Factors affecting traveling-wave tube power capacity

In some of the early traveling-wave tubes relatively high efficiencies and powers were observed. As more tubes were made, however, these results were not repeated and it became apparent that extremely wide variations in operation were obtained between tubes, with no obvious correspondence to observed structural variations. As a result an experimental program was undertaken to determine those parameters which are important to power output and efficiency, using a demountable continuously pumped tube and a probe technique for measuring the field strength along the length of the helix. As a result of this work it has been concluded that the amount and distribution of attenuation on the circuit has an important effect on the power output capabilities of the tube, and in fact that there is a relatively small latitude of attenuation distribution which will allow the maximum power output to be obtained and still give a good margin against oscillations. On a basis of these measurements empirical design criteria have been developed which specify attenuation distributions resulting in a minimum length of active circuit and a minimum net gain consistent with maximum efficiency. Application of the principles given has resulted in an increase in beam efficiency from 2% to 11% for a 4000 cm traveling-wave tube. By collecting electrons at a low voltage and increasing the total current, an overall “plate” efficiency of about 20% and 8 watts output has been obtained. One would expect that efficiency would be a function of the gain parameter (C), space charge (QC), attenuation (L/C) and the beam radius (γa´). This work is incomplete in that the effect of the last factor has been ignored. It does however give rough empirical relationships between the other factors for certain conditions, which have proven to be sound guides in the practical design of traveling-wave power tubes.