to produce electrons, very efficient cooling of the main cathode 9 may be employed, if desired.
Due to the physical position of the tube 14 the electron gun cathode is not subjected to significant electron bombardment in operation so that over-heating of this cathode does not occur, even although it has a low thermal inertia.
In a modification of the magnetron described above one or more ducts are provided in the rod 10 for the passage of cooling fluid in operation.
It will be appreciated that in a magnetron in accordance with the invention the main cathode is not necessarily run at a relatively low temperature. Thus, in an alternative arrangement in accordance with the invention the main cathode may be run at a high temperature and the heat produced by electron back bombardment dissipated by thermal radiation from the cathode surface. In such arrangement the main cathode preferably comprises a body of an impregnated porous refractory material, for example, porous tungsten impregnated with barium aluminate.
1. A magnetron comprising: a secondary electron emissive main cathode; an anode resonator which defines a cylindrical space in which said main cathode is disposed so as to provide an annular space between the anode and cathode; an electron gun employing a thermionic cathode of low thermal inertia disposed at a position outside said cylindrical space defined by the anode and arranged to direct a stream of electrons into said annular space between the anode and said main cathode to initiate operation of the magnetron, whereafter at least the major part of the electron current required for operation of the magnetron is provided by said main cathode due to electron back bombardment; said thermionic cathode being sufficiently remote from said cylindrical space to not be subjected to significant electron back bombardment.
2. A magnetron according to claim 1 including a solid rod of material of good thermal conductivity, one end of the rod constituting the main cathode of the magnetron and the other end of the rod being provided with cooling means.
3. A magnetron according to claim 1 wherein the main cathode comprises a body of an impregnated porous refractory material.
4. A magnetron according to claim 1 wherein the electron gun is arranged to direct an annular stream of electrons coaxially into said annular space.
5. A magnetron according to claim 4 wherein said electron gun thermionic cathode comprises a thinwalled metal tube having a coating of electron emissive material on its outer surface, the tube being disposed coaxially with said cylindrical space and axially spaced therefrom.
6. A magnetron according to claim 5 wherein said metal tube is arranged to be electrically heated by the passage of an electric current directly through it.
7. A magnetron according to claim 5 including a control electrode in the form of a non-magnetic annular member coaxially surrounding the end of the electron gun cathode adjacent said cylindrical space.
8. A magnetron according to claim 1 including two pole pieces for directing a magnetic field through said cylindrical space in operation, the electron gun cathode being disposed coaxially in a bore extending through one of the pole pieces.
9. A magnetron according to claim 8 including a solid rod of material of good thermal conductivity which extends coaxially through a bore in the other pole piece, one end of the rod constituting the main cathode of the magnetron and the other end of the rod being provided with cooling means.