Measurements of intrinsic shot noise in a 35 GHz gyroklystron

Experimental measurements of electron beam shot noise in a 35 GHz, 225 kW, three-cavity gyroklystron have been obtained from both the input and output cavities. This intrinsic noise was studied in the absence of an applied ca rrier (i.e., at zero drive power). The spectrum of the noise emitted by the input cavity is found to have a Lorentzian shape, with peak noise power de nsities from the input cavity typically reaching 6.3x10(-15) W/Hz (-112 dBm /Hz), and typical 3 dB bandwidths of 160 MHz. The output cavity noise spect rum is found to be equal to the input cavity noise spectrum multiplied by t he measured linear frequency response of the gyroklystron. The measured noi se levels at the input cavity are 0-5 dB lower than theoretical predictions for shot noise unaltered by collective effects. Furthermore, the input cav ity noise power exhibits complex variations as a function of beam current, beam velocity ratio, and circuit magnetic field that are not predicted by p resent theory. Noise-to-carrier ratios expected in the input cavity during full power amplifier operation are inferred from the noise measurements and known values of drive power required to saturate the gyroklystron. The noi se-to-carrier ratio, with typical values of -90 to -80 dBc, is found to be a strong function of the operating parameters. (C) 1999 American Institute of Physics. [S1070-664X(99)04207-X].