Studies of Stability and Amplification in AN Overmoded Gyroklystron.

The performance of four overmoded (TE011 circular), two-cavity gyroklystron amplifier tubes operating in X -band is reported. A summary of the theoretical design procedure is presented, followed by a description of the experimental setup. A detailed description of the drift tube and input cavity design procedures and their practical realizations is given. The stability and amplification properties of the four gyroklystron tubes are subsequently reported. The fourth tube produced 2.0-2.7 MW pulses at 9.87 GHz for 0.5-1.0 microseconds, and exhibited gains of 17-19 dB and efficiencies of 5%. The results were obtained using an electron beam with voltage, current, and computed beam alpha (transverse velocity divided by axial velocity) of 407-425 kV, 115-135 A, and 0.8-1.0, respectively. The presence of instabilities in the gun downtaper and drift tube prevented operation at the original design point (500 kV, 160 A, and alpha 1.5). The effects of input frequency, input power, and beam parameters (voltage, current, magnetic compression) on the amplification are documented. Detailed descriptions of the instabilities observed in the gyroklystrons are presented, and the use of lossy dielectrics to suppress the instabilities is described. The amplifier performance is compared with simulations. Finally, various modifications to improve efficiency and stability are discussed. This work represents an advance in beam power by 2 to 3 orders of magnitude compared with previous stable gyroklystron operation. It led directly to the realization of 20 MW peak output power, 31% efficiency, and 26 dB gain from a subsequent gyroklystron experiment.