Studies of electronic noise in gyroklystrons

Direct measurements of phase noise in a four-cavity, 35 GHz gyroklystron producing 50 mu s pulses of 175-210 kW output power with 50-53 dB saturated gain are presented. The measurements were performed at a 10.7 MHz frequency offset from the carrier, where the noise is expected to be dominated by shot noise and where the extrinsic noise from the electron gun's pulsed power supply is manageable. At an operating point with 70 kV beam voltage, 9 A beam current, and a beam velocity ratio of 1.3, a phase noise of -149 +/- 1 dBc/Hz was measured during the production of 180 kW output power at 50 dB gain. At a higher beam current of 10 A, the measured phase noise was -146 +/- 1 dBc/Hz during production of a 200 kW output power carrier with 53 dB gain. The directly measured phase noise levels were generally within 2-3 dB of the values expected on the basis of carrier-free noise temperature measurements. Overall, the measured gyroklystron noise levels are similar to those of conventional klystrons. Also presented are analytic calculations of the growth rates expected for electrostatic cyclotron instabilities in the region between the electron gun and the input cavity in a previous three-cavity gyroklystron. Very modest perpendicular velocity spreads (rms greater than 2%) are found to completely suppress such noise growth. This lack of significant noise growth above that of bare shot noise is in agreement with experimental results. (C) 2000 American Institute of Physics. [S1070-664X(00)90405-1].