In this paper we present an experimental study of the transmission of a layer of small particles of carbon and magnesium oxide moving with a speed of 1200 m/s under the action of a high-power beam from a cw HF laser. Radiant exitance of laser reached 20 MW/m2 within the beam cross section of 25 x 50 mm. In our experiment we have observed the effect of clearing up of such a medium at enhanced radiant flux density of the beam. We have also determined the value of radiant flux density threshold at which the clearing up is of stationary behavior. In this study we managed to reveal the dependence of the volume extinction coefficient of such a medium on the radiant intensity and mass density of the particles flow. The data obtained in this study show that the mechanism of clearing up of such a medium differs from earlier known models. The latter result demonstrates the necessity of studying this process in more detail.