A plasma–microwave system was proposed by one of the authors for the production of microwave radiation. The device exploits the electromagnetic instabilities of nonequilibrium plasmas. During single‐mode operation of the plasma maser, the resonant particles become trapped and cease to radiate before a substantial fraction of the energy available could be extracted from the plasma in the form of radiation. (This is not true for multimode operation. During multimode operation, the total energy available is distributed over a large number of resonant lines and the amplitudes of the various frequency components of the waves are not at all large enough to cause particle trapping and subsequently saturation. Thus, the linear theory remains valid until the anisotropy disappears due to radiation.) One should, therefore, consider the interaction of the bulk plasma with the nonresonant monochromatic electromagnetic wave in order to estimate the total energy transferred from the plasma to the wave. In the framework of the linear treatment, due to the resonant nature of the instability considered in previous work by one of the authors, the transfer of energy from the nonresonant plasma to the wave is zero. Consequently, the efficiency of the device during single‐mode operation can only be evaluated after considering the nonlinear evolution of the plasma–microwave system. In this paper we describe in some details the nonlinear evolution of the plasma–wave system using a single particle analysis. The current densityJis calculated using the second moment of the particle distribution function (evaluated using Liouville’s theorem of conservation of phase space density along particle orbits), and the energy balance equation is solved numerically.