POWER OUTPUT 615 



lengths around a centimeter or shorter, for accurate focusing becomes more 

 difficult as tubes are made physically smaller. Thus, in getting very high 

 powers at ordinary wavelengths or even moderate powers at shorter wave- 

 lengths, filter type circuits which provide heat dissipation by thermal con- 

 duction may be necessary. We have seen that the impedance of such cir- 

 cuits is lower than that of a helix for the broadband condition (group velocity 

 equal to phase velocity). However, high impedances and hence large values 

 of C can be attained at the expense of bandwidth by lowering the group 

 velocity. This tends to raise the efficiency, as do the high currents which are 

 allowable because of good heat dissipation. However, lowering group velocity 

 increases attenuation, and this will tend to reduce efficiency somewhat. 



It has been suggested that the power can be increased by reducing the 

 phase velocity of the circuit near the output end of the tube, so that the 

 electrons which have lost energy do not fall behind the waves. This is a com- 

 plicated but attractive possibility. It has also been suggested that the elec- 

 trode which collects electrons be operated at a voltage lower than that of 

 the helix. 



The general picture of what governs and limits power output is fairly 

 clear as long as C is very small. If attenuation near the output of the tube is 

 kept small, and the circuit is constructed so as to approximate the require- 

 ment that nearly the same field acts on all electrons, efficiencies as large as 

 40% are indicated within the limitations of the present theory. With larger 

 values of C it is not clear what the power limitation will be. 



The usual traveling-wave tube would seem to have a serious competitor 

 for power applications in the traveling-wave magnetron amplifier, which is 

 discussed briefly in a later chapter. 



