Operation of a three cavity second harmonic coaxial gyroklystron

At the University of Maryland, we have been developing gyroklystrons for advanced electron-positron linear collider applications. We have recently achieved over 80 MW of peak power at 8.6 GHz with a three- cavity first harmonic gyroklystron. The interaction was observed between a 470 kV, 500 A annular, rotating beam and a sequence of TE₀₁₁ coaxial cavities. The efficiency was about 32% and the gain was about 30 dB. We are currently testing a three-cavity tube that is designed to produce over 100 MW of power at 17.14 GHz with an efficiency of 40%. The input cavity is the same TE₀₁₁ cavity used in the first harmonic experiment, but the buncher and output cavities are TE₀₂₁ cavities that operate at twice the cyclotron frequency. In this paper we describe the design of the circuit and detail the hot test results. Finally, we also describe the designs of tubes that are capable of higher gain and higher rep-rate operation