A semi-analytic fluid model has been developed for characterizing relativistic electron emission across a warm diode gap. Here we demonstrate the use of this model in (i) verifying multi-fluid codes in modeling compressible relativistic electron flows (the EMPIRE-Fluid code is used as an example; see also Ref. 1), (ii) elucidating key physics mechanisms characterizing the influence of compressibility and relativistic injection speed of the electron flow, and (iii) characterizing the regimes over which a fluid model recovers physically reasonable solutions.
This report describes the high-level accomplishments from the Plasma Science and Engineering Grand Challenge LDRD at Sandia National Laboratories. The Laboratory has a need to demonstrate predictive capabilities to model plasma phenomena in order to rapidly accelerate engineering development in several mission areas. The purpose of this Grand Challenge LDRD was to advance the fundamental models, methods, and algorithms along with supporting electrode science foundation to enable a revolutionary shift towards predictive plasma engineering design principles. This project integrated the SNL knowledge base in computer science, plasma physics, materials science, applied mathematics, and relevant application engineering to establish new cross-laboratory collaborations on these topics. As an initial exemplar, this project focused efforts on improving multi-scale modeling capabilities that are utilized to predict the electrical power delivery on large-scale pulsed power accelerators. Specifically, this LDRD was structured into three primary research thrusts that, when integrated, enable complex simulations of these devices: (1) the exploration of multi-scale models describing the desorption of contaminants from pulsed power electrodes, (2) the development of improved algorithms and code technologies to treat the multi-physics phenomena required to predict device performance, and (3) the creation of a rigorous verification and validation infrastructure to evaluate the codes and models across a range of challenge problems. These components were integrated into initial demonstrations of the largest simulations of multi-level vacuum power flow completed to-date, executed on the leading HPC computing machines available in the NNSA complex today. These preliminary studies indicate relevant pulsed power engineering design simulations can now be completed in (of order) several days, a significant improvement over pre-LDRD levels of performance.
Extended-MHD simulations of power flow along a pulsed-power transmission line are performed here in a 2-D axisymmetric geometry, in particular looking at the influence of Hall physics for a transmission line coupled to the liner used in a magnetized liner inertial fusion experiment at Sandia National Labs. It was recently shown by the authors that, for a coaxial transmission line, when Hall physics is included, significantly more blow-off occurs from plasma initialized against the anode compared to the cathode. The mechanism of this blow-off was traced to electron Ex B drift modeled by the Hall term. This result is also observed for the present simulations, and it is shown that the anode blow-off significantly delays the coupling of current to the liner. It is also found that Hall MHD and MHD results are sensitive to the treatment of density floors and the plasma-vacuum interface. Although MHD shows more sensitivity than Hall MHD, correct modeling of the transition from plasma to vacuum remains an unsolved problem that must be addressed in order to improve the predictive capability of fluid-based power flow simulations with regard to energy coupling.