A ten-stage electrostatic depressed collector for improving klystron efficiency

A ten-stage electrostatic depressed collector, designed with the aid of an analog computer, was tested on a 1-kW CW 750-MHz klystron. Excellent correlation was achieved between computed and measured performance under varying conditions of RF drive. At full RF power output approximately 60 percent of the spent beam energy was recovered by use of the depressed collector. The net power conversion efficiency of the tube was raised from its undepressed value of 54.3 percent to approximately 70.9 percent. At one-half full power output, a collector efficiency of 70 percent was measured. At zero RF power output collector efficiency was 80 percent. To achieve these results it was necessary to install a small focusing coil between the final drift tube and the collector. No spurious oscillations or instabilities were detected when collector depression was employed, nor was electron backstreaming increased significantly. Intentional short circuiting of adjacent collector electrode pairs was shown to cause only minor degradation in collector performance.