Electron shot noise in gyroklystrons

Abstract

Summary form only given. Expressions for shot noise in a gyroklystron are calculated. The essential calculation is that of the noise in the input cavity, because this is the noise amplified by the entire amplification circuit. If the noise power is expressed in terms of a noise temperature, it is shown that this noise temperature in the input scales roughly as the transverse beam energy. A convenient parameter turns out to be N, the number of electrons passing into the cavity in a time of Q//spl omega/. This is typically about 10/sup 10/ In terms of N, the signal to noise power at the output is given roughly by S/N (dB)/spl ap/10log/sub 10/N-G where G is the amplifier gain in dB. However since the noise is distributed over the entire band and the signal may have a much narrower frequency width, the effective signal to noise power may be larger. Collective effects on the beam may also have a large effect on the noise power. Generally, in linear beam devices, the dielectric shielding of the bare electrons typically greatly reduces the shot noise. However for gyroklystrons, where the beam is not in thermal equilibrium in any reference frame, electrostatic instabilities may greatly increase the shot noise. This amplification depends critically on the magnetic field profile. For one particular profile, we find that the shot noise in the input cavity may increase by 20-40 dB, as the beam current is raised from 2 to 8 amps.