Charging properties of dust grain clusters

The dust grains or their clusters can be frequently found in many space environments - interstellar clouds, tails of comets or planetary rings are only typical examples. Being surrounded by plasma, dust grains are charged by various processes as photoemission, collection of electrons and/or ions, secondary or field emissions. The determination of a charge, which can the grain or cluster of grains gain by these processes, is very important for an estimation of the dusty plasma properties because electrostatic forces highly dominate the gravitation. However, our knowledge of charging processes is inferred from measurements on large planar samples and thus it can be applied on micron-sized grains only with a care and their application on the clusters is under question. Our experimental set-up allows us to trap a single dust grain or cluster of grains in an electrodynamic quadrupole and to expose it by the electron and/or ion beams. Precise measurements of a stepwise change of charge-to-mass ratio induced by the elementary charge allow us to determine the charge and mass of the investigated grain. We have investigated SiO2 spherical grains as well as their clusters. A systematic study reveals that the dependence of the charge acquired by an investigated object (grain or cluster of grains) by the particle beam of given energy depends not only on grain macroscopic characteristics but on the configuration of the grains in a particular cluster