PERFORMANCE OF A UNIPOLAR SQUARE-WAVE DIFFUSION CHARGER WITH VARIABLE NT-PRODUCT

A recently developed unipolar diffusion charger is described in detail and is investigated. A square wave voltage across the charging zone guarantees minimized electrostatic particle losses. Subsequently, ultrafine particles can be efficiently charged employing high ion concentrations. An analytical description of the spatial distribution of ions in the charging zone illustrates the strong influence of the space charge field in the charging zone. The distribution of the charging parameter nt (product of the spatial ion concentration and the residence time of particles in the charging region) is shown. This calculated charging parameter is described by an averaged single ntBAR. An increasing discrepancy between the experimental and theoretical charging efficiency size relation is observed by varying the voltage operation parameter in the charging area. The experimental efficiency size relation is fitted to theory, leading to an effective charging parameter nt(ef). An extrinsic charging efficiency E(ex), which accounts for particle losses, is defined. E(ex) is shown to exhibit a maximum with respect to nt(ef). The maximum E(ex) is high in comparison with conventional charger designs, especially for particles in the ultrafine size range.