This paper is aimed at studying the efficiency of the electromagnetic radiation generation in various nonlinear processes occurring during the beamplasma interaction. The beam and plasma parameters were chosen close to the parameters in the experiment on the GOL-3 facility (BINP SB RAS). The research was conducted by means of the numerical model based on the particles-in-cell (PIC) method. The model of collisionless plasma is described by the system of the Vlasov-Maxwell equations, boundary conditions being periodic. Various scenarios of the nonlinear evolution in a beam-plasma system with a beam of low density were studied. It is shown how these scenarios change under the in uence of an external magnetic field. There is a transfer of energy from one unstable mode to another.