PULSE MODULATOR TUBES 835 



static fields. This type of sparking usually does not clear up and when it 

 becomes serious the tube must be replaced.'' It can be aggravated by spark- 

 ing in the oscillator part of the radar system. There is some evidence to 

 indicate that very high rates of rise of the pulse current drawn from the 

 cathode may tend to produce cathode sparking. At rates of rise in excess 

 of about .SO amperes per microsecond per scjuare centimeter of cathode area 

 a tendency for increased sparking has been noticed. 



Cathode emission, here as in any other tube, is governed by cathode 

 temperature and other considerations such as quantity and kind of gas in 

 the tube, the core material, coating material, and techniques of processing. 

 No attempt will be made to consider these factors in this paper as they are 

 sufficiently complex that no very clear cut dissertation can be given. Stand- 

 ard core materials and coatings were employed with good results. It was 

 found that the double carbonates (Ba, Sr) were less subject to sparking than 

 the triple carbonates (Ba, Sr, Ca). The cleanliness and previous treatment 

 of the other parts of the tube seemed to be the major factor in deteimining 

 the level of emission obtained. 



Primary grid emission, or thermionic emission from the control-grid and 

 screen-grid, was one of the most difficult problems in the development and 

 production of these tubes. Many trials were made using different materials 

 and coatings on the grids, but from all considerations gold was found to be 

 the most satisfactory. The grids in all the tubes described here are gold 

 plated or gold clad molybdenum. It is not considered that the use of 

 molybdenum for the core material is necessary, it being used here mainly 

 because it seemed to be the most economical material that had sufficient 

 stiffness to maintain grid alignment. Materials that tend to alloy with 

 gold easily are not suitable as it was found that gold alloys were not as good 

 as pure gold on the grid surface. The limitation involved in the use of gold 

 is that the temperature of the grid must be kept low enough that evapora- 

 tion of gold is not serious. This temperature limit is probably about 700°C. 

 If gold is evaporated, the grid soon loses its coating and primary emission 

 builds up rapidly. Also, the cathode emission seems to be poisoned by the 

 gold vapor. ^ 



Acknowledgment 



The author wishes particularly to acknowledge the contributions of his 

 immediate associates, Messrs. H. L. Downing, J. W. West, and J. E. Wolfe 

 in the development of this series of tubes. Many otliers also made im- 

 portant contributions. We are also indebted to the M.I.T. Radiation 

 Laboratory for data from life tests which they conducted on many of these 

 tubes. 



