EFFECT OF MASS FLUCTUATION ON COLLECTIVE MODES IN A DUSTY PLASMA

Abstract The grains immersed in a plasma get eroded by the ions falling onto them. As a consequence, they exhibit self-consistent mass fluctuations due to perturbations in the plasma charging currents. The grain mass is then a dynamical variable. The modifications in the plasma dielectric properties are investigated and new ultra-low frequency modes, are shown to exist in the plasma. Introduction Due to the dynamical character of the dust grain electric charge, dusty plasmas that contain dust grains of high charge and mass, exhibit new and interesting aspects with respect to already known negative-ion plasmas. Indeed, dust grains when immersed in a plasma acquire their charge via numerous and competing processes, such as photoelectric effect, thermo-ionic effect, secondary emission and plasma particles capture by the grains. As a matter of fact, the electrons are captured by the grain, whereas the ions exchange charge with it, and hence the mass of the grain do not increase appreciably. Any wave motion in the plasma induces particle density perturbations, which in turn affect self-consistently the particle currents falling onto the grains. Consequently, as a response, the grain charge would fluctuate, giving rise to many collective effects 1,2 . So far, the grain mass has been considered constant. Although, in realistic environments, e.g., astrophysical situations, the grain mass is variable. Indeed, the grain mass may grow due to particle accretion or grain-grain agglomeration. The grain may be eroded as well, under the effect of numerous processes such as, sputtering that is ejection of surface atoms of the grain due to striking ions, chemical sputtering, grain-grain collisions and grain disruption 3 . The grain mass is then a dynamical variable coupled to other variables such as particle density. The effect of grain sputtering on self-gravitating dusty plasmas dielectric properties has been investigated, and it has been shown that grain mass fluctuation is at the root of a new instability 4 . In this note we propose to investigate the effect of self-consistent grain mass fluctuations that are a response to perturbations in the plasma charging currents on wave propagation along with instability processes. New modes in the ultra-low frequency range are found. We consider a dusty plasma consisting of electrons, ions and dust grains that acquire their charge by electrons and ions attachment. During this process, dust grains are being eroded by striking ions. The governing equations are cast as follows