MAGNETRON AS GENERATOR OF CENTIMETER WAVES 271 



not being known at the time, and in its general features was much like that 

 shown schematically in Fig. 1. 



The output circuit of the British magnetron was also similar to that of 

 Fig. 1. It had no particular transformer properties designed into it. The 

 vacuum seal, made of copper, glass, and tungsten, was incorporated in the 

 output coaxial line in very much the same manner as that shown in Figs. 60 

 and 61. 



The cathode was a plain, oxide coated, nickel cylinder, 0.3 cm. in radius. 

 It had nickel end disks of 0.5 cm. radius and was mounted on radial leads 

 passing through glass vacuum seals like those shown in Fig. 61. The 

 leads are placed diametrally across the resonator hole to minimize RF flux 

 linkage to the cathode structure. Preliminary British results indicated that 

 Ihc cathode could be activated properly and would possess a reasonable life- 

 lime under the original operating conditions. 



The British magnetron had been designed for use with a magnet liaving 

 a gap of about 1.75 in. and a pole face diameter of 1.25 in., producing a 

 magnetic field of about 1500 gauss. 



Several of the constructional features of the British magnetron were new. 

 The cylindrical block of copper into which the resonator system was m.a- 

 chined was used as the vacuum envelope. It was closed at either end by 

 copper disk cover plates. The vacuum seal was made during the pumping 

 and baking process by the alloying at the baking temperature of gold rings 

 lietween the cover plate and block. The alloying was done at high pressure 

 I)rovided by a clamp bolted across the magnetron. Although no getter 

 was used, satisfactory vacuum conditions could be maintained after seal-off. 



By mid-November of 1940, a number of working reproductions of the 

 British magnetron had been supplied in our Laboratories and to the Radi- 

 ation Laboratory at M. I. T., and a program of study of the magnetron 

 oscillator commenced. The work thus started was continued, on the one 

 hand, to put the new magnetron into production, and on the other hand, 

 to attempt to understand it, improve upon it, and extend its range of useful- 

 ness. 



13. Magnetrons for Wavelengths of 20 to 45 Centimeters 



13.1 The 700A-D Magnetrons: After the British 10 cm. magnetron 

 had been successfully reproduced and an emergency program of research and 

 development of multicavity magnetron oscillators commenced, the question 

 immediately was asked: Can a multicavity magnetron be designed to operate 

 near 40 cm. in the pulsed radar set under development in the Whippany 

 radio laboratory? Clearly there now existed the possibility of much greater 

 power than was possible with triodes at this wavelength with reasonable 



