AbstractThe chaotic relaxation mechanism of a monoenergetic electron beam in a strongly dissipative plasma is investigated in the framework of the single wave theory. An information theoretical quantity is introduced that describes the information loss of the beam system in course of time. It is shown that the numerical calculation of a discrete macroparticle approximation of the continuous beam system loses its physical relevance if the information loss exceeds a critical value which is determined by the number of macroparticles. Moreover, a method is proposed in order to quantify the degree of chaotic behaviour of the beam‐plasma system with a minimum of additional numerical effort.