Non fluid features in z pinch implosions

Summary form only given, as follows. For given driving conditions, a Z pinch implosion can exhibit non fluid features when the preimploded plasma density is below a certain critical value. Below this value, the ion mean free path is comparable to or greater than the plasma size and the ion self collision time is comparable to or greater than the implosion time. As the strength of the drive increases, the post shock conditions are more energetic and this critical density increases. This situation is studied with a "hybrid" simulation code in which the ions are treated as a collisionless species modeled by the Vlasov equation, while the electrons are treated as a charge neutralizing fluid to eliminate the timescales corresponding to the high frequency electron kinetics, so that simulations of macroscopic timescales can reasonably be achieved. The collisionless imploding shock pushes ion beams in front of it, altering the shock dynamics, the stability properties, and the expected experimental profiles of the physical parameters when compared with an MHD simulation with the identical initial conditions and driving conditions. These differences will be quantified and compared with experimental measurements. In particular, enhanced stability is found in the "hybrid" simulations compared to the MHD simulations having identical initial conditions and driving conditions. Although the interface between the plasma and magnetic field develops a wavelike structure, the reflected ion beams hit the axis and travel to the side opposite the region where they were created before the plasma instability becomes so strong that the magnetic field penetrates to the center. In this way the time at which the disruption of the Z pinch occurs is increased significantly. The presence of the reflected ion beams during the implosion also alters the density profiles, creating a "foot" in the profile ahead of the shock.