Measurement of the shot-noise reduction factor

A new method of measurement of the shot-noise reduction factor at microwave frequencies is described. In this method, the reduction factor is measured by using a traveling-wave phototube with oxide-coated cathode, on which an amplitude-modulated laser light is projected. The photodetected outputs, with and without the space-charge cloud near the cathode, are compared. The shot-noise reduction factor is calculated from the ratio of both outputs. Measuring frequencies (1320 and 2640 MHz), beam current density, cathode temperature, and beam-forming-electrode voltage are varied in this measurement. The shot-noise reduction factor tends to small values at lower frequency, and goes to unity at higher frequency than the plasma frequency at the potential minimum. The experimental results agree in general with the theoretical calculations done by Whinnery, Tien, and Moshman. The physical meaning of these results is interpreted as a plasma oscillation at the potential minimum. Also, the relation between the "Currie gun" and the shot-noise reduction is discussed.