Modeling low-density regions in power flow experiments using MHD codes

Low-density regions are difficult to model in traditional MHD, single-fluid codes. First, the classical Spitzer resistivity is independent of density so that in extrapolating toward vacuum, the Ohmic current is carried by fewer charge carriers moving at unrealistic speeds. Additional physics such as the Hall term or lower-hybrid drift turbulence can lead to an effectively larger resistivity which enhances diffusion of the magnetic field. Traditionally, this harder-to-model physics is crudely approximated by the use of a density floor below which the resistivity is arbitrarily increased to some large value in an ad hoc approximation of the vacuum. Here, we demonstrate how differing treatments of the vacuum-to-plasma transition can lead to qualitatively different behavior in simulations. This is done in the context of a coaxial transmission line pulsed with 20 MAmps of current over 100 ns [1] to capture physics relevant to the magnetically insulated transmission lines (MITLs) on Sandia’s Z pulsed power facility.

[1] N. D. Hamlin and C. E. Seyler. Submitted to Phys. Plasmas (2018).