This book consists of three major components: gas kinetic studies of vacuum chamber facility effects on the background flows; theoretical studies on collision-less free plume flow out of different exits. These analytical solutions are validated with numerical methods; and simulations of plasma and neutral plume flows from a cluster of Electric Propulsion (EP) Hall thrusters into low pressure background with the direct simulation Monte Carlo (DSMC) and Particle-In-Cell (PIC) methods. The last part adopts two electron models are used, the Boltzmann relation and a detailed electron model. A a...