Computer simulations of low noise states in a high-power crossed-field amplifier

A large body of experimental data has been accumulated over the past 15 years or so on the remarkable ability of both magnetrons and CFA's to operate under certain conditions at noise levels comparable to those achieved in linear beam tubes. SAIC's MASK code, a 2 1/2 -D particle-in-cell code, has been able to compute total, integrated noise power to an accuracy of /spl plusmn/ a few dB in a high-power CFA, operating with a typical intra-pulse spectral noise density of /spl sim/47-50 dB/MHz. Under conditions that produced low noise (/spl sim/60-100 dB/MHz) in laboratory experiments, the MASK code has been, until now, unable to reproduce similar results. The present paper reports the first successful production of a very low noise state in a CFA simulation using the MASK code. The onset of this low noise state is quite sudden, appearing abruptly as the current is raised to a point near which the cathode operates as nearly emission limited. This behavior is similar to that seen in an experimentally observed transition between low noise and high noise operation in the SFD-266, a Varian [CPI] low noise CFA. Some comments are made concerning the nature of the noise as observed in the simulation and in the laboratory.