Ion collisions and the Z-pinch precursor column

During the early stages of a wire array Z-pinch implosion, low density plasma streams toward the axis by virtue of the Lorentz force. This streaming precursor plasma may initially be highly collisionless with respect to ion–ion collisions and therefore cannot be modeled using standard fluid theory. The hybrid method in this paper models both collisional and collisionless behavior with ions exchanging energy and momentum with other ions via a Monte Carlo algorithm equivalent to a small-angle kinetic solution and with an electron fluid via a frictional force. It is shown that the axial stagnation of the plasma flow occurs once the density becomes sufficiently high to initiate a nonlinear rise in electron–ion energy exchange, resulting in the thermal equilibration between radiatively cooling electrons and hot, thermalized ions. This then gives rise to a dense, long-lived precursor column on axis, as observed experimentally. The column is held in place by the kinetic pressure of the streaming precursor plasma, which is balanced by the thermal pressure of the plasma in the column at the column’s edge.