1 N ov 2 00 4 The magnetized discharge with dust : negative and positive space charge modes

The structure of a discharge across a magnetic field in a dusty plasma is analysed. The dust macroparticles are negatively charged, but are unmagnetized because of their high mass. The electrons are highly magnetized, and the ions have intermediate magnetization. This results in different transport rates of the different species across the magnetic field. Depending on the size of the magnetic field, and the relative charge on the different species, the dust grains can be the dominant current carrier. The space charge clouds near the electrodes will then be determined by the relative mobility of the different species. The discharge can operate in one of two modes, a positive space charge (PSC) mode, characterized by a strong cathode fall, and a negative space charge (NSC) mode, characterized by a broad anode fall. Features unique to the dust particles can also play a role in the structure of the discharge, such as the variable equilibrium charge on the grains, dependent on the local potential and species temperatures, the effect of gravity on the grain dynamics, and the rate of charging of the grains. The dust grains can also form an ordered structure, the dust-plasma crystal. A fluid model of the different species is used to calculate the structure of the resulting discharge, incorporating the above effects. The transition from the PSC mode to the NSC mode as the magnetic field, pressure and dust properties are varied is demonstrated.