Studying Effects of Non-Equilibrium Radiative Transfer Via HPC
Abstract
A goal across many physics and engineering disciplines is to accurately simulate physical systems with radiation-hydrodynamics codes in which the assumption of local thermodynamic equilibrium (LTE) is predicted to be violated, such as laser driven hohlraums, stellar coronae, and supernova ejecta. In such regimes, the computational costs to capture relevant physics have been observed to be 3 orders of magnitude higher over the LTE problem. This thesis discusses innovations in non-LTE (NLTE) calculations by leveraging high performance computing (HPC), load balancing, and algorithmic improvements to solve problems that were insoluble with existing techniques. We also present results demonstrating where NLTE is most important in simulation results. In particular, this capability was applied to 3 demonstration problems: a supernova (SN), a 1D problem meant to capture dynamics present in inertial confinement fusion (ICF) hohlraums, and a modified version of the blastwave diagnostic experiment performed at Sandia National Laboratory’s (SNL) Z machine.
SN modeling was accomplished with a two stage process, first a radiation hydrodynamics (RH) simulation to model explosive dynamics followed by an offline post-processing stage for calculating quantities analogous to those measured by observers. The use of NLTE physics enabled by this work were important in both stages of this modeling, ultimately resulting qualitatively different results between LTE and NLTE.
The 1D hohlraum problem analysis indicates that the quantity of interest (QOI), the analogue of laser entrance hole (LEH) closing time, was mildly sensitive to the use of NLTE physics, but relatively insensitive to atomic model complexity, indicating some detail is needed in the treatment of auto-ionizing states. For non-integrated quantities like the radiation spectral energy density, some modest differences were observed.
The Z machine blastwave diagnostic resulted in comparisons between LTE and NLTE, yielding some differences associated with the early dynamics of simulation, particularly with the hohlraum. More detailed comparisons corroborate the importance of NLTE modeling near hohlraum walls, but also support the idea that LTE is a reasonable assumption at late times, demonstrating that work done by experimental designers to avoid NLTE effects was successful.
Citation
Holladay, Daniel Alphin (2018). Studying Effects of Non-Equilibrium Radiative Transfer Via HPC. Doctoral dissertation, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /173367.