The effect of tapered circuits on efficiency for high power traveling wave tubes

Data on the VA-125, an S-band traveling wave tube, will be presented. This tube uses a cloverleaf interaction circuit which is characterized by a relatively high interaction impedance. The beam used is magnetically confined at 1.85 micropervs beam perveance. Interaction occurs over a fairly large voltage range and data has been obtained from 90 kilovolts to 130 kilovolts beam voltage. Efficiency data obtained on five models of the VA-125 type having differing numbers of reduced length sections at the circuit output will be shown. Efficiencies of 28% obtained with a uniform circuit are shown to be increased to 47% with four reduced height sections. In addition, the power bandwidth of the tube has been increased from 7% to above 14%. The performance of this tube at S band has resulted in peak powers in excess of 4.0 megawatts and average powers of 8.0 kilowatts. The efficiencies quoted compare very favorably with klystron efficiency at these frequencies while having bandwidths double that obtainable with currently available stagger tuned klystrons.