Theory of intermodulation and harmonic generation in traveling-wave masers

Under high power conditions, a nonlinear response in the spin system of a maser is predicted by an analysis based on the density matrix formalism or the Bloch equations of motion. The coupling of the nonlinearities to the traveling-wave maser circuit is considered and the measurable power output at intermodulation and second harmonic frequencies is computed. For a typical traveling-wave maser with ruby maser material and a comb slow-wave structure, the computation yields P(2ω1) = 2P(ω1) - 103.5 and P(2ω1- ω2) = 2P(ω1) + P(ω2) - 92 where, for example, ω1= 2π×4140 Mc, ω2= 2π×4110 Mc and the respective power levels are measured in dbm. These levels are well below noise under most conceivable operating conditions for masers used as low-noise receiver preamplifiers and, in this sense, the maser is a "better linear amplifier" than others. The theoretical predictions are in satisfactory agreement with experimental results.