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The Cavity Magnetron*

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The Cavity Magnetron* THE CAVITY MAGNETRON* By H. A. H. BOOT, Ph.D.,I and J. T. RANDALL, D.Sc., F.R.S.,T Associate Member. (The paper was frst received l5th February, and in revised form 3rd July, 1946.) ST'MMARY mid-plane sectional drawing (at right angles to the cylindrical This paper gives an account of the discovery of the cavity magnetron axis) ofFigs. 1(a) and 1(ô). The general type ofdesign illustrated in the University of Birmingham by the authors, and of subsequent developments in that laboratory including the discovery by Sayers of what is now known as strapping. From October, 1939, to August, 1941, the work was carried out by the authors who, from June, 1940, received much technical help from S. M. Duke. Sayers began work on the magnetron in the summer of 1941 and his ideas on modes led almost immediately to the introduction of straps. Sayers's detailed work will later be the subject of a separate paper. (1) INTRODUCTION (à) As the group of workers originally engaged on the production Fig. l.-End view of resorlator systens. of centimetre waves in the Physics Department of the Type (a) is based on a Hertzian dipole while (à) is based on a short{ircuited quarter University wave Iine, of Birmingham has been gradually disbanded since the autumn of 1943, it may not be inapt to recall the inception of this work in Fig. 1(a) was the first to be used in Birmingham, and a and the.early ideas concerning the magnetron. Initially, under photograph of one of the first six anode blocks to be made is Oliphant, a considerable attempt was made by Sayers and others given in Fig. 2. to improve the klystronl as a centimetre-wave power oscillator, (2.2) and in fact a good deal was achieved in this connection. At the Construction same time an attempt was made by the authors to use the Fig. 2 also males clear how problem (ô) was overcome. The Barkhausen-Kurz valve as a detector; before long, however, interest moved towards the central problem ofthe production of high power at or below ten centimetre wavelengths. It appeared that the major clifficulty in the way of any attempts to improve the klystron by a large factor was that of getting sufficient power into the electron beam, necessarily of small cross-sectional area; it was realized by the authors that a magnetron would be free from this defect. One of the outstanding advantages of the klystron was the use of internal resonators such as had been described by Hanson2 and Rayleigh.3 Resonators of this type, turned from solid copper, give low h.f. losses, high frequenry- stability, and are capable oflarge hèat dissipation. The problern was therefore to design a magnetron, capable of giving a large anode current, and which also incorporated a resonator system of suitable properties. The main investigation therefore initially resolved itself into (a) the design of a suitable type of resonator, the determination of its size, and the method of grouping a number of resonators in a device of cylindrical symmetry. (ô) A method of construc- Fig, 2.-One of the first six anode blocks made. tion which would ensure high electrical and thermal conductivity. (c) The design of an h.f. output-circuit in relation to the chosen whole anode resonator system was constructed from a single resonator system. machined copper block, the central axial cavity forming the anode,/cathode space. The resonator cavities were grouped (2) GENERAL REQLITREMENTS around this, heat being dissipated by conduction along the . (2.1) Type of Resonator webs or segments between neighbouring resonators. The Hanson papers were by this time available in the laboratory, but it was clear that the designs (hollow spheres, (2.3) Resonator Size cubes, and "doughnut"-shaped cavities) could not be assoiiated' The problem of resonator size for an intended wavelength of with the cylindrical symmetry of the magnetron. It was decided, 10 cm was more difficult. The experiments of Hertz (confirmed therefore, to use either a three-dimensional extension of the well- theoretically by MacDonald) had shown that the wavelength to known Hertzian wire loop or a corresponding extension of a be expected from a simple loop of this wire is 7'94 times the shortæircuited quarter-wave line. That either type lends itself diameter of the loop. It was assumed with some justiflcation to a symmetrical magnetron construction may be seen from the that the wavelength to be expected from a single, slotted, copper * Radiosectionpaper. f universityofBirmingham. I universityof London. cylinder would be largely independent of the cylinder length, te28l BOOT AND RANDALL: THE CÀVTTY and the unpredictable effect of electromagnetic and electrostatic 0.75 mm in diameter was used (as being more