258 BELL SYSTEM TECHNICAL JOURNAL 



problems will here be in order. It has been seen earlier that, in scaling all 

 magnetron dimensions by a factor a, the magnetic field changes by a factor 



— while the current and voltage remain unchanged. That this places severe 

 a 



requirements on the cathode may be seen by considering the scaling of a 

 10 cm. magnetron down to 1 cm. The operating current may be 20 amperes 

 in both cases. If this corresponds to a current density of 5 amperes, per 

 sq. cm. at 10 cm., 500 amperes per sq.cm. will be required at 1 cm. What 

 is more, the back bombardment in watts per sq. cm. is increased by a factor 

 of 100. Both of these requirements are completely unreasonable and pre- 

 clude direct scaling in this instance. Consequently, an attempt is made 

 in such cases to decrease the current density by increasing both the cathode 

 length and diameter. Increasing the latter usually involves increasing the 

 anode diameter and the number of resonators. Even so, current densities 

 may exceed 50 amperes per sq. cm., as stated earher. 



The pulsed magnetron cathode at the shorter wavelengths (less than 

 10 cm.) in the centimeter band is a limiting factor in magnetron design. In 

 CW magnetrons the small size of the interaction space has made ihe cathode 

 an important and difficult design problem throughout the centimeter wave- 

 length region. Considerable effort has been expended in a number of 

 laboratories not only to understand the physics of the operation of the mag- 

 netron cathode but to find suitable materials and constructional designs. It 

 has become clear that a good magnetron cathode which will meet the special 

 conditions of high current density and high voltage gradient and the con- 

 siderable electron back bombardment are these: (1) sufficient primary 

 emission to enable the magnetron to start and to supply a part of the re- 

 quired operating current; (2) sufficient secondar}^ emission to supply the 

 remainder (it may be practically all) of the required current density through 

 whatever mechanism is involved; (3) sufficient active material to permit 

 satisfactory Hfe; (4) some mechanical means of holding the active material 

 on the cathode surface; (5) sufficiently low electrical resistance of the coating 

 to permit large bursts of current without undue local heating and , high back 

 bombardment without excessive coating temperature; and (6) satisfactory 

 over-all heat dissipation characteristics, conductive and/or radiative, to 

 keep evaporation of active material to a minimum. 



In pulsed magnetrons of wavelength 10 cm. or greater it has generally 

 been possible to use plain oxide coated cathodes. Nickel base material is 

 generally used and the active material is the usual double carbonate coating 

 (reduced to the oxides during activation). At wavelengths of 3 cm. and 

 shorter the development of satisfactory pulsed magnetrons would have 

 been impossible without the development of special cathodes. In the main, 



