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Electrothermal (ET) plasma discharges have application to mass acceleration technologies relevant to military ballistics and magnetic confinement fusion reactor operation. ET plasma discharges are initiated in capillary geometries by passing large currents (order of tens of kiloamperes) along the capillary axis. A partially ionized plasma then forms and radiates heat to the capillary walls inducing ablation. Ablated particles enter the capillary plasma source and cause a pressure surge that can propel pellets to velocities exceeding 2 km/s. These devices present several advantages over other mass accelerator technologies due to their simple design and ability to achieve high projectile launch frequencies. In order to investigate the operation of ET plasma discharges in more detail than previously possible, a 2-D, multifluid model has been developed to simulate the plasma-fluid dynamics that develop in these devices during operation. In this paper, the 2-D simulation model is used to investigate the effect of source geometry and current density on discharge characteristics. Peak pressure and electric field magnitudes for pulsed discharge operation are shown to scale well with theoretical and empirical scaling laws for steady-state discharge operation. The pulse shape of the source internal pressure is shown to change significantly with increasing source radius. The behavior of other plasma parameters is investigated. In addition, observations of the departure from the ablation-controlled arc regime are presented. This analysis suggests that, for the current pulse length investigated, source radii higher than 4 mm require significantly more current density in order to produce sufficient ablation to stabilize the plasma discharge. [ABSTRACT FROM AUTHOR]