suitable for c.w.' coupling between neighbouring resonators was neglected' operation), it was also realized from the French publications available that the use of an oxideæoated cathode was desirable. The subsequent introduction ofi such cathodes by both the (2.4) Number of Resonators and Anode Hole t: Research Laboratories of the General Electric Company, and the was not known at the time to what extent the operation I It Birmingham group is referred to below. of the valve would be affected by small departures from sym- I metry; it witl be noticed that, in Fie. 2, the anode hole is the (2.O ôutPut Circuit Later ,1 same diameter as that of the surrounding resonators. The output circuit is of some interest as it was destined to be principles in the design experiments showed that the adopted first the standard for a considerable period, exgept in cases of very outstanding 1 were of veiy general application. One of the high power (or very sholt wavelengths), when wave-guide output features of the design at this stage was the use of an enclosed circuits proved more suitable. The method adopted initially i resonator-anode system, and in this feature it differed radically was to utilize a coaxial-transmission line, the inner conductor from ali earlier magnetrons known to the authors. The chief terminating in a loop,'in such a way that the magnetic flux I dimensions of the first caùity magnetron were:- parallel to the resonator axis passed through that fraction ofthe .t cavity section embraced by the loop. The ratio of the loop area Anode lensrh, slot di-"',ion,, to cavity area in the early experiments was estimated to be l I -'''c.;.-^'.â*i;. ] I I:aq:l_"-tresonators I ---.Â- -' cm I I .- I I "ff!$.t:, between 0'4 and 0'5. The ftanSmission line lies in a mid.section t plane at right angles to the cylin\er axis, and being sealed by a glass-metal seal, became a part ol\he valve vacuum system. 4.0 t-2 I 0.1 x0.1 I 6 1.2 It is perhaps useful to summari2\ the main conclusions which had been established by the trst experiments outlined above. f (i) A magnetron with a completely enclosed resonator system (2.5) Operating Conditions had been successfully opeqatèh at a wavelength of 10 cm at an The calculations on which the operating conditions of the efficiency (10-15%) comparable with that of other known first cavity magnetrons wêre based were of the simplest nature. high-frequency oscillators such as the klystron. The chief assumption related.to orbits in which the electrons (ii) The use of a number of closely coupled resonators in the + pass from one iesonator slot to the next at grazing incidence same oscillator apparently offered no difficulty. in a time equal to half a period of oscillation under an applied (iii) The use of a combined copper anode block and resonator ï anode poteritial of Y volts and an axial magnetic field of system ensured adequate heat dissipation, low electrical losses, f/ oersteds with a phase diflerence of n'* between neighbouring and the potentialities of simple manufacture. segments. Such calculations give rise to expressions lor V and H (iv) The use of a single, simple, output circuit to draw h.f. J in terms of À, the expected wavelength, D the anode diameter, power from the system was established and it was clear that such z the number ofresonators, and known physical constants. On an output circuit would enable power to be fed in a comparatively this basis it appeared that 16 kV and I 000 oersteds would be easy manner to aerial systems and wave guides. required to opèiate a magnetron of the above dimensions' The detailed expressions are referred to later. (3) FURTHER DE1IELOPMENTS $ Facilities for making sealed-off valves were not immediately The following steps in the development of the valve were available, and the ûrst cavity magnetron was completed in de- obvious but necessary consequences ofthe eariy experiments. 3 mountable fashion with waxed glass-metal seals. A view of the (a) The completion of a seaied-off version of the valve. valve removed from the pump is shown in Fig. 3. The first (à) The testing of other designs-e.g. different numbers and sizes of resonators. 't (c) The introduction of large cathodes to provide the high nç anode current necessary for high-power operation. (d) The introduction of 4 modulated power supply. (e) The adequate measurement of r.f. power. r In the achievement of (a) considerable help was obtained, J{- directly and indirectly, Ëy collaboration with the Research Laboratories of the G.E.C., Wembley, with ;vhom inlormation was exchanged in April, 1940. In this way a great deal was learned about the technique of sealing giass to copper, and of joining copper to copper by means of a gold wire under pressure $ between the two surfaces at temperatures below 500" C.
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