Klystron-TWT hybrid efficiency improvement

This final report covers effort from 16 December 1974 through 30 June 1976. The effort was expended as Phase II of a theoretical and experimental investigation of methods of improving the conversion efficiency of relatively broadband linear-beam amplifiers operating at approximately 1 MW of peak power. The Phase I 'short-stack' intense-interaction cloverleaf structure design was modified. Extensive cold testing was performed with an emphasis on selective loading of the higher-order modes that supported oscillations in the Phase I tube. Computer-aided calculations predicted 43% minimum efficiency over a 10% bandwidth with a midband efficiency of 56%. Performance was limited by an efficiency dip near the low end of the band, a gain hole in band, an RF drive-induced oscillation near saturation at some combinations of beam voltage and current, and a beam deduced to be of smaller diameter than was assumed in the calculations. Under the above conditions, the best saturation performance was 42% midband efficiency with a 1.5 dB bandwidth of 10%, and 50% midband efficiency with a 1.5 dB bandwidth of 6%. Constant RF drive results included 200 MHz of bandwidth with minimum efficiency of 33% and 36 dB gain at the 800 kW level, and 200 MHz of bandwidth with minimum efficiency of 34% and 37 dB gain at the 1100 kW level.